Abstract
Introduction
When following patients with metal-on-metal (MoM) hip replacements, current evidence suggests that orthopaedic surgeons should avoid reliance on any single investigative tool. In 2014, guidelines for stratifying patients with MoM hip replacement into groups of low, medium, and high risk of failure based on multiple criteria were published. However, such risk stratification guidelines can be difficult to interpret due to the numerous risk factors related to MoM hip replacements. This is especially true for patients with various (high and low) risk levels for different criteria within the guidelines.
The first purpose of this study was to assess if a scoring system can be applied to the current MoM guidelines. The second purpose was to test, using this scoring system, how the contemporary guidelines would classify a cohort of patients with a recalled MoM hip replacement system.
Methods
The study population consisted of 1301 patients (1434 hips) enrolled from September 2012 to June 2015 in a multicenter follow-up study of a recalled MoM hip replacement system at a mean of 6.2 (range 2.4 – 11.2) years from index surgery. Eleven required scoring criteria were determined based on existing follow-up algorithm recommendations and consisted of patient factors, symptoms, clinical status, implant type, metal ion levels, and radiographic imaging results. Criteria considered ‘low’ risk were given 1 point, 2 points for ‘moderate’ risk, and 3 points for the ‘high’ risk group. Forward stepwise logistic regression was conducted to determine the minimum set of predictive variables for risk of revision and assign variable weights. The MoM risk score for each hip was then created by averaging the weighted values of each predictive variable.
Results
Only two of the proposed eleven criteria were found to be significant predictors of revision in our logistic regression model; clinical status (as measured by the Harris Hip Score) and blood metal ion levels.
By September 2015, 85 patients had been revised. The median MoM risk score for unrevised hips (2.15) was lower compared to revised hips (4.49) (p < 0.001). Receiver operating characteristic analysis yielded good discrimination between all revised and unrevised hips, with an area under the curve of 0.82 (p < 0.001) (Figure 1). A high MoM risk score had 5.8-fold increased odds for revision relative to the moderate risk group (p < 0.001) and a 21.8-fold increased odds for revision compared to the low risk group (p < 0.001).
Discussion and Conclusion
The MoM risk score is an effective tool for applying the current risk stratification guidelines to a cohort of patients with a MoM hip replacement. This scoring system is one way to simplify the interpretation of current risk stratification guidelines for patients with MoM hip replacements.
