Biological basis

Etiology and pathogenesis

The tibial nerve arises from the larger, medial portion of the sciatic nerve; it usually grossly separates from the sciatic nerve at the level of the middle to distal third of the thigh. With the common peroneal nerve, the other derivative of the sciatic nerve, it lies deep to the hamstrings. In the proximal thigh, it supplies all the hamstrings (except the biceps femoris short head). In the popliteal area, it usually lies posterior to the popliteal artery and vein. Sensory branches typically leave the tibial nerve in the proximal calf before major motor branches arise. The nerve tends to run beneath the gastrocnemius-soleus muscle, supplying branches to the plantaris, popliteus, and tibialis posterior as well as the gastrocnemius-soleus. A deep or posterior tibial branch (or nerve) arises from it and accompanies the tibial artery and vein to run through the leg, medial and posterior to both the tibia and to the intermuscular septum separating anterior from posterior compartments. The posterior tibial nerve carries fibers destined for the foot, but it gives off motor branches in the more proximal leg to supply the flexor digitorum and flexor hallucis longus. As the posterior tibial nerve approaches the ankle, it courses inferior to the medial malleolus and branches into the medial and lateral plantar nerves as it passes beneath the medial collateral ligament. The medial plantar nerve supplies the intrinsic foot muscles on the medial aspect of the foot and sensation to the medial aspect of the foot. The lateral plantar nerve runs deep in the instep and supplies the foot intrinsic muscles on the lateral aspect of the foot as well as sensation to the lateral portion of the sole of the foot. A medial calcaneal branch can often be found arising either proximal to these nerves or arising from the medial one. It provides sensation to the heel.

Unilateral nerve injury can cause profound, long-lasting, nerve branch-specific loss of distal innervation both contralaterally and ipsilaterally (21). This study was conducted in rats, but it does lend credence to the fact that homologous nerves may be affected when there is unilateral injury.

Epidemiology

Tibial nerve entrapment is most common in the tarsal tunnel. Tarsal tunnel syndrome is, however, very rare.

Prevention

Most causes of tibial neuropathies are not preventable by any specific action that can be easily predicted. Common sense is the best advice. Proper shoe fitting and care with avoidance of compression is important. There was a case report of a hockey player in Canada who suffered from a neuropathy due to an inflatable hockey skate; the patient improved clinically after discontinuing use of the skate (29).

Differential diagnosis

The differential diagnosis of tibial neuropathy includes a partial sciatic nerve lesion, sacral plexopathy, and S1 or S2 radiculopathies. Extensive investigations of more proximal muscle groups, especially gluteal and hamstrings, may be helpful.

In a retrospective chart review of 109 patients with electrodiagnostically confirmed sciatic neuropathies, a third of the cases were related to hip replacement surgery and less than 10% were due to inflammatory sciatic neuropathy (04). The electrophysiology demonstrated mostly axonal pathophysiology that in 5.5% of cases was predominantly in the tibial division. Favorable outcome was associated with young age, lack of severe initial weakness, and presence of tibial compound muscle action potential or sural sensory nerve action potential.

Diagnostic workup

As with all aspects of neurology, a good history and physical examination are important. It is important during the examination to evaluate the gluteal muscles and hamstrings. The popliteal fossa should be palpated for masses and the knee and foot for Tinel sign. In tarsal tunnel syndrome a Tinel sign producing paresthesias on the bottom of the foot is usually present inferior to the medial malleolus and sometimes proximally or distally to the instep.

As an example of sensory innervation, the posterior tibial nerve supplies sensation to the foot, giving rise to calcaneal (heel) and medial and lateral plantar nerves. The sural nerve supplies the lateral side of the foot and the saphenous nerve a patch below the medial malleolus. Complete tibial nerve injury results in sensory loss on the sole and heel of the foot. In terms of electrodiagnostic evaluation, most lesions are axon loss in type; consequently, the amplitude of the tibial motor conduction studies recording both abductor hallucis and abductor digiti quinti pedis is often affected. Also, with lesions proximal to the popliteal fossa the amplitude of the direct muscle response (M component) of the H-reflex is often low, and the H-wave unattainable. Lesions proximal to the popliteal fossa will also affect the amplitude of the sural response, but usually this is normal because the lesions are in the popliteal fossa or more distal. With S1 radiculopathies the sural nerve conduction study's response is normal, whereas it is usually low in amplitude or unelicitable with sacral plexopathies or sciatic neuropathies. Sensory nerve responses can also be obtained for medial calcaneal nerve; conduction latencies, amplitudes, and sensory nerve conduction velocities are significantly less in patients with plantar fasciitis (03). The needle electrical examination can be helpful for localization by revealing which muscles are affected and which are not. The needle electrode examination assessment must also include limb muscles innervated by the common peroneal as well as the tibial nerve (03).

