Abstract
Introduction
The pelvis is not a static structure. It rotates in the sagittal plane depending upon the activity being performed. These dynamic changes in pelvic tilt have a substantial effect on the functional orientation of the acetabulum. The aim of this study was to quantify the changes in sagittal pelvic position between three functional postures.
Methodology
Pre-operatively, 1,517 total hip replacement patients had their pelvic tilt measured in 3 functional positions – standing, supine and flexed seated (point when patients initiate rising from a seated position). Lateral radiographs were used to define the pelvic tilt in the standing and flexed seated positions. Pelvic tilt was defined as the angle between a vertical reference line and the anterior pelvic plane (defined by the line joining both anterior superior iliac spines and the pubic symphysis). In the supine position pelvic tilt was defined as the angle between a horizontal reference line and the anterior pelvic plane. Supine pelvic tilt was measured from computed tomography.
Results
The mean supine pelvic tilt was 4.2°, with a range of −20.5° to 24.5°. The mean standing pelvic tilt was −1.3°, with a range of −30.2° to 27.9°. Mean pelvic tilt in the flexed seated position was 0.6°, with a range of −42.0° to 41.3°. The mean absolute change from supine to stand, and supine to flexed seated was 6.0° (SD = 3.8°) and 10.7° (SD = 8.1°) respectively. 6% of patients rotated posteriorly by more than 13° from supine to stand, consequently putting them at risk of excessive functional anteversion in extension. 11% of patients rotated anteriorly by more than 13° from supine to seated, consequently retroverting their cup and putting them at risk in flexion. Therefore, 17% of patients had sagittal pelvic rotations that could lead to functional cup malorientation even with a supposedly ideal orientation of 40°/20°. Factoring in an intraoperative delivery error of ± 5° extends this risk to 51% of patients.
Conclusions
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The position of the pelvis in the sagittal plane changes significantly between functional activities. The extent of change is specific to each patient.
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17% of patients had sagittal pelvic rotations that could lead to functional cup malorientation in functional flexion or extension, even with an apparently perfectly-orientated component. This number extended to 51% when an intra-operative delivery error of ± 5° was considered.
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Planning and measurement of cup placement in the supine position can lead to large discrepancies in orientation during more functionally relevant postures.
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Optimal cup orientation is likely patient-specific and requires an evaluation of functional pelvic dynamics to pre-operatively determine the target angles.
