Anchor-based predictive modeling and receiver operating characteristic curve estimates of patient acceptable symptom state for the forgotten joint score in total knee arthroplasty patients stratified by age and gender

Objectives

This study aimed to estimate patient acceptable symptom state (PASS) thresholds in the Forgotten Joint Score (FJS) 1 year following primary total knee arthroplasty (PTKA) while investigating the impact of patients’ characteristics on PASS thresholds.

Study Design and Setting

This cohort study used data from patients who underwent PTKA at a public hospital in Scotland between April 2021 and December 2022. Assessment of FJS (0-100, high-low knee awareness) was completed 1 year postoperatively. A single-item question about satisfaction with the operated knee was completed at 1 year and served as the anchor for estimating PASS thresholds. Anchor-based predictive modeling (adjusted and unadjusted) and receiver operating characteristic (ROC) curve methods were used to determine PASS thresholds. The impact of patient characteristics on PASS threshold values was investigated by calculating stratified PASS values based on gender and age groups.

Results

A total of 1832 PTKAs were performed between April 2021 and December 2022, of which 1359 (74%) had complete data comprising the study cohort. The median age and body mass index of patients included in the study were 70 years and 31.2 ${\text{kg}/m}^2$, respectively, with 54% being females. The proportion of satisfied patients was 84%. A moderate positive correlation between FJS and patient satisfaction was found (r = 0.64, P < .001), which supports the validity of the external anchor. PASS thresholds for the entire cohort were 31 (ROC method) and ∼33 points (predictive modeling method). Larger PASS values were found for male patients and patients aged ≥70 years compared to their female and younger counterparts. The adjusted predictive modeling estimate was 15.3; given that the data do not meet the assumption of normal distribution, we consider this threshold might be biased and must be interpreted with circumspection.

Conclusion

A postoperative FJS of ≥33 points can be used as a reference guide to evaluate successful achievement of a “forgotten joint” in a Scottish population. Patients’ characteristics impact PASS estimates and should be considered when interpreting outcome scores.

Plain Language Summary

After knee replacement surgery, patients want their new knee to feel natural. The FJS measures how much a person notices their artificial knee in daily life. The score goes from 0 to 100, with higher numbers meaning the knee feels more natural. We studied data from over 1300 patients in Scotland using modern statistical methods. Our results show that most patients need a score of at least 33 to feel satisfied with their knee 1 year after surgery, while men and people aged ≥70 years seem to need higher scores.