Introduction. The pelvis moves in the sagittal plane during functional activity. These movements can have a detrimental effect on functional cup orientation. The authors previously reported that 17% of total hip replacement (THR) patients have excessive pelvic rotation preoperatively. This increased pelvic rotation could be a risk factor for instability and edge-loading in both flexion and/or extension. The aim of this study was to investigate how gender, age and lumbar spine stiffness affects the number of patients at risk of excessive sagittal pelvic rotation. Method. Pre-operatively, 3428 patients had their pelvic tilt (PT) and lumbar lordotic angle (LLA) measured in three positions; supine, standing and flexed-seated, as part of routine planning for THR. The pelvic rotation from supine-to-standing and from supine-to-seated was determined from the difference in pelvic tilt measurements between positions. Lumbar flexion was determined as the difference between LLA standing and LLA when flexed-seated. Patients were stratified into groups based upon age, gender and lumbar flexion. The percentage of patients in each group with excessive pelvic rotation, defined by rotation ≥13° in a detrimental direction, was determined. Results. Posterior pelvic rotation from supine-to-stand increased with age and decreasing lumbar flexion. This was more pronounced in females. Similarly, anterior pelvic rotation from supine-to-seated increased with age and decreasing lumbar flexion. This was more pronounced in males. Notably, 30% of elderly females had excessive pelvic rotation. Furthermore, 38% of patients with lumbar flexion <20° had excessive pelvic rotation. Conclusions. Excessive pelvic rotation was more common in older patients and in patients with limited lumbar flexion. This might be a factor in the increased dislocation rate in the elderly population. A more stable articulation might be a consideration in patients with limited lumbar flexion (<20°). This constitutes 5% of the THR population. The large range of pelvic rotation in each group supports individual analysis on all patients undergoing THR

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. The position of the pelvis in the sagittal plane changes significantly between functional activities. The extent of change is specific to each patient. 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. Planning and measurement of cup placement in the supine position can lead to large discrepancies in orientation during more functionally relevant postures. Optimal cup orientation is likely patient-specific and requires an evaluation of functional pelvic dynamics to pre-operatively determine the target angles