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Dynamic multi-detector CT in the assessment of patellarinstability: how can I do it, what should I measure and why

Poster No.: C-0056

Congress: ECR 2014

Type: Educational Exhibit

Authors: S. Rapisarda, A. Cipriani, F. Ferrara, N. Magarelli, C. Dell'atti, A.Leone, L. Bonomo; Rome/IT

Keywords: Musculoskeletal joint, CT, Computer Applications-Detection,diagnosis, Education and training

DOI: 10.1594/ecr2014/C-0056

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Learning objectives

To illustrate the clinical indications for performing a dynamic multi-detector CT (MDCT)examination of the knee, its technical principles, post-processing of data, measurementsachievable, possible diagnoses and their therapeutic outcomes.

Background

Patellar instability is a widespread cause of pain and functional limitation in youngpatients, particularly athletic women. Plain film, even at different grades of knee flexion,as well as dynamic ultrasonography or static MR examination of the knee, in some casesare not capable of demonstrating all the different causes of patello-femoral instability.Dynamic MDCT can give a substantial incremental value to the examination of patello-femoral instability.

Findings and procedure details

In this exhibit are described:

• Clinical assessment of patellar instability• Dynamic MDCT acquisition protocol: field of view, slice thickness, image

reconstruction algorithms• Patient collaboration: lower limbs in extension and 20° flexion, each at rest

and under contraction of quadriceps femorismuscle• Post-processing of data: single images superimposition, multi-planar (MPR)

and volume rendered (VR) reconstructions• Measurements obtainable: trochlear groove-tibial tuberosity (TG-TT) distance,

type of patella (according to Wiberg's classification), depth and angle ofthe trochlear groove, femoro-patellar congruence (Merchant) angle, femoro-patellar tilt angle, Laurin angle, patellar tendon-patella (Insall-Salvati) ratio

• Influence of knee position and muscle contraction on measurements• Ruling out differential diagnoses• Advising the clinician on possible surgical strategies

Clinical assessment of patellar instability

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Patellar instability is a disabling condition predominantly affecting young patients [1,2] and including patellar dislocation, patellar subluxation and symptomatic instability ingeneral[3]. Certain anatomical or functional factors, such as trochlear dysplasia, patellarmorphology, tibial tuberosity location[4, 5], soft tissues disruption (in particular a ruptureof the medial retinaculum or medial patellar-femoral ligament) , hypotrophy of the vastusmedialis oblique and generalized ligamentous laxity, have been associated with primarypatellar dislocation or recurrent secondary instability[6-11].

Several clinicaltests are used to diagnose and assess patellar instability [12], the mostcommon of which are:

Apprehension test

patient supine, knee relaxed, 30° flexion

the examiner uses one hand to push the patella laterally

a positive sign is when the maneuver reproduces the patient's pain or causes verbalexpression of anxiety and/or involuntary quadriceps muscle contraction

Patellar positioning

patient initially supine, then retested in sitting position with knee initially relaxed in fullextension

the examiner observes for eventual patellar tilt, lateralized position of patella, patella baja(excessively distal to the normal), patella alta (excessively proximal to the normal) whilethe knee is actively moved from full extension to full flexion

Q-angle

patient supine

a line is drawn from the anterior superior iliac spine, to the center of patella

a second line is then drawn from the center of patella to the tibial tubercle

the angle comprised between these lines is the Q-angle (normal value from 10° to 15°for men and from 15° to 20° for women) [13]

an increased Q-angle may enhance the laterally directed force of the extensormechanism, predisposing the patella to mal-positioning and instability

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Clinical examination is an essential diagnostic tool which, associated to radiologicalassessment, can frequently assist to choose among differential diagnoses and direct thedecision-making process.

Dynamic MDCT acquisition protocol

The patient is examined supine, with knees positioned in the center of the gantry,feet affixed and toes pointing upwards in order to minimize leg movement throughoutscanning. Basing a CT scout on the coronal and sagittal planes, both knees are includedin the field of view (FOV) in order to allow a comparative evaluation of the inferior limbs.The scan interval always includes the myo-tendinous junction of the quadriceps femoristendon cranially, and the distal insertion of patellar tendon on the tibial tuberosity caudally.The CT scanner to employ preferably is a 16 rows multi-detector CT (MDCT) or superior.The unit employed in our institution is a 64 slices LightSpeed VCT (GE Healthcare, Wis,USA). Images are obtained through a spiral acquisition. No intravenous iodinated contrastadministration is required.

