Common Entrapment Neuropathies of the Lower Extremity
Dr. Gregory Rubin
For the purposes of this review, we will follow the course of the posterior tibial nerve. The tibial nerve originates from L4-S3 and is a division of the sciatic nerve (Mark Jobe, 2017). The tibial nerve will course through the posterior thigh, through the popliteal fossa, and finally under the soleus until it reaches the foot (Mark Jobe, 2017). The four distal branches of the tibial nerve are the medial plantar nerve, lateral plantar nerve, inferior branch of lateral plantar nerve, and medial calcaneal nerve (David Del Toro, 2018).
Due to the large area of distribution of the tibial nerve and its branches, there are many locations where the nerve can become entrapped. As a result, entrapment neuropathy can cause a wide constellation of symptoms. The difficulty in diagnosis for providers lies in teasing out whether the etiology of foot pain is due to an entrapment neuropathy or one of a wide variety of other pathologies affecting the foot like plantar fasciitis. This review will focus on the tibial nerve and its most commonly affected branches.
Tarsal Tunnel Syndrome
The tarsal tunnel is the distal portion of the deep posterior compartment of the lower extremity and is located behind the medial malleolus (John Lau, 1999; Norman Espinosa, 2019). There are two tarsal tunnel syndromes, with one being anterior tarsal tunnel syndrome and the other known as the tarsal tunnel syndrome (Luis Beltran, 2010). Anterior tarsal tunnel syndrome is due to compression of the deep peroneal nerve and the tarsal tunnel syndrome is due to compression of the posterior tibial nerve (Luis Beltran, 2010). Most commonly, the posterior tibial nerve is compressed at the level of the medial malleolus as it passes under the flexor retinaculum (David Del Toro, 2018). The role of the posterior tibial nerve is to innervate the muscles of the foot and provide sensation to the plantar surface (Luis Beltran, 2010). Compression of the nerve can occur due to bone spurs, fractures, ganglia cysts, tumors, or foot deformities. Foot deformities, like having a hindfoot valgus, can also stretch the tibial nerve causing symptoms (Norman Espinosa, 2019). Diabetic neuropathy can also lower the threshold for patients to develop tarsal tunnel syndrome (Norman Espinosa, 2019).
Patients will complain of paresthesias along the medial ankle that radiate into the plantar foot (John Lau, 1999; Norman Espinosa, 2019). They also may describe their symptoms as burning or tingling in the foot that is worse with standing and walking (John Lau, 1999). In order to detect nerve entrapment on physical exam, providers should test two point discrimination on patient’s plantar surface (John Lau, 1999). A tinel’s test can be done with repeated pressure of the nerve behind the medial malleolus (John Lau, 1999). In 2001, Dr. Kinoshita published a study looking at the accuracy of a dorsiflexion-eversion test to help diagnose tarsal tunnel syndrome (M Kinoshita, 2001). They found that, by placing the ankle in dorsiflexion with heel eversion and maximally dorsiflexion the toes, the symptoms of tarsal tunnel syndrome could be reproduced (Figure 1) (M Kinoshita, 2001).
If history and physical suggest tarsal tunnel syndrome, an EMG can help confirm the diagnosis. EMG testing should include testing of both the motor and sensory function (David Jackson, 1991). Studies have showed that the sensitivity of an EMG is close to 90% (Norman Espinosa, 2019). In a review based on the usefulness of EMG for diagnosing Tarsal Tunnel published in Muscle and Nerve in 2005, they found that sensory nerve conduction studies were more sensitive than motor nerve conduction studies in confirming the diagnosis (Atul Patel, 2005).
Ultrasound has also played a role in the evaluation for peripheral nerve entrapment. Initially when a nerve is entrapped, it will look hypoechoic and then can enlarge proximally to the site of entrapment (Jon Jacobson, 2016). Standing radiographs can also show osseous abnormalities that can be compressing the tibial nerve (Norman Espinosa, 2019).
First line treatment is typically non-operative. Oral anti-inflammatories, activity modification, and orthotics may aid in decreasing inflammation and relieving some of the pain from neuropathy (David Jackson, 1991). Over 90% of patients treated with non-operative means will improve and not require surgery (Troy Watson, 2002). For the other <10% who fail first line therapy, they can consider a tarsal tunnel release (Eric Ferkel, 2015; Troy Watson, 2002). In a study done by Gondring, he found that following tarsal tunnel release, only 51% of patients had symptom relief despite 85% of them having improvement in nerve conduction velocity (Eric Ferkel, 2015).
