A 47-year-old male patient who complained of pain and numbness in the medial aspect of the sole of his foot for two months, and alteration of the electromyography in that location.
On the lateral ankle radiograph we can see the talocalcaneal C sign (Fig 1), therefore MRI is performed to complete the study.
MRI showed an overgrowth of the posteromedial subtalar facet and posterior-subtalar joint space narrowing associated. These findings were consistent with non osseous posteromedial subtalar coalition. MRI also revealed an increase of soft tissue inside the tarsal tunnel appearing hypointense on T1-weighted images and slightly hyperintense on STIR/T2 fat-satured weighted images, which means inflammation. We can also observe the presence of small cystic lesions inside the tunnel, better seen on STIR/T2 fat-satured weighted images, which are compatible with ganglions. Abductor hallucis is seen hyperintense on the axial T2 fat-saturated image, related to medial plantar nerve denervation oedema, secondary to compression by the posteromedial subtalar coalition.
The tarsal tunnel is a fibro-osseus canal bordered by medial malleoulus, the talus and calcaneus laterally; and the flexor retinaculum medially. The posterior tibial nerve travels within the tarsal tunnel, along with the posterior tibial, flexor digitorum longus, and flexor hallucis longus tendons and the posterior tibial vessels. This nerve bifurcates, in this region, into the medial and lateral plantar nerves [1]. The major mechanisms responsible for tarsal tunnel syndrome are:
- Compression neuropathy, secondary to trauma (fracture, surgery, and scarring), space-ocuppying lesions (tumour, ganglions, varicosities, coalition, anomalous muscles...), and foot deformities (hindfoot valgus and, less typically, hindfoot varus, with forefoot pronation, pes planus, and tarsal coalition).
-Tension neuropathy, which is significantly increased in ankles in dorsiflexion and hindfoot eversion [1, 2].
Patients commonly complain of numbness, burning pain and paresthesias in the toes, sole of the foot, or medial heel aggravated by weight bearing [3]. The most helpful signs are sensory loss along the plantar aspect of the foot and a positive Tinel sign at the tarsal tunnel, while the failure of motor function is more rare and late [1].
Magnetic resonance imaging and high-resolution ultrasonography have been used as secondary tools to confirm the presence of nerve entrapment or compression, to identify the cause of neuropathies. The clinical history, physical examination, and electrodiagnostic study, including electromyographic and nerve conduction studies are all the basis of the diagnosis. The characteristic signal intensity patterns of acute and subacute muscle denervation at MR imaging include high signal intensity for denervated muscle on images obtained with fluid-sensitive sequences, such as T2-weighted or STIR images, and normal signal intensity on T1-weighted images. Atrophy and fatty replacement occur in chronically denervated muscles [2].
Only a few series of tarsal tunnel syndrome have been reported, and the report of the talocalcaneal coalition as a cause of the syndrome is more uncommon. Posterior talocalcaneal coalition is more often associated with tarsal tunnel syndrome and the medial plantar nerve is predominantly involved [4].
Surgical decompression is the treatment of choice for tarsal tunnel syndrome when conservative treatment (anti-inflammatory medications, injection of corticosteroids into the area around the nerve, orthotics and changes in footwear) are insufficient [5]. Decompression should be performed early to prevent nerve fibrosis [6].
MR Imaging is an excellent modality for imaging the tarsal tunnel and identifying potential causes of posterior tibial nerve entrapment. Space-occupying lesions can be well depicted at MR imaging.
Posteromedial subtalar coalition causing tarsal tunnel syndrome and abductor hallucis denervation
1. X-ray (Lateral View of the Foot): An abnormal shape of the articular surfaces between the talus and calcaneus in the posterior region is visible, suggesting a possible partial or complete talocalcaneal coalition. The posterior portion of the subtalar joint space appears indistinct, with some trabecular abnormalities or signs of fusion. No apparent fracture line is observed in the overall bony structure.
2. MRI (Sagittal and Coronal Sequences):
- A low-signal, bridge-like structure is observed at the talus-calcaneus junction, consistent with bony or fibrous coalition. The local joint space is narrowed or absent.
- No obvious swelling lesion in the soft tissue around the subtalar joint. However, slightly increased signal intensity is seen in the soft tissues along the medial plantar aspect on T2/STIR sequences, suggesting possible chronic inflammation or perineural reactive edema.
- No obvious tumor lesion or other mass effect is identified. Overall muscle morphology of the foot is acceptable, without significant atrophy or fatty degeneration, though a small area of increased fluid signal is noted near the pathway of local nerves.
Considering the patient’s age (47 years), main complaint (medial plantar numbness and pain for 2 months), electrodiagnostic findings indicating nerve conduction abnormalities in the corresponding area, and imaging evidence suggesting a possible posterior talocalcaneal coalition, the most likely diagnosis is: Tarsal Tunnel Syndrome Caused by Posterior Talocalcaneal Coalition.
Disclaimer: This report serves as a reference analysis and does not replace in-person consultations or professional medical advice. Specific diagnoses and treatments should be determined by a clinician, taking into account the patient’s actual condition.
Posteromedial subtalar coalition causing tarsal tunnel syndrome and abductor hallucis denervation
