Abstract
Introduction
The resultant cup orientation depends upon the orientation of the pelvis at impaction. No studies to date have assessed whether patient-position during total hip arthroplasty (THA) has an effect on cup orientation. This study aims to 1) Determine the difference in pelvic position that occurs between surgery and radiographic, supine, post-operative assessment; 2) Examine how the difference in pelvic position influences subsequent cup orientation and 3) Establish whether pelvic orientation, and thereafter cup orientation, differences exist between THAs performed in the supine versus the lateral decubitus positions.
Patients/Materials & Methods
This is a retrospective, multi-surgeon, single-centre, consecutive series. 321 THAs who had intra-operative, post-cup impaction, AP pelvic radiograph, in the operative position were included; 167 were performed with the patient supine (anterior approach), whilst 154 were performed in the lateral decubitus (posterior approach). Cup inclination/anteversion was measured from intra- and post-operative radiographs and the difference (Δ) was determined. Change in pelvic position (tilt, rotation, obliquity) between surgery and post-operatively was calculated from Δinclination/anteversion using the Levenberg-Marquardt algorithm.
Results
The mean post-operative inclination and anteversion were 40°±8 and 23°±9 respectively. 74 had either Δinclination and/or Δanteversion±10° (21%). Intra-operatively (compared to post-operative), the pelvis was on average 4°±10 anteriorly tilted; 1°±10 internally rotated and 1°±5 adducted. Having a Δinclination and/or Δanteversion ±10° was associated with an odds ratio of 3.5 in having a cup orientation outside the target. A greater proportion of cases had Δinclination and/or Δanteversion > ±10° amongst the hips operated in the lateral decubitus (54/153) compared to the supine position (8/167) (p<0.001). The pelvis was significantly more anteriorly tilted (p<0.001) and the hemi-pelvis was significantly more internally rotated intra-operatively in the lateral position (p=0.04) compared to supine.
Discussion
Pelvic movement is significantly less in supine position, which leads to more consistent cup orientation. Significant differences in pelvic tilt and rotation were seen in the lateral position, illustrating the difficulties for surgeons to consistently position the pelvis.
