In total hip arthroplasty (THA), incorrect cup orientation markedly increases the risk of dislocation, wear, and impingement. The primary method of cup placement is based on the surgeon's interpretation of orientation within a "safe zone" defined by Lewinnek almost 40 years ago. This approach has undergone little change despite consistent dislocation rates and a large proportion of dislocations occurring in patients with cups within the safe zone. One reason for this may be that it does not account for a patient's natural pelvic posture, which affects the functional cup anteversion. Some modern techniques of cup placement assess a functional anteversion based on supine and others on standing orientation. However, these methods do not account for lumbar mobility and the effects on spinopelvic mechanics when in the seated position, the most common position in which dislocation occurs. The investigators have shown that patients with lumbar fusion have a higher rate of THA dislocation. This risk also correlated with length of fusion construct. This suggests that decreased lumbar mobility, whether a result of arthrodesis, disease or age related degeneration, may play an important role in the risk of THA dislocation. Some studies have measured change in pelvic orientation and cup anteversion between sitting and standing. Others have used modeling to estimate change in functional anteversion based on pelvic tilt. Presently, however, there are no studies that have investigated the quantitative relationship, in vivo, between patient-specific changes in spinopelvic parameters and cup anteversion between sitting and standing. Within this exploration, identification of an appropriate safe zone in individuals with mobility restrictions of the lumbar spine is not possible.