A Closer Look At Tarsal Tunnel Syndrome
Symptoms associated with compression of the posterior tibial nerve and its branches first appeared in the literature in the early ‘60s.1-3 Since these early reports over 40 years ago, tarsal tunnel syndrome has become one of the most written about and discussed foot and ankle pathologies. Yet, even with the vast amount of literature on the subject, tarsal tunnel syndrome often remains somewhat elusive in regard to diagnosis and treatment. When inspecting the anatomy of the posterior tibial nerve, it is easy to appreciate why compression neuropathy may occur. Entrapment may occur proximally where the nerve lies between the deep and superficial fascia of the leg. As it courses distally closer to the medial malleolus, the fascia thickens and forms the flexor retinaculum, which overlies the nerve at this area. The nerve is further bound within its tunnel in this location by the medial surface of the tibia, talus and calcaneus. Fibrous septae from the retinaculum separate the nerve and posterior tibial artery from the flexor hallucis longus, flexor digitorum longus and posterior tibial tendons. The posterior tibial nerve divides into its two terminal branches (medial plantar nerve and lateral plantar nerve) deep to the flexor retinaculum in over 90 percent of cases. The medial calcaneal branch usually originates just proximal to or deep to the retinaculum. However, this branch may originate further distally. The medial calcaneal branch divides from either the posterior tibial nerve or lateral plantar nerve and is usually, although not always, a single branch.4 The medial and lateral plantar nerves then continue distally, deep to the abductor hallucis. The first branch of the lateral plantar nerve originates distally and is also a potential site of nerve entrapment between the deep fascia of the abductor hallucis and medial caudal margin of the quadratus plantae.5 Although the tarsal tunnel has been traditionally thought of as being only deep to the flexor retinaculum, it may extend anatomically both proximally and distally. Over 25 different etiologies are commonly discussed in the literature as contributing to the causation of tarsal tunnel syndrome. Some of the more common ones include overpronation or a valgus rearfoot, space occupying lesions including benign or malignant tumors, neuropathy (idiopathic or disease-related), trauma, arthritis, diabetes, various accessory muscles and posterior tibial tendon dysfunction. One of the more overlooked etiologies is the “double crush syndrome.”6 In this setting, a more proximal nerve injury may result in diminished axoplasmic flow, predisposing the nerve to be more easily compressed distally. What You Should Look For In Terms Of Key Clinical Findings The patient with tarsal tunnel syndrome may present with a variety of symptoms and it is this set of concurrent symptoms, which truly defines compression in the tarsal tunnel as a “syndrome.” Symptoms include combinations of burning, tingling, numbness over variable areas plantarly and/or sharp pain. More often than not, patients will describe the pain as diffuse but may be fairly specific in cases of space occupying lesions that produce isolated nerve compression. Extended standing or walking may aggravate symptoms. Conversely, some patients may experience night pain and find relief by becoming ambulatory. When examining a patient with tarsal tunnel syndrome, he or she may have a positive Tinel’s sign or Valleix phenomenon with percussion. It is usually somewhat difficult to actually demonstrate areas of discrete numbness plantarly. A significant decrease in two-point discrimination between the involved and uninvolved side has been reported in the literature.7,8 One may appreciate a motor deficit of the short flexors or abductor hallucis atrophy. You would also assess the deep tendon reflexes in order to help determine if there is proximal causation. Proceed to perform venous and arterial examinations in order to rule out venostasis or arterial insufficiency. Simple observation or palpation will reveal the presence of most space occupying lesions. One should inspect the patient’s stance and gait for excessive pronation or valgus deformities of the rearfoot.9 A Review Of Diagnostic Studies While electrodiagnostic studies remain controversial for detecting possible tarsal tunnel syndrome, the testing technique is still the most commonly used objective test for evaluating compression of the posterior tibial nerve and its branches. Electrodiagnostic tests of the tarsal tunnel can be separated into two main areas: sensory conduction of the medial and plantar nerves, and motor conduction of the posterior tibial nerve. A third area of testing, mixed nerve conduction of the plantar nerves, may also be beneficial. One may consider testing of the common peroneal nerve to evaluate for the presence of a peripheral neuropathy. You can test sensory conduction by placing surface electrodes for recording proximal to the flexor retinaculum. Place distal electrodes at the hallux when evaluating the medial plantar nerve and at the fifth toe when evaluating the lateral plantar nerve. Byank, et. al., found the average sensory distal latency to be 6.4 ms in normal subjects and 6.8 ms in patients with tarsal tunnel syndrome (1 SD from normal).10 The sensitivity for this test has been reported to be as high as 93 percent, but specificity is not as good with false positives reported from 4 percent to 8 percent.11 Nevertheless, sensory conduction remains the best objective test for diagnosing tarsal tunnel syndrome. A standardized method of measuring motor nerve latencies consists of stimulating the posterior tibial nerve proximal to the tarsal tunnel, 10 cm proximal to the recording electrode. It is critical to include temperature controls with this test. The standard motor latency of the medial plantar nerve at the abductor hallucis, using the above 10-cm distance, averaged 3.8 seconds. Likewise, the standard latency of the lateral plantar nerve at the abductor digiti minimi averaged 3.9 seconds.12 Values beyond 1 SD are considered suggestive of tarsal tunnel syndrome. Unfortunately, motor conduction studies show low sensitivity with only as high as 50 percent of symptomatic patients showing abnormal results. Many electromyographers feel a study of motor-evoked potentials and a search for fibrillation potentials yield a more sensitive motor study. Mixed nerve studies have also been described and standardized.13 These studies were designed to improve the accuracy of sensory conduction studies and increase the sensitivity of motor conduction studies. This study places the recording electrodes plantarly over the medial and lateral plantar nerves and records a larger amplitude of signal over a distance of 14 cm. In patients