Is Femoral Head to Acetabular Cup Ratio Associated With Dislocation After Total Hip Arthroplasty?

Background: Dislocation remains a common complication following total hip arthroplasty (THA). Previous literature has shown that the femoral head-to-neck ratio is essential in hip motion, function, and stability. While large femoral heads and dual mobility bearings have been developed to improve stability, it remains unknown if the ratio between femoral head size to acetabular cup size also plays a role in stability. This study aimed to investigate if the relationship between the femoral head size and the acetabular cup size factors into the risk of hip dislocation. Methods: 3155 patients who underwent primary THA for the indication of osteoarthritis between 2016 and 2019 were identified. Patient demographic, radiographic, implant and procedural data were analyzed. The primary outcome was determining head-to-cup ratios calculated by dividing the femoral head size by the acetabular cup size. Head-to-cup ratios were then compared in patients who suffered a dislocation after THA to those who did not. Univariate and logistic regression was conducted to determine additional dependent variables for hip dislocation. Results: Patients in this study had an average age of 64 ± 11 years. Females comprised 51% of the cohort, and BMI was 29.2 ± 5.2. A total of thirty-six (1.1%) patients sustained a dislocation at a median of 51 days following surgery. The dislocation cohort was similar to the non-dislocation cohort in age, gender, BMI, CCI, and ASA scores. The head-to-cup ratio was also identical between the dislocation (0.64 ± 0.05 mm) and non-dislocation (0.64 ± 0.05 mm) patients, P=0.317. Logistic regression adjusting for age, sex, BMI, and surgical approach demonstrated that the head-to-cup ratio was not associated with an increased risk of dislocation (p=0.728). Conclusion: Head-to-cup ratio was not found to be predictive of dislocation. Hip dislocation is complex and multifactorial, and it is difficult to isolate specific risks, such as the head-to-cup ratio analyzed in our study. Further investigation is needed to define groups benefiting from specialized implants such as large-diameter or dual mobility heads and the optimal head-to-cup size ratio.