Compressive neuropathies of the foot and ankle

COMPRESSIVE NEUROPATHIES OF THE FOOT AND ANKLE DONALD E. BAXTER, MD

Subtle compressive neuropathies of the foot and ankle can cause weakness in the foot that affects athletic performance, especially iri runners or jumpers. Detection of these syndromes requires a knowledge of the sensory distribution and anatomic course of the peripheral nerves in the foot and ankle. Most compressive neuropathies can be treated with standard conservative measures; however, if chronic pain causes significant disability, surgical treatment may be indicated. KEY WORDS: entrapment syndromes, interdigital neuroma, tarsal tunnel syndrome, treatment

Tarsal tunnel syndrome usually produces burning, aching, and cramping sensations in the forefoot as well as plantar pain and paresthesia. These symptoms often are a§gravatedby activity, but night pain also may occur.!: Etiologies of tarsal tunnel syndrome include tenosynovitis of the posterior tibial tendon, varicosities, benign tumors, bony impingment of the medial aspect of the talus, postural tension on the nerve from excessive pronation of the foot, and fibrosis surrounding the posterior tibial nerve behind the medial malleolus.f The importance of careful examination is illustrated by the athlete who came to my office with typical symptoms of tarsal tunnel syndrome. Compression of the nerve behind the medial malleolus produced pain that radiated in a fashion typical of tarsal tunnel syndrome. However, nerve conduction studies showed that the compression was in the deep posterior compartment at the lower edge of the gastrocnemius. The symptoms resolved after release of the deep posterior compartment. Tarsal tunnel syndrome distal to the medial malleolus can be caused by isolated compression of either the medial or lateral plantar nerves or by compression of both nerves by the superior aspect of the abductor hallucis muscle fascia (Fig 1). Most frequently the medial plantar nerve is compressed at the knot of Henry ("jogger's foot'": Fig 2). Symptoms include "giving way" of the foot, radicular pains, and instability. Careful physical examination is essential for diagnosis of distal tarsal tunnel syndrome. Often the athlete has a history of ankle sprain or hypermobility at the talonavicular joint that causes excessive migration of the navicular OJ} the head of the talus and stretches the medial plantar nerve at the knot of Henry. One of our patients, a world-class runner, had vague foot pain that had persisted for 3 years and that caused his foot to give way on the second or third lap of a fast race. Careful examination suggested a compressive disorder in the medial foot in the area of the From Baylor College of Medicine, Houston, TX. Address reprint requests to Donald E. Baxter, MD, 7500 Beechnut St, No. 175, Houston, TX 77074. Copyright © 1994 by W. B. Saunders Company 1060-1872/94/0201-0003$05.00/0

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medial plantar nerve, and release allowed the runner to return to world competition. Etiological factors in this athlete were instability of the talonavicular joint from a previous lateral ankle and subtalar strain and hyperflexibility of the midfoot and subtalar joints.

Treatment Initial treatment for tarsal tunnel syndrome at the ankle and distally is conservative, including the use of a longitudinal arch support or a medial sale and heel wedge built into the athletic shoe.f This medial wedge should be no more than 3/16-inch thick. The longitudinal arch support or medial wedge functionally inverts the foot, taking some of the stretching effect away from the tibial nerve and its branches. The longitudinal arch support also may eliminate the knifelike pinching of the medial plantar nerve at the knot of Henry. If problems persist despite a decrease in activity and support of the medial foot structures, nerve conduction studies should be performed. I -3•s Conduction should be tested across the tarsal tunnel at the ankle. The medial and lateral nerves also should be tested to determine if a distal tarsal tunnel syndrome is present. Nerve testing also will determine if neuritis or a double-crush syndrome (pressure on the nerve in two places) exists. If nerve conduction studies suggest nerve compression, surgical release should be considered, but only after conservative measures have failed.

Technique With the use of a regional anesthetic at the ankle, a small incision no longer than 3 or 4 inches is made behind the medial malleolus to allow release of the tibial nerve. Release of the superior edge of the abductor muscle and the deep fascia underneath the abductor hallucis muscle is imperative, because this is the area in which the tibial nerve is most commonly entrapped. If a more distal tarsal tunnel syndrome exists at the knot of Henry, a more isolated release of the medial plantar nerve is carried out. Usually, if compression is iso-

Operative· Techniques in Sports Medicine, Vol 2, No 1

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as a lower back disorder) should be ruled out before making this diagnosis. If entrapment is present in this area, release of the distal abductor muscle in the area of nerve exit will eliminate symptoms.

