Entrapment of the common peroneal (fibular) nerve can cause a myriad of symptoms, ranging from low back pain to drop foot. This author provides a guide to diagnosing the nerve entrapment, offers pearls on performing neurolysis/decompression and discusses the potential impact it can have for patients.
Common peroneal (fibular) nerve entrapment is a more common pain generator than we recognize clinically.1,2 This nerve entrapment is easy to diagnose most of the time and surgical neurolysis is highly efficacious.3-5 However, we either frequently miss this diagnosis or we may make the diagnosis and do nothing for the condition. There is a significant prevalence of subclinical entrapment of the common peroneal (fibular) nerve in runners, and with scrutiny and suspicion, the astute practitioner can pick these up very early.6
Entrapment of the common peroneal (fibular) nerve can cause a wide range of symptoms from one patient to another with the same etiology. Most apparent and almost universally recognized is when there is such a degree of nerve damage that a drop foot has occurred. Sadly, when the condition has reached this point, there may be limited success with peripheral nerve decompression.
Mild symptoms can range from continued pain after a knee replacement surgery (very common and almost never recognized by the orthopedic surgeon who has done the surgery) to low back pain or sometimes just a feeling of weakness in the foot. Patients may complain of the usual nerve symptoms such as burning or numbness, but may often complain of a feeling of tightness in their leg or “ankle pain.” Common peroneal (fibular) nerve entrapment is frequently associated with low back involvement in a double crush phenomenon.
Anatomists now refer to the common peroneal nerve as the common fibular nerve with divisions into the deep and superficial fibular nerves. The sciatic nerve originates from the L4, L5, S1, S2 and S3 nerve roots in the sacral plexus of the lower back. The sciatic nerve divides variably, ranging from proximal division patterns in the upper thigh to distal divisions down into the level of the popliteal fossa, into its two terminal branches: the tibial nerve and the common peroneal (fibular) nerve.8
As the common peroneal (fibular) nerve courses around the lateral aspect of the leg, it lies against the fibular neck with the fascia of the peroneus (fibular) longus muscle superficial, which forms a very tight tunnel. At this level, there are numerous motor branches that innervate the peroneus (fibular) longus nerve, which divides into its superficial and deep branches. This bifurcation can be variable at the level where it actually divides and the nerve has very little capability of excursion at this level in contradistinction to its laxity in the popliteal fossa. Often, there is a very dense band of the deep fascia of the peroneus (fibular) longus muscle at the most posterior portion of the muscle, which binds the nerve down onto the surface of the fibular neck.
The combination of these two anatomical factors makes this nerve especially susceptible to entrapment. An example of this is how frequently this nerve becomes injured with an inversion ankle sprain.9 The patient may complain of “ankle” pain for months after the injury and it is difficult for the practitioner to look proximally to this level for the diagnostic answer. The common peroneal (fibular) nerve has two terminal branches: the deep peroneal (fibular) nerve and the superficial peroneal (fibular) nerve. The deep peroneal (fibular) nerve provides the muscular innervation for the extensor hallucis longus tendon, the tibialis anterior tendon and the extensor digitorum longus tendon. The superficial peroneal (fibular) nerve innervates the peroneus longus (fibular) nerve and the peroneus (fibular) brevis nerve.
The common peroneal nerve and its two terminal branches provide for a large surface area of skin neurosensory sensation with a corresponding large area on the cortical map. This can prove difficult when a portion or all of this nerve distribution is denervated by either injury or surgery.
Diagnosis of entrapment of the common peroneal (fibular) nerve is very easy for the experienced practitioner but the inexperienced doctor with little lower extremity peripheral nerve experience and awareness almost never recognizes the diagnosis.
This should not be the case as there are three simple things that even the inexperienced provider, from the standpoint of peripheral nerve surgery, can use to be virtually assured that an accurate and timely diagnosis has occurred. First, complete a thorough history of the present illness. Second, perform specific motor strength testing. Finally, consider the presence of a local provocation/and or Tinel’s sign at this known site of entrapment.10,11
One can perform additional diagnostic testing with nerve conduction velocity (NCV) and the Pressure Specified Sensory Device (PSSD, Sensory Management Services), or via measurement of galvanic skin response.12-14
As with any additional testing modality, the astute practitioner knows that these tests do not make a diagnosis but help augment the diagnosis with expert clinical examination. These tests also give a measure of the level of nerve involvement, which can serve a baseline for future evaluation after interventional treatment. The PSSD and galvanic skin response exams are non-painful tests, and one can use them postoperatively to evaluate nerve response after intervention. In my clinical experience, the electrodiagnostic testing has been falsely negative up to 50 percent of the time.
