Tönnis angle and acetabular depth ratio: predictors of dislocation following hemiarthroplasty for hip fracture

Despite efforts to mitigate possible modifiable risk factors, dislocations following hemiarthroplasty for hip fracture still occur, with reported incidence rates ranging from 1.5 to 11%. The aim of this study was to investigate whether acetabular dysplasia is a significant contributing factor to dislocation, and since non-modifiable, whether it should affect patient selection for this treatment option. This is a multicentre nested case-control study of patients treated at 2 separate centres over a 10-year period from January 2011 to December 2020. All cases of hemiarthroplasty dislocation following hip fracture were identified from local databases, and 4 random controls were selected for every case. Tönnis angle (TA) and Acetabular-depth-ratio (ADR) was measured on the injured side using AP pelvis radiographs. Patients with TA > 10° and/or ADR < 250 were considered to have abnormal acetabular morphology. 35 cases of dislocation were identified and 140 random controls were selected. Cases of dislocation had a larger mean TA (8.8° vs. 5.5°, p < 0.001) and lower mean ADR (254 vs. 289, p < 0.001) than controls. 20 out of 35 (57%) cases were considered to have abnormal acetabular morphology, compared to 19 out of 140 (14%) controls. The odds of dislocation is 8.5 times greater (odds ratio = 8.49, 95%CI = 3.73, 19.39) in patients with abnormal TA and/or ADR. This study demonstrates that TA > 10° and ADR < 250 are useful criteria to identify patients at greater risk of hip hemiarthroplasty dislocation due to acetabular dysplasia. Special consideration to preoperative planning should be taken in these patients. Future research should focus on methods to minimise risk in this subset of patients, including evaluating total hip arthroplasty with dual mobility component vs hemiarthroplasty.