**1. Introduction**

Hip arthroplasty remains one of the most successful surgeries offered today; however, with a prosthetic hip component, a unique possibility of dislocation arises [1]. The incidence of instability within revision hip arthroplasty is estimated from 17 to 25% with a mean cost of care at \$77.851.24 [2, 3]. With rates rates of primary hip arthroplasty increasing, the projected financial burden on the healthcare system remains large [4–6]. Historically, implant designs featured smaller femoral head articulations, such as the 22 mm femoral heads of the Charnley hip, which were associated with instability rates as high as 4.8% [7, 8]. Since the landmark Morrey article in 1982, improved implant designs and surgical techniques have evolved lowering the dislocation rate from 3.2% to less than 2% [9, 10]. Although a large percentage of instability can be managed nonoperatively, instability remains the most common indications for revision arthroplasty within the United States [4, 11]. Numerous risk factors exist including patient demographic variables, approach, surgeon learning curve, spinopelvic relationship, and indication for surgery. These factors should be

#### **Figure 1.** *Femoral offset and subsequent abductor moment arm [13].*

taken into consideration when projecting a specific patients' potential risk of subsequent instability.

A stable THA relies on understanding the biomechanics of femoral head size and center of rotation (COR) [12]. Briefly, hip offset is defined as the linear distance from the femoral COR to the axis of the femoral shaft. A medial shift in the center of rotation decreases the moment arm of the abductors, thereby changing abductor tension and increasing potential risk of instability (**Figure 1**). Conversely, an increase in femoral offset adds to abductor tension and reduces potential instability [14]. Hip stability can also be achieved through modulation of femoral head size and consequently jump distance. The linear distance required for the femoral head to travel prior to dislocation is directly proportional to the femoral head size. By increasing the size of the femoral head a larger displacement is required prior to dislocation (**Figure 2**). Surgical manipulation of hip anatomy through biomechanics is central towards optimizing patient stability.