The history of present illness is very important as it gives the provider a wealth of information, which can sometimes mean the difference between a successful outcome and even saving a patient’s life.
While it sounds extreme, the history of a sudden drop foot in a young otherwise healthy patient with no metabolic disease and without a recent history of trauma gleaned from the interview must make the most experienced practitioner extremely wary of the probability that a central nervous system lesion exists. If there is no provocation sign at the fibular neck, send the patient immediately for a neurological consultation, which includes spine and brain magnetic resonance imaging (MRIs). This can save a patient’s life.
Most symptoms of common peroneal (fibular) nerve entrapment are slow in their onset, even in patients with a history of a severe ankle sprain. Symptoms appear in middle age or later, and are often associated with an underlying metabolic disease such as diabetes, pre-diabetes, metabolic syndrome or other metabolic diseases.
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.
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.
For patients who come in with only the complaint of ankle pain, in addition to the examination I described above, there should be radiographic and clinical evaluation of the ankle joint. Before making the diagnosis of sinus tarsi syndrome, which frequently coexists with proximal nerve entrapments of the common peroneal (fibular) and superficial peroneal (fibular) nerves, the provider must rule these out.
If there are no clinical signs of proximal entrapment, no motor weakness, no decrease in sensation to light touch and two-point discrimination, then one can usually make a reliable sole diagnosis of sinus tarsi syndrome but physicians often miss this. However, if it appears that both or one of these etiologies are present, then confirmation of the diagnosis with peripheral nerve blocks can be extremely reliable.
The use of peripheral nerve blocks with just lidocaine (Lidoderm, Endo Pharmaceuticals) is an extremely powerful and helpful adjunct in the diagnosis of peripheral nerve entrapments. In the case of the common and superficial peroneal (fibular) nerves, I recommend blocking the more distal known site of entrapment (the superficial peroneal (fibular) nerve) at the foraminal level with just 1 cc of the local anesthetic. If this eliminates all the “ankle” pain, then the physician can feel assured that this is the level of the pain generator. However, if in this same situation, the practitioner had blocked the more proximal common peroneal (fibular) nerve site and the pain went away, he or she would not know if there really was a more distal entrapment causing or at least contributing to the pain in the sinus tarsi.
A neurosensory testing report will shows an elevated two-point discrimination threshold with axonal degeneration for only the distribution of the common peroneal (fibular) nerve in a patient with an isolated single entrapment. The report shows widespread loss of sensation in a patient with diabetic peripheral neuropathy—both one point and two point.
Decompression of the common peroneal (fibular) nerve is a technically easy surgery for the experienced peripheral nerve surgeon, and a difficult and dangerous one for the inexperienced surgeon. Surgeons who are not formally trained in lower extremity peripheral nerve surgery should not perform this surgery as the potential complication of nerve injury resulting in a permanent drop foot condition is disastrous for the unfortunate patient.
However, for the trained surgeon who can provide judicious technique, this is one of the most immediate and gratifying surgeries for the lower extremity.
Patient positioning and incision placement are critical to the success of any surgery. This is especially the case with decompression of the common peroneal (fibular) nerve. One should ensure the patient is in a supine position with the knee flexed and a weight placed on the OR table holds the knee in that position. When placing the incision, the surgeon marks the level of the fibular neck, which is easily palpable in patients with a normal body mass index (BMI).
However, in those patients with high BMIs, this can be difficult to palpate. In these patients, one can palpate this anatomic marker with the leg extended. In very serious cases in which there is no osseous palpation of the fibula, one can use intraoperative fluoroscopy. As most patients tend to abduct their lower extremities when they are relaxed (as in general anesthesia), the OR table can tilt slightly away from the surgeon, holding the leg in better position.
The use of bipolar cautery is mandatory for hemostasis and avoids the potential of thermal nerve injury, which is problematic with monopolar cautery. As there is no way to know to what depth of thermal injury occurs with monopolar cautery, even in cases in which the surgeon “just likes to use it to make the skin incision,” this nerve can lie just millimeters deep to the dermis. There is no way to know how superficial this nerve is this until surgical exposure. Additionally, there is nothing from a hemostatic standpoint that the surgeon cannot address adequately with bipolar cautery, thus obviating any need for the potential risk associated with monopolar cautery. The use of monopolar cautery in lower extremity peripheral nerve surgery is dangerous and below the standard of care.