ENTRAPMENT OF THE FIRST BRANCH OF THE LATERAL PLANTAR NERVE Posterior tibial n.

Medial plantar n.

Nerve to abductor digiti quinti muscle Distal entrapment Lateral plantar n.

Fig 1. The tarsal tunnel. (Reprinted with permission from Baxter DE: Functional nerve disorders in the athelete's foot, ankle, and leg. AAOS Instruct Course lect 42:185-194,1993.)

lated, some of the fat in the area will be compressed and cause an indention on the nerve. I do not strip the remaining fat from the nerve but simply decompress the tight fascial structures from the nerve in the medial aspect of the superior abductor hallucis muscle fascia. Rarely, the medial plantar nerve may be entrapped as it exits the distal abductor muscle before it migrates underneath the tibial sesamoid. Entrapment at this location is difficult to diagnosis with electro diagnostic studies, and diagnosis usually depends on clinical examination. A more proximal lesion and double-crush syndrome (such

Another common compression syndrome is caused by entrapment of the first branch of the lateral plantar nerve underneath the proximal portion of abductor hallucis muscle in the heel 6 ,7 (Fig 3). This is a mixed nerve with both sensory and motor fibers that ultimately innervates the abductor digiti quinti muscle. It most commonly is entrapped where it migrates over the sharp edge of the quadratus plantae medially and underneath the tight fascia of the abductor hallucis muscle (Fig 4). Pronation of the foot, thickening of the plantar fascia, or bony formation within the proximal portion of the flexor brevis muscle can compress the nerve and cause neurogenic syxnptoms. Pain usually occurs on the medial aspect of the heel at the origin of the plantar fascia and migrates proximally up into the ankle and across the inferior heel into the distal lateral aspects of the foot. Application of compression produces a characteristic pain along the medial heel structures, especially under the abductor hallucis muscle.

Treatment This syndrome usually responds to conservative treatment, including appropriate longitudinal arch support and heel padding, stretching of the Achilles tendon and plantar fascia, and decreased activity." If neurogenic pain persists despite 12 months of conservative treatment, decompression of the nerve should be considered.

ENTRAPMENT OF THE DEEP FERONEAL NERVE The deep peroneal nerve passes deep to the superior and inferior retinaculum and bifurcates into medial and lateral terminal branches. Compression of the deep peroneal

Posterior tibial n.

Fig 2. Medial planar nerve entrapment. (Reprinted with permission from Baxter DE: Functional nerve disorders in the athelete's foot, ankle, and leg. AAOS Instruct Course lect 42:185-194, 1993.) COMPRESSIVE NEUROPATHIES

Fig 3. Release of the deep fascia of the abductor hallucls muscle. (Reprinted with permission from Baxter DE: Functional nerve disorders In the athelete's foot, ankle, and leg. AAOS Instruct Course lect 42:185-194,1993.)

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surgery may consist of joint stabilization, nerve release, or both.:' Posterior tibial n.

Treatment

Abductor hallucis m.

Heel spur

Nerve to abductor digiti quinti muscle

Fig 4. Branches of the posterior tibial nerve. (Reprinted with permission from Baxter DE: Functional nerve disorders in the athelete's foot, ankle, and leg. AAOS Instruct Course Lect 42:185-194, 1993.)

nerve causes pain, numbness, or paresthesia in the first web space, discomfort at the site of compression, and often night pain. The usual site of compression is beneath the taut inferior extensor retinaculum, but the nerve may be compressed anywhere along its course by an osteophyte, a ganglion, or an accessory ossicle (Fig 5). Local trauma is another etiological factor. 8 •9 For example, the jogger who places his or her car key in the lace of a running shoe risks neuroplexia of the deep peroneal nerve, which can result in significant disability. If neurogenic pain is located in the sinus tarsi, the inferior extensor retinaculum may be compressing the lateral branch of the deep peroneal nerve. This may be mistaken for a sinus tarsi bony abutment or a subtalar strain. Often entrapment of the lateral branch of this nerve occurs in conjunction with a joint instability, and

