MRI OF TARSAL TI]NNELPATHOLOGY

Micbael S.

INTRODUCTION

Recently, magnetic resonance imaging (MRI) hastaken a prominent role in the evaluation of mus-culoskeletal disease of the extremities. MR imag-ing of the wrist has been shown to provide aneffective means of imaging the carpal tunnel, andhas become a valuable tool in the preoperativeevaluation of carpal tunnel syndrome. Similariy,several recent papers have suggested that MRimaging may be of comparable value in imagingthe tarsal tunnel.'3 MR imaging can accuratelydepict the contents of the tarsal tunnel, includingthe posterior tibial nerve and its terminal branch-es. Pathological entities affecting these neuralstructures including soft tissue masses, varicosi-ties, fibrosis, tendon sheath thickening andtenosynovitis, as well as secondary intrinsicchanges such as swelling and inflammation, canbe readily identified.

NORMAL MRI ANATOMY

Detailed anatomic MR images of the normalankle have been achieved and reported in all thebody planes." Images obtained from cadavericspecimens and healthy volunteers are quite simi-lar with the exception of vascular flow voids andenhancement phenomena found in live subjects.In general, the transverse (axial) and sagittal (lon-gitudinal) planes offer the most easily appreciatedand readily interpreted information. The frontal(coronal) plane images are more difficult to inter-pret, and therefore, are least helpful.

Transverse (Axial) Plane

The transverse (axial) plane images are favoredby most radiologists, and provide the best cross-

Doumey, DPM

sectional depiction of the tarsal tunnel's bound-aries and contents. The roof of the tarsal tunnel,the flexor retinaculum (or laciniate ligament), is

usualiy identified as a continuation of the deepfascia. It appears as a thickened low intensityband radiating from the medial malleolus in a

fan-iike manner to insert and blend with theperiosteum of the calcaneus posteriody and theabductor hallucis inferiorly. The floor of the tarsaltunnel is thus formed by the medial wall of thecalcaneus and ta1us.

The tarsal tunnel is further divided by verythin low intensity fibrous septa which run fromthe undersurface of the flexor retinaculum to theperiosteum of the calcaneus. These septa formfour separate compartments for the underlyingstftictures. From anterior-medial to posterior-later-al, the structures within these compartmentsinclude Tom, Dick, ANd, Harry, namely the Tib-ialis posterior tendon, the flexor Digitorumlongus tendon, the posterior tibial neurovascularbundle ({rtery, venae comitantes, and Nerve),and the flexor Hailucis longus tendon. The cross-sections of the tendons appear as low signalintensity (darD ovals, while the nerve is an inter-mediate signal intensity (gray) round stl-ucture. Atthe proximal portion of the tarsal tunnel, one cansee all these structures embedded in high signalintensiry fat (FiB. 1,A., 1B).

The posterior tibial nerve typically branchesinto the medial and lateral plantar nelves beneaththe flexor retinaculum. Occasionally, this divisionwill occur proximal to the retinaculum, butalthough anatomic texts describe it, there havebeen no surgically documented cases of the sepa-ration occurring distal to the retinaculum. Themedial calcaneal nerve is the final terminalbranch and tnay arise from the posterior tibial

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Fig. 1A. Transverse (axial) section of lower 1eg(i.e., just proximal to the ankle joint) in normalsubject. T1-weighted image. [KEY: fr : flerorretinaculum; t = tibialis posterior tendon; d =flexor digitorum longus tendon; h : flexor hal-lucis longus tendon; ANv = posterior tibialartery, neffe, and venae comitantes.

Fig. 18. T2-weighted image.

nerve pdor to its bifurcation or the lateral plantarnelve after bifurcation. In the former case, it maycourse proximal to the flexor retinaclllum, and inthe latter case it generally pierces the retinaculuminferiorly.

The ability to image these terminal nervesvaries and requires good MRI resolution and con-trast. Due to their smailer size, these netves mayblend with the adjacent vasculature and be diffi-cult to image. Usually however, the medial andlateral plantar nerves will be visualized lateral tothe vascular structures. The medial plantar neryeis most easily identified as it maintains a fairlyconsistent course along the medial aspect of theflexor hallucis longus tendon.

