A shortened Knee Injury and Osteoarthritis Outcome Score (KOOS) is sufficient for measuring change in a cohort of patellofemoral instability patients

Abstract

Introduction: Various knee-related patient-reported outcome measures (PROMs) have been used for patients who undergo surgical treatment for patellofemoral instability. There has been limited evaluation of their suitability and no agreement about an optimal set of measures.

Objectives: To evaluate the Knee injury and Osteoarthritis Outcome Score (KOOS) and published shorter versions to determine their suitability for patients with patellofemoral instability.

Methods: The study cohort consisted of patients who underwent surgical stabilization with medial patellofemoral ligament reconstruction for recurrent lateral patellar dislocations. The full KOOS was administered preoperatively and the 6, 12, and 24 months postoperative. Four short form versions were calculated: KOOS-12, KOOS-Physical Function, KOOS-Joint Replacement, and KOOS-Global. Floor and ceiling effects were determined (threshold >15%). Individual items were evaluated to identify where >67% of patients had preoperative responses of "≥ moderate" (demonstrating item relevance). The standardized response mean (SRM) was calculated to assess responsiveness from baseline to each postoperative assessment time, as well as between postoperative assessments.

Results: 289 patients completed the full KOOS at least once. No ceiling effects were present at any time point for the quality of life (QoL) or symptoms subscales, KOOS-12, or KOOS-global. Only 11 of the items passed the >67% threshold for item relevance. These included all Sport/Recreation (SR) and QoL items, as well as one item (knee stiffness after sitting/resting later in the day) from the symptom subscale and one item (pain frequency) from the pain subscale. Given these individual item results, a two-subscale "composite" score was calculated that included the 9 items of the KOOS SR and QoL subscales (KOOS-SR ​+ ​QoL). Excellent responsiveness was seen for all preoperative to postoperative comparisons, especially the composite SR ​+ ​QoL measure (SRM >1). Between 6- and 12-month assessments, the composite SR ​+ ​QoL score was also shown to be the most responsive measure.

Conclusion: The composite KOOS SR ​+ ​QoL score may be an improved option for measuring patient-related outcomes in this more active population, as it focuses on the most relevant constructs for this patient group. The full 42-item KOOS was shown to have limited applicability in this patient group.

Level of evidence: Level II.

Keywords: KOOS; MPFL; Outcomes; Patella femoral instability.