Magnetic resonance imaging of lower extremity injuries

Magnetic Resonance Imaging of Lower Extremity Injuries By David A. Rubin, Murray K. Dalinka, and J. Bruce Kneeland

T HE ACCURATE diagnosis and treatment of lower-extremity trauma requires a syn-

thesis of clinical and imaging data. Knowledge of the mechanism of injury coupled with a thorough physical examination is indispensable in predicting the likely abnormality and ex- pected sequela and in determining a tailored imaging approach to the patient. Conventional radiographs often suffice for diagnosis: however, at times, advanced imaging techniques are necessary to evaluate the extent of injury and determine the treatment options.

Magnetic resonance imaging (MRI) is a widely available, noninvasive technique ideally suited to this task. Its multiplanar capability and high- contrast rendition result in an unparalleled depiction of the osseous and soft-tissue structures. The exquisite sensitivity of MRI for abnormalities within cancellous bone allows detection of symptomatic, previously occult injuries. With the continued evolution of MRI hardware and software, the list of musculoskeletal applications grows daily.

This review will highlight the MRI appearance of commonly encountered lower extremity injuries. It will also address several technical considerations for successful imaging of lower- extremity trauma.

ABBREVIATIONS

ACE anterior cruciate ligament; AVN. avascular necrosis; CT, computed tomography; FOV, field-of- view; MRI, magnetic resonance imaging; PCL, posterior cruciate ligament; STIR, short-tau inversion recovery.

From the Department of Radiology, Hospital of the Univer- sity of Pennsylvania, Philadelphia, PA.

Address reprint requests to Murray K~ Dalinka, MD, Profes- Sor of Radiology, and Chief, Section of Musculoskeletal Radiology, Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce St, Philadelphia, PA 19104.

Copyright �9 1994 by W.B. Saunders Company 0037-198X/94/2902-000955.00/0

INJURIES TO THE HIP

Technique

MRI of the hip is routinely performed with the body coil, which enables use of a field-of- view (FOV) large enough to visualize both hips and the surrounding musculature. Our routine examination consists of a coronal Tl-weighted spin-echo pulse sequence supplemented by a T2-weighted axial sequence. We add a short-tau inversion recovery (STIR) image, usually in the coronal plane, to increase the sensitivity of the examination by enhancing subtle areas of edema. The STIR sequence uses an inversion pulse timed to null the signal from fat; prolonged T1 and T2 relaxation times both serve to increase the resultant signal. 1 A relatively large FOV and thick slices increase the relatively poor signal-to- noise in the STIR images but do not detract from the study because the goal of this sequence is to exploit differences in signal, not to provide high-resolution detail.

High-resolution images can be achieved with paired surface coils or a coil array, and in these cases the FOV is reduced to include only the involved hip. Occasionally imaging in the sagittal plane will better define abnormalities of the femoral head and articular surfaces of the hip.

Osseous Injuries

Fractures of the femoral neck. particularly in elderly patients, are associated with high morbidity and mortality. 2 In the presence of osteoporo- sis, conventional radiographs may be normal or equivocal in acute nondisplaced fractures. 3,4 In the past, bone scans have been used to diagnose suspected fractures in these patients; however, 20% of acute fractures are not visible on bone scans performed within the first 24 hours after injury and 5% to 10% may not be visible at 72 hours. 5,6 Patients with suspected occult fractures of the hip were often prophylactically hospitalized with bed rest until the diagnosis was established or disproved. This practice was costly and placed elderly patients at risk for hospital-acquired complications4; delay in treat-

194 Seminars in Roentgenology, Vol XXIX, No 2 (April), 1994: pp 194-222

MRI OF LOWER EXTREMITY INJURIES 195

ment of hip fractures also is associated with higher morbidity and mortality. 2,7 Finally, even when the bone scan is positive, additiOnal studies such as computed tomography (CT) were often required to delineate the exact location and orientation of the fracture. 3

Because of its sensitivity for changes in cancellous bone, MRI is an excellent study in the diagnosis of occult hip fractures. The normal proximal femur in adults contains mostly fatty marrow and appears bright on Tl-weighted

images, s Fractures are identified as well-defined linear or serpiginous bands of very low signal intensity on T1- and T2-weighted images (Fig 1). 3 The cause of the dark signal is uncertain, but it may represent condensation of fractured trabeculae along the fracture line2 The fracture line is usually surrounded by a more diffuse area of marrow edema, which has low signal on Tl-weighted images and high signal on T2- weighted images?

Stress fractures become visible on MRI be-

Fig 1 . Occult femoral neck fracture. (A) Plain film is normal. (B) The fracture line appears as a band of low signal intensity on Tl-weighted coronal image. (C) On T2-weighted image the low-signal fracture line (arrow) is easily distinguished from surrounding high-signal edema.

196 RUBIN, DALINKA, AND KNEELAND

fore plain films and appear similar to occult fractures; they may depict an area of juxtacorti- cal high signal on T2-weighted images, which is believed to represent subperiosteal hemorrhage or edema (Fig 2). 1~

MRI can detect fractures in the first 24 hours after injury, including those that are missed by CT polytomography, 3 or bone scan. 7 MRI also accurately characterizes the extent of the fracture, obviating the need for additional imaging. 3 Several investigators have found MRI to be 100% sensitive and specific in diagnosing these proximal femoral fractures in patients with normal or indeterminate radiographs. 3,4,7 A limited study can be performed using a single coronal Tl-weighted sequence; it can be com- pleted in approximately 7 rain at a cost compa- rable with three-phase bone scanning or CT. 4

MRI is also useful for demonstrating avascular necrosis (AVN), a sequela of hip fracture or dislocation. 11,12 The incidence of AVN is increased when the fracture is displaced or when there is a delay in reduction of a dislocation. 2 MRI is highly sensitive to the early changes of AVN. 13 The MRI appearance of femoral AVN is rather variable but appears most commonly as normal-appearing marrow fat in the anterior- superior femoral head surrounded by a curvilin- ear band of low signal. On T2-weighted images,

a parallel inner line of high signal is found within the low-signal area, which has been called the "double-line sign". 14 Attempts to use MRI to predict which fractures will go on to develop AVN unfortunately have been unsuc- cessful. 15 A recent preliminary report in patients with femoral neck fractures suggests that the presence of an impaired blood supply can be detected by a lack of the normal femoral head enhancement after intravenous gadolinium ad- ministration. 12 If this observation holds true, it may be possible to predict which patients will develop AVN and to alter the initial treatment accordingly.

Soft- Tissue Injuries

The soft tissues are best evaluated with T2- weighted or STIR images; they enhance perspi- cuity of edema, which has increased signal intensity. Trauma to the hip can occasionally tear the acetabular labrum, which is triangular in cross-section. In patients with labral tears, the normal shape of the labrum is disrupted and high signal can be detected within the disrupted structure3 6

Avulsion injuries are common in young athletes. Several powerful muscles have attachment sites around the hip and pelvis. Typical injuries include avulsions related to the attach-

Fig 2, Femoral neck stress fracture. T2-weighted coronal image. The subperiosteal high signal adjacent to the cortical disruption is characteristic (arrow). The plain film (not shown) was normal.


ments of the sartorius, rectus femoris, iliopsoas, hamstring, and gluteal and adductor muscle groups. 17 When a piece of bone is avulsed, the diagnosis can be made on conventional radiographs. If the injury occurs within the substance of the muscle or tendon (the myotendinous unit), however, plain radiographs are normal. In these cases, MRI can demonstrate changes in the muscle or tendon directly (Fig 3). Findings include diffusely increased intrasubstance signal with discrete foci of increased signal intensity on T2-weighted and STIR images, and partial or complete discontinuity of muscle or tendon fibers. High signal within the muscle on

Tl-weighted images represents hemorrhage. Edema in the subchondral bone underlying the site of avulsion may be present.

INJURIES TO THE KNEE

The rapid evolution of MRI technology has increased our sophistication in the diagnosis and evaluation of knee injuries. MRI can detect a wide spectrum of pathological conditions, many of which could previously be diagnosed at arthroscopy or open surgery. Some injuries, particularly those within cancellous bone, are occult even at surgery. 18A9 The ability to find such lesions has led to an increased understand-

Fig 3. Right hamstring avulsion in a waterskier. Conven- tional radiograph (not shown) was normal. (A) Coronal T1- weighted image shows normal attachment of left hamstring muscles to the ischium (curved white arrow). The tendinous attachment is absent on the right, replaced by high-signal hemorrhage (curved black arrow). Also note extensive intramuscular hematoma (straight black arrow). (B) The disrupted tendons are identified on adjacent image (ar- rowsl. (Courtesy of R. Herzog, Philadelphia, PA)


ing of the mechanisms of injury and, in some cases, has modified therapy. In the setting of suspected meniscal tear, MRI may be cost- effective by decreasing the number of diagnostic arthroscopies. 2~

Technique

MRI examination of the knee is performed with a local coil to provide the necessary resolution for accurate diagnosis. The easiest approach is the use of a cylindrical transmit/ receive coil. The patient lies supine with the knee extended and centered within the coil. The knee is immobilized in a comfortable position, which is usually 10 to 15 ~ of external rotation. This position is useful because it aligns the anterior cruciate ligament in a near sagittal plane with respect to the bore of the magnet. Typical imaging is performed with a 16-cm FOV and 3-mm thick slices with a 1-mm interslice gap. Thicker sections (4 or 5 mm) also can be used; these provide increased signal-to-noise but decreased spatial resolution due to increased partial-volume artifacts. We use a long repetition time (TR), dual-echo ("proton- density" and "T2-weighted") spin-echo sequence in the sagittal, oblique coronal, and transverse

plane in all cases. The coronal sequence is graphically prescribed so that the images are parallel to the long axis of the tibial plateau. We believe that the spin-echo sequences provide excellent resolution and contrast, and we do not use gradient-recalled sequences or fast spin- echo sequences in the knee. We also routinely obtain a sagittal STIR sequence that may detect subtle areas of edema in subchondral bone or the surrounding soft tissues, which are some- times easily missed on the spin-echo images.