Determining appropriate treatment depends on careful evaluation, because there are several areas of potential compression. If a bony exostosis is visible on lateral radiograph, excision of the exostosis is indicated. If hypermobility of the talonavicular joint causes functional compression, removal of the sharp dorsal edge of the talus and release of the retinacular ligament are appropriate. When possible, only a portion of the retinacular ligament should be released to avoid making the extensor tendon mechanism unstable. If an accessory bone in the area of the first and second metatarsal bases is causing symptoms, removal of the bone usually will relieve them. If the bony exostosis is at the first metatarsal-tarsal joint, some of the bony prominence in the area of compression should be removed along with release of the nerve. During the recovery phase the lacing pattern of the shoe should be altered to remove pressure from the site of compression.

ENTRAPMENT OF THE SUPERFICIAL PERONEAL NERVE The superficial peroneal nerve is a branch of the common peroneal nerve that pierces the fascia 10 to 12 cm proximal to the tip of the lateral malleolus, where it divides into two cutaneous branches (Fig 6). Entrapment of this nerve causes numbness and tingling over the dorsum of the ankle and foot and pain in the distal third of the leg at the site of compression. Entrapment of the superficial peroneal nerve may be caused by fascial compression, muscle herniation, exertional compartment syndrome, local trauma, or recurrent ankle sprains. 10-12

Treatment Inferiorextensor retinaculum and tendon

01extensorhallucis longusm. pressing on peroneal n.

Fig 5. Deep peroneal nerve compression. (Reprinted with permission from Baxter DE: Functional nerve disorders In the athelete's foot, ankle, and leg. AAOS Instruct Course Lect 42:185-194,1993.)

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The most difficult part of treating superficial peroneal nerve compression is making the diagnosis. This pain syndrome may go undiagnosed for long periods of time, during which the athlete is treated for ankle sprain or other ankle disorders. I have seen athletes who have had arthroscopic ankle procedures before this syndrome was diagnosed. Treatment of superficial peroneal nerve compression usually is surgical nerve release. The nerve may be compressed in a local area of muscle herniation where the superficial nerve migrates from underneath the fascia of the lateral compartment or it may be compressed within a more lengthy tunnel. If significant ankle instability is an etiological factor, ligament instability should be corrected at the time of nerve release. Usually nerve release can be performed with the use of local anesthetic and without a tourniquet. The incision is made no longer than is necessary to release the area of compression. The recovery phase generally occurs in 3 to 4 weeks, and excellent results can be expected in approximatslo 85% of patients. Persistence of problems DONALD E. BAXTER

~

-1 Superficial peroneal n.

Achilles tendinitis, although they were slightly more proximal than those normally found in Achilles tendinitis (Fig 7). The pain was more of a burning type of pain in a localized area, with some occasional radicular symptoms. After conservative treatment failed, local decompression was performed to release the sheath of the Achilles tendon. When the exact location of the symptoms was exposed, the sural nerve was found to be bound by adhesions to the posterior aspect of the Achilles tendon sheath. Release of the sural nerve and place-

.

\

Intermediate dorsal cutaneous n.

Fig 6. Superficial peroneal nerve entrapment. (Reprinted with permission from the Baxter DE: Functional nerve disorders in the athelete's foot, ankle, and leg. AAOS Instruct Course Lect 42:185-194, 1993.)

may be caused by a more proximal lesion such as radiculopathy from the back or from a lateral knee disorder. One of our patients who improved after nerve release but who continued to have pain was found to have a herniated disc that was responsible for her pain (double-crush syndrome).

ENTRAPMENT OF THE SURAL NERVE The sural nerve runs 1 em posterior to the peroneal tendon sheath and divides 2 em above the ankle. One branch innervates the lateral foot; the other often anastomoses with the superficial peroneal nerve. Symptoms of sural nerve entrapment include paresthesia in the sural nerve distribution and numbness along the lateral aspect of the foot; these symptoms are exacerbated by' certain positions of the foot or specific activities. The etiologies of this syndrome include displaced fractures of the calcaneus or fifth metatarsal, osteophytes, and ganglions. 13 •14 It also may be associated with instability of the lateral ankle ligaments.

