What Are the Minimum Clinically Important Difference, Substantial Clinical Benefit, and Patient-Acceptable Symptom State Thresholds for the Modified Harris Hip Score and International Hip Outcome Tool 12 Among Patients Who Undergo Periacetabular Osteotomy?
Authors: Trotzky Zachary A. , Smolarsky Ryan G. , Madjarova Sophia J. , Jochl Olivia M. , Ricciardi Benjamin F. , Lyman Stephen , MacLean Catherine H. , Nwachukwu Benedict U. , Sink Ernest L. 
Published: February 2025

Background
The utility of patient-reported outcome measures (PROMs) has been well established, but their interpretation relies on population-specific definitions of meaningful improvement. As such, the minimum clinically important difference (MCID), substantial clinical benefit (SCB), and patient-acceptable symptom state (PASS) thresholds have become prominent metrics in the orthopaedic evidence to ascribe clinical relevance to numeric PROM scores. Studies assessing outcomes of periacetabular osteotomy (PAO) relative to the MCID and PASS have previously evaluated patients against thresholds defined for hip arthroscopy for the treatment of femoroacetabular impingement or distribution-based MCID calculations. These scores may not accurately reflect the status or expectations of patients with a different symptom profile undergoing open hip preservation surgery.

Questions/purposes
For patients treated with PAO, we sought to (1) define the MCID, SCB, and PASS threshold values for the mHHS (modified Harris hip score) and International Hip Outcome Tool 12 (iHOT-12) using anchor-based methods; (2) assess the validity of MCID and SCB estimates against minimal detectable change (MDC) values; and (3) determine the proportion of patients who achieved a clinically meaningful threshold.

Methods
Between February 2011 and May 2023, a total of 690 patients underwent PAO for symptomatic acetabular dysplasia at one institution and were included in a longitudinally maintained hip preservation registry. The cohort used to define and validate MCID, SCB, and PASS threshold values consisted of those with a completed postoperative anchor questionnaire, which yielded 456 patients as potentially eligible. An additional 139 patients were excluded because of missing mHHS or iHOT-12 scores during the eligibility window (1 to 2 years postoperatively), leaving 70% (317 of 456) of patients to define and validate MCID, SCB, and PASS at a mean ± SD of 1.0 ± 0.3 years of follow-up. A minimum 1-year follow-up was chosen to reduce recall bias. The cohort for defining MCID, SCB, and PASS (94% [298 of 317] women, mean ± SD age at time of surgery 27 ± 8 years) included 21% (68 of 317) of patients with prior ipsilateral surgery. From those registry patients without complete anchor questionnaires, 37% (137 of 373) were identified with pre- and postoperative PROM scores at a mean ± SD of 1.0 ± 0.9 years of follow-up to form the sample for assessing the proportion of patients achieving a clinically meaningful threshold. The MCID, SCB, and PASS thresholds for the mHHS and iHOT-12 were calculated through an anchor-based approach, using area under the receiver operating curve to determine cut points that best identified positive responses, according to quality of life–based anchor questions. The MDC was calculated with confidence intervals (CIs) reflecting 80%, 90%, and 95% certainty to determine the smallest change in the PROM scores that can be considered above the level of measurement error. The validity of MCID estimates was assessed by confirming that they exceeded corresponding MDC values. The validity of SCB estimates were assessed by confirming that they exceeded corresponding MCID values. The proportion of patients achieving a clinically meaningful threshold was determined by calculating the percentage of patients who met the defined anchor-based scores.

Results
The MCID, SCB, and PASS thresholds for the mHHS were 18, 23, and 71, respectively. The MCID, SCB, and PASS thresholds for the iHOT-12 were 26, 42, and 65, respectively. The MDC ranged from 8 to 12 for the mHHS and 10 to 16 for the iHOT-12. The MCID values for the mHHS and iHOT-12 exceeded corresponding values of the MDC at all CIs. The SCB thresholds exceeded all corresponding MDC and MCID values. Across the mHHS and iHOT-12, the proportion of patients who achieved an MCID at the first time point ranged from 60% to 73%, the proportion of patients who achieved the SCB ranged from 49% to 56%, and the proportion of patients who achieved the PASS threshold ranged from 55% to 79%. Among the cohort for defining MCID, SCB, and PASS, the proportion of patients achieving any MCID, SCB, or PASS was 79%, 66%, and 81%, respectively. Among the sample for assessing the proportion of patients achieving a clinically meaningful threshold, the proportion achieving any MCID, SCB, or PASS threshold was 74%, 58%, and 72%, respectively.

Conclusion
We found that using a sample of patients undergoing PAO, the anchor-based values for the MCID and SCB were generally larger than previous distribution- and anchor-based scores that have been defined for hip preservation, whereas PASS threshold scores were similar. All MCID and SCB thresholds exceeded corresponding MDC values, confirming these scores to be valid estimates. These metrics provide more rigorous, procedure-specific definitions for the evaluation of treatment success and failure after PAO. As anchor-based metrics are defined based on patients’ perceptions, they should be used preferentially for postoperative assessment over distribution-based scores.

Level of Evidence
Level III, therapeutic study.