Osseous and Osteochondral Injuries

Fractures about the knee may be subtle or simply not visible on conventional radiographs, particularly nondisplaced or minimally depressed fractures of the tibial plateau or supra- condylar area of the femur) 8,19 These injuries are easily demonstrated with MRI as linear or serpiginous bands of very low signal intensity extending to the cortical surface of the bone (Fig 4). 18 The fracture line is usually surrounded by an area of edema or hemorrhage in the cancellous bone, identified by its low signal on Tl-weighted images and high signal on T2- weighted or STIR images. 19

Injuries confined to the medullary bone have

Fig 4. Tibial plateau fracture. (A) The fracture is quite subtle on the conventional radiograph (arrow). (B) Tl-weighted coronal image shows true extent of fracture, which involves both medial and lateral plateaus.


received much attention. These injuries have a variety of appearances and can be observed as relatively well-defined or poorly defined areas of decreased signal intensity on T1- and proton- density-weighted images and high signal on T2-weighted or STIR images. 18,19 Their appearance and location may well affect management, although this is not yet proven. These lesions have been called occult intraosseous or subcor- tical fractures, 9,21 bone contusions, or bone bruises. The overlying cartilage is frequently normal but may be depressed or thinned. ~8 When the cartilage is intact, the intraosseous abnormalities cannot be detected at arthroscopy. 18,19 It is hypothesized that the changes in the medullary bone represent hemorrhage and edema that result from trabecular microfrac- tures. 19 In cases where follow-up MRI examinations have been performed, certain lesions have completely resolved by 3 months, 9,19 whereas lesions that were contiguous with the adjacent cortical bone have demonstrated thinning or absence of the overlying articular cartilage. 2j

Whereas bone contusions may occur as isolated events, there is a high association with ligamentous injuries,in the knee (see Figs 10 and 11), particularly tears of the anterior cruciate ligament (ACL) 18Ag,22-z6 and less commonly collateral ligament tears. ~8,~9 These contusions provide corroborative evidence of ligamentous injury, indicate the direction of instability at the time of injury, and suggest recent as opposed to remote trauma (see later section on cruciate ligament injuries). The presence of these bone contusions may explain why some patients continue to have pain on weight bearing? 9,22

Acute osteochondral fractures are a result of shearing or impaction forces. These injuries may involve only cartilage (chondral fractures) or include a piece of underlying cortical bone. 18 They typically occur on the articular surface of the patella after lateral patellar dislocation (Fig 5), 27 and in the anterior aspect of the lateral femoral condyle overlying the lateral menis- cusJ 8 The latter injuries have a high association with ACL tears. MRI can demonstrate defects in the hyaline cartilage as well as in the displaced chondral or osteochondral fragments. The T2-weighted images are best for demonstrating loose fragments, particularly when an effusion is present. In the absence of a displaced

Fig 5. Osteochondral fracture after reduction of a patellar dislocation, High-signal fluid (large curved arrow) is present within a defect in the low-signal cortical bone (small straight arrow) and replaces a portion of the intermediate signal cartilage (large straight arrow) on this T2-weighted sagittal image.

fragment, T2-weighted or STIR images may depict thinning of the articular cartilage or depression of the cartilage with the underlying cortical bone. With depressed osteochondral fractures, contusion of the subjacent cancellous bone is common. 18

Osteochondral lesions that are not the result of a single recognized traumatic episode fall under the rubric of osteochondritis dissecans. Although controversy exists, many authors favor a traumatic origin for these lesionsfl 7,28,29 In the knee, osteochondritis dissecans most commonly involves the femoral condyles, classically the lateral aspect of the medial condyle.~V

The MRI appearance of osteochondritis dissecans is similar to that of an acute osteochondral fracture, and often the two are distinguished only on the basis of clinical history. ~s Management depends on the stability of the osteochondral fragment: stable fragments are treated conservatively, and loose fragments re- quire surgery for removal or fixation. 28 On


Tl-weighted images, the lesion appears as an area of low-to-intermediate signal intensity adjacent to the articular surface, which is demar- cated from the underlying bone by a low-signal- intensity rim. 28 The most reliable sign of an unstable fragment is displacement from its na- tive position. 2s,29 T2-weighted images showing high signal at the interface between the lesion and parent bone also suggest a loose fragment (Fig 6), 29 although in our experience it is not specific unless there is high signal fluid seen beneath the base of the fragment extending up to the joint. The MRI depiction of disrupted cartilage over an osteochondral fragment has not correlated well with fndings at arthroscopy.28, 29

Meniscal Injuries

The ability to accurately and noninvasively demonstrate meniscal tears has popularized the use of MRI examination in the diagnosis of internal derangements of the knee. The reported sensitivity for meniscal tears ranges from 87% to 97% with specificities ranging from 91% to 95%. 30-33 The negative predictive value of MRI for meniscal tears is approximately 95%. 34 MRI examination is an effective screening test for meniscal tears and has been an effective method in decreasing unnecessary diagnostic arthroscopies.

The normal menisci are composed of fibrocar- tilage with few mobile protons accounting for their low signal intensity on all pulse sequences. 32 Cross-sectional images through the menisci demonstrate the anterior and posterior horns as separate triangular structures. The menisci appear bow-tie-shaped on cross-sectional images through the periphery. Intrameni- scal signal corresponding to histological mucoid or myxoid degeneration is common, especially in older patients. 35,36

There are two major MRI criteria for diagnosing meniscal tears: (1) abnormal signal extending to the meniscal surface and (2) abnormal morphology. The increased signal seen with meniscal tears is believed to represent adsorp- tion of water to exposed macromolecules at the edges of the torn meniscus. 32 Most meniscal tears do not contain free water, and their signal usually is less prominent on T2-weighted images. Thus T1- or proton-density-weighted images are critical for meniscal evaluation. 31 To confidently diagnose a meniscal tear, signal must be seen extending to one or both articular surfaces (Fig 7). 37'38 Cases in which the intra- meniscal signal equivocally contacts the surface almost always prove to be not torn at arthroscopy.39, 40

Although a meniscal tear may occasionally be seen only on one image, the chance that signal

Fig6. Osteochondritis dissecans. {A) Proton-density-and (B) T2-weighted coronal images demonstrate high signal (arrows) at the boundary between the fragment and underlying bone, a sign that has been purported to indicate a loose fragment.


Fig 7. Meniscal tear. On this proton-density-weighted sagittal image, linear high signal (arrow) extends to the inferior surface of the posterior horn of the medial meniscus.

contacting the meniscal surface represents a true tear increases if it is found on two or more contiguous images? ~ Depending on its orientation, a meniscal tear may only be visualized in one plane, 4~ so both sagittal and coronal series must be carefully examined in all cases. The coronal plane is also indispensable for examin- ing meniscal morphology. Blunting or amputation of the inner edge of the meniscus or irregularity in the meniscal contour are subtle but suggestive findings of a torn meniscus (Fig 8A). In one series, tears of the free edge--so- called parrot beak tears--accounted for 44% of false-negative errors. 3~

A circumferential vertical meniscal tear with inward displacement of the central fragment is called a bucket-handle tear. The coronal plane is ideal for demonstrating the displaced fragment and the foreshortened peripheral piece of meniscus (Fig 8B). 31 The displaced fragment often becomes interposed between the posterior cruciate ligament (PCL) and the medial tibial eminence; on sagittal images, the appearance has been likened to a second PCL (Fig 8C).41,42

MRI can also detect meniscal cysts. These are cystic collections that occur at the joint line and are invariably associated with meniscal tears.

They are seen on T2-weighted sequences as fluid-filled masses in contiguity with an underlying horizontal or complex meniscal tear. 43

Several pitfalls exist for the MRI diagnosis of meniscal tears. The majority occur when a normal low-signal anatomic structure lies in close proximity to one of the menisci, mimicking a displaced fragment. These pitfalls are recognized by tracing the course of the suspected fragment on adjacent images until its true na- ture becomes clear. The structures commonly involved include the meniscofemoral ligament and the popliteus tendon, which can mimic a tear of the posterior horn of the lateral meniscus (Fig 9), 44'45 the lateral inferior geniculate artery, which can simulate a tear of the anterior horn of the lateral meniscus, 46 and the transverse geniculate ligament, which may produce a false impression of a tear of either anterior horn. 44,46 Volume averaging of the normal concave surface at the outermost margin of the meniscus can produce a horizontal band of increased signal that should not be mistaken for a meniscal tear. 46 Occasionally a loose body, a thick ligament of Humphrey, or a torn ACL may lie within the intercondylar notch and mimic the "double PCL" sign, suggesting a bucket-handle tear. 4~ In these cases, identification of normal


meniscal morphology helps to avoid an interpre- tation error.

