A 16-year-old club player presented approximately two weeks after an inversion injury to his right foot, sustained during a slide tackle. He was able to limp off the field but was unable to continue. Over the next few days he developed progressive weakness in his right foot. On examination he was unable to dorsiflex the ankle, extend the toes, invert the foot, or evert against gravity. There was loss of light touch perception in the distribution of the deep peroneal nerve and mild alteration of perception to light touch in the distribution of the superficial peroneal nerve. There was moderate tenderness over the common peroneal nerve at the fibular neck and quite significant tenderness in the junction of the proximal and middle thirds of the anterior compartment of the leg.
Ultrasound imaging performed shortly after the injury (not shown here) showed a tear in the peroneus longus muscle. Subsequent MRI images further defined the injury to the proximal peroneus longus. There is a ‘grade 3’ muscular tear with a 4 cm intramuscular haematoma and retraction of the central tendon. The haematoma is associated with bulging of the peroneus musculature within the axial plane, which likely causes mechanical pressure on the common peroneal nerve at the fibular neck. The nerve appears otherwise anatomically normal.
This patient was treated with a temporary ankle-foot orthosis (AFO) to manage his foot drop. Consideration was given to aspirating the haematoma however this was not done as it was not certain that this would be either possible or effective. His strength progressively recovered over a three month period and he was able to return to football without any ongoing disability.
The common peroneal, superficial peroneal, and saphenous nerves are the most common nerves at risk of compression (or other injury) in the lower leg. This is most commonly related to trauma. Common peroneal nerve entrapment can be caused by external compressive sources which include poorly fitting plaster casts and knee braces. “Internal” compressive sources include osteophytes, proximal tibiofibular joint ganglion cysts and haematomas (as seen in this case). The nerve can also be affected following an ankle sprain/inversion injury where a traction-type mechanism occurs. The common peroneal nerve can also be injured after repetitive exercises involving inversion and eversion, such as running and cycling. Finally, knee surgery is a documented cause of injury.
Imaging studies can be useful to assess for a mechanical cause of compression. MRI and ultrasound are the most commonly used modalities for this assessment. Nerve conduction studies can also be useful where the diagnosis is in doubt, or to assess the potential for recovery when the symptoms have been more prolonged. This type of test is generally not useful during the first 4-6 weeks post-injury.
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