More distally, the medial plantar nelve con-tinues in close proximity to the flexor hallucislongus tendon. It passes through the "abductorhiatus" or canal between the abductor hallucismuscle and flexor digitorum brevis muscle. Thelateral plantar nelve may occasionally be identi-fied as it passes between the abductor hallucismuscle and the quadratus plantae muscle enroute to the plantar vault. The calcaneal branch isvery difficult to image in this plane, except nearits origin.

Sagittal (Longitudinal) Plane

Although the boundaries of the iarsal tunnel arenot readily demonstrated on the sagittal (iongitu-dinal) plane images, the longitudinal course ofthe nerwe and its branches are most easily seen inthese sections. With thin sections (e.g., 3-mm sec-tions) and no gap between the sections, the levelof division of the posterior tibial nerve and originof the medial calcaneal branch may be seen inthis plane.

As in the transverse plane, the sagittal planeMRi appearance of the tendons is as low intensity(dark) structures. The nerve appears as an inter-mediate intensity (gray), curvilinear, branchingstructure just posteriorlateral to the tibialis poste-rior and flexor digitorum longus tendons, and justmedial to the flexor hallucis longus tendon (Fig.

2A-D).In the normal patient, the posterior tibial

artery appears as a low signal intensity structuredue to vascular flow void (i.e., blood with a

velocity of greater than 5 cm per second will notgenerate a signal as the excited nuclei within thebiood vessel have moved from the imaging planeby the time data is collected).' The venae comi-tantes will usually demonstrate a high signalintensity on T2-weighted images due to flowenhancement. Thus, the intermediate signal

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Ftg.2A- Ftg. 28.

Sagittal (longitudinal) sections of tarsal tunnel. T1-weighted A. and T2-rl''eighted, B. Images of medial tarsal tunnel. The tibialis posterior tendonand flexor digitorum longus tendons can be noted passing just posterior to the tibia and medial malleolus. The posterior tibial artery and venaecomitantes are serpiginous structures readily appreciated just posterior to these tendons. Note the vessels continue and pass deep to the abduc-tor hallucis muscle.

T1-weighted C. and T2-r,eighted, D. Images of lateral tarsal tunnel. The flexor hallucis longus tendon is noted passing through the posteriortalar tubercles and under the sustentaculum tali of the calcaneus. The posterior tibial nen,e is notecl just medial to this tendon. Note the divisionof the nen'e just posterior to the talus. IKEY: A = zlfiery.i \' = \'enae con-ritantes; N = neruel

intensity nerve and its terminal branches can usu-ally be distinguished.

More distally, as the medial and lateral plan-tar nerves turn obliquely to enter the plartatcompartments of the foot, the nerwes are tracedonly with difficuiry. Frontal (coronal) plane sec-tions may have some use in evaluating theseneryes distal to the tarsal tunnel (e.g., if entrap-ment of the medial plantar nerve is suspected inthe plantar vault of the foot) (Fig. 3).

PATHOLOGIC MRI FINDINGS

Since the posterior tibial nerue is confined withina fibro-osseous tunnel, tarsal tunnel syndrome

may occur when the nerve is compressed eitherintrinsically or extrinsically. Further, biomechani-cal tension placed upon the nerve may causesymptomatology. Two recent reports havedescribed the wide variety of pathology whichmay cause tarsal tunnel syndrome.10 1i To attemptbetter differentiation of this pathology, two stud-ies have specifically evaluated the use of MRimaging in patient's with suspected tarsal tunnelsyndrome.l'2

In 7990, Erickson et a1.1 repofied 5 patientswith symptoms suggestive of tarsal tunnel syn-drome studied with MRI. They found causes ofcompression in all cases including 2 neurilemo-mas, 1 ganglion arising from the flexor hallucis

Fig. 2C. Fig.2D.

