Mal-rotation of pelvis on the sagittal plane, which is common in patients with fixed spinal kyhposis, for example, ankylosing spondylitis, can cause error in cup positioning when hip arthroplasty is performed. The present study was performed to quantify the effects of sagittal pelvic mal-rotation on the final cup position and to evaluate different methods of cup positioning to compensate for the mal-rotation. Three-dimensional reconstruction of computer tomograms of 15 sets of full pelvi was performed. Two methods of cup insertion were simulated and compared: a method mimicking genuine surgery (anatomical positioning) and one that compensates for the sagittal pelvic mal-rotation (functional positioning). Sagittal pelvic mal-rotation of more than 20 degrees , if ignored, resulted in a cup with an anterversion of more than 30 degrees and an inclination of more than 55 degrees. Half of the cup surface was not in contact with host bone when the cup position was maintained at 20 degrees anteversion and 45 degrees inclination in a patient with 50 degrees sagittal pelvic mal-rotation. The usual method of cup positioning may need to be modified in patients with sagittal pelvic mal-rotation in order to maintain the desired cup position. For each 10 degrees of sagittal pelvic mal-rotation beyond 20 degrees of mal-rotation, the cup needs to be put in such that it is 5 degrees less inclined and anteverted.
(c) 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc.