The development of an algorithm that provides accurate individualised estimates of revision risk could help patients make informed surgical treatment choices. This requires building a survival model based on fixed and modifiable risk factors that predict outcome at the individual level. Here we compare different survival models for predicting prosthesis survivorship after hip replacement for osteoarthritis using data from the National Joint Registry for England, Wales, Northern Ireland and the Isle of Man. In this comparative study we implemented parametric and flexible parametric (FP) methods and random survival forests (RSF). The overall performance of the parametric models was compared using Akaike information criterion (AIC). The preferred parametric model and the RSF algorithm were further compared in terms of the Brier score, concordance index (C index) and calibration. The dataset contains 327 238 hip replacements for osteoarthritis carried out in England and Wales between 2003 and 2015. The AIC value for the FP model was the lowest. The averages of survival probability estimates were in good agreement with the observed values for the FP model and the RSF algorithm. The integrated Brier score of the FP model and the RSF approach over 10 years were similar: 0.011 (95% confidence interval: 0.011–0.011). The C index of the FP model at 10 years was 59.4% (95% confidence interval: 59.4%–59.4%). This was 56.2% (56.1%–56.3%) for the RSF method. The FP model outperformed other commonly used survival models across chosen validation criteria. However, it does not provide high discriminatory power at the individual level. Models with more comprehensive risk adjustment may provide additional insights for individual risk.
Longer hip axis length (HAL) has been shown to be associated with the risk of hip fracture. We examined whether HAL is associated with hip fracture risk in elderly community dwelling women in England, unselected for osteoporosis. We undertook a case-controlled study of women aged ≥ 75 years enrolled to a large, randomised controlled study looking at the effect of a bisphosphonate, clodronate, in the prevention of hip fractures. Cases comprised those women who sustained a radiologically verified hip fracture during follow-up (median 3. 1 years). Two age, height and weight-matched controls were selected for each case. Baseline total hip bone mineral density (BMD) was measured using a Hologic 4500 QDR and the HAL was measured using the densitometer’s automated software. 92 of 4347 women (2. 1%) sustained a hip fracture, but two women had not received a baseline BMD assessment and were excluded, leaving 90 women with hip fractures and 180 matched controls. The mean age of the cases and controls was 81. 9 ± 4. 9 years. The mean hip BMD was significantly lower (0. 65 ± 0. 31 vs 0. 72 ± 0. 13, P<
0. 001) and the mean HAL was significantly longer in the hip fracture women than the controls (11. 1 ± 0. 6 vs 10. 9 ± 0. 6, P=0. 03). The increase in the HAL was significantly associated with the risk of hip fracture (Odds Ratio (OR) per 1 standard deviation increase was 1. 33, 95% CI 1. 02–1. 72; p=0. 03) remaining significant after adjustment for hip BMD (OR 1. 32, 95% CI 1. 01–1. 71; p=0. 04). An increase in hip axis length is associated with an increased risk of hip fracture in elderly English women independent of hip BMD, although the odds ratio appears somewhat lower than that reported in other studies.