Cruciate Ligament Injuries

The normal ACL attaches to the posteromedial surface of the lateral femoral condvle and courses inferiorly, anteriorly, and medially to insert on the anterior tibial plateau, just anterior and lateral to the anterior tibial spine, 24 although the precise tibial insertion can vary. The ACL prevents anterior translation and internal rotation of the tibia and also limits hyperextension.25,47

The ACL is frequently injured in sports

Fig 8. Bucket-handle meniscal tears. (A) A coronal image shows amputation of the inner margin of the lateral meniscus (arrow). (B) Further posteriorly the displaced inner fragment of the lateral meniscus is identified (arrow). (C) In a different patient, the displaced fragment (straight arrow) from a bucket-handle medial meniscus tear lies beneath the PCL (curved arrow), giving the appearance of two PCLs,

injuries with skiing and football accounting for many ACL tears. 24 Most tears involve the mid- substance of the ligament. Occasionally the ligament is avulsed at its femoral or tibial insertion. 24 Several mechanisms can lead to disruption of the ACL. but the most common scenario involves internal rotation and abduc- tion in a flexed knee. often combined with a valgus stress. 25

MRI has very high accuracy for diagnosing ACL tears. Sensitivity and specificity of greater than 90% have been reported in multiple series. 25"31"33'34"48 On sagittal images, t h e normal ligament is a straight low-signal-intensity struc-


Fig 9. Meniscofemoral ligament mimicking a lateral meniscus tear, (A) A low-signal structure (arrow) adjacent to the posterior horn of the lateral meniscus might be mistaken for a displaced meniscal fragment, The structure could be traced on sequential images until (B) it became clear that it represented the ligament of Wrisberg (arrow). Note the appearance of the normal PCL and patellar tendon in this patient. The ACL is torn.

ture in the intercondylar notch running posterior to anterior as it extends from the femur to the tibia. Its course is parallel to the femoral intercondylar roof on an approximate 45 ~ slope. 25 Areas of increased signal may be seen within its substance, especially near the tibial insertion, in asymptomatic subjects. Slight external rotation of the knee usually allows the complete ligament to be imaged on a single sagittal slice. 24,31

The most direct sign of an ACL tear is focal discontinuity or fragmentation of its fibers. On T2-weighted images, increased signal representing edema is usually present at the site of acute tears (Figs 10 and 11). 49 Laxity of the ligament in the extended knee, an abnormal course or angulation, or an abrupt change in caliber also suggest a deficient ACL (Fig 11).25 Nonvisualiza- tion of the proximal ACL due to volume averaging with the lateral femoral condyle can be problematic on sagittal images. In these cases, demonstrating an intact ligament on the coronal images increases diagnostic accuracy and confi- dence (see Fig 14A). 24,4s The coronal and axial series also can confirm discontinuity of the ligament or detachment at its osseous insertions (see Fig 15B). 24,25

Occasionally the ACL is incompletely dis-

rupted. A partial tear can be suggested on MRI when disruption of only some of the ACL fibers is seen, but we have found this to be a difficult diagnosis. Diffusely increased signal throughout a thickened but continuous ACL may indicate interstitial tearing within an intact synovial sheath. Chronic tears of the ACL can at times appear deceptively normal. The ligament may heal by scarring to adjacent structures such as the PCL. 48,49 When the chronically torn ACL lies in an abnormal position, the diagnosis is obvious. However, in 30% of cases, the ligament may scar down in near anatomic position and mimic an intact normal ligament. 49

In patients with ACL tears and resultant instability, many secondary findings have been described. The presence of one or more of these indirect signs should prompt a critical analysis of the ACL; however, the diagnosis of ACL tear should be reserved for cases where findings are present in the ligament itself. A deficient ACL may allow the tibia to sublux anteriorly with respect to the femur. The anterior translation (anterior "drawer" sign) occasionally can be visualized directly on sagittal MRI through the midplane of the lateral femoral condyle, 5~ or indirectly as an "uncovered lateral meniscus"


Fig 10. ACL tear. (A} T2-weighted sagittal image depicting focal disruption of ACL fibers with high signal (arroW) in the defect. (B) There are bone contusions (arrows) in the anterior lateral femoral condyle and posterior lateral tibial plateau--so-called "kissing contusions," which are characteristic of acute ACL tears.

where the posterior horn of the lateral meniscus projects beyond the posterior-most cortex of the lateral tibial plateauY The translation also produces laxity of the PCL manifest by acute anguiation or buckling. 25,31,49

The most consistently observed secondary signs of ACL disruption are associated bony injuries. These occur at the time of the initial trauma, and they provide insight into the mechanism of injury. The most common force leading to an ACL tear produces internal rotation of the tibia, often with a valgus stress. 25 Once the ACL tears, anterolateral rotatory instability develops allowing the knee to torque around the axis of the medial collateral ligament. 22 As a result, the anterior lateral femoral condyle can impact against the posterolateral proximal tibia, producing contusions of one or both bones (Figs 10 and 11), Additionally, as the knee returns to its normal position, tension may increase sud- denly in the posterior joint capsule, a manifesta- tion of the pivot-shift phenomenon. 26 The capsule can avulse from the posterior lateral tibial plateau, again evident on MRI as edema in the subjacent cancellous bone. Lateral compartment contusions are observed in 44% to 94% of MRI examinations in the presence of acute ACL tears. 2225

Other osseous injuries can be seen in association with acute ACL tears. Trauma involving a varus strain can avulse the lateral tibial rim at the insertion of the deep portion of the lateral capsular ligament producing a Segond fracture. A third mechanism of ACL tear is forced hyperextension. In these cases, contusions may be visible in the anterior aspects of both the femoral condyles and the tibial plateau. 24 The presence of bony contusions also helps date the ACL injury. In one study, no ACL tear more than 9 months old demonstrated bony contusions. 25

The PCL is the fundamental stabilizer of the knee. 47 The normal PCL appears as an arcuate low-signal-intensity structure extending from the anterior intercondylar notch to the posterior tibial plateau. MRI examination consistently demonstrates the thick PCL on several adjacent sagittal images and has high sensitivity for complete PCL tears with a 100% negative predictive value. 51 Tears of the PCL are much less common than those of the ACL. PCL injuries are more devastating and are often associated with other internal derangements. Tears, which may be partial or complete, are demonstrated as areas of anatomic disruption of the ligament fibers (Fig 12). Occasionally an


Fig 11. ACL tear. (A) Sagittal T2- weighted image shows gap between proximal (curved arrow) and distal (straight arrow) portions of the torn ACL. Note the abnormal course of the distal ACL as it is no longer parallel to the intercondylar roof. {B) Tl-weighted and (C) STIR images show contusion (arrows) in femoral condyle.

avulsed fragment of the posterior tibia may be identified. 5~

Collateral Ligament Injuries

Injuries to the collateral ligaments can usually be diagnosed clinically; however, in the acute setting, physical examination may be limited owing to pain and swelling. Focal tenderness at the origin or insertion sites is a sign suggestive of collateral ligament trauma, but this finding can also be due to muscular injury, osseous infraction, or meniscal tear. Addition- ally, there is wide variation in the accuracy of the clinical examination for instability, even among experienced orthopedists, s2

The medial collateral ligament has a deep and superficial component. The deep layer, called the medial capsular ligament, represents the true joint capsule and is tightly attached to the medial meniscus. 53 The superficial component, the tibial collateral ligament, may be separated from the capsular ligament by a bursa. 54 The posterior third of the tibial collateral ligament is composed of oblique fibers, which become lax when the knee is flexed. The oblique fibers fuse with the capsular ligament and the semimembranosus tendon sheath at the posteromedial corner. 53 The middle third of the tibial collateral ligament consists of parallel fibers that can attach to the medial tibia as far as


Fig 12. Complete PCL tear. The sagittal image demonstrates a high-signal gap within the substance of the PCL (curved arrow). Compare with Figure 9B,

7 cm below the joint line. 54,55 These fibers remain taut in flexion and provide the primary restraint against valgus stress as well as external rotation of the tibia. Anteriorly, the fibers of the tibial collateral ligament fuse with the more superficial surrounding soft tissues and the medial patellar retinaculum. 55 The anterior fibers contribute little if anything to medial stability. 47

Ligament injuries or sprains are graded by physical examination, and treatment is dependent on grade of injury 55 as well as associated injuries, especially those of the ACL or menisci. A grade I ligamentous sprain is secondary to stretching or microscopic tearing of ligament fibers, which usually produces localized tenderness without instability. A grade II sprain represents a gross tear of some ligamentous fibers. These patients often have generalized tenderness and mild instability with an endpoint on physical examination. Complete disruption of all the fibers constitutes a grade III injury, usually accompanied by instability without an endpoint on physical examination. 47

The coronal plane is the most helpful for demonstrating the collateral ligaments of the knee. The deep portion of the medial collateral ligament (the medial capsular ligament) is less

important functionally than the superficial component (tibial collateral ligament). It is closely adherent to the medial meniscus, and it is attached to the femur and tibia by the meniscofemoral and meniscotibial ligaments. The tibial collateral ligament appears as a thin continuous low-signal-intensity structure extending from the medial femur to the medial tibia at a variable distance distal to the tibial plateau (see Fig 15B). Posteriorly there may be a small amount of fat or a bursa between it and the joint capsulel The anterior fibers of the tibial collateral ligament are separated from the medial meniscus, and this appearance should not be mistaken as evidence for a meniscocapsular separation. 55

Whereas no studies have specifically correlated the clinical grades of collateral ligament injuries with their MRI appearance, several general rules apply. Grade I injuries are manifest as edema in the peritendinous soft tissues, often with a thickened but intact tibial collateral ligament. Chronic or healed higher-grade injuries can have a similar appearance (Fig 13). 55 In these circumstances, calcification, which may have variable signal intensity, can be present in the ligament, which is the so-called Pellegrini- Stieda "disease." Higher-grade sprains will show


Fig 13. Chronic or grade I sprain of medial collateral ligament, The tibial collateral ligament is markedly thickened (arrow). Compare with Figure 15B.

focal discontinuity in the tibial collateral ligament, which may develop a wavy or irregular contour (Fig 14). 55 It may be difficult to distinguish between grade II and grade III injuries by MRI. Osseous contusions of the lateral joint compartment may be seen in conjunction with high-grade sprains because of the valgus force

usually responsible for the medial collateral ligament injury. 19

If the injury is isolated to the medial collateral ligament, surgical repair and conservative management have equal results) 6 However, concomitant tears of the ACL and/or menisci may change the treatment from conservative management to surgical repair, and these associated injuries should be carefully searched for on the MRI examination.

