Associate Professor Emory University Atlanta, Georgia, United States
Case Diagnosis: Right Foot Drop in the Setting of Sciatic Neuropathy
Case Description: 71 year old female who underwent recent total hip arthroplasty (THA) via anterior approach presented for electrodiagnostic testing after developing right foot drop with lower lateral leg and dorsal foot numbness post-operatively. She had been using an ankle foot orthosis (AFO) daily and attending physical therapy for hip and ankle strengthening with minimal improvement. The patient was referred for electrodiagnostic testing of the right lower extremity to evaluate for fibular vs sciatic neuropathy. The study revealed ongoing denervation in muscles innervated by the common fibular division of the sciatic nerve, as well as absent fibular compound motor action potentials and sensory nerve action potentials suggesting a sciatic neuropathy. This was later confirmed with magnetic resonance neurography showing Sunderland Grade II injury involving the common fibular nerve bundle within the sciatic nerve.
Discussions: With THA via the anterior approach, a femoral nerve injury is considerably more common, with the sciatic nerve theoretically further from harm. Typically, the sciatic nerve is at increased risk of injury via the posterior approach, with incidence of 1.5% reported in the literature. However, in one specific study reviewed, 0.05% of patients undergoing THA via the anterior approach developed sciatic neuropathy. Potential reasons a sciatic neuropathy could theoretically develop include compression from a hematoma, ischemia to the nerve, or excess traction. Further, the presence or absence of fibular compound muscle action potential (CMAP) amplitudes can help guide prognostication of recovery.
Conclusions: Ultimately, the femoral nerve is more commonly injured post-THA via the anterior approach compared to the sciatic nerve. However, while the relative incidence of sciatic neuropathy after THA via anterior approach is very low, it must remain high on the list of differential diagnoses in the setting of post-operative foot drop.
