Keys To Detecting And Treating Entrapment Neuropathies

   Entrapment neuropathy of the tarsal tunnel and its terminal branches is a painful condition, which can be challenging to diagnose given the possibility of concurrent conditions with overlapping symptoms. Heel spur syndrome/plantar fasciitis and entrapment neuropathy often present at the same time. However, it is important to differentiate clearly between the two pathologies as entrapment neuropathy has a distinct history and clinical presentation.

   The anatomic outline of the posterior tibial nerve and its branches is pertinent to the appropriate diagnosis and management of the entrapment neuropathy. Following the course of the posterior tibial nerve will lead you to the site of entrapment. The posterior tibial nerve courses posterior to the medial malleolus in a fibroosseous compartment and just deep to the flexor retinaculum, which is one potential site of entrapment. At the distal end of the tarsal tunnel, the tibial nerve branches into the larger medial plantar nerve and smaller lateral plantar nerve. At the level of bifurcation, a fascial septa separates the nerves, leading them into their respective courses that innervate the plantar foot.

   This fibrous band is another potential site of nerve entrapment. The medial plantar nerve courses superficial to the abductor hallucis muscle and sends motor and sensory branches to the plantar medial aspect of the foot. The lateral plantar nerve travels through a fibrous opening of the abductor hallucis.

   The first branch of the lateral plantar nerve, Baxter’s nerve, arises at this location. It transverses laterally and passes anterior to the medial calcaneal tuberosity (often the site of a heel spur) to innervate the abductor digiti quinti muscle.1,2 The Baxter’s nerve is often entrapped at the abductor hallucis muscle and one must release this nerve surgically.

What To Look For In The Clinical Exam

   Often mistaken for plantar fasciitis with heel spur syndrome, entrapment neuropathy of Baxter’s nerve is a painful condition that can lead to chronic heel pain. Typically, a patient will present with pain on the plantar medial aspect of the heel that exponentially increases in severity the longer the individual is weightbearing.

   The discomfort begins as an ache, progresses to a burning pain and eventually leads to numbness. These sensations can travel proximal toward the medial ankle along the tarsal tunnel, the Valleix sign or across the plantar aspect of the foot laterally toward the base of the fifth metatarsal. Often the patient complains of pain shooting laterally on the plantar aspect of the heel. Unless Baxter’s nerve is entrapped and occurs in conjunction with plantar fasciitis, the pain does not present as post-static dyskinesia.

   When it comes to diagnosing entrapment neuropathy of Baxter’s nerve, the physician should ensure that the clinical examination incorporates complete and systematic rearfoot examination. The patient will point directly to the site of pain at the proximal level of the abductor hallucis muscle. This position is approximately 5 cm anterior to the posterior border of the heel at the intersection of the plantar and medial skin.3 One will elicit pain with palpation at the exact location, which is usually not the medial calcaneal tubercle nor the ligamentous band of the plantar fascia.

   The rearfoot examination includes evaluation of the Achilles tendon course and insertion. One should also check for posterior and inferior calcaneal pathology, medial and lateral compression (calcaneal squeeze test), tarsal tunnel entrapment, medial calcaneal nerve symptoms and plantar fascial pain. Physicians should also perform a gait analysis to check for for pronatory or supinatory forces causing compression on the abductor hallucis muscle.

   Radiographs and magnetic resonance imaging (MRI) will only be helpful to rule out other pathologies. In one prospective study, researchers found that MRI revealed fatty atrophy of the abductor digiti quinti in only 7 percent of patients with nerve entrapment related symptoms.4 An electromyography/nerve conduction velocity (EMG/NCV) study may be beneficial. However, it is crucial that the physician perform the test under dynamic conditions. In regard to the standard EMG/ NCV test, the patient is usually non-weightbearing and the static study may present a false negative outcome.

Can Conservative Care Have An Impact?

   Conservative therapy consists of: a short course of oral anti-inflammatory medication; padding and offloading of the entrapment site; steroid injection therapy; stretching exercises; and physical therapy. Physical therapy is beneficial if the patient has concurrent entrapment of Baxter’s nerve and plantar fasciitis.

   Orthotic management and control may be beneficial. However, orthotics often exacerbate symptoms. Vertical ground/orthotic reaction forces against an entrapped site lead to further compartment compression and burning pain ensues.

Emphasizing The Benefits Of Preoperative Planning

   When attempted conservative therapy fails, surgical intervention is warranted. It is recommended to perform a medial band plantar fascial release in conjunction with Baxter’s nerve release. If tarsal tunnel syndrome is present, one must address this condition as well. A clear differential is necessary as Baxter’s nerve entrapment can cause referred pain in the tarsal tunnel but isolated tarsal tunnel entrapment may not be present.5

   Preoperative incision planning is essential. Patient feedback, palpation of the artery and marking the incision preoperatively will allow for visualization of all pertinent structures. Ask the patient to identify the site of intense pain and mark the entrapment area with an “X.”

