Tarsal tunnel syndrome (TTS) is a common pathology encountered by foot and ankle surgeons. It is characterized by numbness, neuropathic pain, and burning sensation in the tibial nerve distribution largely resulting from compression of the tibial nerve because it descends into the sole of the foot beneath the flexor retinaculum.1 Many etiologies of this neuropathy have been described, which can be due to intrinsic and extrinsic causes. Extrinsic sources are generally synonymous with compression, while intrinsic causes can include tendinopathy, perineural fibrosis, inflammatory conditions, hypertrophic retinaculum, ganglion cysts, venous dilatation, aberrant vasculature, and lipomas.1–5 Although management of TTS may begin conservatively, surgical treatment may be warranted if the inciting cause is discovered to be secondary to an extrinsic mass.1

Bone abnormalities have been previously described as inciting factors in TTS, but they remain a rare cause, with few examples reported in the literature.3,6 Of the skeletal abnormalities that present near the tarsal tunnel, two notable structures include an os trigonum and a bipartite talus. An os trigonum is an accessory ossicle of the foot and ankle that can be found posterior to the talus in 7 to 25% of the population.7 This accessory ossicle is derived from the failure of the fusion of a secondary ossification center with the talus leading to an adjacent ossicle that shares a synchondrosis with the talus.8 The presence of an os trigonum has be noted radiographically in many asymptomatic pediatric patients aged between 9 and 10 years, with the fusion of the os occurring in most by 17.9 One of the pathological consequences of a persistent os trigonum is posterior ankle impingement syndrome, which results from trauma to the ossicle secondary to repetitive plantar flexion. This syndrome is characterized by posterior ankle pain, primarily with plantar flexion and posterior ankle swelling.8 In the few reported cases, talus bipartitus has presented similar to an os trigonum with ankle pain, along with decreased range of motion and ankle swelling.10,11 The etiology of a bipartite talus currently remains debated, with theories postulating it arises because of talar nonunion after trauma, repetitive trauma during development preventing secondary talar ossification center fusion, or occurs because of idiopathic failure of fusion of a secondary ossification center similar to an os trigonum.10–12 Lateral radiographs are one of the most valuable modalities to visualize these bony structures, yet MRI imaging remains a vital imaging modality to assess bone marrow edema of the talus or ossicles and assess surrounding soft-tissue injury in the setting of posterior ankle pain.7,13 Similar to TTS, the initial treatment of a symptomatic bipartite talus or os trigonum syndrome may begin conservatively with activity modification, rest, and anti-inflammatories, with progression to surgical treatment should conservative management fail.8,10–12 In this unusual case, we describe an initial TTS presentation in an older patient secondary to a large bony structure within the posterior ankle. This structure, originally thought to be a large os trigonum, was identified as a bipartite talus, which was successfully managed with tarsal tunnel decompression and removal of the posterior talar fragment.

Case

A 64-year-old woman presented to the orthopaedic clinic with six months of numbness and tingling on the plantar aspect of her left foot. Medical history was notable for chronic lower back pain managed by pain management, lumbosacral spinal stenosis, and bilateral carpal tunnel release undergone more than 10 years prior. Her arrival to this appointment was preceded by a presentation to the emergency department 1 month earlier with an outpatient follow-up with a neurologist after a mechanical fall on her staircase. This fall was stated to be secondary to her left foot slipping and difficulty walking secondary to numbness in her foot. At her appointments after the emergency department visit, she described worsening numbness, particularly with activity. The numbness was stated to involve much of the plantar surface of her left foot, beginning at her heel before passing into the rest of her foot. These symptoms were accompanied by left foot and ankle pain and chronic swelling in her left lower extremity.

Clinical Findings

Clinical examination revealed left ankle swelling with pitting edema. The range of motion around the left ankle joint was normal, with 15° of dorsiflexion and 45° of plantar flexion in both active and passive range of motion of the ankle joint. No pain was elicited on palpation of the ankle or with inversion or eversion of the foot. On neurologic examination, tenderness and numbness were noted along the tarsal tunnel with decreased pinprick sensation in the left lateral sole and hyperesthesia of the left medial sole. The Tinel test of the left medial ankle was positive, with numbness and tingling radiating from the medial ankle toward the plantarmedial and plantarlateral aspects of the foot on tibial nerve percussion. The rest of the neurologic examination and contralateral foot and ankle examination were normal. The Tinel test of the contralateral medial ankle was negative.

