The Effect of Spinopelvic Motion on Implant Positioning and Hip Stability Using the Functional Safe Zone of THR

Accurate and precise component positioning in total hip arthroplasty is a ubiquitous goal amongst hip surgeons and an important topic of research. Early studies defined a “safe zone” for placement of the acetabular cup, and divergence from the defined safe zone was shown to predispose patients to dislocation [1]. However, ideas regarding the position of the acetabular cup have continued to evolve beyond the initial description by Lewinnek in 1978 [1]. Murray et al. [2], in 1993, defined anatomic, operative, and radiographic parameters for inclination and anteversion. DiGioa et al. expanded upon this work by describing functional cup position, rather than just anatomical inclination and anteversion, as being the angles of the acetabulum that correlated to the axis of the body, using lateral radiographic measurements of the spine, pelvis, and hip [3]. This expanded definition was the first to consider spinal parameters as part of a functional spine–pelvis–hip relationship. Lazennec et al. [4], in France, used a new imaging modality (EOS, Biospace Med, Paris, France) to clearly demonstrate the interrelationship of spinal mobility to acetabular position during postural change of sitting to standing (Fig. 12.1). This research increased our understanding that spine–pelvis–hip motion is synchronized for the purpose of allowing the normal hip to move freely through its arc of motion without the greater trochanter impinging on the pelvis, or the lesser trochanter on the ischium. As hip surgeons learned more about the anatomy of this spine–pelvis–hip relationship, research shifted to studying the effect of this relationship on outcomes following total hip arthroplasty. The cumulative effect of that work has been to redefine the safe zone for acetabular component positioning (functional safe zone) and the influence of sagittal hip motion by taking into account spinopelvic motion. This chapter will focus on the evolving definition of an acetabular safe zone in the context of the spine–pelvis–hip construct, as well as how to personalize and optimize component positioning based on patient-specific spinopelvic parameters.