Our knowledge with regard to his-tology, anatomy and pathology ofthe talocalcaneal joints and sinustarsi remains limited, despiteadvances in imaging of this region.The purpose of this article is to illus-trate bony and soft tissue anatomyof the subtalar joint, to demonstratevariations in bony anatomy of thesubtalar joint, and to show patholog-ical findings as imaged with MDCT.

Technical considerations

For this study we used 9 pairs ofbone specimens (talus-calcaneus),and obtained anatomical slices in 2fresh cadaver specimens. Photo -graphs of dissection available atthe anatomy department were alsostudied. MDCT images wereobtained in 120 patients with a sus-picion of talocalcaneal and anklepathology. A Siemens imaging sys-tem was employed (SiemensSomatom Plus, Erlangen, Germany).Scan parameters were as follows:collimation, 0.75; mAs, 130;kV, 120:feed/rotation, 6.8 mm> Recon struc -tions were performed as follows:slice thickness, 0.75; kernel, sharp;bone window; reconstruction incre-ment, 0.5 mm.MR can also demonstrate subtalar

joint anatomy and is more accuratein demonstrating the ligamentousstructures. Also fractures that aresubtle and occult may be apparenton MRI as well as delicate subchon-dral changes in the absence of frankerosions.

JBR–BTR, 2010, 93: 20-27.

CONTINUING EDUCATION

THE TALOCALCANEAL UNIT: PICTORIAL REVIEW OF ANATOMY ANDPATHOLOGIC CONDITIONS ON MULTI DETECTOR CTA. Ayoob1, M. De Maeseneer2, M. Shahabpour3, P. Van Roy4, E. Barbaix4, S. QIng4

Normal anatomy of the subtalar joints is illustrated in cadaveric specimens. Variations of the subtalar joints includethree facets, fused middle and anterior facet, and missing anterior facet. Patients were studied by means of MultiDetector Computed Tomography (MDCT) to provide clinical examples of subtalar pathology including traumatic andnon traumatic disorders.

Key-word: Foot, CT.

From: 1. Department of Radiology, University of Michigan, Ann Arbor, MI, USA,2. Division of Radiologic Sciences, Wake Forest University, Winston Salem, NC, USA,3. Department of Radiology, Vrije Universiteit Brussel, Brussels, Belgium,4. Department of Experimental Anatomy, Vrije Universiteit Brussel, Brussels, BelgiumAddress for correspondence: Dr M. De Maeseneer, MD, Department of Radiology,Wake Forest University, Medical Center Boulevard, Winston Salem, NC 27157, USA; e-mail: mdemaes2@wfubmc.edu

Fig. 1. — A. Sagittal MDCT imageshows middle subtalar joint (whitearrow). Note small subchondral geode(long black arrow) and osteochondrallesion of talus (arrowheads). B. SagittalMDCT image shows posterior (thickarrow) and anterior (thin arrow) subtalarjoints. Note sinus tarsi (curved arrow). C.Sagittal MDCT shows middle (thickarrow) and posterior (thin arrow) subtalarjoints, and tarsal canal (curved arrow).

Fig. 2. — Lateral radiograph in speci-mens. Note posterior (thin arrows), andmiddle (thick arrows) subtalar joints.

A B

C

TALOCALCANEAL UNIT — AYOOB et al 21

Bony anatomy

The subtalar joint represents thearticulation between the talus andcalcaneus. While variations exist,typically the subtalar joint isdescribed as consisting of threefacets: anterior, middle, and posteri-or (Figs. 1, 2) (1, 2). The anterior facetis concave in shape and articulateswith the head of the talus. It can becongruent with the middle facet. Themiddle facet is also slightly concaveand is situated on the sustentaculumtali. The posterior facet is the largestfacet and is convex. Three variationsof the bony anatomy of the subtalarjoints exist: three joint facets, miss-

by the tarsal sinus and tarsal canal.The position of the joint facets givesthe tarsal sinus its trumpet shapewith a wide opening at the lateralaspect, and a narrower opening atthe medial aspect of the hindfoot(Figs. 4, 5). The medial portion is alsotermed the tarsal canal.

ing anterior joint facet, and fusedanterior and middle joint facets(Fig. 3). It is unknown whether cer-tain types predispose to subtalarinjury, or are associated with differ-ences in subtalar ligament anatomy.The anterior and middle facet are

separated from the posterior facet

Fig. 3. — A. Superior view of calcaneus. Three subtalar facets are seen, the anterior (A), middle (M), and posterior (P) facet. B.Superior view of calcaneus. Two facets are seen. The anterior and middle facet are fused (F).Note posterior facet (P). C. Superior viewof calcaneus. The anterior facet is missing, and only a middle (M), and posterior (P) facet are seen.