Diagnostic Assessment

Ankle radiographs taken from the date of the emergency department visit demonstrated a large bone fragment posterior to the talus with degenerative changes at the left tibiotalar joint (Figure 1). Notable findings from the foot and ankle MRIs taken shortly after discharge included the large posteromedial lying bone fragment without bone marrow edema of the talus or the ossicle, surrounding fluid extending into the tarsal tunnel, and moderate subtalar joint arthritis (Figure 2). The os was measured to be 19.3 mm in the AP direction and 13.5 mm in the superior-inferior direction on the sagittal MRI view, and 29.9 mm transversely on the axial MRI views. Bilateral electromyography/nerve conduction velocity (NCV) (EMG/NCV) studies obtained before orthopaedic consultation are presented in Tables 1, 2, and 3. NCV revealed a reduced amplitude of the motor component of the left tibial nerve, while left plantarmedial and plantarlateral sensory nerves generated no response (Table 1). EMG evaluation of the left abductor hallucis and abductor digiti quinti revealed the muscles to have increased insertional activity, moderately increased spontaneous activity, and reduced recruitment (Table 2). All other EMG results, NCV studies, F-wave latencies, and H-Reflex latencies were within normal limits (Tables 1–3). The studies demonstrated evidence of a moderate to severe left distal tibial neuropathy at the ankle consistent with TTS. Duplex ultrasonography acquired because the chronic left ankle swelling was negative for deep vein thrombosis.

Figure 1:

Lateral, AP, and oblique radiographs of the left ankle demonstrating the posterior fragment of the bipartite talus (white arrow) and degenerative changes at the left tibiotalar joint.

Figure 2:

T1-weighted MRI of the left ankle in the sagittal, coronal, and axial planes. Red asterisk (*) denotes the posterior fragment of the bipartite talus, and the red arrow denotes the neurovascular bundle of the and posterior tibial artery and tibial nerve.

Table 1 - Bilateral Nerve Conduction Velocity Studies of Sensory Nerves and Motor Components of the Tibial and Peroneal Nerves of the Foot and Ankle Nerve Stimulation Site Left Right Onset (ms) Amplitude (motor = mV) (sensory = µV) Velocity (m/s) Onset (ms) Amplitude (motor = mV) (sensory = µV) Velocity (m/s) Motor Peroneal motor (EDB) Ankle 2.7 4.6 46 3.4 2.2 41 Below fibular head 10.2 4.1 53 11.9 4.6 44 Popliteus fossa 11.9 3.8 13.6 4.6 Tibial motor (FHB) Ankle 4.8 1.6 43 4.4 4.5 39 Popliteus fossa 13.4 1.1 14.2 2.9 Sensory Plantarlateral Lateral sole NR NR NR 2.7 4.8 56 Medial malleolus NR NR NR 2.5 4.6 Plantarmedial Medial sole NR NR NR 2.3 3.6 Medial malleolus NR NR NR 2.1 3.0 Superficial peroneal (anterior lateral malleolus) Lower leg 2.3 5.4 52 2.3 5.3 52 Anterior lateral malleolus 2.1 5.0 2.0 5.5 Sural sensory Calf 2.0 8.0 55 2.0 7.3 58 Lateral malleolus 2.0 8.2 2.1 8.0

EDB = extensor digitorum brevis, FHB = flexor hallucis brevis, ms = milliseconds, mV = millivolts, NR = no response, µV = microvolts, m/s = meters/second