Acquisition parameters:

·pitch 1

·tube rotation 0.6 s-1

·FOV 40 mm2

·slice thickness 1.25 mm

·acquisition kernel standard

·reconstruction slice thickness 0.6 mm

·reconstruction kernel bone

·total median dose length product (DLP) 2895.88 mGy/cm

Patient collaboration

Knees are initially scanned with the inferior limbs in full extension and muscle relaxation(Fig. 1, Fig. 3). Then the patient is asked to contract the quadriceps femoris muscle anda second scan is performed (Fig. 2). At this point a dedicated wedge-shaped foam padin inserted below patient's knees in order to obtain a 20° flexion of the inferior limbs (Fig.4). After acquiring a second CT scout in the coronal and sagittal planes, both knees arescanned first at rest and subsequently under muscular contraction.

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Post-processing of data

A superimposition of the axial slices passing through the tibial tuberosity, at the pointof its greatest prominence, and the trochlear groove, at its maximum depth is obtainedfor the measurement of the trochlear groove-tibial tuberosity (TG-TT) distance. A sagittalmulti-planar reconstruction (MPR) passing through the middle of patella is employed forthe assessment of the Insall-Salvati ratio. All remaining measurements are obtained onaxial images.

Measurements obtainable

Trochlear groove-tibial tuberosity (TG-TT) distance

The trochlear groove-tibial tuberosity (TG-TT) distance is a measurement of the lateralpull on patella. When the tibial tuberosity is in an excessively lateral position, the patellais shifted laterally during knee flexion[14, 15]. The TG-TT distance is measured bysuperimposing the axial images passing through the apex of the intercondylar notch andthe tibial tubercle, respectively, on MDCT scans obtained with the inferior limbs in fullextension and without contraction of the quadriceps femoris muscle (Fig. 5). When thedistance comprised between the tibial tuberosity and the deepest point of the trochleargroove (measured parallel to the posterior condylar axis) is greater than 20 mm, it isconsidered abnormal and always associated with patellar instability[14, 15].

Type of patella

The assessment of patellar morphology, according to commonly used Wiberg'sclassification [16, 17], may be another key point to address the clinician :

·type I: patellar facets are concave, symmetrical and of equal size (ideal configuration)

·type II: the medial facet (eventually flat or slightly convex) is rather smaller than the lateralone, this latter being concave (the most common patellar anatomy)

·type III: the medial facet (convex) is markedly smaller than the lateral one.

Trochlear depth

The trochlear depth is measured as the distance between the deepest point of thetrochlear groove and a line traced tangential to the anterior margins of the medial andlateral femoral condyles, taken perpendicularly to it (Fig. 6). A trochlear depth of less than3 mm is considered abnormal [18].

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Trochlear groove angle

The angle of the trochlear groove is formed by the intersection of the lines connectingthe highest points of the femoral condyles to the lowest point of the trochlear groove (Fig.7). Normal values are 125°-145° [19].

Femoro-patellar congruence angle

The femoro-patellar congruence or Merchant angle is a measurement of the lateralsubluxation of patella. It is comprised between a line bisecting the sulcus angle anda second line joining the apex of trochlear groove with the apex of patella (Fig. 8). Anormal congruence angle is -8°±6°. Positive values are associated with recurrent lateraldislocation [20].

Patellar tilt angle

The patellar tilt angle is formed by a line paralleling the lateral facet of patella and a secondline tangential to the posterior aspects of lateral and medial femoral condyles (Fig. 9).

The normal angle is greater than 8°. An abnormal patellar tilt angle may be associatedwith lateral pressure syndrome [21].

Laurin angle

It is the angle between the lines drawn along the lateral patellar facet and the anteriormargins of femoral trochlea (Fig. 10). It should be open laterally, normal values beingcomprised between 8° and 13°. When it assumes negative values (open medially), itindicates lateral subluxation of patella (Fig. 11) [22].

Patellar tendon-patella (Insall-Salvati) ratio

The Insall-Salvati ratio is useful to evaluate, in full extension, the height of patella. It iscalculated by dividing the length of patellar tendon (from the apex of patella to its insertionon the tibial tuberosity) by the longest supero-inferior diameter of patella (Fig. 12). AnInsall-Salvati ratio greater than 1.50 is consistent with patella alta [15]. A ratio less than0.74 indicates patella baja [23].