Muscular injury may produce clinical findings similar to medial collateral ligament tears. Avul- sion of the central tendon of the semimembranosus tendon may show bone marrow edema or cortical avulsion of the posteromedial corner of the tibial plateau. 57 Muscle strains will produce edema of the involved muscle that is visible as increased signal on T2-weighted or STIR images.

The supporting structures on the lateral aspect of the joint are more complex. The lateral collateral ligament complex is divided into thirds. Stabilizing the posterolateral corner is the posterior third or arcuate complex, which consists of the fibular collateral ligament, arcuate ligament, and aponeurosis of the popliteus tendon. 58,5~ Supplementing the ligaments are the lateral head of the gastrocnemius, biceps femoris, and popliteus muscles. 47 The iliotibial band

Fig 14, Grade III medial collateral ligament tear, T2-weighted coronal images through the {A) mid- and (B) posterior joint show disruption of the middle and posterior thirds of the tibial collateral ligament (straight arrows). The iliotibial band (small arrowheads) and fibular collateral ligament (large arrowheads) are well demonstrated. Also note the coronal appearance of the normal ACL (curved white arrow).


and tract function as the main lateral dynamic and static stabilizers, respectively, and together with the lateral capsular ligament, comprise the middle third. 59 Similar to the medial side, the anterior third of the lateral complex is composed of the anterior fibers of the capsular ligament and the lateral patellar retinaculum, which is a lateral extension of the quadriceps tendon. 59

Injuries to the lateral collateral ligament complex are less common but more disabling than those on the medial side. The lateral ligaments are under tension during standing and walking, when they are at or near maximal extension, s9 The force necessary to produce lateral ligament injury is usually greater than that required for medial injury, 55 which partially explains the high frequency of associated injuries accompanying lateral collateral ligament tears. The usual mechanism of injury is hyperextension with varus stress, frequently accompanied by a direct blow or rotation? 8 Occasionally a valgus force, after rupturing the medial collateral ligament, ACL, and PCL, will continue through the lateral ligament complex, producing a complete knee dislocation.

On the coronal images the longitudinal extent of the lateral structures are well depicted; they can also be traced on sequential transverse sections. The fibular collateral ligament is consistently demonstrated. It originates from the lateral epicondyle of the femur, above the groove for the popliteus tendon, and courses posteriorly, inferiorly, and laterally (see Fig 14B). It fuses with the biceps femoris muscle and tendon forming the conjoint tendon, which attaches to the fibular head. 55 The iliotibial band arises as a thickening of the fascia lata. Its posterior fibers are called the iliotibial tract, 59 and they appear as a vertically oriented, linear low-signal-intensity structure that inserts on Gerdy's tubercle in the anterolateral tibial cortex just below the tibial plateau (see Fig 14A). 55 A groove in the lateral femoral condyle gives rise to the popliteus tendon, which proceeds posteriorly, medially, and inferiorly into its muscle belly lying posterior to the proximal tibia. 55 The course of the popliteus muscle and tendon can be traced in its entirety on both the sagittal and axial images.

The majority of lateral collateral ligament tears occur at or near the distal attachments. 55

They can be graded in a fashion analogous to the medial injuries. Grade I sprains are depicted as thickened but intact ligaments. Grade II or III injuries are characterized by interrup- tion of some or all of the ligament fibers, which usually have an irregular contour or may be so distorted that they are unrecognizable (Fig 15). The conjoint tendon and iliotibial tract should be analyzed independently as either or both may be injured depending on the mechanism of trauma and the force involved. An avulsion fracture of the lateral tibia (the Segond fracture) represents an avulsion of the lateral capsular ligament from the tibia. 26 On MRI, edema occurs in the lateral tibia at the insertion site; occasionally the avulsed cortical fragment may be demonstrated. Segond fractures are commonly associated with ACL tears and anterolateral rotatory instability. 6~

As on the medial side, other injuries can present with lateral pain and tenderness. Poplit- eus muscle strains and tears are a common chronic injury in runners, 55 but they may also occur in conjunction with injuries producing acute posterolateral rotatory instability (Fig 16). 58 Injuries to the lateral head of the gastrocnemius muscle, biceps femoris muscle, and iliotibial band also have been reported with this scenario. 58 The traumatized muscles may increase in size with increased signal intensity on T1- and T2-weighted images representing a combination of hemorrhage and edema (Fig 16). Lateral pain may also be due to the iliotibial band friction syndrome, an overuse condition seen mainly in runners and cyclers. It is secondary to friction between the iliotibial band and the lateral femoral epicondyle. 55 On MRI, patients with this clinical syndrome may demonstrate interstitial edema deep to the iliotibial band. 6~

Patellofemoral and Extensor Mechanism Injuries

Complete disruption of the extensor mechanism of the knee is usually obvious clinically. It is an injury that occurs in frail elderly patients after relatively minor trauma, as well as in high-performance athletes who suffer more vio- lent injuries. The disruption can occur at any level from the quadriceps muscles and tendon through the patella and patellar tendon. Partial disruption may be a difficult diagnosis on clinical examination, requiring imaging for confirma-


Fig 15. Avulsion of conjoint tendon. T2-weighted coronal images. (A) The fibular collateral ligament (large arrowheads) is thickened and wavy. The inferior aspect of the conjoint tendon is seen separated from the fibular head. (B) On a slightly more anterior image, there is retraction of the biceps femoris muscle and tendon (straight arrow). The ACL is also torn at its femoral insertion where fluid occupies the usual position of the ACL (curved arrow). Note the normal tibial collateral ligament (small arrowheads).

tion. MRI provides excellent demonstration of the entire extensor mechanism and is an ideal modality for diagnosing pathology in this system.

The normal quadriceps tendon appears lami- nated on sagittal and axial MRI. 62 Complete tear of the tendon involves transection of all layers. Incomplete ruptures are confined to one


Fig 16. Acute muscle strain. There is high-signal edema within the popliteus muscle belly (curved white arrow) and at the fibular origin of the soleus muscle (curved black arrow), The ferromagnetic artifact in the anterior tibia is from prior ACL repair. Same patient as Figure 17B.

or more layer but spare at least one tendon layer (Fig 17A). MRI study aids in treatment planning for partial tears because a disrupted layer can be traced back to its muscle or muscles of origin. 62 A contact sports injury may produce a hematoma within one of the quadriceps muscles without gross disruption of the muscle fibers. MRI is sensitive to intramuscular hematomas and can show the extent of muscle abnormality with great precision. 63

Patellar tendon rupture is a straightforward diagnosis by MRI. The normally thin, straight low-signal tendon appears thickened and wavy, often containing inhomogeneous increased signal. 64 Focal discontinuity of the tendon is seen on both sagittal (Fig 17B) and axial images. Although in rare cases a normal tendon can tear after severe trauma, rupture commonly occurs as an end stage of patellar tendinitis. Patellar "tendinitis" occurs in athletes whose knees undergo repeated sudden extension. "Tendi- nitis" in this situation is probably a misnomer because the condition really represents chronic microscopic partial tears along with mucoid degeneration and fibrinoid necrosis; inflamma- tion occurs during healing. 64 On MRI, the abnormal tendon is thickened with an anterior-

posterior dimension measuring greater than 7 mm on axial images. The thickened segment is usually proximal. There is increased intrasubstance signal on both T1- and T2-weighted sequences, and the margins of the tendon may be indistinct. 64 "Jumpers knee" refers to a spectrum of patellar tendon and extensor mechanism abnormalities that result from chronic repetitive stress64; in addition to changes in the proximal patellar tendon, there may be fragmentation or edema in the inferior pole of the patella (Fig 18).

Lateral dislocation of the patella is usually an athletic injury resulting from a valgus stress combined with flexion and external rotation. 27 The injury produces predictable findings in the patella and its supporting structures. In nearly all cases, there is disruption or sprain in the medial patellar retinaculum due to stretching of this structure as the patella subluxes laterally. 27.65 The injured retinaculum is recognized on axial images as a thickened or discontinuous structure that may contain abnormally increased signal intensity (Fig 19). Contusions of the anterior lateral femoral condyle commonly occur as the patella reduces and impacts against the condyle. The same impaction may cause an osteochondral fracture of the medial facet of the patella (see Fig 5). A careful search of the T2-weighted axial images should be made to detect osteochondral fragments loose in the joint. In cases where the dislocation is transient and not recognized by the patient, the secondary findings in the bone and retinaculum provide an important clue to the diagnosis.