Tumors may require MRI using gadolinium enhancement. Soft tissue tumors in the popliteal space may be seen with plain film radiographs, ultrasonography, or computerized axial tomography scanning. Arthrograms and arteriography can be useful for Baker cysts and popliteal aneurysms.

In a preliminary report, high-resolution MR neurography (MRN) was promising and yielded accurate morphologic information about the location and extent of nerve injury after failed tarsal tunnel release (05). MRN studies were reviewed in nine patients (10 legs) referred for failed tarsal tunnel release. Nine studies (90%) had findings of nerve re-entrapment related to focal fibrosis, with MRN sensitivity of 77% for posterior tibial nerve, 100% for medial plantar nerve, and 100% for lateral plantar nerve injury. The sensitivity of MRN was 67% for neuroma detection. Normal values for the ratio of fractional anisotropy to apparent diffusion coefficient on diffusion tensor imaging (DTI) has been established in 25 normal controls, and more is to be learned about the usefulness of this novel method in diseased sural nerves (14). In another study using DTI tractography to image 26 cases of soft tissue peripheral nerve tumors, DTI was found to be possibly helpful in the assessment of peripheral nerve infiltration by malignant tumors, but more studies are needed (12).

Management

Management depends on the problem. In cases of compression, removal of the compressive force is the first step. Baker cysts can be removed surgically as can many nerve sheath tumors. Tarsal tunnel syndrome treatment, at least initially, should be conservative. When conservative measures fail, surgical release may be considered. Krishnan and colleagues described a novel technique of endoscopic in situ decompression in eight patients with moderate to severe compression of the tibial nerve at the tarsal tunnel who had failed conservative management (15). All patients had an outcome rated good to excellent with neurodiagnostic normalization in six of eight patients. A more extensive surgery may be needed to effectively decompress the tibial nerve and its branches in tarsal tunnel syndrome (23). In addition to roof incision, this involves decompression of the medial plantar and lateral plantar nerves and the excision of the inter-tunnel septum. There is an increasing interest in acupotomy for the treatment of tarsal tunnel syndrome. This modification of acupuncture uses a modified needle that is bladed and with a flat head and cylindrical body. Acupotomy is widely used as part of traditional Chinese medicine. A systematic review is in progress of acupotomy for the treatment of tarsal tunnel syndrome (27). In a systematic review of the management of tarsal tunnel syndrome, the authors suggest that surgical treatment should be reserved to a very select group of cases refractory to medical or minimally invasive options (28).

The unique combination of plantar fasciitis, posterior tibial tendon dysfunction, and tarsal tunnel syndrome is referred to as “heel pain triad.” Labib and colleagues found that those who manifested this triad improved after surgery 85% of the time (16). Surgical success is debatable and many patients have multiple surgeries with worsening symptoms. Though similar improvement by objective measurements was noted in a series of 68 surgeries, 51% of patients reported persistent symptoms following tarsal tunnel release (09). Superficial peroneal nerve is a valuable nerve donor site, particularly when multiple long grafts are needed for large motor and sensory defects (01). Physical therapy and orthotic devices should also be investigated when deemed appropriate.

Initial management of Morton neuromas is also conservative, consisting of shoe modifications and steroid injections. Though the difference in improvement between steroid injections with shoe modification and shoe modification alone was not statistically significant at 12-month follow-up, Saygi and colleagues reported a trend toward better outcome and a significant difference in patient satisfaction in those receiving steroid injections (25). Haddad-Zebouni and colleagues reported near complete regression of Morton neuroma on MRI following steroid injection (10). Fifty-eight subjects diagnosed with Morton neuroma and foot pain of at least four were locally injected with capsaicin 0.1 mg or blinded placebo (02). At weeks 1 and 4, there was statistically significant pain reduction in the capsaicin group as compared to placebo, and at weeks 2 and 3, a trend towards capsaicin benefit was observed. The capsaicin-treated group had improvements in functional interference scores and reductions in oral analgesic use. This study provides preliminary evidence that local capsaicin injection may be a short-term therapy effective in reducing pain of patients with Morton neuroma. A study showed no difference in outcomes between patients injected with a corticosteroid plus a local anesthetic as compared to a local anesthetic alone (17). In a long-term study of the treatment of Morton neuroma, ultrasound-guided injections yielded better outcome than blind injections for up to three years (24).