The study reports on the linguistic and cross-cultural translation and the psychometric properties of the Polish version of the KOOS in patients having had ACL reconstruction. To improve the quality of the report, cross-cultural translation of the KOOS for use in the Polish language was reported in accordance with the COSMIN checklist for cross-cultural validation [31, 32]. The COSMIN checklist was easy to work with and can be recommended for future reports of cross-cultural translation and validation processes .
The results indicate that the Polish version of KOOS questionnaire is a reliable, valid and responsive tool for use in groups of patients having ACL reconstruction.
In this study Cronbach’s alphas ranged from 0.92 to 0.97 indicating very high internal consistency. These values are higher than in previous KOOS validation studies [6, 9, 11–13]. One possible explanation is the relative homogeneity of the group examined. We evaluated reliability at one year postoperatively when patients likely constitute a more homogenous sample compared to pre-operatively, a time point frequently used by others for assessment of reliability.
We found the test-retest reliability to be excellent with ICCs ranging from 0.86 to 0.93. It revealed satisfactory stability and reliability of all the KOOS subscales over time in examined subjects. The ICCs observed in our study were higher than in previous studies in patients with knee injuries [6, 7] and osteoarthritis [9, 13]. Explanations include test-retest reliability being assessed postoperatively in this sample while others commonly use preoperative samples. ICCs comparable to ours were observed by de Groot et al. in validation of the Dutch version of the KOOS in patients with different stages of osteoarthritis (OA) . They found that the highest ICCs occurred in subjects with moderate OA. Since the patients with mild OA as well as those after revision total knee replacement had lower ICCs, especially in the KOOS subscale Sports and Recreation Function, they suggested that the questions about sport were less relevant in these groups. Such a phenomenon was not observed in our study.
The excellent test-retest reliability translated into smallest detectable changes of 3 points or less for the different subscales. Being able to detect a difference of 3 points indicates that the currently suggested minimal clinically important change of KOOS of 8-10  is well detectable in groups examined. However on an individual level, greater changes are needed (10.9 to 20.2) for the different subscales to be reliably detected. This means that despite excellent reliability the KOOS is better used for monitoring groups of subjects.
We confirmed content validity at the pre-test evaluation in that the original questionnaire items were relevant for young active individuals undergoing ACLR in Poland. Considering the large number of KOOS translations available in countries with a similar cultural context, we did not ask patients to add items to the existing questionnaire. The construct validity of the KOOS questionnaire was determined by setting up a priori hypotheses and comparing the KOOS subscales with the subscales of the SF–36. We compared the correlations between respective subscales measuring similar or dissimilar constructs. As hypothesized, the highest correlations were observed between SF–36 subscales and KOOS subscales measuring similar constructs while low correlations were seen when comparing subscales measuring dissimilar constructs. Correlation coefficients were comparable to those previously seen by Roos et al.  and Goncalves et al. .
The results of the responsiveness assessment confirmed that KOOS is able to detect clinical improvement in subjects undergoing ACLR. Further we confirmed the a priori set hypotheses including QOL being the most responsive subscale following ACLR. The pattern found was similar to data reported by Lind et al. in a 2-year follow-up study of 5000 patients having had primary ACLR included in the Danish knee ligament reconstruction registry .
There are limitations to be acknowledged. Since psychometric properties of a questionnaire may depend on the characteristics of the patients included, our findings apply to young adults having ACLR only and not necessarily to elderly with OA or those having other interventions. Further validation of the Polish version of the KOOS in patients with other knee complaints including osteoarthritis is therefore advised.