CORR Insights®: The Effect of Postural Pelvic Dynamics on the Three-dimensional Orientation of the Acetabular Cup in THA Is Patient Specific

Abstract

Where Are We Now? Dislocation continues to be a common complication after THA. In a recent study of 51,345 revision hip arthroplasties in the United States, dislocation was the most-common cause of revision, accounting for nearly one-fourth of those revisions [3]. Many factors affect the likelihood of dislocation after THA, including soft-tissue laxity, surgical approach, component position, patient factors, component designs, and intraoperative stability testing results. A recent study described how sagittal pelvic tilt changes during activities of daily living [9]. These changes in pelvic tilt influence the acetabular component’s position relative to the femur. In the seated position, functional anteversion and abduction increase as the pelvis tilts posteriorly, which serves to uncover the femoral head anteriorly. Posterior pelvic tilt is thought to prevent anterior impingement by the femoral component. Additionally, a history of lumbar fusion is a risk factor for dislocation and revision after THA [2]. With lumbar fusion, the absence of compensatory pelvic tilt leads to loss of functional anteversion of the acetabular component and anterior impingement of the femoral component during sitting, contributing to posterior dislocation. Acetabular cup positioning is one of the most-important technical factors affecting dislocation, postoperative ROM, impingement, liner fracture, and long-term wear. The safe zone of Lewinnek is widely accepted by surgeons, but it does not correlate well with dislocation [1, 17]. Understanding the cup’s functional position will help to explain the findings of previous studies [1, 17] that dislocation is not correlated with the historical safe zone, based on static AP radiographs of the pelvis. These studies raised the question of how much the position of the acetabular component changes with postural changes, based on different combinations of cup abduction and anteversion achieved during surgery. Two studies reported the impact of pelvic tilt on the acetabular component’s position; that is, the change in functional anteversion was a constant, linear response to change in pelvic tilt, but the change in functional abduction was a nonlinear response [6, 9]. Placing the acetabular component without considering the patient’s pelvic tilt may lead to impingement, dislocation, and long-term wear. Snijders et al. [10] have presented a mathematical model that calculates the acetabular component’s three-dimensional position with changing sagittal pelvic tilt, validated the mathematical model by comparing it with a computer model, and described changes in the acetabular component’s position during functional pelvic tilt. The correlation between the measured and mathematically calculated angles was excellent. A detailed analysis revealed that the changes in the acetabular component’s three-dimensional position during functional pelvic tilt differed substantially between cups with different initial positions, and the tested mathematical model was incorporated into an easy-to-use tool. Of note, the change in cup anteversion was marked in cups with low cup abduction, and the change in cup abduction was marked in cups with high cup anteversion. By using an easy-to-use tool, it is possible for surgeons to know the functional cup position using several radiographs. For example, surgeons could evaluate the coronal, sagittal, and transverse acetabular component position with the patient in the sitting position. Where Do We Need to Go? We need a more-comprehensive understanding of the functional cup position, particularly an elucidation of the mechanisms of dislocation, in order to reduce the risk of dislocation. The information presented in the current study [10] should be used to define the functional safe zone; as the authors suggest, the correct position of the acetabular component may not be the same for each individual. The functional safe zone should be individualized based on functional pelvic tilt, particularl