Fig. 4. — Photograph of calcaneus and talus. The sinus tarsi (S)is seen. Also note middle (thick arrow) and posterior (thin arrow)subtalar joints.

Fig. 5. — Drawing of funnel shaped appearance of sinus tarsi(arrows). Note anterior (A), middle (M), and posterior (P) facets.

A B C

22 JBR–BTR, 2010, 93 (1)

Soft tissue anatomy of the subtalarjoint

Extrinsic stabilizers of the subtalarjoints include medial and lateralankle tendons and the variousextrinsic ligaments connecting thebones of hind- and midfoot. Thesinus tarsi contains fat, nerves, ves-sels, capsular reinforcements, andligaments (3, 4). The soft tissue vari-ations of the sinus tarsi and functionof the various ligaments are incom-pletely understood. Ligaments in thesinus tarsi are extracapsular andinclude (from medial to lateral) a lig-ament in the canalis tarsi, capsularreinforcements, the interosseous lig-ament, the cervical ligament, and afiber bundle connecting to the exten-sor retinaculum (Fig. 6). The cervicalligament extends from the inferolat-eral talar neck to the dorsal calca-neus. The primary function of thecervical ligament is thought to belimiting hindfoot inversion. Theinterosseous ligament extendsobliquely between the talus and cal-caneus. Although smaller than thecervical ligament, the interosseousligament is also felt to play a role inhindfoot stability by limiting inver-sion (Figs 7-9). At the lateral aspectof the sinus tarsi, the inferior exten-sor retinaculum is anchored to thetalus and calcaneus by roots that arelocated in the sinus tarsi. Tears of thesinus tarsi ligaments are often asso-ciated with ankle distortion. MR and

Fig. 7. — Coronal anatomical slice. Centrally interosseous lig-ament is seen (thick black arrows), Also note tarsal canal exten-sion (white arrow), and medial and lateral tendon groups (thinblack arrows).

Fig. 6. — A. Sagittal anatomical slice through sinus tarsi (S).Note posterior subtalar joint (arrow). B. Sagittal anatomical slicethrough tarsal canal (white arrow). Note middle (thin arrow) andposterior (thick arrow) subtalar facets. C. Sagittal anatomicalslice through sustentaculum tali (white arrow). Note flexor hal-lucis longus tendon (black arrows).

C

A B

TALOCALCANEAL UNIT — AYOOB et al 23

CT may show soft tissue promi-nence, fiber interruptions and bonyavulsions. The clinical and prognos-tic significance of these findingsremains obscure, however.

Subtalar joint pathology

The subtalar joint is frequentlyinvolved in traumatic injuries to thehindfoot, namely fractures of the cal-caneus and talus (Fig. 10) (5). Giventhe complex anatomy of the subtalarjoint facets, radiographs are oftenlimited in both the detection andcharacterization of these injuries.

the detection and characterization ofthese entities.

Traumatic conditions

The calcaneus is the most fre-quently fractured tarsal bone withcalcaneal fractures accounting forabout 60 % of all major tarsalinjuries. The majority of fractures areintraarticular with subtalar jointinvolvement (Figs. 11-13) (6, 7). Cal -caneal fractures result when axialforces applied to the foot cause thecalcaneus to be driven into the later-al process of the talus. The result is a

MDCT is useful in establishing thepresence of fractures, in detectingadditional fractures not visible onradiography, and in the assessmentof joint involvement, joint incon-gruity, and fracture comminution.These are important factors in boththe treatment and prognosis forthese injuries and therefore MDCTplays a vital role in the clinical evalu-ation of these patients. The subtalarjoint can also be involved in a varietyof arthritides, including degenera-tive, inflammatory, infectious, andcrystalline processes. As for trau-matic injuries, MDCT is invaluable in

Fig. 8. — A. Sagittal PD weighted MR image. Note ligament oftarsal canal (thin black arrow) and anterior and posterior (thickblack arrows) subtalar joints. B. Sagittal PD weighted MR image.Note cervical ligament (black arrow). Also note fiber bundle(curved arrow) extending to extensor retinaculum (E).