   Proceed to identify the posterior tibial artery. The tibial nerve will lay just posterior to the artery. For a complete tarsal tunnel release, start the incision roughly 3 cm proximal to the medial malleolus and halfway between the medial border of the Achilles tendon and the medial malleolus.

   The tibial nerve courses distally and will bifurcate at the level between the medial malleolus and the calcaneus. Follow this distally to the site of the “X.” Extend the incision distally over the medial border of the anterior calcaneus. Preoperative identification of the medial calcaneal nerve through percussion will not only prevent unintentional neurectomy but helps to identify a crucial area of surgical release.

Pertinent Pearls On Performing Appropriate Releases

   Ensure the patient is in a supine position and apply a thigh tourniquet. Flex the hip and ensure it is externally rotated while the knee is flexed. This allows direct visualization to the medial ankle and foot.

   In order to only address the Baxter’s nerve and the medial plantar fascia, make a medial linear incision from the proximal plantar aspect of the abductor hallucis to the anterior level of the medial calcaneal tubercle. This incision is anterior to the medial calcaneal nerve. The surgeon should not encounter the medial calcaneal nerve but be aware of the anatomical variation of other calcaneal branches.

   Deep to the subcutaneous tissue lies the superficial abductor fascia, which you can proceed to release. Retraction of the abductor hallucis muscle superiorly will allow for release of the thick deep fascia of abductor hallucis and medial border of the quadratus plantae. An adequate release will liberate Baxter’s nerve from entrapment at this site.

   Proceed to perform a partial fasciotomy of the medial one-third of the plantar fascia. If a spur is present, perform an exostectomy but take care to avoid damage to the nerve coursing anteriorly or laterally.

   If you combine a Baxter’s nerve release with a tarsal tunnel release, the previously marked anatomy is the path of the incision. Employ the use of vessel loupes to facilitate the identification of nerve structures and careful dissection. Four critical sites of entrapment the surgeon can release include:

   • the tibial nerve under the flexor retinaculum;
   • the fascial septa between the medial and lateral plantar nerves at the porta pedis;
   • the medial calcaneal nerve; and
   • Baxter’s nerve.

   After performing these releases, proceed to perform superficial closure of the subcutaneous tissue and skin.6

   Apply a compression bandage and allow protective weightbearing in a walking boot. Remove the sutures after two weeks and have the patient initiate active range of motion. You can have the patient start weightbearing in a sneaker at four weeks. Anti-inflammatory medication or a steroid dose pack will help with postoperative edema and pain.

Final Notes

   In conclusion, physicians often overlook Baxter’s entrapment neuropathy or misdiagnose it as plantar fasciitis. Given the amount of overlap of symptoms between Baxter’s nerve entrapment and plantar fasciitis, the surgeon may combine the release of Baxter’s nerve with a partial plantar fasciotomy or heel spur resection. In some cases, one should also perform a complete tarsal tunnel release. A thorough preoperative clinical analysis will help identify and isolate these pathologies to allow for accurate surgical management.

Dr. Schoenhaus is a foot and ankle specialist in Boca Raton, FL. She is a Fellow of the American College of Foot and Ankle Surgeons. She is board-certified in foot surgery and reconstructive rearfoot and ankle surgery by the American Board of Podiatric Surgery.

Dr. Gold is a podiatrist in Boynton Beach, FL. He is a Fellow of the American Board of Foot and Ankle Surgery, board-certified in foot surgery and reconstructive rearfoot and ankle surgery by the American Board of Podiatric Surgery.

Dr. Burks is a Fellow of the American College of Foot and Ankle Surgeons, and is board-certified in foot and ankle surgery. He is in private practice in Little Rock, Ark.

For further reading, see “How To Address Baxter’s Nerve Entrapment” in the November 2004 issue of Podiatry Today or “A Guide To The Differential Diagnosis Of Heel Pain” in the May 2009 issue.

References:

1. Sarrafian SK. Nerves. In: Anatomy of the Foot and Ankle. Sarrafian SK (ed). Lippincott, Williams and Wilkins, Philadelphia, pp 381, 1993. 2. Baxter DE. Release of the nerve to the abductor digiti minimi. In: Master Techniques in Orthopaedic Surgery of the Foot and Ankle. Kitaoka HB (ed). Lippincott, Williams and Wilkins, Philadelphia, pp 359, 2002. 3. Schon LC, Baxter DE. Heel pain syndrome and entrapment neuropathies about the foot and ankle. In: Operative Foot Surgery. Gould JS (ed). WB Saunders, Philadelphia, pp 192-208, 1994. 4. Recht MP, Grooff P, Ilaslan H, et al. Selective atrophy of the abductor digiti quinti: an MRI study. Am J Roentgenol 2007; 189(3):W123-7. 5. DiGiovnni B, Abuzzahab F, Gould J. Plantar fascia release with proximal and distal tarsal tunnel release: a surgical approach to chronic, disabling plantar fasciitis with associated nerve pain. Techniques Foot Ankle Surg 2003; 2(4):254-261. 6. Dellon L. Tarsal Tunnel Syndrome Release of the Four Medial Ankle Tunnels. DVD, 1998.