--

Treatment Treatment includes identifying the source of compression and removing the offending pressure. One of our patients, a "half-miler," had symptoms that suggested COMPRESSIVE NEUROPATHIES

Fig 7. Sural nerve compression.

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ment in a different position relieved symptoms and allowed the athlete to return to running. Another patient, a golfer, was referred for subtalar joint fusion because of persistent pain after a calcenael fracture. Careful evaluation localized the pain at the lateral subtalar joint. Surgical exploration of this area showed a small bony spur rubbing against the sural nerve. After removal of the spur and anterior relocation of the sural nerve, the symptoms subsided, and the patient was able to resume golfing.

INTERDIGITAL (MORTON'S) NEUROMA Although the etiology of interdigital neuroma (Morton's metatarsalagia) is not completely understood, it probably is caused by an entrapment neuropathy involving the common digital nerve and is predominantly characterized by the deposition of an amorphous eosinophilic material followed by a slow degeneration of the nerve fibers. Even the term neuroma is not completely accurate, because the haphazard proliferation ofaxons seen in traumatic neuro~as is not found. The pathological process probably is degenerative rather than proliferative, with repetitive trauma against the deep transverse intermetatarsal ligament being the most likely cause. Acute trauma, such as fall, crush injury, or stepping on a sharp object, occasionally results in an interdigital neuroma. Interdigital neuromas are approximately four times more common in women than in men. The most common symptom of interdigital neuroma is pain localized to the plantar aspect of the foot between the metatarsal heads (Fig 8). Patients usually describe the pain as a burning sensation that radiates to the toes of the involved interspace. Pain is increased by foot activities or when the patient puts on a tight-fitting, highheeled shoe. About half of patients describe some

numbness in the toes or foot, and some complain of pain radiating up the leg or a cramping sensation in the foot and leg. In a few patients a small, moveable mass may be palpated on the plantar aspect of the foot.

Treatment Conservative management consists of the use of a wide, soft, laced shoe with a low heel to allow the foot to spread and relieve pressure on the metatarsal head area. A soft metatarsal support just proximal to the metatarsal region further relieves pressure. Steroid injections occasionally may be helpful but rarely produce long-lasting results. IS Although most patients obtain some initial relief with these measures, approximately 70% eventually elect to have surgery because of persistent symptoms or a desire for different types of footwear. IS An interdigital neuroma may be excised through either a dorsal or a plantar incision. The dorsal approach has as its main advantage the prevention of scar formation on the plantar aspect of the fOOt. I 6

Technique Under tourniquet control an incision is made in the dorsal aspect of the foot, beginning in the involved web space and continuing proximally for approximately 3 em to the level of the metatarsal head (Fig 9). It is important to keep the incision directly in the midline, because deviation to either side may result in cutting of one of the dorsal digital nerves, which could cause a painful neuroma. The incision is deepened through the soft tissue to the level of the metatarsal heads. A retractor is placed between the metatarsal heads to spread them apart, which places the transverse metatarsal ligament under significant tension. A neurological freer is used to dissect out the contents of the interspace, allowing identifi-

Neurofibroma -trT\-l~"'l'

Communicating -~o---I~ branch Lateral pIa nta r --+---"lI~' nerve Medial plantar nerve

Fig 8. Most common location of Interdigital neuroma (plantar and dorsal views). (Reprinted with permission from McElvenny RY: Morton's neuroma: The etiology and surgical treatment of Intractable pain about the fourth metatarsophalangeal joint (Morton's toe). J Bone Joint Surg [Br] 25:675-679,1943.)

Tibial nerve

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DONALD E. BAXTER

Postoperative Management The compression dressing is removed after 18 to 24 hours, and a firm forefoot dressing is applied. Ambulation is permitted in a postoperative shoe. The dressing is worn for 3 weeks, after which active and passive rangeof-motion exercises are begun.

