Background: Late dislocations after total hip arthroplasty (THA) are challenging for the hip surgeon because the cause is often not evident and recurrence is common. Recently, decreased spinopelvic motion has been implicated as a cause of dislocation. The purpose of this study was to assess the mechanical causes of late dislocation, including the influence of spinopelvic motion. Methods: Twenty consecutive patients were studied to identify the cause of their late dislocation. Cup inclination and anteversion were measured on standard pelvic radiographs. Lateral standing and sitting spine-pelvis-hip radiographs were used to measure pelvic motion, femoral mobility, and sagittal cup position by assessing sacral slope, pelvic-femoral angle, and cup ante-inclination. Spinopelvic motion was defined as the difference between the standing and sitting sacral slopes (Δsacral slope). A new measurement, the combined sagittal index, which measures the sagittal acetabular and femoral positions, was used to assess the functional motion of the hip joint and risk of impingement. Results: There were 9 anterior dislocations (45%) and 11 posterior dislocations (55%) at a mean of 8.3 years after a primary THA. Eight of the 9 patients with an anterior dislocation had spinopelvic abnormalities such as fixed posterior pelvic tilt when standing, increased standing femoral extension, and an increased standing combined sagittal index. Ten of the 11 patients with a posterior dislocation had abnormal spinopelvic measurements such as decreased spinopelvic motion (average Δsacral slope [and standard error] = 9.0° ± 2.4°), increased femoral flexion, and a decreased sitting combined sagittal index. For every 1° decrease in spinopelvic motion, there was an associated 0.9° increase in femoral motion and, in some patients, this resulted in osseous impingement and dislocation. Conclusions: Patients with a late dislocation have abnormal spinopelvic motion that precipitates the dislocation, especially when combined with cup malposition or soft-tissue abnormalities. Spinopelvic stiffness is associated with increased age and increased femoral motion, which may lead to impingement and dislocation. Lateral spine-pelvis-hip radiographs may predict the risk and direction of dislocation. Level of Evidence: Therapeutic Level IV . See Instructions for Authors for a complete description of levels of evidence.

Computer navigation and robotic assistance technologies are used to improve the accuracy of component positioning in total knee arthroplasty (TKA), with the goal of improving function and optimizing implant longevity. The purpose of this study was to analyze trends in the use of technology-assisted TKA, identify factors associated with the use of these technologies, and describe potential drivers of cost. The Nationwide Inpatient Sample database was used to identify patients who underwent TKA using conventional instrumentation, computer navigation, and robot-assisted techniques between 2005 and 2014. Variables analyzed include patient demographics, hospital and payer types, and hospital charges. Descriptive statistics were used to describe trends. Univariate and multivariate analyses were performed to identify differences between conventional and technology-assisted groups. Our analysis identified 6,060,901 patients who underwent TKA from 2005 to 2014, of which 273,922 (4.5%) used computer navigation and 24,084 (0.4%) used robotic assistance. The proportion of technology-assisted TKAs steadily increased over the study period, from 1.2% in 2005 to 7.0% in 2014. Computer navigation increased in use from 1.2% in 2005 to 6.3% in 2014. Computer navigation was more likely to be used in the Western United States, whereas robot-assisted TKAs were more likely to be performed in the Northeast. Increased hospital charges were associated with the use of technology assistance ($53,740.1 vs $47,639.2). The use of computer navigation and robot-assisted TKA steadily increased over the study period, accounting for 7.0% of TKAs performed in the United States in 2014. Marked regional differences in the use of these technologies were identified. The use of these technologies was associated with increased hospital charges.