Incidence, risk factors and prognosis of transient pseudosubluxation after total hip arthroplasty.

INTRODUCTION: Pseudosubluxation is a transient separation of the prosthetic femoral head from the acetabular liner during the anaesthetic period after total hip arthroplasty (THA). However, little is known about the frequency, pathomechanism, risk factors or natural history.

METHODS: To determine the incidence, direction of the displacement, risk factors and subsequent instability of pseudosubluxation, we evaluated 1099 primary cementless THAs (943 patients), which were performed during 8 years at one institution. Immediately after THA, postoperative radiographs were obtained for all hips. If a subluxation was noted, the operated hip was examined by fluoroscope within 1 hour of the detection and repeat radiographs were taken on the postoperative day 1.

RESULTS: The pseudosubluxation was identified in 2.6% (28/1059). All of the 28 heads subluxed anteriorly and reduced in flexion-internal rotation. In multivariate analysis: (1) operative decrease of the femoral offset (odds ratio; 1.161 (95% confidence interval [CI]; 1.077-1.251), p = 0.001); and (2) medialisation of acetabular cup (odds ratio; 3.402 (95% CI; 1.482-7.813), p = 0.009) were found as risk factors for pseudosubluxation. None of the 28 hips dislocated during 3- to 8-year follow-up.

CONCLUSIONS: Our results provide information about the incidence, risk factors and natural history of pseudosubluxation after primary THA. Surgeons should be aware of the risk of pseudosubluxation when performing THA in patients with coxa vara, who have a large femoral offset, and in those with dysplastic or deficient acetabulum, who have a lateral centre of rotation.