Fig. 9. — Sagittal MDCT image. Note fiber bundle (arrow) outof sinus tarsi extending to extensor retinaculum.

Fig. 10. — Sagittal MDCT image. Note intraarticular fracturefragment (white arrow) and posterior malleolus fracture (blackarrow). Arrowhead points to donor site.

B

A

24 JBR–BTR, 2010, 93 (1)

primary fracture line oriented in ananteromedial to posterolateral fash-ion. Secondary fracture lines com-monly occur. Also there are varyingdegrees of comminution. The man-agement of intraarticular calcanealfractures remains controversial.Decision making regarding treat-ment depends heavily on imagingfindings.Extra-articular fractures aregenerally treated in a closed manner.Intraarticular fractues may also betreated closed but are usually treat-

fractures. Osteonecrosis is a com-mon complication of talar neck frac-tures and is a major determinant ofoutcome. Fractures of the talar body,although less common than talarneck fractures, involve the posteriorfacet of the subtalar joint and may beassociated with a subtalar disloca-tion (Fig. 17). Lateral process frac-tures are a subgroup of talar bodyfractures that are rare and oftenoverlooked (8). The mechanism ofthese fractures is dorsiflexion and

ed with open reduction,osteotomy,internal fixation and arthrodesis. Thegoal of the radiologist is to accurate-ly describe the fracture pattern suchthat the surgeon can match the frac-ture anatomy with the appropriatesurgical approach.The integrity of the talus is also

critical to the normal function of thesubtalar joint. Talus fractures are rel-atively uncommon injuries (Figs. 14-16) (7). Talar neck fractures accountfor approximately 50 % of all talus

Fig. 11. — Sagittal MDCT image. Fracture of the calcaneus(arrowhead) involving the middle subtalar joint.

Fig. 12. — Sagittal MDCT image. Fracture of the calcaneusinvolving the anterior (bold arrow) and posterior (thin arrow)subtalar joints.

Fig. 13. — Sagittal MDCT image. Fracture through the pos -terior talus (bold arrow) and fracture of the calcaneus withfragment (thin arrow) in the sinus tarsi.

Fig. 14. — Sagittal MDCT image. Fracture (arrow) through themiddle aspect of the talus. Note small bony fragment in thecanalis tarsi.

TALOCALCANEAL UNIT — AYOOB et al 25

inversion and they often occur dur-ing snowboarding. A loss of talarblood supply may lead toosteonecrosis, which is a seriouscomplication. The risk of AVN can beestimated by the Hawkins sign.Normally a subtle subchondrallucent line develops at the talardome. Areas that remain scleroticare at risk for AVN. If the fracturefragments are not displaced a closedtreatment may suffice. If the frag-ments are displaced, a fixation withscrews or pins is indicated.

Non traumatic conditions

Tarsal coalition is a frequentcause of painful flatfoot (9).Talocalcaneal and calcaneonavicular

calcaneal coalition may occur at anyof the three facets, the majority ofosseous fusions involve the middlefacet. Osseous coalitions are charac-terized by complete cortical bridgingbetween the calcaneus and talus.Non osseous coalitions are charac-terized by joint space narrowing andirregularity sometimes accompaniedby cortical hypertrophy and sclero-sis.

coalition are the most commontypes (Figs. 18, 19). While calcaneon-avicular coalitions are usually wellseen on conventional radiography, acoalition between the talus and cal-caneus may be difficult to detect. Onradiography the C sign may bedemonstrated, but often CT or MR isnecessary for accurate diagnosis(10). Coalitions can be osseous, carti-laginous or fibrous. Although a talo-

Fig. 15. — Sagittal MDCT image. Com -minuted fracture through the middleaspect of the talus (arrow).

Fig. 16. — Coronal MDCT image. Fracture of the lateralprocess of the talus (arrow).

Fig. 18. — Lateral radiograph of the foot. Note C sign (arrows)of subtalar coalition.

Fig. 17. — Coronal MDCT image. Healed fracture of the lateral process of the talus(arrow).

