Ortho Dx: Valgus deformity of the knee

People with severe valgus deformities of the knee are at an increased risk for peroneal injury and resulting postoperative foot drop following total knee arthroplasty (TKA). The common peroneal nerve arises from the fourth and fifth lumbar roots and the...

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People with severe valgus deformities of the knee are at an increased risk for peroneal injury and resulting postoperative foot drop following total knee arthroplasty (TKA).

The common peroneal nerve arises from the fourth and fifth lumbar roots and the first and second sacral nerves. It descends from the popliteal fossa, wraps laterally around the fibular head, and then divides beneath the peroneus longus muscle into the superficial and deep peroneal nerves. The superficial branch provides motor innervation to the peroneus longus and brevis muscles and sensation to the lateral leg and dorsum of the foot. The deep peroneal nerve supplies movement to the foot and toe dorsiflexors and sensation to the dorsal first web space.

Injury to the peroneal nerve after correction of valgus TKA typically presents with foot drop and catching of the toes when ambulating. Numbness is usually present over the lateral leg and dorsum of the foot.^1,2

Correction of valgus knee and/or preoperative flexion contracture expands the lateral joint line, resulting in stretching of the peroneal nerve and surrounding soft tissue. Nerve damage has been shown to occur in the axons after just 4% to 11% elongation,¹ and it is hypothesized that stretch may impair the blood supply to the nerves.

 

If peroneal palsy is identified after TKA, all dressings should be removed to relieve compression, and the knee should be placed in 20 to 30 degrees of flexion to ease stretch on the nerve. Physical therapy is initiated in an attempt to activate the foot dorsiflexors and stretch the contralateral muscle groups. An ankle-foot orthosis may be necessary if foot drop persists or interferes with gait.

The prognosis for peroneal palsy or foot drop following total knee replacement is guarded; approximately 50% of patients experience complete recovery with nonoperative care. Recovery usually occurs in cases of partial peroneal palsy; however, recovery from complete palsy is variable. Electromyography and/or nerve conduction studies help determine the severity of conduction loss and the need for surgical intervention. If foot drop does not improve after 3 months, operative exploration and decompression should be considered.²

Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).

References

1. Nercessian OA, Ugwonali OF, Park S. Peroneal nerve palsy after total knee arthroplasty. J Arthroplasty. 2005;20(8):1068-1073.
2. Poage C, Roth C, Scott B. Peroneal nerve palsy: evaluation and management. J Am Acad Orthop Surg. 2016;24(1):1-10.