Abstract
INTRODUCTION
The restoration of the anatomical hip rotation center (HRC) has a major influence on the longevity of hip prostheses. Deviations from the HRC of the anatomical joint after total hip arthroplasty (THA) can lead to increased hip joint forces, early wear or loosening of the implant. The contact conditions of acetabular press-fit cups after implantation, including the degree of press-fit, the existence of a polar gap and cup orientation, may affect the HRC restoration, and therefore implant stability. The aim of this study was to determine the influence of acetabular press-fit, polar gap and cup orientation on HRC restoration during THA.
METHODS
THAs were performed by an experienced orthopaedic surgeon in full cadaveric models simulating real patient surgery (n=7). Acetabular cups with a Porocoat™ (n=3) and Gription™ surface coating (n=4) were implanted (DePuy Synthes, Leeds, UK). Computed tomography (CT) scans prior to surgery, as well as after reaming and implantation of press-fit cups were used to calculate the HRC displacement. After aligning the pelves in the anterior pelvic plane, 3D reconstruction of the HRC at each stage was performed by fitting spheres to the femoral head, the reamed cavity and the inserted cup. 3D surface models of the cups were generated using a laser scanner and were registered to the CT images. The effective press-fit was calculated using the diameters of spheres, fitted to the cavity prior to cup insertion and to the outer cup coating. The polar gap was defined as the difference between the outer cup surface and the subchondral bone at the cup pole. Anteversion and abduction angles were calculated as difference between the cup planes and the sagittal and transverse plane, respectively.
RESULTS
A medial (6.4±1.6mm), superior (5.1±1.5mm) and posterior (3.0±1.4mm) displacement of the HRC after reaming was measured. A significant inferior shift of the HRC could be measured after cup implantation (p=0.043). No significant influence of the coating design on the HRC shift could be observed. The shift of the HRC back towards the anatomical HRC was highly correlated to the degree of polar gap (R2=0.928, p<0.001) and a trend towards an association with effective press-fit was observed (R2=0.536, p=0.061). The cup angles had no influence on the shift of the HRC, but a high variability in cup anteversion (20.7° to 61.8°) was observed.
DISCUSSION
The study suggests that increasing the press-fit and polar gap improves the restoration of the anatomical HRC. Since increasing the degree of press-fit could also lead to higher stresses and an increased fracture risk, future work will study how the acetabular contact conditions influence both primary implant stability and fracture risk, in order to establish an optimal HRC reconstruction to maximize implant longevity.