Prior to making the actual skin incision, I recommend preemptive local anesthesia with lidocaine to prevent the potential for centralization of nociception. Depending on the morphology of the patient, there can be virtually no subcutaneous fat to dissect bluntly or a very thick amount in the high BMI patient, making the surgery very difficult. Then identify the deep fascia and lift it off the tissues below, which includes the nerve itself.
One area of caution is that once through this tissue plane, the nerve can appear like a fatty globule. The surgeon cannot safely excise anything from this area and must only use blunt dissection to release the nerve from the surrounding fat due to longstanding entrapment. In patients with metabolic disease such as diabetes, this nerve can take on the appearance of subcutaneous fat. Inexperienced surgeons have sadly mistaken this nerve as a lipoma only to discover their mistake after they removed it.
After delineating the nerve at this level, the surgeon can perform proximal dissection to release the nerve from any fascia in the popliteal fossa. The surgeon can safely use a finger to dissect this proximally into the fossa but should never perform this maneuver distally as the potential to harm motor innervation is high. In all reality, this is usually not the level of nerve entrapment as the true entrapment is beneath the peroneus (fibular) longus muscle belly as it courses into the lateral compartment.
The next step is to perform a fasciotomy of the superficial fascia of the muscle belly, and then retract the muscle anteriorly and medially. This will leave the surgeon with good visualization of the true site of entrapment as the nerve can be visible before and after entering the lateral compartment. There is often an “hourglass” deformity at this level of entrapment.
At this point, one can perform neurolysis of the nerve distally with extreme care to prevent any damage to the motor branches coming off under the muscle. There can be a 1 to 2 cm length of entrapment medially from where the nerve enters the lateral compartment that one must release in order to ensure a complete neurolysis. At this time, the nerve decompression is essentially complete with the only remaining step to remove or shrink any remaining redundant fascia with bipolar cautery, which will only serve to increase fibrosis. The surgeon must again take care to lift this tissue as far as possible from the nerve when using the bipolar cautery or excising the tissue.
Closure of the skin usually entails both an intradermal interrupted suture such as 4-0 or 5-0 Monocryl (Ethicon) followed by 5-0 nylon sutures of the surgeon’s choice. One can inject bupivacaine (Marcaine) in the skin edges prior to closure so as to avoid any local anesthesia coming in contact with the common peroneal nerve itself. This allows for postoperative pain management but at the same time allows the surgeon to immediately evaluate the motor function of the patient in the post-anesthesia care unit.
Patients may be fully weightbearing immediately after surgery and one should encourage them to begin walking modestly that day. In order to avoid the potential for hematoma, advise patients to walk only to the point of necessity the first 48 hours. Strongly recommend that they begin flexion and extension of the knee repeatedly.
As with all lower extremity peripheral nerve surgery, nerves need to slide and glide. Nothing can destroy the ultimate outcome of peripheral nerve surgery as much as immobilization of the patient after a neurolysis.
One can usually remove sutures at 14 days but in patients with metabolic disease, sutures can stay in as long as one month to prevent dehiscence.
Entrapment of the common peroneal (fibular) nerve is common and we frequently only recognize it when there is severe motor loss, except in the case of the experienced practitioner who has a high degree of suspicion and clinical diagnostic acumen. Patients who present with seemingly unrelated symptoms should always have evaluation for entrapment of this nerve as physicians so commonly miss the diagnosis.
Dr. Barrett is an Adjunct Professor in the Arizona Podiatric Medical Program at the Midwestern University College of Health Sciences. He is a Fellow of the American College of Foot and Ankle Surgeons. Dr. Barrett is a Fellow and the incoming President of the Association of Extremity Nerve Surgeons.
The author discloses that he is a shareholder in Sensory Management Services, which manufactures the Pressure Specified Sensory Device (PSSD).
This article is excerpted from a chapter in the textbook “Practical Pain Management For The Lower Extremity Surgeon,” (in press) with the permission of the publisher, Data Trace.
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Editor’s note: For further reading, see Dr. Barrett’s DPM Blogs, “Overcoming Podiatric Dogma On Neuromas And Peripheral Nerve Surgery” at http://tinyurl.com/lpjnrkt , “Avoiding Crippling Cases Of Drop Foot With Early Diagnosis” at http://tinyurl.com/lrqzer3  or “Peering Into The Crystal Ball: When Will The Podiatry Profession Shake Free Of Dogma When It Comes To Nerves?” at http://tinyurl.com/l7zd74q  .