Pertinent Insights On Surgical Decompression For Common Peroneal Nerve Entrapment

Stephen L. Barrett, DPM, MBA, FACFAS, FAENS

   It is not unusual for patients with a common peroneal (fibular) nerve entrapment to relate only symptoms of pain in their ankle or on the top of their foot, and have no idea that their lesion is proximal at the fibular neck. Patients often get a misdiagnosis of sinus tarsi syndrome when in reality they have a proximal nerve entrapment. This is very easy to diagnose.

Keys To The Clinical Examination

The presence of a Tinel’s sign or provocation sign at the level of the fibular neck is highly suggestive of entrapment of the common peroneal (fibular) nerve.15 Since there is a frequent association of other nerve entrapments co-existing with this entrapment, one must evaluate all areas of known entrapment in the lower extremity. This would include the superficial peroneal (fibular) nerve at the foraminal level, the deep peroneal (fibular) nerve on the dorsum of the foot, the tarsal tunnel (which consists of the tibial nerve, medial and lateral plantar nerves in the porta pedis, and the medial calcaneal nerve(s)) at the medial ankle, and the proximal tibial nerve at the level of the soleal sling.

   Manual testing of sensation both with light touch and two-point discrimination can provide the physician with information about the usual innervation patterns of these nerves. Additional testing with the PSSD is much more sensitive and specific than with manual testing, and can provide a wealth of information about all the nerve distributions that have testing.

   By the time a patient exhibits a loss of one-point discrimination with the 5.07 monofilament, there is severe nerve damage. In my opinion, this test is almost virtually worthless when evaluating peripheral nerve injury and entrapment. Sadly, testing with this monofilament can give the provider the impression that there is no nerve pathology when the patient can discriminate it and this delays necessary treatment because of the mistaken impression that the nerve is functioning because the patient can “feel” it.

   Testing of motor strength is critical for several reasons. First, loss of motor strength of either the extensor hallucis longus or tibialis anterior is an ominous sign that there is a high level of nerve damage with corresponding entrapment. This loss of motor strength gives a high level of confidence to the practitioner that the diagnosis of common peroneal (fibular) nerve entrapment is accurate.

   Secondly, this evaluation can provide a baseline for the surgeon to measure against after peripheral nerve surgical neurolysis. While 0-5 muscle grading strength has been in use for decades in neurological examination, there are inexpensive dynamometers such as the microFET2 (Hoggan Scientific). This dynamometer can accurately measure the strength of individual muscles or groups with extreme accuracy, which eliminates the “guesswork” of what grade to rate a patient. This device will give a measurement in pounds or Newtons of strength, which one can rely upon as an accurate, reproducible measurement to compare against the postoperative patient. Even more importantly in unilateral presentations, dynamometry can accurately compare the affected leg to the non-affected leg without subjectivity.

   Finally, if there is a measurable loss of motor strength, this indicates that definitive surgical intervention may be imminent to prevent drop foot and potentially permanent lower extremity impairment. Conduct motor testing for all groups of muscles in the entire lower extremity to rule out other entrapment or motor lesions, and eliminate the possibility of a proximal centralized etiology. In patients with good hip flexor function, the provider can feel more assured that there is not a proximal entrapment as this muscle group is innervated by the femoral nerve and could be affected by this more proximal entrapment if present.

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