Knee Injury and Osteoarthritis Outcome Score (KOOS) is available in over 30 languages and a commonly used Patient-Reported Outcome (PRO) for assessment of treatment effects following knee surgery. The aim of the study was to report the linguistic translational process and evaluate the psychometric properties of the Polish version of the KOOS questionnaire. We translated and culturally adapted the KOOS according to current guidelines for use in Poland. Patients who had undergone anterior cruciate ligament reconstruction (ACLR) completed the KOOS and Short Form 36 Health Survey (SF-36). We evaluated floor/ceiling effects, reliability (using Cronbach’s alpha, intraclass correlation coefficients (ICC) and measurement error), convergent and divergent construct validity (using four a priori stated hypotheses) and responsiveness (using data obtained prior to and one year after ACLR and described by both effect size (ES) and standardized response mean (SRM)). The clinical study population consisted of 72 subjects (mean age 29.8, 28% women). We did not observe floor effects in any KOOS subscales neither pre- nor postoperatively. As expected, ceiling effects were found postoperatively for the subscales Pain and ADL in this cohort assessed on average 1.3 year after surgery as more than 15% reported no pain or limitations in daily activities. The Cronbach’s alpha was above 0.9 for all subscales indicating excellent internal consistency. The test-retest reliability of all KOOS subscales at one-year postoperatively was excellent with ICCs exceeding 0.86 for all subscales. The minimal detectable change on group level ranged from 1.3 to 2.4, and on an individual level from 10.9 to 20.2. Responsiveness was demonstrated since the expected pattern of effect sizes between subscales following ACLR was found. We found the Polish version of the KOOS to be a valid and reliable instrument for use in patient groups having ACLR. We caution against monitoring individual patients since the smallest change considered clinically relevant cannot reliably be detected.

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Objective: The retrospective investigation was carried out to assess whether subjects who fulfilled our proposed recruitment criteria responded more favorably to anterior cruciate ligament reconstruction (ACLR) than those who did not. Methods: We retrospectively analyzed 109 skeletally mature subjects (78 men and 31 women) according to the following proposed criteria of recruitment: (1) pre-injury Tegner activity score ≥7 and a wish to return to a professional sports activity, (2) residual knee instability following injury and/or (3) age <20 years at the operation. The primary outcome was an improvement between assessment A (before operation) and B (mean follow-up of 1.6 years) in the average score for four of the five Knee injury and Osteoarthritis Outcome Score (KOOS) subscales, covering pain, symptoms, difficulty in sports and recreational activities, and quality of life (KOOS₄). Results: The proposed recruitment criteria for ACLR were met by 58 subjects (53%). There were 49 subjects (45%) who improved between assessment A and B. Subjects who met proposed recruitment criteria were more likely to improve clinically after ACLR (OR 5.7, 95% CI 2.5–13.3). Conclusions: Fulfillment of proposed recruitment criteria was a strong predictive factor for outcome improvement in short- to medium-term follow-up after ACLR. Level of evidence: Case-control study. Level of evidence 3.