Influence of knee position and muscle contraction on measurements

The additional value of MDCT with respect to conventional axial radiographic projectionsof the knees at 30° flexion, consists in allowing the assessment of patello-femoral joint inthe axial plane, with the inferior limbs in extension and in early degrees of flexion [24]. ACT scan performed with the knees in at least 20° flexion allows to evaluate the behavior

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of patella when it engages in the femoral trochlea, identifying the most serious cases inwhich the patella is dislocated in flexion.

However, patellar instability is a dynamic phenomenon, due in part to functionalabnormalities of quadriceps femoris muscle. Thus, a CT examination obtained duringmuscle contraction increases the sensitivity of this modality in demonstrating a lateraldeviation of patella, already observed in static conditions and accentuated by musclecontraction (Fig. 2), as well as in revealing an eventual patellar misalignment not evidentat rest conditions (Fig. 1). CT scan performed under muscle contraction can identify thosepatients in whom the action of quadriceps femoris does not exacerbate a poor patellaralignment and for which the lysis of lateral retinaculum, when executed, would cause theonset of post-operative medial subluxation.

Ruling out differential diagnoses

The most common symptom associated to patellar instability is anterior knee pain.However, this kind of pain may be determined by several pathologies. The most commoncauses in young athletes are trauma, osteochondrosis, patellar insertional peritendinitisor tendinosis and synovial impingement. Less common sources of patellar pain areosteochondritis dissecans or tumors. It is always important to rule out eventual underlyinghip pathologies such as osteochondrosis or infections. A careful physical examinationand an appropriate MDCT scan in extension and 20° flexion of the knees, each at restand under contraction of quadriceps femoris muscle, can point to the correct diagnosisof patellar instability in the majority of cases and rule out the differential diagnoses [25].

Advising the clinician on possible surgical strategies

Non-operative treatment techniques include physical therapy, focusing on strengtheningthe gluteal or vastus medialis oblique muscles, and patellar taping or bracing. Acutemedial-sided repair of patella may be indicated when there is an osteochondral fracturefragment or a retinacular injury. Recent literature does not support the use of an isolatedlateral release for the treatment of patellar instability. The measurements provided by anMDCT scan of the knee, such as TG-TT distance and the type of patella, are of greathelp to the clinician in determining the correct therapeutic procedure and, possibly, themost appropriate type of surgery.

A patient with recurrent instability, with or without trochlear dysplasia, who has a normalTG-TT distance and a normal patellar height, may be a candidate for a reconstructionof the medial patello-femoral ligament with an autograft or allograft. Differently, distalrealignment procedures are used in patients with an increased TG-TT distance or patellaalta. The degree of anteriorization, distalization, and/or medialization of tibial tuberosityduring surgery mainly depends on associated arthrosis of the lateral patellar facet or thepresence of patella alta. An associated medial or proximal patellar osteochondrosis is acontraindication to distal realignment because the tissues are already degenerated [26].

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Images for this section:

Fig. 1: CT scan in full extension at the rest. These images demonstrate an externalpatellar tilt in both knees.

Fig. 2: In the same patient, CT scan in full extension with quadriceps femoris musclecontraction. Muscle contraction increases the external.

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Fig. 3: CT scan in full extension at the rest. External patellar tilt in both knees.

Fig. 4: In the same patient, CT scan in 20° flexion shows a reduction of the externalpatellar tilt.

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Fig. 5: TG-TT. Border-line values: 13 mm to the right and 15 mm to the left. PathologicTG-TT: # 20 mm.

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Fig. 6: Trochlear depth. Normal value > 3mm.

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Fig. 7: Trochlear groove angle. Normal values: 125°-145°.

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Fig. 8: Femoro-patellar congruence angle. This congruence angle is 21°. Normal values:-8°±6°.

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Fig. 9: Patellar tilt angle. The normal angle is greater than 8°.

Fig. 10: Laurin angle. It should be open laterally, so this measurement is normal.

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Fig. 11: Laurin angle. It is pathological Laurin angle. When it assumes negative values(open medially) it indicates lateral subluxation of patella.

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Fig. 12: Insall-Salvati ratio. This is a normal value. An Insall-Salvati ratio greater than1.50 is consistent with patella alta. A ratio less than 0.74 indicates patella baja.

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Conclusion

Dynamic MDCT of the knee is a fast, non-invasive technique, requiring little collaborationfrom the patient, through which radiologists can rule out almost all causes of patellarinstability, functional or structural, advising the clinician about possible surgical strategies.

Personal information

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