INJURIES OF THE FOOT AND ANKLE

Technique High-resolution MRI of the ankle requires

the use of a local coil: we use a cylindrical transmit/receive extremity coil and a 12- to 14-cm FOV. The patient ties supine with the ankle immobilized in neutral position (sole of foot perpendicular to the table). Other investigators have advocated examination of the ankle in full plantar flexion and dorsiflexion to study specific ankle ligaments. 66 Although reposition- ing the ankle is occasionally useful to clarify equivocal findings, we have not found it routinely necessarv. Furthermore, excessive plantar flexion may cause artifactual buckling and


Fig 17. Rupture of the extensor mechanism. (A) Sagittal T2-weighted image shows partial quadriceps tendon rupture with disruption and retraction of part of the tendon (curved arrow), but two layers are still visible in the intact part of the tendon (straight arrow). (B) In another patient, there is gross discontinuity of the patellar tendon (arrows). Com- pare with Figure 10A.


Fig 18. Jumpers" knee. In addition to chronic thickening of the patellar tendon, there is focal edema (arrows) in the inferior pole of the patella seen on the sagittal Tl-weighted sequence.

thickening in the posterior ankle structures, particularly the Achilles tendon. A combination of Tl-weighted and dual-echo long TR spin- echo sequences are obtained in the three planes orthogonal to the tibiotalar joint. This protocol

requires graphic prescription of the sagittal and coronal series from the axial data set. We also supplement the examination with a set of STIR images in the sagittal plane.

The forefoot can be studied with the patient supine or prone. We perform supine examinations with the extremity coil; for prone studies the foot is placed in maximum plantar flexion and a local surface coil is positioned over the area of interest. 67,68 We have also developed a cylindrical forefoot coil to allow a smaller FOV and higher anatomic resolution. Tl-weighted, dual-echo proton-density/T2-weighted, and STIR images in orthogonal planes constitute our routine protocol for the forefoot.

Osseous and Osteochondral Injuries

As is the case for the hip and knee, MRI easily demonstrates occult fractures and stress fractures in the foot and ankle. 69 The latter occur commonly in the calcaneus, 67 tarsal navicular, v~ and metatarsal bones (Fig 20). In smaller bones, the medullary edema often involves the entire width of the bone.

Osteochondral fractures in the ankle usually affect the talar dome. These injuries are a common finding in patients who have continued chronic disability after injuries initially attrib- uted to simple ankle sprains; in these circumstances MR! can detect osteochondral injuries

Fig 19. Lateral patellar dislocation. (A) Axial image shows thickened torn medial patellar retinaculum (straight arrow). (B) There is a focal hematoma within the vastus medialis muscle on this Tl-weighted image (curved arrow). The patella has relocated but appears to be tilted laterally.


Fig 20. Metatarsal stress fracture. Plain film at the time of MRI examination showed no abnormality. (A) Tl-weighted image demonstrates edema within the medullary space of the third metatarsal bone (arrow) and some increased signal in surrounding soft tissues. (B} Conventional film several weeks later shows callus formation around healing stress fracture (arrow),

that are not seen on conventional radiographs or nuclear medicine studies. 71

Treatment of talar dome osteochondral injuries varies depending on whether the fragment is attached or loose. A partially attached fragment may demonstrate a thin irregular line of high signal intensity on T2-weighted images at the interface between the talus and the fracture fragment. This finding likely represents loose granulation tissue. 72 A completely detached osteochondral fragment is diagnosed either when fluid entirely surrounds it or when the fragment is visualized distant from its parent site. The signal within the fragment is not a helpful finding: both attached and loose fragments may show low, intermediate, or high signal on T2- weighted images.

MRI can also identify cartilage defects overlying talar dome osteochondral infractions. Al- though this finding is often associated with loose fragments, it can be seen with healed ones. 72 The cartilage is best evaluated on coronal images (Fig 21). The normal cartilage demonstrates a smooth contour with intermediate signal intensity on both T1- and T2-weighted images. Focal thinning, absence, or disruption

of the cartilage, as well as increased signal within it, indicate pathology. 73

Ligamentous Injuries Ligamentous sprains constitute the most com-

monly encountered ankle injuries. Three liga- m e n t s - t h e anterior talofibular, calcaneofibular, and posterior talofibular--provide the majority of lateral ankle support. The anterior talofibular ligament courses horizontally from the anterior aspect of the distal fibula to the lateral talar neck. 74 It appears as a homoge- neously dark structure in 100% of normal subjects on axial images at the level of the malleolar fossa of the fibula (see Fig 24). 74-77

The calcaneofibular ligament is the strongest of the three lateral ligaments. It extends from the tip of the lateral malleolus to a small tubercle on the lateral aspect of the calcaneus, crossing both the ankle joint and the posterior subtalar joint. It lies deep to the peroneal tendons. 66,74 Because of its vertical orientation, the calcaneofibular ligament is difficult to identify on axial images; it may be seen on coronal images or axial images when the foot is in plantar flexion. 66,74-76


Fig 21. Talar dome osteochondral lesion. (A) Tl-weighted sagittal and (B) T2-weighted coronal images. Overlying the osteochondral fragment there is a focal defect in the articular cartilage (arrows), which is replaced by high-signal fluid on the T2-weighted image.

The posterior talofibular ligament runs horizontally, extending from (he distal aspect of the malleolar fossa on the fibula to a lateral tubercle on the posterior talus. 74 The ligament separates the ankle from the subtalar joint. 66 On MRI, the ligament may demonstrate inhomogeneous signal intensity. 74,77 Due to its slight obliq- uity, volume averaging may cause the superior edge of the posterior talofibular tendon to appear frayed; this irregularity is a normal appearance that is seen in asymptomatie subjects. 7s

The deltoid ligament provides medial support for the ankle. Its shape is triangular with its apex on the medial distal tibia above the medial malleolus and its broad base attached to the talus, navicular, and calcaneus and to the spring (plantar calcaneonavicular) ligament. Tibiona- vicular, tibiospring, and tibiocalcaneal components make up a superficial layer with the anterior and posterior tibiotalar ligaments con- stituting a deep layer. 66 The tibiotalar, tibiospring, and tibiocalcaneal components are seen on coronal MRI; the tibionavicular ligament is not routinely visualized but may be demonstrated on axial views with the foot positioned in full plantar ftexion. 66,76 Like the posterior talofibular ligament, the deep deltoid ligament normally has a striated mixed signal intensity on

M R I . 77,78 Very little has been written concern- ing the MRI appearance of medial ankle sprains.

Inversion injuries produce lateral sprains. Ankle sprains can be graded using the same criteria described for the collateral ligaments of the knee. Complete tears may involve one or more ligaments. The anterior talofibular ligament is most commonly torn. 74,79 Tears of the calcaneofibular ligament are usually accompanied by anterior talofibular ligament tears, and the posterior talofibular ligament almost never ruptures without concomitant injury to the other two lateral ligaments. It may be difficult clinically to distinguish isolated single ligament injuries from those involving multiple ligaments, s~ Peroneal tenography followed by ankle arthrography has been successful in differentiating isolated anterior talofibular ligament tears from combined anterior talofibular and calcaneofibular ligament tears. 81 However, the technique of tenography is invasive, and it cannot diagnose tears of the posterior talofibular ligament or other associated bone or soft-tissue injuries.

MRI examination of the ankle can diagnose sprains of single or multiple ligaments 8~ and can show unsuspected additional injuries. Grade III ligament sprains demonstrate focal disruption on MRI (Fig 22). With lesser degrees of injury,


Fig 22. Torn anterior talofibular ligament. The axial T2- weighted image shows disruption of the ligament (arrows) with surrounding edema. Compare with Figure 24. Note the normal size difference between the posterior tibial tendon (large arrowhead) and the flexor digitorum Iongus tendon (small arrowhead).

the injured ligament may appear attenuated, thickened or lax; longitudinal splits may be identified within the substance of the ligament. There may be peritendinous edema present or an effusion in an adjacent joint or tendon sheath when the injury is acute. 74,8~ When a ligament is completely disrupted, one should exercise care not to misinterpret normal nearby structures for the absent ligament. For example, the anterior tibiofibular ligament runs an oblique course parallel to but above the anterior talofibular ligament, for which it may be mistaken. This pitfall can be avoided by noting the configuration of the fibula: the anterior tibiofibular ligament attaches above the malleolar fossa of the fibula, and it is seen on axial images where the medial border of the fibula is convex. In contra- distinction, the anterior talofibular ligament is present on images when the medial fibular border is concave. 74

Although MRI can accurately diagnose

sprains of the lateral ligaments, its role in this capacity is limited in the setting of acute trauma. Because long-term outcome is the same for conservative treatment as it is for primary surgical repair of torn lateral ligaments, surgery is rarely indicated for acute lateral ankle sprains and precise anatomic delineation of the injury is not important. 79 MRI likely does have a function in patients with persistent symptoms after a lateral sprain where the study can demonstrate such sequelae as osteochondral fractures, 7~ peroneal tendon abnormalities, 82 and findings suggestive of the sinus tarsi syndrome. 75,83

Tendon Injuries

The tendons that cross the ankle are subject to a large amount of chronic repetitive trauma. The commonly injured tendons include the Achilles tendon posteriorly, the posterior tibial tendon medially, and the peroneus longus and brevis tendons laterally.

