A 31-year-old man experienced pain at the medial side of his left talus during plantar flexion of his left foot. Symptoms started three months before when he began playing basketball. His symptoms were attributed to tarsal tunnel syndrome (TTS) and he was referred for an MRI.
An MRI of his left ankle joint was performed on a 1.5 Tesla scanner. There was a soleus muscle with a lower than usual tendon insertion into the achilles tendon [Fig.1-3]. Besides that, a volume of tissue was disclosed encircling the belly of the flexor hallucis longus (FHL) muscle at the level of the ankle joint [Fig. 1b], where the belly of the FHL is expected to end. This tissue, distally to the ankle joint, seemed abutting the FHL tendon, and followed the tendon in front of the flexor retinaculum (FR) into the tarsal tunnel until the sustentaculum tali [Fig. 2, 3]. The nerves and vessels of the tarsal tunnel were displaced medially and backwards [Fig. 1b]. The signal intensity equalled that of muscle in all sequences [Fig. 2, 3]. There were no low signal intensity structures in this volume of tissue resembling tendons and/or vessels. The findings were interpreted as a low extending FHL muscle belly causing TTS.
The tarsal tunnel is bounded ventrally by the FR and anteriorly by the distal medial cortex of the tibia, and the medial aspect of the talus and calcaneus. In the tunnel run tendons (FHL, flexor digitorum longus, tibialis posterior), vessels (posterior tibial artery and veins), and nerves (posterior tibial nerve and its terminal branches). The entrapment of these nerves in the tarsal tunnel causes tarsal tunnel syndrome (TTS) [1]. The TTS is idiopathic in about 50% of cases [1]. In the remaining cases various lesions have been described as causes. They include bone deformities after injury, tenosynovitis of the flexor tendons, tumours and tumour-like lesions, synovial hypertrophy and accessory or anomalous muscles [2].
Accessory muscles that may cause TTS are [3]: a) Flexor Digitorum Accessorius Longus (FDAL): FDAL has a prevalence of about 7%. The tendon descents in close relation to the posterior tibial nerve into the tarsal tunnel, beneath the FR, causing forward displacement of the nerve. In our case the neurovascular bundle was situated just beneath the FR excluding the presence of FDAL [Fig. 1]. b) Peroneocalcaneus Internus (PCI): PCI is uncommon. The tendon descends posteriorly and laterally to the FHL, displacing it medially, and adheres to the calcaneus. In our case there was no tendon attaching to the calcaneus [Fig. 1]. c) Accessory Soleus (AS): AS has a prevalence of 0.7-5.5%. It is located in front of the achilles tendon behind the FHL. Its insertion may be anywhere between the calcaneus and the gastrocnemius tendon [4]. It may compress the posterior tibial nerve, if hypertrophied. In our case there was a soleus muscle with a low-lying belly [Fig. 1-3], located away from the neurovascular bundle [Fig. 1, 2, 3]. d) Tibiocalcaneus Internus (TCI): TCI is rarely found. Its tendon descends deep to the FR and posterior to the neurovascular bundle, inserting onto the medial cortex of the calcaneus. In our case the neurovascular bundle was displaced medially and backwards, and there was no tendon insertion to the calcaneus [Fig. 1].
Anomalous muscles have been reported in the literature that may cause TTS [5, 6]. In these patients with TTS a hypertrophied belly of the FHL muscle was depicted extending into the tarsal tunnel. CT was used in the first case and MRI in the second. In our case MRI depicted accurately both the low belly of soleus muscle and the low extending belly of the FHL. The TTS was attributed to this latter finding.
TTS due to low extending FHL muscle belly.
1. The MRI shows soft tissue signal proliferation or abnormalities near the medial malleolus (medial aspect of the talus), which corresponds to the area where the patient reports pain in a plantarflexed position.
2. Within the tarsal tunnel (formed by the medial aspects of the talus and calcaneus), there is an abnormally low extension of the flexor hallucis longus (FHL) muscle belly.
3. In the posterior muscle group of the lower leg, the soleus muscle belly is situated lower than usual, but it is relatively distant from the neurovascular bundle, and no significant signs of compression are noted.
4. The neurovascular bundle (posterior tibial nerve and its branches, the posterior tibial artery, and accompanying veins) is partially displaced anteriorly or medially within the tarsal tunnel compared to its normal anatomical location, lying closer to the flexor retinaculum (FR).
5. No distinct signs of bony destruction or fracture are observed, and common local tumors or significant inflammatory lesions are excluded.
Considering the patient’s symptoms, increased physical activity over the past three months (basketball), and MRI findings of a distinctly low-lying FHL muscle belly within the tarsal tunnel, the tunnel appears narrowed, causing compression of the posterior tibial nerve.
The most likely diagnosis: Tarsal Tunnel Syndrome (TTS) caused by the low-lying flexor hallucis longus muscle belly.
1. Conservative Treatment:
- Reduce weight-bearing activities: Limit or decrease high-load foot and ankle activities such as basketball, especially avoiding excessive plantarflexion or intense rotational movements of the foot.
- Medications: Non-steroidal anti-inflammatory drugs (NSAIDs) can help relieve pain and inflammation; short-term local corticosteroid injections may reduce inflammatory edema around the nerve.
- Orthotic devices: Use arch supports or ankle braces to reduce stress within the tarsal tunnel, helping to “decompress” the nerve.
- Physical therapy: Modalities such as heat therapy, ultrasound, low-frequency electrical stimulation, and iontophoresis can improve local blood circulation and reduce edema, promoting tissue repair.
2. Surgical Treatment:
- If conservative treatment for 3–6 months shows poor results or if symptoms worsen, surgical exploration and decompression (tarsal tunnel release) may be considered. This can include repair or modification of any abnormal muscles/tendons (e.g., partial resection or revision of the FHL muscle belly) to relieve pressure on neurovascular structures.
3. Rehabilitation/Exercise Prescription (FITT-VP Principle):
- Frequency (F): 3–4 times per week, starting with low-intensity active exercises for the ankle joint.
- Intensity (I): Based on pain tolerance, maintain low to moderate intensity; increase gradually if no significant pain occurs.
- Time (T): Each session lasts 20–30 minutes, performed in 2–3 sets with appropriate rest intervals.
- Type (T): Emphasize stretching and stability exercises, such as small-range ankle dorsiflexion/plantarflexion stretches, Achilles tendon and plantar fascia stretching, and ankle stability/balance training (e.g., balance board).
- Progression (P): As pain decreases and function improves, gradually introduce resistance band work, single-leg support, or light jogging. If discomfort worsens, adjust or pause the exercise program promptly.
- Special considerations: Discontinue training and seek medical assessment if severe swelling or worsening radiating nerve pain occurs. Appropriate footwear is recommended to protect the ankle joint, and regular follow-up with a physician or rehabilitation specialist is advised.
This report is based on currently available data for analysis and is provided for clinical reference and patient education only. Actual treatment requires an in-person clinical examination, further auxiliary tests, and advice from a specialized physician. This report does not replace in-person consultations or professional medical guidance.
TTS due to low extending FHL muscle belly.