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FiB. 3. T1-weighted fronral (coronal) imagethrough midtarsal joint area. The mediai plan-tar artery and nerve are seen in the abdtrctorhiatus (small arrou). [KEY: Cbd : cuboid; abH= abdllctor hallucis muscle; Qp : quaclratusplantae mllscle; FDB = flexor digitorum brevismuscle; MK = master knot of Henryl

longus tendon sheath, 2 cases of post-traumaticfibrosis (1 with an associated neuroma), and 7

case of extensive tenosynovitis involving all threetendons within the tarsal tunnel. Except for thecase of tenosynovitis, which responded to con-selvative treatment, all the cases were confirmedsurgically. These researchers concluded that MRimaging may be used to "identify those patientswith a definite space-occupying lesion" and mayhelp "before surgery by providing a diagnosisand an operative map."

In 7991, Kerr and Frey' reported a largerstudy involving 33 feet in 27 patients with tarsaltunnel syndrome. MR imaging demonstrated a

mass in 5 feet (2 hemangiomas, 1 ganglion, 1

neurilemoma, and 1 neurofibrosarcoma), dilatedveins or varicosities in 8 feet, fracture or soft tis-sue injury in 5 feet, post-traumatic fibrosis in 2

feet, flexor hallucis longus tenosynovitis in 6 feet,and abductor hallucis muscle hypertrophy in 1

foot. In 5 feet, MR imaging was determined to benormal. The presence and extent of the patholo-gy as shown by MR imaging was confirmed in 17of 19 patients that went to surgery. The missedlesions were a small ganglion and venous vari-cosities. The investigators summarized their studyby stating they "found MR imaging useful for

localizing a variety of compressive and traction-producing lesions and to determine the extentand relationship to the posterior tibial nerve andits branches. The information provided by MRimaging enhances surgical planning by indicatingthe extent of decompression required."

It is the author's current recommendationthat an MR imaging study of the tarsal tunnelinclude both T1-weighted (TR 600/TE 2O') andT2-weighted (TR 2,000/TE B0) transverse (axial) andsagittal (longitudinal) plane images of the lowerleg and rearfoot. Ideally, the images should beobtained in 3-mm or less slice thicknesses withno interspace gap. This protocol will maximizecontrast, resolution, and sensitivity of the study.The T1-weighted image provides the greatestanatomic detail while the T2-weighted image aidsprimarily in the identification and/or charactetza-tion of soft tissue masses, inflammation, and fluidaccumulations.

Lesions which are primarily fibrous in origin(e.g., posttraumatic fibrosis, neurofibromas) willgenerally demonstrate low signal intensity onboth T1-weighted and T2-weighted images. Con-versely, fluid containing lesions (e.g., ganglions,hemangiomas, neurilemomas, varicosities,tenosynovitis) will typically demonstrate low orintermediate signal intensity on T1-weightedimages and comparatively high signal intensity onT2-weighted images (Fig. 4A,48),

Perhaps one of the greatest values of MRI inevaluating suspected tarsal tunnel syndrome is itspotential to document varicosities or dilated veinswithin the tarsal tunnel. Venous insufficiency isoften a contributing factot to tarsal tunnel syn-drome and tarsal tunnel decompression surgeryoften includes ligation of enlarged, tortuousveins.

In his review of the literature, Cimino'o tabu-lated varicosities as causing tarsal tunnel syn-drome in 16 of the 722 cases (730/o) with reportedetiologies. In their study of 87 cases, Grumbine eta1." listed 7 cases (B%) as being directly causedby venous insufficiency. However, cases withanatomical abnormalities, tumors, or multiplepathologies were excluded from their study.Many of these may have included venous insuffi-ciency. Demonstrating the potential value andsensitivity of MR imaging, Kerr and Frey' founddilated veins or varicosities to be the most com-mon MRI finding - 8 of 27 positive findings O}VA- in their study. Although not previously reported,

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Fig. 4A. Ganglion of the flexor hallucis longustendon sheath. Transverse (axial) sections per-formed in the low-er leg (i.e., just proximal tothe ankle joint). T1-weighted irnage.

