Responsiveness and ceiling effects of Knee Injury and Osteoarthritis Outcome Score following anterior cruciate ligament reconstruction: Evidence from the UK National Ligament Registry

PURPOSE: Multiple versions of the Knee Injury and Osteoarthritis Outcome Score (KOOS) are used in anterior cruciate ligament (ACL) populations, but their responsiveness and ceiling effects remain uncertain. This study aimed to evaluate the floor and ceiling effects, responsiveness and minimal important difference (MID) using the United Kingdom's National Ligament Registry (NLR). It was hypothesised that ceiling effects would increase over time, but would vary by KOOS version. METHODS: A secondary analysis of the NLR (2012-2024) was performed including patients undergoing ACL reconstruction. We quantified floor and ceiling effects for KOOS5, KOOS4, KOOS-ACL, KOOSglobal. Responsiveness was assessed using Cohen's d and standardised response mean comparing pre-operative scores and follow-up scores (6 months, 1 year, 2 years and 5 years). MIDs were estimated using a distribution-based method (half of the baseline standard deviation). RESULTS: 14,660 participants completed baseline scores and were followed up. Large improvements were observed from baseline to 12 months (KOOS5 1.44 [95% confidence interval, CI, 1.40-1.48], KOOS4 1.54 [95% CI 1.50-1.59], KOOS-ACL 1.45 [95% CI 1.42-1.49] and KOOSglobal 1.81 [95% CI 1.74-1.87], followed by increased ceiling effects). At 12 months, ceiling effects (90-100 definition) were present for KOOS5 (30.46%), KOOS4 (24.60%) and KOOS-ACL composite (44.01%), and increased at 24 and 60 months. KOOSglobal demonstrated lower ceiling effects: 5.37% at 12 months and 13.38% at 60 months. Pre-operative floor effects were highest for sport/recreation (20.32%) and KOOS-ACL sport (19.68%). Estimated MIDs were 9.0 for KOOS5, 9.0 for KOOS4, 9.6 for pain, 9.7 for symptoms, 10.3 for activities, 13.6 for sport/recreation, 9.5 for quality of life, 10.1 for KOOS-ACL composite, 10.0 for KOOS-ACL function, 12.5 for KOOS-ACL sport, and 5.1 for KOOSglobal. CONCLUSIONS: Ceiling effects were observed for KOOS5, KOOS4 and KOOS-ACL from 12 months, suggesting a limited ability to distinguish between higher-functioning patients. KOOSglobal appears less susceptible to ceiling effects. LEVEL OF EVIDENCE: Level III, cohort study.