Characterizing “Peroneal Nerve” Injury Clinicoradiologic Patterns with MRI in Patients with Sciatic Neuropathy and Foot Drop After Total Hip Replacement
Kitty Wu, MD, MSc, FRCSC1, Kimberly K. Amrami, MD2, Kirsten Hayford, RN1 and Robert J. Spinner, MD3, 1Mayo Clinic, Rochester, MN, 2Department of Radiology, Mayo Clinic, Rochester, MN, 3Department of Neurologic Surgery, Mayo Clinic, Rochester, MN
Introduction: Sciatic nerve injury following total hip arthroplasty (THA) predominantly affects the personal division, often causing a foot drop. This can result from a focal etiology (hardware malposition, prominent screw, post-operative hematoma) or non-focal/traction injury. The objective of this study was to compare the clinico-radiologic features and define the extent of nerve injury resulting from these two distinct mechanisms.
Materials & Methods: Patients who developed a post-operative foot drop within one year following primary or revision THA with a confirmed proximal sciatic neuropathy based on MRI or electrodiagnostic studies were retrospectively reviewed. Patients were divided into two cohorts: Group 1 (Focal) including patients with an identifiable focal structural etiology and Group 2 (Non-Focal) including patients with a presumed traction injury. Patient demographics, clinical examination, subsequent surgeries, electrodiagnostic study results, and MRI abnormalities were noted. Student’s t-tests were used to compare time to onset of foot drop and time to secondary surgery.
Results: Twenty-one patients, treated by one surgeon, met inclusion criteria (8 men, 13 women, 14 primary, 7 revision THA). Group 1 had significantly longer time from THA to onset of foot drop, with a mean of 2 months, compared to immediately post-operative in Group 2 (p = 0.02). Group 1 had a consistent pattern of localized focal nerve abnormality on imaging. In contrast, the majority of patients in Group 2 (n = 11) showed a long continuous segment of abnormal size and signal intensity of the nerve, while the other three patients had a segment of less abnormal nerve in the mid-thigh. All patients with a long continuous lesion had M0 dorsiflexion prior to secondary nerve surgeries compared to one of three patients with a more normal mid-segment.
Conclusions: There are distinct clinicoradiologic findings in patients with sciatic injuries resulting from a focal structural etiology versus a traction injury. While there are discrete localized changes in patients with a focal etiology, those with traction injuries demonstrate a diffuse zone of abnormality within the sciatic nerve. A proposed mechanism involves anatomic tether points of the nerve acting as points of origin for traction injuries, resulting in an immediate post-operative foot drop (Figure 1). In contrast, patients with a focal etiology have localized imaging findings but a highly variable time to the onset of foot drop.
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