Table 2 - Bilateral Electromyography Results Side Muscle Nerve Root Insertional Activity Fibrillation Potentials Positive Spike Waves Amplitude Duration Polyphasic Potentials Recruitment Interference Pattern Right Anterior tibialis Deep branch peroneal L4-5 Normal 0 0 Normal Normal 0 Normal Complete Right Medial gastrocnemius Tibial S1-2 Normal 0 0 Normal Normal 0 Normal Complete Right Vastus medialis Femoral L2-4 Normal 0 0 Normal Normal 0 Normal Complete Right L4-5 paraspinal Rami L4-5 Normal 0 0 Normal Complete Right L5-S1 paraspinal Rami L5-S1 Normal 0 0 Normal Complete Right Abductor digiti quinti Plantarlateral S1-2 Normal 0 0 Normal Normal 0 Normal Complete Right Abductor hallucis Plantarmedial S1-2 Normal 0 0 Normal Normal 0 Normal Complete Left Anterior tibialis Deep branch peroneal L4-5 Normal 0 0 Normal Normal 0 Normal Complete Left Medial gastrocnemius Tibial S1-2 Normal 0 0 Normal Normal 0 Normal Complete Left Vastus medialis Femoral L2-4 Normal 0 0 Normal Normal 0 Normal Complete Left L4-5 paraspinal Rami L4-5 Normal 0 0 Normal Complete Left L5-S1 paraspinal Rami L5-S1 Normal 0 0 Normal Complete Left Abductor digiti quinti Plantarlateral S1-2 Increased 2+ 2+ Normal Normal 0 Reduced Complete Left Abductor hallucis Plantarmedial S1-2 Increased 2+ 2+ Normal Normal 0 Reduced Complete
Table 3 - Bilateral F-Wave and H-Reflex Latency Studies. F-Latency Reported for F-Wave. H-latency Reported for H-Reflex. Nerve Left
Latency (ms) Right
Latency (ms) Latency Norm (ms) F-wave studies  Peroneal (EDB) 45.33 45.33 <56  Tibial (FHB) 52.67 52.67 <56 H-reflex studies  Tibial (Gastrocnemius) 29.74 29.74 <34

EDB = extensor digitorum brevis, FHB = flexor hallucis brevis, ms = milliseconds

Therapeutic Intervention

Until the orthopaedic visit, the patient had been managed conservatively with activity modification, ibuprofen, and acetaminophen. Given the failure of nonsurgical management, clinical symptoms of TTS confirmed by EMG/NCV, and a source of tibial nerve compression established by the identification of sizeable medial lying bone structure, the patient was scheduled for tarsal tunnel release and excision of the bone piece. Tarsal tunnel decompression was done with the patient in the supine position under general anesthesia with a pneumatic tourniquet placed on the left thigh. The incision was made on the posterior medial ankle, overlying the tarsal tunnel. Dissection was carried down to the flexor retinaculum, which was incised to release the tibial nerve in the tarsal tunnel. A complete release of the nerve was done both proximally and distally. The flexor digitorum longus tendon and the neurovascular bundle were identified. During the procedure, it was observed that the neurovascular bundle, including the tibial nerve, was draped over a sizeable and prominent bony process of the posterior bone fragment and quite visibly compressed between the flexor retinaculum and the bone fragment. The attachments of the bony fragment to the surrounding structures were dissected away to excise it in its entirety. After removing the fragment, the visible distortion of the previously overlying tibial nerve resolved.

Outcomes

Postoperatively, the patient was placed in a lower extremity AO splint and remained non–weight bearing for 4 weeks. She was transitioned to a pneumatic walking boot at 2 weeks, which was discontinued at 6 weeks after surgery. Beginning 2 weeks postoperatively, the patient began physical therapy (PT). Over the course of 10 weeks, she attended 17 PT sessions. At the conclusion of her PT program, she was documented to have made good progress in foot and ankle range of motion, strength, and functional mobility. At her 12-month follow-up, the physical examination demonstrated improvement in left plantar sensation. Subjectively, the patient reported notable improvement in her left ankle pain and gait. She noted previous episodes of chronic left ankle swelling to have decreased. Regarding sensation, the patient reported residual numbness along the sole of her foot, yet it remained much improved from when she initially presented. However, no objective scoring measure was undertaken preoperatively or postoperatively.