CONCLUSION

Fig 9. Dorsal approach to Interdigital neuroma. (Reprinted with permission from Richardson EG: Neurogenic disorders, In Crenshaw AH led]: Campbell's Operative Orthopaedics led 8]. St Louis MO, Mosby Year-Book, 1992.)

cation and transection of the transverse metatarsal ligament.. The retractor is removed and placed deeper between the metatarsal heads to expose the contents of the web space. The neurological freer again is used to allow identification of the common digital nerve in the proximal portion of the wound. The nerve is traced distally to its bifurcation, where a significant amount of soft tissue may be: present around the nerve. If possible this tissue should be removed to allow the nerve to be followed past the bifurcation. If the adhesions are too great, all of this material is removed with the nerve rather than taking time to carefully dissect it out. The interspace should be carefully explored for any accessory branches from underneath the metatarsal heads. In the proximal portion of the wound, the common digital nerve is cut proximal to the metatarsal head, dissected out distally past the bifurcation, and excised with as little plantar fat as possible. If a significant accessory nerve branch passes to the common nerve either medially or laterally, the consequences of cutting this nerve and allowing it to retract under the metatarsal head must be carefully considered. If the nerve trunk appears to be larger than 2 mm, rather than resecting the neuroma proximal to the metatarsal heads, the common nerve should be cut just proximal to its bifurcation, which also is just proximal to the thickening usually observed in the nerve distal to the transverse metatarsal ligament. The distal portion of the nerve is removed. The cut end is sutured to the side of the metatarsal 'or to one of the intrinsic muscle so that it will not drop onto the plantar aspect of the foot. Placing the nerve along the side of the metatarsal, off the bottom of the foot, keeps the stump neuroma from being in a weight bearing position. The skin is closed in a single layer, and a compression dressing is applied.

COMPRESSIVE NEUROPATHIES

In the evaluation of athletes with unusual symptoms about the foot and ankle, the possibility of a nerve disorder should be considered. A knowledge of the exact anatomy of the nerves to the foot and ankle and of the common areas of compression often will allow the physician to make the diagnosis and to correct the problem with a simple surgical procedure that rapidly restores function. The possibility of a more proximal cause of the ankle and foot symptoms, such as disc herniation or knee disorders, also should be considered. Though rare in athletes, other possible causes of neuropathy include metabolic conditions, reflex sympathetic dysfunction, and nutritional deficiency.

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agnostic and surgical correlation. J Bone Joint Surg {Am] 63:96-99, 1981 Schon Le, Baxter DE: Neuropathies of the foot and ankle in athletes. Clin Sports Med 9:489-509, 1990 Mann RA, Baxter DE: Diseases of the nerves, in Mann RA, Coughlin MJ (ed): Surgery of the Foot and Ankle (ed 6). St Louis, MO, Mosby Year-Book, 1993, pp 543-573 Rask MR: Medial plantar neuropraxis (jogger's foot): Report of 3 cases. Clin Orthop 134:193-195, 1978 Mann RA: Tarsal tunnel syndrome. Orthop Clin North Am 5:109115,1974 Baxter DE, Thigpen CM: Heel pain: Operative results. Foot Ankle 5:16-25, 1984 Henricson AS, Westlin NE: Chronic calcaneal pain in athletes. Entrapment of the calcaneal nerve? Am J Sports Med 12:152-154, 1984 Borges LF, Halle 11M, Selkoe OJ, et al: The anterior tarsal tunnel syndrome: Report of two cases. J Neurosurg 54:89-92, 1981 Gesseni L, [andolo B, Peitrangel A: The anterior tarsal tunnel syndrome: Report of four cases. J Bone Joint Surg [Am] 66:786-787, 1984 Kernahan J, Levack B, Wilson IN: Entrapment of the superficial peroneal nerve: Three case reports. J Bone Joint Surg [Br] 67:60-61, 1985 Lowdon IMR:Superficial peroneal nerve entrapment: A case report. J Bone Joint Surg [Br] 67:58-59, 1985 McAuliffe TB, Fiddian NJ, Browett JP: Entrapment neuoropathy of the superficial peroneal nerve: A bilateral case. J Bone Joint Surg [Br] 67:62-63, 1985 Gould N, Trevino S: Sural nerve entrapment by avulsion fracture at the base of the fifth metatarsal bone. Foot Ankle 2:153-155, 1981 Stem OS, Joyce MT: Tarsal tunnel syndrome: A review of fifteen surgical procedures. J Foot Surg 28:290-295, 1989 Mann RA, Reynolds JD: Interdigital neuroma: A critical clinical analysis. Foot Ankle 3:238-243, 1983 Beskln JL, Baxter DE: Recurrent pain following interdigital neurectomy-A plantar approach. Foot Ankle 9:34-39, 1988

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