26 JBR–BTR, 2010, 93 (1)

Fig. 23. — Sagittal MDCT image. Note erosions of posteriorsubtalar joint (arrow) in inflammatory arthritis.

The subtalar joint can be involvedwith a variety of degenerative,inflammatory, and crystalline arthri-tides. Involvement of the subtalarjoints is likely underestimated due tolimited visualization on radiography.Degenerative changes may beidiopathic or more commonly sec-ondary to trauma and malformation(Figs. 20-22). Manifestations ofosteo arthrosis are the same as inany joint and include asymmetric

Fig. 19. — Sagittal MDCT image. Note bony coalition of sub -talar joint (long arrow) with C sign (thick arrows).

Fig. 20. — Sagittal MDCT image. Early sclerosis (arrow) as asign of osteoarthritis is seen.

Fig. 21. — Sagittal MDCT image. Note large posterior processof the talus (thin arrow). Also note subchondral sclerosis andgeodes (thick arrow) of the posterior subtalar joint.

Fig. 22. — Sagittal MDCT image. Note os trigonum (arrow-head) and reactive sclerosis (long arrow) in the posterior talus.

TALOCALCANEAL UNIT — AYOOB et al 27

joint space narrowing, sclerosis, andsub chondral cysts. Rheumatoid andinflammatory disorders may involvethe subtalar joints (Fig. 23) (11,12).Neuropathic arthropathy may occurin the diabetic foot, although themidfoot is more commonly affected.Gout may involve the mid- and hind-foot, in addition to the first metatar-sophalangeal joint. Erosions areseen often with an overhangingedge and may be accompanied bysoft tissue masses. Infection mayoccur in diabetics with infected softtissues, and in a postoperative set-ting. The joint surfaces appear nar-rowed and eroded. Fragmentationmay occur.

In conclusion, we described thenormal anatomical variations of thebony anatomy of the subtalar joint.Pathological conditions of the subta-lar joint may be imaged with MDCT,allowing for a more accurate diagno-sis than radiography.

6. Zwipp H., Rammelt S., Barthel S.:Calcaneal fractures, open reductionand internal fixation (ORIF). Injury,2004, 35: SB46-54.

7. Early J.S.: Management of fracturesof the talus: body and head regions.Foot Ankle Clin, 2004, 9: 709-722.

8. Mukherjee S.K., Pringle R.M.,Baxter A.D.: Fracture of the lateralprocess of the talus. J Bone JointSurg, 1974, 56; 263-273.

9. Beckly D.E., Anderson P.W.,Pedegana L.R.: The radiology of thesubtalar joint with special referenceto talo-calcaneal coalition. ClinRadiol, 1975, 26: 333-341.

10. Lateur L.: Subtalar coalition: diagno-sis with the C sign on lateral radi-ographs of the ankle. Radiology,1994, 193: 847-851.

11. Barthelemy C.R., Nakayama D.A.,Carrera G.F. et al.: Gouty arthritis: aprospective radiographic evaluationof 60 patients. Skeletal Radiol, 1984,11: 1-8.

12. Antoniou D., Connor A.N.: Osteo -myelitis of the calcaneus and talus.J Bone Joint Surg, 1974, 56: 338-345.

References

1. Barbaix E., Van Roy P., Clarys J.P.:Variations of anatomical elementscontributing to subtalar joint stability:intrinsic risk factors for posttraumaticlateral instability of the ankle.Ergonomics, 2000, 43: 1718-1725.

2. Uygur M., Atamaz F., Servet C.,Pinar Y.: The types of talar articularfacets and morphometric measure-ments of the human calcaneal boneon Turkish race. Orthop Surg, 2009,129: 909-914.

3. Lektrakul N., Chung C.B., Lai Ym et al.:Tarsal sinus: arthrographic, MR imag-ing, MR arthrographic, and patholog-ic findings in cadavers and retrospec-tive study data in patients with sinustarsi syndrome. Radiology, 2001, 219:802-810.

4. Beltran J., Munchow A.M., Khabiri H.et al.: Ligaments of the lateral aspectof the ankle and sinus tarsi: an MRimaging study. Radiology, 1990, 177:455-458.

5. Morvan G., Laredo J.D., Wybier M.:Imagerie Osteoarticulaire. Paris. Flam -marion Médecine-Sciences 1998.