Ftg. 48. T2-weighted image. Note the gangliondemonstrates comparatively low signal intensi-ty (dark gray) on the T1-weightecl image andhigh siglnal intensity (white) on the T2-weight-ed image. The neurovascular bundle is locatedjust meclial to the FHI tendon,

if varicose veins are suspected as the etiology ofa larsal tunnel syndrome, MR imaging may also

be performed with compression of the superficialvenous system. Prior to the imaging study, a

pneumatic tourniquet may be applied around themid-calf area and inflated to a pressure whichoccludes the superficial venous system withoutimpeding either the arterial system or deepvenous system (.i.e., a pressure of 30 to 50mmHg). In this manner, compression of thesuperficial venous system (i.e, the great and smallsaphenous veins and their tributaries) will cause

engorgement of any incompetent veins within thedeep venous system including the posterior tibialveins. The MR imaging study performed with thesuperficial venous system occluded will be evenmore sensitive to deep venous insufficiency (Fig.

5A_D)Some caution must be given regarding MRI

interpretation. Although MR imaging is consid-ered to be very sensitive, it is NOT always veryspecific. The ability to differentiate between dif-ferent pathologic processes (e.g., between a gan-glion and tenosynovitis) is not always possibie.Additionally, MR imaging of the tarsal tunnelnecessitates a clinician and/ot radiologist familiarwith the anatomy of the lower extremity and thetarsal tunnel (i.e., the best MR imaging study is

always subject to possible interpretation error).As the aforementioned researchers concluded,MR imaging is optimally suited for identifyinglesions within the tarsal tunnel and determiningthe relationship of the lesion to the posterior tib-ia1 nerve and its branches.'''

CONCLUSIONS/SUMMARY

Despite the correlative value of MR imaging ofthe tarsal tunnel, it remains only a supportive test

for tarsal tunnel syndrome. The diagnosis of tarsal

tunnel syndrome continues to be primarily found-ed on a sound historical interyiew and thoroughphysical evaluation including a good neurologicalexamination. Like eiectromyography and nerveconduction velocities, MR imaging may be help-fu1 in supporting or confirming the diagnosis. Aclinical diagnosis of tarsal tunnel syndromeshould not be reversed based solely on negativeMRI findings, as it should not be excluded basedsolely on normal electro-diagnostic findings.

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Fig. 5A. Ftg. 58.

Patient with suspected tarsal tunnel synclrome seconclary to \renous insufficiency (i.e., varicosities). T2-weighted lransverse(axial) sections perfbrmecl just proximal (A-B) and distel (C-D) to the ankle loint. In1ages A and C are prior tr ) tourniquerocclusion, The posterior tibial artery is noted as a low intensity (black) structure clue to signal voicl. The postelior tibial r'enae

comitantes are noted as high intensrty structures clue to flow enhancement. Hox,ever, the venae comitantes :rre barell, visillleon these vien's. lmages B ancl D are perfbmed while the superficial venous system is occluded tith a mid-calf toumiqlietinflated to 5Omrn Hg. Note the veins are mucl'r more noticeable on either side of the artery. This strongly sllEa€lests thlt venolispath.rloEiy may be the etiobgy of tarsal tunnel syndrome. In this case, er.tensive varicosities nere confimed at the time ofsur€1ery.

Fig. 5C.

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Fig.5D.

Second, the information provided by MRimaging may be helpful if surgical decompressionof the tarsal tunnel is contemplated. Both extrin-sic and intrinsic lesions can be identified andtheir boundaries determined. Previously unsus-pected lesions or causes of tarsal tunnel syn-drome may be identified. Further, variabTe pat-terns of nelve branching and the relationship ofthe nerue to the artery and venae comitantes maybe determined.

In summary, MR imaging is ideally suited toevaluation of the tarsal tunnel due to its excellentsoft tissue contrast, ability to demonstrate neu-rovascular and musculotendinous structures, andsensitivity to soft tissue pathology. As the tech-nology of these scanners continues to improvewith better resolution, narrower anatomic sec-tions, and even greatff contrast, the resultantimages created will be even more valuable inhelping to confirm the diagnosis and eriology oftarsal tunnel syndrome.

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