By M. Jordan Brassell, DPM, Benjamin Marder, DPM, AACFAS and Jason R. Miller, DPM, FACFAS.
Ankle sprains are one of the more common problems foot and ankle surgeons address. About one million ankle injuries occur each year and 85 percent of them are sprains.1 Ankle sprains can result in a wide variety of injuries depending on the severity of the inciting event and the age of the patient. There are many common injuries that occur with ankle sprains, including ligament tears, avulsion fractures, talar dome lesions and peroneal tendon tears. One of the less common or less diagnosed concomitant injuries is common peroneal nerve traction injury.
The mechanism of a peroneal nerve traction injury in ankle sprains involves inversion of an already plantarflexed ankle. This position applies traction to the peroneal nerve at the fibular head due to stretching of the peroneal muscles.2
The common peroneal nerve branches from the tibial nerve at the popliteal fossa. It then courses laterally toward the fibular head and divides into the superficial and deep peroneal nerves. The superficial peroneal nerve courses laterally down the leg to then provide sensory innervation to the majority of the dorsum of the foot while the deep peroneal nerve, as its name suggests, courses deep in the anterior compartment of the leg.3
Clinicians often classify nerve injuries with Seddon’s classification (neuropraxia, axonotmesis, neurotmesis) or Sunderland’s classification (first through fifth degrees). It can be difficult to classify a traction injury due the nature of the injury. The force of the pulling and stretching is not uniform along the entire course of the nerve. Therefore, this results in varying levels of nerve damage at different areas of the nerve. Most common peroneal traction injuries can be classified as either neuropraxia or axonotmesis.4
A nerve traction injury results from a stretching force on the nerve and more frequently occurs in the brachial plexus. It most commonly happens to the peroneal nerve in knee, hip or spinal injuries, but also occurs with ankle sprains and proximal fibular fractures. Additionally, iatrogenic nerve injury can occur from the improper placement of a cast resulting in compression of the nerve at the fibular head. There are also reported cases of injury to the peroneal nerve following knee arthroplasty.5,6
One can identify the signs and symptoms of this pathology through a thorough physical exam. In the acute setting, most practitioners will find pain out of proportion to the actual orthopedic ankle injury. The patient may describe burning, pulsating or radiating pain in the lateral ankle with or without proximal radiation along the lateral leg. Clinicians may misdiagnose this as a peroneal tendon injury but one can often rule this out with magnetic resonance imaging or musculoskeletal ultrasound that is negative for peroneal tendon pathology.
Many of the patients diagnosed in our office have had predominantly cutaneous symptoms but there may be motor symptoms as well. Early musculoskeletal symptoms can include weakened dorsiflexion of the ankle or the digits, or weak eversion of the rearfoot. Late musculoskeletal symptoms may result in drop foot or an acquired equinovarus foot deformity.
During the physical exam, it is important to palpate the peroneal nerve just distal to the fibular head. Tapping or compression over this area may reproduce dysesthesia in the lateral calf, anterolateral ankle or foot. Other physical exam findings might include paresthesia along the nerve’s area of sensory innervation and bruising along the course of the common peroneal nerve. While symptoms can be indicative of a peroneal nerve traction injury, one can make a more definitive diagnosis with the use of electromyography (EMG) within two weeks of the injury. Eighty-six percent of patients with grade III sprains and 17 percent of patients with grade II sprains had electrodiagnostic evidence of peroneal nerve injury on needle examination.7
Pain along the lateral aspect of the leg is the earliest symptom in peroneal neuropathy and may be the most difficult to treat.8 Currently available agents for neuropathic pain include topical lidocaine, capsaicin, selective serotonin reuptake inhibitors (SSRIs), antiepileptics, opioids and μ-receptor agonists. As these provide symptomatic relief only, the choice of medication depends on comorbidities and possible adverse effects.9 Additionally, Edwards and colleagues have suggested iontophoresis for the relief of pain in peroneal neuropathy.10 For these patients, one should strongly consider physical therapy modalities focused on nerve and pain reduction strategies, and employ these modalities as tolerated.
Weakness in peroneal neuropathy may lead to functional gait impairment. Muscle atrophy can become clinically apparent as early as two weeks after injury. All patients with weakness should stretch daily. If they are unable to stretch on their own, they should work with a qualified physical therapist on passive stretching to prevent contracture of the Achilles tendon. Equinovarus foot deformity is a common complication of ankle dorsiflexion weakness. It is essential that a patient maintain his or her range of motion to have the ability to ambulate.9 Modalities such as heat and ice can also provide effective pain relief. However, one should carefully observe patients with sensory loss during the use of these modalities in order to prevent skin damage.
If the patient has weakness of the toe extensors only as one may observe in those with distal deep peroneal neuropathy, sturdy footwear may be all that is necessary to optimize gait. If the patient has isolated superficial peroneal nerve palsy, he or she may benefit from a shoe insert with a lateral wedge to prevent over-supination of the foot due to weak evertors. If the patient has neuropathy of the proximal deep peroneal nerve or at the level of the common peroneal nerve, he or she may be unable to dorsiflex the ankle. In this case, an ankle foot orthosis (AFO) would help maintain the foot in a neutral position so the patient can ambulate normally.9
One should employ surgical interventions in cases that involve early diagnosis and when the surgeon is prepared to decompress the common peroneal nerve through its various muscular fascial strictures. In my experience, surgical decompression has yielded very favorable results when examination and/or EMG have demonstrated the potential efficacy for nerve release.
While persistent peroneal nerve traction injuries are less common, a thorough physical examination and good knowledge of patient clinical presentation can lead to timely diagnosis and rapid determination of a successful treatment plan.
Dr. Miller is the Director of the Pennsylvania Intensive Lower Extremity Fellowship at Premier Orthopaedics in Malvern, PA.
Dr. Brassell is a first-year resident at Phoenixville Hospital/Tower Health residency program.
Dr. Marder is a Fellow at the Pennsylvania Intensive Lower Extremity Fellowship in Malvern, Pa.