Defining the Minimal Clinically Important Difference in Athletes Undergoing Arthroscopic Correction of Sports-Related Femoroacetabular Impingement: The Percentage of Possible Improvement

BACKGROUND: Measures of clinically meaningful improvement in patient-reported outcomes within orthopaedics are becoming a minimum requirement to establish the success of an intervention. PURPOSE: To (1) define the minimal clinically important difference (MCID) at 2 years postoperatively in competitive athletes undergoing hip arthroscopic surgery for symptomatic, sports-related femoroacetabular impingement utilizing existing anchor- and distribution-based methods and (2) derive a measure of the MCID using the percentage of possible improvement (POPI) method and compare against existing techniques. STUDY DESIGN: Cohort study (diagnosis); Level of evidence, 2. METHODS: There were 2 objective outcome measures—the modified Harris Hip Score (mHHS) and 36-Item Short Form Health Survey (SF-36)—administered at baseline and 2 years postoperatively. External anchor questions were used to determine the MCID through mean change, mean difference, and receiver operating characteristic (ROC) techniques. Distribution-based calculations consisted of 0.5 SD, effect size, and standard error of measurement techniques. The POPI was calculated alongside each technique as an achieved percentage change of maximum available improvement for each athlete relative to the individual baseline score. The impact of the preoperative baseline score on the MCID was assessed by assigning athletes to groups determined by baseline percentiles. Statistical analysis was performed, with P < .05 considered significant. RESULTS: There were 576 athletes (96% male; mean age, 25.9 ± 5.7 years). The MCID score change (and POPI) for the mHHS and SF-36 ranged from 2.4 to 16.7 (21.6%-63.6%) and from 3.3 to 24.9 (22.1%-57.4%), respectively. The preoperative threshold value for achieving the ROC-determined MCID was 80.5 and 86.5 for the mHHS and 70.1 and 72.4 for the SF-36 for the patient-reported outcome measure (PROM) score– and POPI-calculated MCID, respectively. Through the commonly used mean change method, 40.0% (mHHS) and 42.4% (SF-36) of athletes were unable to achieve the MCID because of high baseline scores and PROM ceiling effects compared with 0% when the POPI technique was used. A highly significant difference for the overall MCID was observed between preoperative baseline percentile groups for the mHHS (P = .014) and SF-36 (P = .004) (improvement in points), while there was no significant difference between groups for either the mHHS (P = .487) or SF-36 (P = .417) using the POPI technique. CONCLUSION: The MCID defined by an absolute value of improvement was unable to account for postoperative progress in a large proportion of higher functioning athletes. The POPI technique negated associated ceiling effects, was unrestricted by the baseline score, and may be more appropriate in quantifying clinically important improvement.