Discussion

Os trigonum is a relatively common orthopaedic phenomenon of an accessory ossicle that occasionally appears in the pediatric population but may persist into adulthood due to failure of fusion of the ossicle with the talus.9 It is often found incidentally on imaging or is discovered on investigating posterior ankle pain and swelling.8,9 In our case, our patient was initially assumed to have a very large os trigonum. This was primarily due to the location on radiographs. In a small case series of patients diagnosed with a bipartite talus, a patient had been previously diagnosed with an os trigonum under similar circumstances of ankle pain after an ankle inversion injury.10 In other documented patients with a bipartite talus, many have also presented similarly with symptoms that overlap with os trigonum syndrome, such as posterior ankle pain and ankle swelling.11 Initial radiographs may also lend themselves to diagnostic confusion because a posterior bipartite talar fragment may appear to be in a similar location to an os trigonum, making more extensive imaging with CT or MRI necessary to make a bipartite talus diagnosis.6,10,11 In differentiating the two entities, an os trigonum is more likely to appear posterolateral to the talus. By contrast, a bipartite talus will involve the posteromedial and posterolateral tubercles of the talus and appear as a much larger bony fragment, with a division occurring within the coronal plane of the talus separating it into anterior and posterior components with both segments having contributions to the articular surface of the ankle joint.10,11 In this patient, the bone fragment was much more medial and larger than previously documented os trigonums at close to 3 cm on the axial imaging. By contrast, in a retrospective study of foot and ankle axial CT scans of 204 patients with os trigonums, the majority measured less than 1.5 cm.14 On additional inspection, articular cartilage was also noted on the removed specimen, and MRI imaging demonstrated possible articulation with the posterior facet of the calcaneus. These details provided evidence that the structure in this patient was most likely a posterior fragment of a bipartite talus rather than an os trigonum.

There were several confounding factors when determining the role of this bipartite talus as the causative etiology for this patient's TTS. TTS beginning after an acute ankle sprain or trauma has been reported, yet this patient's MRI failed to show evidence of collateral damage typically associated with notable ankle sprains. MRI also demonstrated a lack of bone edema within the talar fragment and the talar body, no evidence of trauma to the surrounding tarsal tunnel, and TTS symptoms preceding this traumatic event by several months suggested a different etiology than acute trauma. There was the possibility of a double crush syndrome with the possible contribution of her spinal stenosis to her overall symptoms. However, EMG/NCV results in Tables 1–3 provided evidence of unilateral abnormalities originating from the tibial nerve at the tarsal tunnel. Although the nerve appeared to lack tethering or flattening at the time of surgery, the lack of weight-bearing radiographs prevents the exclusion of long-standing pes planus, possibly contributing to the delayed presentation in this patient. In addition, there is a described potential association of TTS with a history of carpal tunnel syndrome.15

Given evidence against other common causes of TTS, the bony process of the posterior fragment of the bipartite talus was believed to be contributing to TTS through compression of the tibial nerve. Surgical management was pursued to remove the bone fragment, release the flexor retinaculum, and decompress the tarsal tunnel. As in a previously described study, patients with a compressive structure as the cause of their tibial neuropathy treated with separation of the flexor retinaculum along with the removal of the space-occupying mass were found to have a greater rate of symptom improvement when compared with patients who received surgical treatment for TTS secondary to venous dilatations or of idiopathic origin.16 Other studies also demonstrate good postoperative recovery for patients with TTS after the removal of an extrinsic compressive etiology, with a positive Tinel sign indicative of a good prognosis, particularly with early surgical management.4,5 In this patient, the presence of TTS secondary to the fragment made removing it essential. In the management of symptomatic bipartite talus, preoperative planning must consider the extent of articular involvement, with more articular involvement potentially contributing to subtalar instability after removal. In cases of potentially unstable subtalar joints, posterior fragments have been fused with the talar body and subtalar arthrodesis has been used.10–12 However, in smaller fragments, surgical removal without additional fixation has also been shown to have satisfactory outcomes.6,10,12 In this case, the patient did not require additional subtalar fixation because of the small articulation with the posterior facet and subtalar stability after removal. In addition, due to its location and role in the patients TTS, fixation of the posterior talar fragment could not be pursued. After a review of the literature, few cases have previously described a skeletal abnormality as a cause of TTS.2,3,4,5,6,16 Although one such case has described a similar bipartite talus as a cause of TTS, this case highlights how a relatively common entity in an os trigonum may seem similar to a bipartite talus and adds to the literature surrounding this rare entity, its pathological consequences, and prognosis after its removal.6

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