Hip stability after total hip arthroplasty predicted by intraoperative stability test and range of motion: a cross-sectional study

BACKGROUND: Dislocation continues to be a common complication following total hip arthroplasty (THA). A larger intraoperative range of motion (ROM) is believed to minimize dislocation risk, and intraoperative stability tests have been used to assess the ROM. However, it is not clear whether or not intraoperative stability tests can predict hip stability after THA. It is also unclear which angles are required in intraoperative stability tests. We investigated the usefulness of intraoperative stability tests, and other risk factors to predict hip stability after THA. METHODS: Patients operated by single surgeon at one hospital from June 2009 to December 2013 were evaluated. This study included 185 hips with 32 mm metal femoral head. The range of internal rotation with 90° hip flexion (IR angle) was measured as an intraoperative stability test. The variables studied as risk factors included age, height, weight, gender, cerebral dysfunction, preoperative diagnosis, history of previous hip surgery, and IR angle. RESULTS: Mean IR angle was statistically different between patients with dislocation and patients without dislocation (59.5° vs 69.6°: p = 0.006). Cerebral dysfunction and a history of previous hip surgery were statistically related with prevalence of dislocation (p = 0.021, and p = 0.011). The receiver-operating characteristic curve analysis suggested that the cutoff points for IR angle were 51° and 67°. Dislocation rate in larger IR angle group was significantly lower than the rate in smaller IR angle group when patients were divided by 51° (p = 0.002). Logistic regression analyses showed that significant risk factors were cerebral dysfunction (OR: 5.3 (95%CI 1.1–25.9); p = 0.037), history of previous hip surgery (OR: 8.6 (95%CI 1.2–63.0); p = 0.035), and IR angle (OR: 10.4 (95%CI 1.9–57.1); p = 0.007). CONCLUSIONS: The results showed that intraoperative stability test, especially the IR angle, was a useful method to predict hip stability after THA, and a larger intraoperative ROM reduced the likelihood of dislocation. 51° and 67° were indicated as cutoff points for IR angle. Cerebral dysfunction and a history of previous hip surgery are also risk factors for the incidence of dislocation after THA. TRIAL REGISTRATION: This is a retrospective study, not a clinical trial defined by the World Health Organization (WHO).