The ability of the Knee Osteoarthritis Outcome Score to detect changes over time is limited in patients with patellar...

The purpose of the current study was to evaluate important aspects of interpretability (floor and ceiling effects) for the Knee injury and Osteoarthritis Outcome Score (KOOS) in patients with patellar instability. Secondarily, the study aims to provide minimal important clinical difference (MICD) values for all subscales in this patient category.

Patients undergoing patella stabilising surgery with an individualised approach based on anatomic deviation were prospectively included if (1) ≥13 years of age at the time of surgery, (2) fluent in Norwegian and (3) able to understand and complete the questionnaires. Patients were excluded if they had concomitant bony and/or knee ligament injuries. KOOS was completed before and 6 months after surgery. Interpretability of the KOOS was evaluated according to recommendations from COnsensus-based Standards for the selection of health Measurement INstruments. A floor or ceiling effect is considered to be present if the number of patients that had a score in the lower (0-10) or upper (90-100) end of the scale exceeded 15%. This was identified with a distribution-based approach with standard deviation (SD) of the change score between pre- and postoperative scores using the following equation: MICD = 0.5 × SD.

A substantial ceiling effect was present in the KOOS subscales pain and activities of daily living (ADL) measured prior to surgery, and in all, except the quality of life subscale, 6 months after surgery. KOOS ADL demonstrated the highest number of patients, 46% preoperatively and 72% postoperatively with a ceiling effect. In addition, 32% of patients had the best possible score on the pain subscale 6 months after surgery. The only subscale that displayed a floor effect was the preoperative KOOS Sport/Rec. The MICD for the different subscales ranged from 7.6 to 12.4.

The substantial ceiling effect in the current implies that the KOOS is not suited to evaluate the long-term effect of treatment in patients with patellar instability.

Level II.