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Validation of a Risk Calculator for Conversion of Hip Arthroscopy to Total Hip Arthroplasty in a Consecutive Series of 1400 Patients

Background: A previously published calculator used age, preoperative modified Harris Hip Score, femoral anteversion, preoperative lateral center-edge angle, revision surgery, and acetabular and femoral cartilage damage to provide risk estimates for conversion rate of hip arthroscopy to total hip arthroplasty (THA). Validation for this calculator has not been established. The purpose of this study is to (1) validate the previously published hazard ratios for the predictor factors in a new cohort of hip arthroscopies with minimum 2-year follow-up and (2) determine the accuracy of the calculator at determining conversion rates to THA at 2 and 4 years of follow-up.

Methods: Hazard ratios for THA conversion were calculated using data between February 2008 and November 2016 and compared to the previously published results, which comprised the training set. Actual conversion to THA data was used to evaluate the accuracy of the calculator.

Results: Of the 1400 patients examined, THA conversion occurred in 101 (7.2%) patients at an average of 28.4 ± 22.9 months (0.2-115.8) after hip arthroscopy. The hazard ratios for the validation set compared to the training set were as follows: age 1.06 versus 1.06; modified Harris Hip Score 0.97 versus 0.98; femoral anteversion 0.99 versus 0.97; lateral center-edge angle 0.98 versus 0.93; and revision surgery 1.77 versus 2.40. Accuracy of the risk calculator at 2 years was 75% (Harrell C-statistic 0.806) and at 4 years was 73% (C-statistic 0.797).

Conclusions: This study found 75% and 73% accuracy at 2 and 4 years respectively in calculating risk of conversion of hip arthroscopy to THA using a previously published calculator. As this calculator relies on intraoperative data, the major benefit it provides is information regarding patient prognosis postoperatively. Furthermore, it could potentially enable the surgeon, after receiving proper surgical consent, to decide on immediate conversion to THA.

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