Normal tendons contain few mobile protons and should have inherently low MRI signal intensity. 77 In chronic tendon degeneration, an increase in intratendinous signal intensity on T1- and proton-density-weighted images may be seen, 84 but many tendons in asymptomatic subjects also show increased signal. 85 One of the causes of increased signal intensity is the "magic angle" phenomenon. T2 relaxation times of tendons is strongly dependent on the orientation of the collagen fibers to the main magnetic field. When the angle of the tendon in the field approaches 55~ so-called "magic a n g l e " - increased signal appears with relatively short echo times (< 20 to 25 msec). The signal becomes less prominent with the echo time used for T2 weighting. The lateral (peroneal) tendons are most dramatically affected as they curve around the lateral malleolus, but the effect may also occur in the tendons medial to the ankle (flexor hallucis longus, flexor digitorum longus, and posterior tibial tendons) or anterior to the ankle (extensor hallucis longus and extensor digitorum longus tendons). 86 The Achilles tendon orientation is such that it is not subject to this phenomenon. It has also been suggested that some of the foci of increased signal represent subclinical degeneration within the tendon, as discussed next.

Chronically traumatized tendons demon-


strate a spectrum of pathological findings that correlate to their appearance on MRI studies. Normal tendons are composed of bundles of dense collagen fibers. With overuse, microscopic tears develop within the tendon substance, which subsequently may undergo partial healing. 85 The eventual result of cyclical injury and incomplete repair is intratendinous degeneration, s4 which is perhaps best referred to as "tendinosis," although the term "tendinitis" is so well entrenched that it has become an ac- cepted expression for this condition. 87,8s Eventu- ally, macroscopic partial or complete tears may develop in the weakened, degenerated tendon. 84,89,9~ It is rare for a complete tendon rupture to occur in a previously healthy tendon.

Caution is required in the MRI diagnosis of tendon abnormalities, particularly if the only finding is abnormal signal intensity within the tendon. The true hallmark of chronic tendon degeneration or partial tearing is tendon thickening, 84,89 with or without intratendinous signal. In patients in whom there is a question of increased tendon signal, one can repeat the sequence with increased plantar flexion of the foot. The change in tendon orientation within the static magnetic field will eliminate any increased signal caused by the magic angle phenomenon and will not affect true tendon signal increases. 77,86

The commonly used surgical grading system for tendon ruptures classifies mild partial tears with increased tendon girth and slight intratendinous degeneration as type I. Type II tendons are partially tears where a segment of the diseased tendon is stretched and attenuated. Surgical type III abnormalities are complete tendon tears. On MRI, type I abnormalities are characterized by enlarged tendons containing increased signal intensity with or without longitudinal fissures within the tendon. Type II tendons are attenuated and may contain increased intratendinous signal. In patients with type III rupture, a gap within the tendon seen on T2-weighted images is often associated with tendon retraction. 91

The MRI evaluation is not only accurate at staging these abnormalities, but in one study, the preoperative MRI findings were more accurate than surgical staging in predicting failure of reconstructed posterior tibial tendons because

MRI was better at depicting intratendinous degeneration. 9: This ability is also the reason that MRI is more sensitive than CT, which often underestimates the degree of tendon injury. 91 Tenography, an invasive technique, cannot demonstrate intratendinous pathology and corre- lates poorly with surgical findings. 84,93 Ultra- sound may demonstrate abnormalities within the substance of an injured tendon; however, it is very operator dependent, can only be applied to superficial tendons like the Achilles, and is limited in its depiction of anatomic detail. 69,84,94

Specific Tendon Abnormalities. The Achilles tendon is the common tendon of the gastrocnemius and soleus muscles, forming in the calf and inserting on the posterior calcaneus. Achilles tendinitis is a common overuse syndrome in joggers. 95 Partial and complete tears are seen in "weekend athletes" who stress the tendon inter- mittently 89 and in trained athletes in sports that involve constant plantar flexion and pushing off from the toes. 67

Rupture can occur at the calcaneal insertion, at the myotendinous junction, or in the muscle bellies themselves, but there is a predilection for tears to occur 4 to 5 cm above the tendon insertion, in an area of relative hypovascularity, where the majority of degeneration takes place. 95 Sagittal images demonstrate the entire length of the Achilles tendon and depict focal areas of thickening and increased signal in patients with chronic tendon degeneration. On axial images, the anterior margin of the tendon loses its usual flat or concave shape. 89 T2-weighted sagittal sequences are the most helpful in Achilles tendon tear. These images show irregular linear areas of increased signal intensity in partial tears (Fig 23) and complete discontinuity of all fibers in complete tears. 96 Surrounding soft- tissue edema and hemorrhage are often present acutely. 97 Rerupture of the Achilles tendon is a complication of both conservative and surgical treatment, and MRI can be used to follow-up healing Achilles tendon tears. 69

The posterior tibial muscle originates from the posterior margins of the tibia, fibula, and interosseous membrane. Its tendon passes posterior to the medial malleolus and deep to the flexor retinaculum. As it turns towards the foot, it forms multiple tendon slips that insert on the navicular, cuboid, cuneiform, and metacarpal


increased girth of the posterior tibial tendon are seen in partial and complete tears, but they are not specific, s5 The most reliable sign of partial tendon rupture is a longitudinal split within the tendon, best seen on T2-weighted images (Fig 24). In patients with complete rupture, the tendon itself is disrupted and may be retracted proximally. 91 Indirect findings of posterior tibial tendon tears also have been described. On sagittal images, the long axis of the talus may fail to bisect the navicular bone; this abnormal relationship reflects the altered biomechanics that result from a torn tendon. An accessory navicular bone or anomalous prominence of the medial navicular (identified as asymmetry of the bone on axial images) also have a high association with tendon tears. 85 These two signs likely represent preexisting conditions that predispose to tendon rupture.

Pitfalls in the evaluation of posterior tibial tendon abnormalities include increased intratendinous signal seen secondary to the magic angle

Fig 23. Partial Achilles tendon tear. The tendon (arrow) is markedly thickened with a convex anterior border. A high- signal rent within it on this T2-weighted image does not involve all fibers.

bones. There is a zone of relative hypovascularity beginning 1 to 1.5 cm distal to the medial malleolus and extending approximately 1.5 cm beyond, 92 which may explain the common site of rupture near the navicular insertion. 85

Posterior tibial tendon tears are often the result of repetitive microtrauma leading to tendon degeneration, tendinitis, and eventually tendon rupture. This tendon helps support the lateral arch of the foot, and tears frequently present as acquired, progressive, painful flat- foot deformities, most commonly in elderly womenY Partial or complete tendon tears can also result from acute inversion injury. 87

The normal posterior tibial tendon is demonstrated on both axial and sagittal images, s7 On axial images, it usually appears twice as large as the adjacent flexor digitorum longus tendon. Comparison of the size of the two tendons can be used to detect abnormalities. 91

A small amount of fluid can be present in the tendon sheath with both intact and torn tendons. 78,85 Increased intratendinous signal and

Fig 24. Partial posterior tibial tendon tear. The tendon (curved arrow) is swollen. (Compare with the adjacent flexor digitorum Iongus tendon and with Figure 22.) The high-signal tear within its substance was oriented longitudinally on the sagittal images. Note normal anterior talofibular ligament (arrowheads).


Fig 25. Compartment syndrome after an ankle fracture. (A) Axial Tl-weighted image shows high-signal (arrows) hemorrhage within the muscles of the anterior ankle compartment as well as bulging of the contour of the compartment. (B) T2-weighted image demonstrates extensive intramuscular edema largely confined to the anterior compartment. Anterior subcutaneous edema also is present,

effect, the normal heterogeneity in the tendon's signal intensity near its navicular insertion, % and the division of the tendon into separate slips before its insertions. 7v Tracing the distal branches of the tendon to their respective insertion sites on sequential images will avoid mistaking the normal anatomic appearance for

a tendon tear. Thin sections and three-dimensional-volume-acquired acquisitions with multiplanar reformatting also may be helpful in equivocal cases by reducing the effects of partial volume averaging. 9~ In patients with chronic tears and tendon retraction, the low-signal cortex of the medial malleolus can at times be


misinterpreted as a normal tendon when the tendon is in fact absent. 91 A sesamoid bone within the tendon should not be confused with an area of focal discontinuity. 77

Injuries to the other medial tendons are unusual. Flexor hallucis longus tendinitis and rupture due to constant plantar flexion 67,87 have been reported in ballet dancers. The tendon usually ruptures just posterior to the talus, s7 These tears can be detected with MRI using the same criteria as those for posterior tibial tendon injuries. 97

Peroneal tendon rupture is rare but is recognized with increasing frequency. 67,82 Longitudi- nal ruptures can be a cause of persistent swelling, popping, and retrofibular pain after lateral sprain injuries. With plantar flexion and inversion, the peroneus longus tendon impinges on the tip of the fibula and the peroneus brevis tendon can impinge against the lateral wall of the peroneus groove in the fibula or against the adjacent peroneus longus tendon. This mechanism may explain the incidence of peroneus strains and tears after inversion injuries. 82 The MRI diagnosis of these injuries can be made using the criteria previously outlined for the posterior tibial tendon. 67 Axial and sagittal images are the most helpful for identifying the peroneal tendons. 76