The tibial nerve will divide into the lateral and medial plantar nerve just inferior to the medial malleolus (Mark Jobe, 2017). Baxter neuropathy is typically caused by trapping of the first branch of the lateral plantar nerve between the abductor halluces muscle and quadratus plantae muscle (LF Llanos, 1999; Gregory Pomeroy, 2015). It is at this level that the lateral plantar nerve will change direction from going inferior to now a more lateral direction (LF Llanos, 1999). Multiple reasons for entrapment of this nerve have been evaluated with no clear consensus. The nerve can be compressed by local edema due to an inflammatory process at the heel, local trauma, an enlarged abductor halluces muscle, thickened plantar fascia, or hypermobile foot (Volkan Oztuna, 2002; Luis Beltran, 2010). Compression of the lateral plantar nerve has been estimated to cause 15-20% of chronic plantar heel pain (Figure 2) (Ali Alshami, 2008).
Patients typically complain of pain along the medial portion of the heel. Compression of the first branch of the lateral plantar nerve is sometimes difficult to discriminate from Plantar Fasciitis, as they both can cause medial heel pain (David Jackson, 1991). The pain associated with nerve entrapment will typically have an after burn, which is pain that continues after a period of rest (Eric Ferkel, 2015). Diagnosis can be aided with the use of an EMG.
Similar to the other entrapment neuropathies, a total contact orthotic is considered first line therapy (Gould, 2014). Physicians recommend 12 to 20 months of conservative therapy prior to operating (Eric Ferkel, 2015).
Compression of the medial plantar nerve is known as Jogger’s foot. The medial calcaneal nerve is a pure sensory nerve and provides innervation to the medial portion of the heel and plantar surface of the first three toes (David Del Toro, 2018). Entrapment typically occurs at the space between the abductor halluces muscle and the knot of Henry (Figure 3) (Luis Beltran, 2010). The knot of Henry is the site where the flexor digitorum longus and flexor halluces longus tendons cross over (Luis Beltran, 2010).
The most common causes of compression of the medial plantar nerve within the tarsal tunnel is trauma, varicose veins, a varus heel, and fibrosis (John Lau, 1999). The syndrome is also seen in sports that involve repetitive motion like jumping and sprinting (Mitsuo Kinoshita, 2006). Patient’s will typically complain of pain and numbness at the medial heel and arch (David Del Toro, 2018). They also may complain of a more shock-like or burning pain in the heel (Ali Alshami, 2008). Physicians will palpate for pain along the medial arch during their physical examination (Norman Espinosa, 2019). Treatment goals are similar to the treatment of the tarsal tunnel syndrome.
Another source of pathology in the foot is a Morton’s neuroma. A Morton’s neuroma is also known as interdigital neuralgia. Histological samples have revealed that it is not a nerve tumor or inflammation of the nerve (P Peters, 2011). A Morton’s neuroma occurs as a result of nerve fibrosis and degeneration because of repetitive microtrauma to the nerve, MTP joint instability or ganglion, or metatarsal ligament thickening (P Peters, 2011).
Patients will typically complain of pain between their metatarsal heads in their forefoot (P Peters, 2011). Symptoms can vary between burning, sharp, or tingling (P Peters, 2011). The most common location for a neuroma is the third web space (P Peters, 2011). Physicians can elicit pain with palpation of the web space or squeezing the metatarsals together (Jon Jacobson, 2016). Weight bearing x-rays are typically done as first line evaluation. The use of an MRI or ultrasound can help differentiate alternative differential diagnosis (P Peters, 2011). On ultrasound, a Morton’s neuroma can be found between the metatarsal heads and is seen as a hypoechoic mass (Jon Jacobson, 2016). In the American Journal of Roentgenology in 2009, they found that the rate of finding a Morton’s neuroma was 79% for ultrasound and 76% for MRI (Lee, 2009).
First line therapy is non-operative and includes well-padded wide based shoes (P Peters, 2011). Corticosteroid injection into the affected web space can also lead to relief of symptoms (Norman Espinosa, 2019). However, results of the injection must be interpreted with caution since a corticosteroid injection will also help the pain from MTP arthritis or plantar plate injury (Norman Espinosa, 2019). Patients can also use a metatarsal pad (P Peters, 2011). Patients who fail conservative therapy should be evaluated for surgical excision (P Peters, 2011).
The tibial nerve and its branches can become entrapped in multiple locations throughout its course into the foot. Symptoms can mimic many other clinical syndromes and lead to treatment challenges. It is important for providers to understand that treatment courses can take upwards of 12 months to take effect. Primary care sports medicine providers are a good first-line for patients with chronic foot pain since most patients with entrapment neuropathies will not need surgery.
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