CORR Insights®: Spinal Fusion Is Associated With Changes in Acetabular Orientation and Reductions in Pelvic Mobility

Abstract

Where Are We Now? In patients with no spine or hip disorders, there is a relationship between acetabular tilt in the sagittal plane and the motion of the sacrum, which is, in turn, related to the motion of the lumbar spine. Two studies [8, 18] originally quantified this using AP, lateral, sitting, and standing radiographs, as well as CT images. Since then, additional studies [7, 13] have shown that lumbar disorders, as well as previous lumbar surgery, can complicate this relationship. Esposito and colleagues [7] measured the standing-to-sitting alignment of the lumbar spine, pelvis, and femur, showing that, on average, patients with degenerative disc disorders experience 10° lower spine flexion and 10° higher femoroacetabular flexion. Stefl and colleagues [13] outlined their method to intraoperatively measure acetabular component position using preoperative spinopelvic mobility, allowing them to avoid impingement in 145 of 160 hips (90%) in 151 patients. The current study by Bernstein and colleagues [1] makes a valuable contribution to this topic by quantifying changes in acetabular orientation under lumbar spine flexion and extension as a function of fusion at L5-S1 or additional spinal levels. Although the current study, among others [1, 7, 13] have been designed to address avoiding hip replacement dislocations, it is important to recognize that acetabular cup impingement may have deleterious effects, even if dislocation never occurs. Specifically, impingement is known to be related to microseparation, edge wear, and damage to the head or neck with hard-on-hard bearing surfaces [2, 4]. With ceramic-on-ceramic implants, edge loading has been associated with squeaking [17], a potentially serious problem that is difficult to address. A more universal issue is that edge loading can have a harmful effect on wear of polyethylene acetabular cups [11]. Since edge wear, by definition, is concentrated on edge of the cup, it cannot be easily detected by wear measurement techniques using clinical radiographs; therefore, the extent of this type of damage over time is somewhat unknown. Beyond polyethylene wear, impingement may result in repetitive stresses on the cup’s fixation interfaces, thus instigating loosening. Consequently, trying to avoid impingement by proper component positioning goes beyond avoiding dislocation; it is not only the small percentage of patients experiencing dislocation who are affected by poor component positioning, but also the larger percentage of patients at risk for impingement. Where Do We Need To Go? Clearly, a major goal is to eliminate dislocations caused by component impingement; even better would be to eliminate impingement among all patients, including those who do not dislocate, since impingement has—as noted above—important adverse effects on wear and loosening. This would require even more accurate calculations and more precise positioning techniques than are now available. One possible solution that deserves greater consideration is the use of dual-mobility acetabular components. Excellent results have been reported in medium-term clinical studies of modern-generation dual-mobility cup designs, including a multicenter prospective study that reported no dislocations after 2 to 4 years followup of 485 primary hip replacements in 452 patients [16]. A systematic review, which included nearly 18,000 total hip replacements with dual-mobility acetabular components, reported dislocation in 0.9% of primary THAs, 3% in revisions, and a risk of intraprosthetic dislocation of 0.7% to 1.3% [5]. Another review found the risk of dislocation in primary dual-mobility THAs was 0.46%, and intraprosthetic dislocation occurred in 1.1% of primary THAs using this device [3]. They further reported 1.3% of primary hips experienced aseptic loosening. These results suggest that problems with the early generations of dual-mobility designs may have been partially related to polyethylene wear, and that the use of highly cross-l