Subluxation or dislocation of the peroneal tendons is a more common injury than tendon tears. 87 The injury is often associated with calcaneal or lateral malleolar fractures 84 and can mimic a lateral ankle sprain clinically. 67 Forced dorsiflexion and inversion of the foot ruptures the superior peroneal retinaculum, allowing the tendons t o slide out from their usual position posterior to the lateral malleolus. This injury is seen most frequently in dancers and skiers. 87 A shallow or absent peroneal notch in the posterior fibula, present as a variant in

20% of the population, may predispose to this injury. 67 The diagnosis is made on MRI by noting the abnormal position of the tendons and the disruption of the peroneal retinaculum. 67

MUSCLE INJURIES

Intramuscular injury in the thigh or calf can result from direct or indirect trauma and can be confused clinically with joint injury. MRI has been very useful for demonstrating the presence and extent of intramuscular hematomas (see Fig 19B)63; the signal characteristics of an intramuscular hematoma vary with the age of the injury. 98 A muscle contusion is manifest by interstitial edema within the muscle without disruption of fibers. The interstitial edema is evident on T2-weighted or STIR images as high signal intensity within the muscles. A contusion is differentiated from an acute muscle strain by the fact that the former is caused by a direct blow, whereas the latter results from a stretch injury; however, on MRI, the appearance of a muscle contusion is similar to an acute muscle strain (see Fig 16). 98

A particularly dangerous type of soft-tissue injury occurs with intramuscular swelling in a deep fascial compartment. In this situation, increased interstitial pressure within a closed space can lead to rapid neurovascular compro- mise--a surgical emergency called a compartment syndrome. 98 MRI can directly demonstrate edema and/or hemorrhage in the affected muscles as well as the extent and precise location of compartmental swelling (Fig 25).

In conclusion, the high soft-tissue contrast and excellent sensitivity of MRI have made it an invaluable imaging modality in patients with lower-extremity trauma. MRI can depict abnormalities that had been previously occult and can confirm clinically suspected but unproven diag- noses.

REFERENCES

1. Bydder GM, Young IR: MR imaging: Clinical use of the inversion recovery sequence. J Comput Assist Tomogr 9:659-675, 1985

2. Barnes R, Brown JT, Garden RS, et al: Subcapital fractures of the femur: A prospective review. J Bone Joint Surg [Br] 58:2-24, 1976

3. Deutsch AL, Mink JH, Waxman AD: Occult fractures of the proximal femur: MR imaging. Radiology 170:113-116, 1989

4. Quinn SF, McCarthy JL: Prospective evaluation of

patients with suspected hip fracture and indeterminate radiographs: Use of Tl-weighted MR images. Radiology 187:469-471, 1993

5. Holder LE, Schwartz C, Wernicke PG, et al: Radionu- clide bone imaging in the early detection of fractures if the proximal femur (hip): Multifactorial analysis. Radiology 174:509-515, 1990

6. Martin P: The appearance of bone scans following fractures, including immediate and long-term studies. J Nucl Med 20:1227-1231, 1979


7. Rizzo PF, Gould ES, Lyden JP, et al: Diagnosis of occult fractures around the hip: Magnetic resonance com- pared with bone-scanning. J Bone Joint Surg [Am] 75:395- 401, 1993

8. Volger JB, Murphy WA: Bone marrow imaging. Radi- ology 168:679-693, 1988

9. Yao L, Lee JK: Occult intraosseous fracture: Detec- tion with MR imaging. Radiology 167:749-751, 1988

10. Lee JK, Yao L: Stress fractures: MR imaging. Radiol- ogy 169:217-220, 1988

11. Jacobs B: Epidemiology of traumatic and nontrau- matic osteonecrosis. Clin Orthop 130:51-67, 1978

12. Lang P, Mauz M, Sch6rner W, et al: Acute fracture of the femoral neck: Assessment of femoral head perfusion with gadopentetate dimeglumine-enhanced MR imaging. AJR 160:335-341, 1993

13. Mitchell MD, Kundel HL, Sieinberg ME, et al: Avascular necrosis of th e hip: Comparison of MR, CT, and scintigraphy. AJR 147:67-71, 1986

14. Mitchell DG, Rao VM, Dalinka MK, et al: Femoral head avascular necrosis: Correlation of MR imaging, radiographic staging, radionuclide imaging, and clinical findings. Radiology 162:709-715, 1987

15. Speer KP, Spritzer CE, Harrelson JM, et al: Magnetic resonance imaging of the femoral head after acute intracap- sular fracture of the femoral neck. J Bone Joint Surg [Am] 72:98-103, 1990

16. Ensign MF: Magnetic resonance imaging of hip disorders. Semin US, CT, MR 11:288-306, 1990

17. Resnick D, Goergen TG, Niwayama G: Physical trauma, in Resnick D (ed): Bone and Joint Imaging. Philadelphia, PA, Saunders, 1989, pp 801-898

18. Mink JH, Deutsch AL: Occult cartilage and bone injuries of the knee: Detection, classification, and assessment with MR imaging. Radiology 170:823-829, 1989

19. Lynch TCP, Crues JV Jr, Morgan FW, et ak Bone abnormalities of the knee: Prevalence and significance at MR. imaging. Radiology 171:761-766, 1989

20. Rowe PA, Wright J, Randall RL, et al: Can MR imaging effectively replace diagnostic arthroscopy? Radiol- ogy 183:335-339, 1992

21: Vellet AD, Marks PH, Fowler PJ, et al: Occult posttraumatic osteochondral lesions of the knee: Preva- lence, classification, and short-term sequelae evaluated with MR imaging. Radiology 178:271-276, 1991

22. Murphy BJ, Smith RL, Uribe JW, et al: Bone signal abnormalities in the posterolateral tibia and lateral femoral condyle in complete tears of the anterior cruciate ligament: A specific sign? Radiology 182:221-224, 1992

23. Kaplan PA, Walker CW, Kilcoyne RF, et al: Occult fracture patterns of the knee associated with anterior cruciate ligament tears: Assessment with MR imaging, Radiology 183:835-838, 1992

24. Remer EM, Fitzgerald SW, Friedman H, et al: Anterior cruciate ligament injury: MR imaging and patterns of injury. RadioGraphics 12:901-915, 1992

25. Tung GA, Davis LM, Wiggins ME, et al: Tears of the anterior cruciate ligament: Primary and secondary signs at MR imaging. Radiology 188:661-667, 1993

26. Stallenberg B~ Gevenois PA, Sintzoff SA Jr, et al: Fracture of the posterior aspect of the lateral tibial plateau:

Radiographic sign of anterior cruciate ligament tear. Radi- ology 187:821-825, 1993

27. Kirsch MD, Fitzgerald SW, Friedman H, et al: Transient lateral patellar dislocation: Diagnosis with MR imaging. AJR 161:109-113, 1993

28. Mesgarzadeh M, Sapega AA, Bonakdarpour A, et al: Osteochondritis dissecans: Analysis of mechanical stability with radiography, scintigraphy, and MR imaging. Radiology 165:775-780, 1987

29. De Smet AA, Fisher DR, Graf BK, et al: Osteochon- drifts dissecans of the knee: Value of MR imaging in determining lesion stability and the presence of articular cartilage defects. AJR 155:549-553, 1990

30. Crues JV Jr, Mink J, Levy TL, et al: Meniscal tears Of the knee: Accuracy of MR imaging. Radiology 164:445~448, 1987

31. Mink JH, Levy T, Crues JV: Tears of the anterior cruciate ligament and menisci of the knee: MR imaging evaluation. Radiology 167:769-774, 1988

32. Crues JV Jr, Ryu R, Morgan FW: Meniscal pathology: The expanding role of magnetic resonance imaging. Clin Orthop 252:80-87, 1990

33. Heron CW, Calvert PT: Three-dimensional gradient- echo MR imaging of the knee: Comparison with arthroscopy in 100 patients. Radiology i83:839-844, 1992

34. Boeve BF, Davidson RA, Staab EV Jr: Magnetic resonance imaging in the evaluation of knee injuries. South Med J 84:1123-1127, 1991

35. Stoller DW, Martin C, Crues JV Jr, et al: Meniscal tears: Pathologic correlation with MR imaging. Radiology 163:731-735

36. Hajek PC, Gylys-Morin VM, Baker LL, et al: The high signal intensity meniscus of the knee: Magnetic resonance evaluation and in vivo correlation. Invest Radiol 22:883-890, 1987

37. Dillon EH, Pope CF, Jokl P, et al: Follow-up of grade 2 meniscal abnormalities in the stable knee. Radiology 191:849-852, 1991

38. Hodler J, Haghighi P, Pathria MN, et al: Meniscal changes in the elderly: Correlation of MR imaging and histologic findings. Radiology 184:221-225, 1992

39. Kaplan PA, Nelson NL, Garvin KL, et al: MR Of the knee: The significance of high signal that does not clearly extend to the surface. AJR 156:333-336~ 1991

40. De Smet AA, Norris MA, Yandow DR, et al: MR diagnosis of rneniscal tears of the knee: Importance of high

signal in the meniscus that extends to the surface. AJR 161~101-107, 1993

41. Singson RD, Feldman F, Staron R, et al: MR imaging of displaced bucket-handle tear of the medial meniscus. AJR 156:121-124, 1991

42. Weiss KL, Morehouse HT, Levy IM: Sagittal MR images of the knee: A low-signal band parallel to the posterior cruciate ligament caused by a displaced bucket- handle tear. AJR 156:117-119, 1991

43. Burk DL Jr, Dalinka MK, Kanal E, et al: Meniscal and ganglion cysts of the knee: MR evaluation. AJR 150:331-336, 1988