with tarsal tunnel syndrome, reduced amplitude and slowed conduction has been seen up to 86 percent of the time. Magnetic resonance imaging (MRI) is best utilized in investigating for the presence and extent of space occupying lesions such as tumors, ganglion cysts, etc. One may see tenosynovitis of adjacent tendons as well as deep varicosities or post-traumatic scar tissue. If you are planning to perform revisional tarsal tunnel surgery, a MRI may be quite valuable in identifying constricting scar tissue or a previous incomplete release of the flexor retinaculem. When Conservative Treatment Is Indicated In most cases of tarsal tunnel syndrome, initial treatment usually consists of nonoperative management. (If there is a mass producing significant nerve compression or if suspected malignancy exists, immediate release, exploration and/or biopsy are obviously recommended.) Nonoperative treatment may include the use of nonsteroidal antiinflammatory drugs, injections of local anesthetic and/or corticosteroids, physical therapy and compression stockings if edema is present. One may also employ local anesthetic injections for their “diagnostic” property. Immobilization in a short-leg walking cast or walking boot may be beneficial. You may emphasize orthotics if you consider overpronation or valgus position of the rearfoot a likely etiology of the patient’s symptoms. Key Pearls For Surgical Decompression If an adequate period of conservative treatment has failed, then surgical treatment is warranted. One may proceed with surgical decompression of the posterior tibial nerve under general or spinal anesthetic if using a thigh tourniquet or with a popliteal or further distal block in the absence of a tourniquet. Place the patient in a supine position with adequate external rotation of the affected limb. Start the incision at least 6 to 7 cm superior to the tip of the medial malleolus, coursing inferiorly about 2 cm posterior to the malleolus and then turning distally into the medial instep over the abductor hallucis muscle belly. Then carry out dissection to the level of the flexor retinaculum and the deep fascia over the abductor hallucis. Proceed to completely release the fascia well proximal to the flexor retinaculum and follow with a complete release of the flexor retinaculum and abductor hallucis fascia. Take care to inspect and preserve the medial calcaneal nerve. At this time, you should have a good view of the neurovascular bundle and be able to inspect the nerve and its branches for remaining constriction from any soft tissue structures. If you feel that local varicosities are pathologic, ligate them. Proceed to perform plantar retraction of the abductor hallucis and carry the release distally in order to include the first branch of the lateral plantar nerve. This necessitates releasing the deep fascia of the abductor hallucis over the quadratus plantae and may include “windowing” the fascia in this area. It is my opinion that one must carry out complete release in tarsal tunnel surgery well proximal and distal to the flexor retinaculem. Failure to do so is one of the most common causes of continued symptoms postoperatively. Take care to preserve the posterior tibial artery and its branches, especially distally where there is often overlap of the arterial structures over the nerve branches. Identifying respective nerve branches and ensuring that no further constriction exists is all that is needed. Avoid excessive dissection and “stripping” of the nerve and its branches. A more limited approach may be feasible in cases in which you need to remove smaller space occupying lesions. At this point, release the tourniquet, if utilized, and obtain hemostasis. You can observe the nerve directly for return of normal coloration. The deep fascia is not closed and you would only use subcutaneous and skin sutures. Apply a well-padded dressing and plaster splint, and keep the patient nonweightbearing for 10 to 14 days. Postoperative care includes removing the skin sutures 14 days after surgery. Emphasize a walking boot and have the patient start range of motion exercises. Utilize physical therapy for edema control and prevention of excessive postoperative scarring. Return the patient to normal shoewear and allow activities as he or she can tolerate them. Other Pertinent Points About Tarsal Tunnel Surgery Endoscopic release of the tarsal tunnel has been reported in the literature.14 Obviously, one cannot use this technique to excise space occupying lesions. Although release of the flexor retinaculum is possible endoscopically, distal release of the abductor hallucis deep fascia and the first branch of the lateral plantar nerve is quite difficult with this technique. One would carry out a revisional tarsal tunnel release as described above. However, this can be extremely challenging due to the fact that significant fibrosis and scarring are usually present. This results in the loss of usual soft tissue landmarks, making surgical dissection quite demanding. Results of tarsal tunnel surgery vary greatly in the literature with good results varying from 44 percent to 100 percent. These results are somewhat specific to etiology.15 In my experience, I have found the best results occur with excision of space occupying lesions. Associated systemic diseases or proximal nerve pathology tend to lower the rate of success of tarsal tunnel surgery. Also, in a limited study, younger patients appeared to gain a higher rate of favorable results than older patients.15 Final Words Tarsal tunnel syndrome remains a diagnostic and treatment dilemma. However, when taking a closer look at this disorder, I believe the following points are paramount to achieving satisfactory outcomes. • When performing the exam on a patient with suspected tarsal tunnel compression, realize that this is truly a syndrome producing a set of concurrent symptoms. • Although the use of electrodiagnostic studies in evaluating for tarsal tunnel syndrome remains controversial, they remain the best objective tests available. It is important that experienced personnel perform them under controlled circumstances. Sensory conduction studies are the most sensitive. • If you proceed to surgery, you must perform a complete release proximally and distally. This includes performing the release well proximal to the flexor retinaculem and distally as far as the first branch of the lateral plantar nerve. • One must emphasize early range of motion and physical therapy to prevent excessive postoperative fibrosis and edema. Dr. Thomas is an Associate Professor within the Division of Orthopaedic Surgery in the School of Medicine at the University of Alabama in Birmingham, Ala. He is on the Board of Directors of the American College of Foot and Ankle Surgeons.
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