Acetabular component positioning is commonly referenced with the pelvis in the supine position in direct anterior approach THA. Changes in pelvic tilt (PT) from the pre-operative supine to the post-operative standing positions have not been well investigated and may have relevance to optimal acetabular component targeting for reduced risk of impingement and instability. The aims of this study were therefore to determine the change in PT that occurs from pre-operative supine to post-operative standing, and whether any factors are associated with significant changes in tilt ≥13° in posterior direction. 13° in a posterior direction was chosen as that amount of posterior rotation creates an increase in functional anteversion of the acetabular component of 10°. 1097 THA patients with pre-operative supine CT and standing lateral radiographic imaging and 1 year post-operative standing lateral radiographs (interquartile range 12–13 months) were reviewed. Pre-operative supine PT was measured from CT as the angle between the anterior pelvic plane (APP) and the horizontal plane of the CT device. Standing PT was measured on standing lateral x-rays as the angle between the APP and the vertical line. Patients with ≥13° change from supine pre-op to standing post-op (corresponding to a 10° change in cup anteversion) were grouped and compared to those with a <13° change using unpaired student's t-tests. Mean pre-operative supine PT (3.8±6.0°) was significantly different from mean post-operative standing PT (−3.5±7.1°, p<0.001), ie mean change of −7.3±4.6°. 10.4% (114/1097) of patients had posterior PT changes ≥13° supine pre-op to standing post-op. A significant number of patients, ie 1 in 10, undergo a clinically significant change in PT and functional anteversion from supine pre-op to standing post-op. Surgeons should be aware of these changes when planning component placement in THA.
The pelvis moves in the sagittal plane during functional activity. This can be detrimental to functional cup orientation. Increased pelvic mobility 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 mobility. Pre-operatively, 3428 patients had their pelvic tilt and lumbar lordotic angle (LLA) measured in three positions; supine, standing and flexed-seated. 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 “at risk” pelvic rotation, defined by rotation ≥13° in a detrimental direction, was determined. There was an increased incidence of “at risk” pelvic mobility with increasing age, and decreasing lumbar flexion. This was more pronounced in females. Notably, 31% of elderly females had “at risk” pelvic mobility. Furthermore, 38% of patients with lumbar flexion <20° had “at risk” pelvic mobility. “At risk” pelvic mobility was more common in older patients and in patients with limited lumbar flexion. Additional stability, such as a dual mobility articulation, might be advisable in patient cohort. However, the majority of patients exhibiting “at risk” pelvic mobility were not older than 75, and did not have lumbar flexion <20°. This supports analysis of pelvic mobility on all patients undergoing THR.