44. Watanabe AT, Carter BC, Teitelbaum GP, et al: Normal variations in MR imaging of the knee: Appearance and frequency. AJR 153:341-344, 1989


45. Vahey TN, Bennett HT, Arrington LE, et al: MR imaging of the knee: Pseudotear of the lateral meniscus caused by the meniscofemoral ligament. AJR 154:1237- 1239, 1990

46, Herman LJ, Beltran J: Pitfalls in MR imaging of the knee. Radiology 167:775-781, 1988

47. Hughston JC, Andrews JR, Cross M J, et al: Classifica- tion of knee ligament instabilities: Part I. The medial compartment and cruciate ligaments. J Bone Joint Surg [Am] 58:159-172, 1976

48. Fitzgerald SW, Remer EM, Friedman H, et al: MR evaluation of the anterior cruciate ligament: Value of supplementing sagittal images with coronal and axial images. AJR 160:1233-1237, 1993

49. Vahey TN, Broome DR, Kayes K, et al: Acute and chronic tears of the anterior cruciate ligament: Differential features at MR imaging. Radiology 181:251-253, 1991

50. Vahey TN, Hunt JE, Shelbourne KD: Anterior trans- location of the tibia at MR imaging: A secondary sign of anterior cruciate ligament tear. Radiology 187:817-819, 1993

51. Grover JS, Bassett LW, Gross ML, et al: Posterior cruciate ligament: MR imaging. Radiology 174:527-530, 1990

52. Noyes FR, Cummings JF, Grood ES, et al: The diagnosis of knee motion limits, subluxations, and ligament injury. Am J Sports Med 19:163-171, 1991

53. Warren LF, Marshall JL: The supporting structures and layers on the medial side of the knee: An anatomic analysis. J Bone Joint Surg [Am] 61:56-62, 1979

54. Brantigan OC, Voshell AF: The tibial collateral ligament: Its function, its bursae, and its relation to the medial meniscus. J Bone Joint Surg 25:121-131, 1943

55. Mink JH: The cruciate and collateral ligaments, in Mink JH, Reicher MA, Crues JV III, et al (eds): MRI of the Knee (ed 2). New York, NY, Raven Press, 1993, pp 170-179

56. Indelicato PA: Non-operative treatment of complete tears of the medial collateral ligament of the knee. J Bone Joint Surg [Am] 65:323-329

57. Yao L, Lee JK: Avulsion of the posteromedial tibial plateau by the semimembranosus tendon: Diagnosis with MR imaging. Radiology 172:513-514, 1989

58. Baker CL, Norwood LA, Hughston JC: Acute posterolateral rotatory instability of the knee. J Bone Joint Surg [Am] 65:614-618, 1983

59. Hughston JC, Andrews JA, Cross M J, et al: Classifica- tion of knee ligament instabilities: Part II. The lateral compartment. J Bone Joint Surg [Am] 58:173-179, 1976

60. Weber WN, Neumann CH, Barakos JA, et al: Lateral tibial rim (Segond) fractures: MR imaging characteristics. Radiology 180:731-734, 1991

61. Murphy BJ, Hechtman KS, Uribe JW, et al: Iliotibial band friction syndrome: MR imaging findings. Radiology 185:569-571, 1992

62. Zeiss J, Saddemi SR, Ebraheim NA: MR imaging of the quadriceps tendon: Normal layered configuration and its importance in cases of tendon rupture. AJR 159:1031- 1034, 1992

63. Dooms JC, Fisher MR, Hricak H, et al: MR imaging of intramuscular hemorrhage. J Comput Assist Tomogr 9:908-913, 1985

64. EI-Khoury GY, Wira RL, Berbaum KS, et al: MR imaging of patellar tendinitis. Radiology 184:849-854, 1992

65. Virolainen H, Visuri T, Kuusela T: Acute dislocation of the patella: MR findings. Radiology 189:243-246, 1993

66. Schneck CD, Mesgarzadeh M, Bonakdarpour A, et al: MR imaging of the most commonly injured ankle ligaments: Part I. Normal anatomy. Radiology 184:499-506, 1992

67. Berquist TH: Magnetic resonance imaging of the foot and ankle. Semin Ultrasound, CT, MR 11:327-345, 1990

68. Erickson S J, Rosengarten JL: MR imaging of the forefoot: Normal anatomic findings. A JR 160:565-571, 1993

69. Mitchell MJ, Sartoris DJ, Resnick D: The foot and ankle. Top Magn Resort Imag 1:57-73, 1989

70. Kiss ZS, Khan KM, Fuller P J: Stress fractures of the tarsal navicular bone: CT findings in 55 cases. AJR 160:111- 115, 1993

71. Anderson IF, Crichton KJ, Grattan-Smith T, et al: Osteochondral fractures of the dome of the talus. J Bone Joint Surg [Am] 71:1143-1152, 1989

72. De Smet AA, Fisher DR, Burnstein MI, et al: Value of MR imaging in staging Osteochondral lesions of the talus (osteochondritis dissecans): Results in 14 patients. AJR 154:555-558, 1990

73. Yulish BS, Mulopulos GP, Goodfellow DB, et al: MR imaging of osteochondral lesions of talus. J Comput Assist Tomogr 11:296-301, 1987

74. Erickson SJ, Smith JW, Ruiz ME, et al: MR imaging of the lateral collateral ligament of the ankle. AJR 156:131- 136, 1991

75. Beltran J, Munchow AM, Khabiri H, et al: Ligaments of the lateral aspect of the ankle and sinus tarsi: An MR imaging study. Radiology 177:455-458, 1990

76. Kier R, Dietz M J, McCarthy SM, et al: MR imaging of the normal ligaments and tendons of the ankle. J Comput Assist Tomogr 15:477-482, 1991

77. Link SC, Erickson S J, Timins ME: MR imaging of the ankle and foot: Normal structures and anatomic variants that may simulate disease. AJR 161:607-612, 1993

78. Noto AM, Cheung Y, Rosenberg ZS, et al: MR imaging of the ankle: Normal variants. Radiology 170:121- 124, 1989

79. Cass JR, Morrey BF, Katoh Y, et al: Ankle instability: Comparison of primary repair and delayed reconstruction after long-term follow-up study. Clin Orthop 198:110-117, 1985

80. Schneck CD, Mesgarzadeh M, Bonakdarpour A: MR imaging of the most commonly injured ankle ligaments: Part II. Ligament injuries. Radiology 184:507-512, 1992

81. Bleichrodt RP, Kingma LM, Binnendijk B, et al: Injuries of the lateral ankle ligaments: Classification with tenography and arthrography. Radiology 173:347-349, 1989

82. Bassett FH, Speer KP: Longitudinal rupture of the peroneal tendons. Am J Sports Med 21:354-357, 1993

83. Klein MA, Spreitzer AM: MR imaging of the tarsal sinus and canal: Normal anatomy, pathologic findings, and features of the sinus tarsi syndrome. Radiology 186:233-240, 1993

84. Cheung Y, Rosenberg ZS, Magee T, et al: Normal anatomy and pathologic conditions of ankle tendons: Cur- rent imaging techniques. RadioGraphics 12:429-444, 1992


85. Schweitzer ME, Caccese R, Karasick D, et al: Poste- rior tibial tendon tears: Utility of secondary signs for MR imaging diagnosis. Radiology 188:655-659, 1993

86. Erickson S J, Cox IH, Hyde JS, et al: Effect of tendon orientation on MR imaging signal intensity: A manifesta- tion of the "magic angle" phenomenon. Radiology 181:389- 392, 1991

87. Trevino S, Baumhauer JF: Tendon injuries of the foot and ankle. Clin Sports Med 11:727-739, 1992

88. Galloway MT, Jokl P, Dayton OW: Achilles tendon overuse injuries. Clin Sports Med 11:771-782, 1992

89. Quinn SF, Murray WT, Clark RA, et al: Achilles tendon: MR imaging at 1.5T. Radiology 164:767-770, 1987

90. Klein MA: Reformatted three-dimensional Fourier transform gradient-recalled echo MR imaging of the ankle: Spectrum of normal and abnormal findings. AJR 161:831- 836, 1993

91. Rosenberg ZS, Cheung Y, Jahss MH, et al: Rupture of the posterior tibial tendon: CT and MR imaging with surgical correlation. Radiology 169:229-235, 1988

92. Conti S, Michelson J, Jahss M: Clinical significance of magnetic resonance imaging in preoperative planning for reconstruction of posterior tibial tendon ruptures. Foot Ankle 13:208-214, 1992

93. Alexander I J, Johnson KA, Berquist TH: Magnetic resonance imaging in the diagnosis of disruption of the posterior tibial tendon. Foot Ankle 8:144-147, 1987

94. Kainberger FM, Engel A, Barton P, et al: Injury of the Achilles tendon: Diagnosis with sonography. AJR 155: 1031-1036, 1990

95. Allenmark C: Partial Achilles tendon tears. Clin Sports Med 11:759-769, 1992

96. Keene JS, Lash EG, Fisher DR: Magnetic resonance imaging of Achilles tendon ruptures. Am J Sports Med 17:333-337, 1989

97. Beltran J, Noto AM, Herman JL, et al: Tendons: High-field-strength, surface coil MR imaging. Radiology 162:735-740, 1987

98. Mink JH: Muscle injuries, in Deutsch AL, Mink JH, Kerr R (eds): MRI of the Foot and Ankle. New York, NY, Raven Press, 1992, pp 281-311

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