Of the 105 included patients, 39 were men and 66 (63%) were women, with a mean age of 71.3 years (range, 43–86). 22 patients had undergone a prior knee replacement of the other knee, and 5 patients had undergone a prior hip replacement. The patients self-reported on average 1.3 co-morbid diseases (median 1, range 1–11). In 56 cases (53%) the right knee was operated, in 43 (41%) the left, and in 6 (6%) both knees were operated simultaneously.
One patient died before the 6 month-follow up, and 7 did not return the questionnaires. Thus 6 month-follow up data were available for 97 (92%) patients. 12 month follow-up data were available for 90 (86%) of the included patients (two patients died between the 6 and 12 month follow-ups, and an additional five did not return the questionnaires).
Missing baseline data
KOOS. Few individual items were missing for the four subscales Pain, Symptoms, ADL and knee-related Quality of Life (126 items of 105 patients × 37 items= 3.2 %). A subscale score could be calculated for 103/105 patients for the subscale Pain, 105/105 for Symptoms, 104/105 for the subscale ADL, and 105/105 for the subscale knee-related Quality of Life. For the subscale Sport and Recreation Function 391 items of 105 patients × 5 items = 74% were noted as "not applicable" and thus treated as missing. A subscore could be calculated for 58/105 patients.
Totally, 54 patients included in the study completed the KOOS twice within one to 23 days. Pre-operatively, test-retest data were available for 28 patients (mean number of days between the two assessments 9.9 ± 3.8 days). At the six-month follow-up, test-retest data were available for another 26 patients (mean number of days between the two assessments 10.2 ± 5.6 days). There were no statistically significant differences of the scores between the first and second assessments with the exception of the subscale Symptoms. When calculated for all 54 patients together, the patients reported on average more symptoms at the second test occasion (60/100 vs. 58/100 points, p = 0.04, Wilcoxons signed-rank test). The intraclass correlation coefficients (ICC 2,1) were all over 0.75 when determined for all 54 patients together (Table ). Bland and Altman plots for the five KOOS scales are given in Figure .
Intraclass correlation coefficients
Bland-Altman plots for the five KOOS subscales
Content validity. Over 90% reported that improvement in the four subscales Pain, Symptoms, Activities of Daily Living, and knee-related Quality of Life was extremely or very important when deciding to have their knee operated on, Table . 51% reported that improvement in functions included in the subscale Sport and Recreation Function such as squatting, kneeling, jumping, turning/twisting and running was extremely or very important when deciding to have their knee operated on. The group reporting items related to Sport and Recreation Function being extremely or very important held more men (48% vs. 30%, p = 0.08) but was similar with regard to age (71 vs. 70, p = 0.6) and preoperative ADL function (41/100 vs. 40/100, p = 0.8).
Content validity. The percentage of patients reporting the importance of the five different koos subscales when deciding to have the operation
Following surgery, patients tended to start doing physical functions that they had not performed pre-operatively. Pre-operatively, 27% rated their degree of difficulty with squatting, 17% with running, 12% with jumping, 42% with twisting/pivoting, and 34% with kneeling. The others reported not performing the function. At six months the percentages of patients reporting doing the functions had increased to 40%, 28%, 23%, and 46% for squatting, running, jumping, and twisting/pivoting. The percentage reporting kneeling had decreased to 26%. These trends were confirmed at the 12 month follow-up.
Construct validity. As expected, high correlations occurred between the SF-36 scales and the KOOS scales that are intended to measure similar constructs (bodily pain vs. pain, rS = 0.62; physical function vs. activities of daily living, rS = 0.48). Generally, higher correlations were seen when comparing KOOS scales to SF-36 scales with a high ability to measure physical health (convergent construct validity), and lower correlations were seen when comparing KOOS scales to SF-36 scales with a high ability to measure mental health (divergent construct validity). The correlations of the KOOS scales to the SF-36 subscale Role Physical were lower compared to the other SF-36 subscales with a high ability to measure physical health (Table ).
Construct validity. Spearman's correlation coefficients (rS) determined when comparing KOOS' five subscales to the SF-36 eight different subscales. N = 103-105 with the exception of Sport/Rec where n = 58.
A significant improvement (p < 0.001) was seen post-operatively in all subscales (Table ). The most responsive subscale was knee-related quality of life (QOL) with an effect size of 2.86 at 6 months and 3.54 at 12 months. The second most responsive subscale was Pain with effects sizes of 2.28 and 2.55 at 6 and 12 months, respectively. The subscale sport and recreation function (Sport/Rec) was the least responsive subscale with effect sizes of 1.18 and 1.08 at 6 and 12 months, respectively. It should be noted that the effect size calculation for the subscale Sport/Rec are based on 29 and 27 patients only. Generally the effect sizes were larger at 12 months, implying improvement occurring between 6 and 12 months (Table ). The calculation of SRM generally yielded somewhat smaller numbers but did not change the interpretation of the data (Table ).
Mean (SD) of the KOOS and WOMAC at baseline and follow-ups at 6 and 12 months. 0–100 worst to best scale
Effect sizes and Standardized Response Mean (SRM) 6 and 12 months post-operatively
Floor and ceiling effects. Pre-operatively, no notable ceiling effects were found. At 6 months, 15% reported best possible pain score and 16% reported best possible sport and recreation score making detection of further improvement impossible. The ceiling effects for the other subscales were lower. At 12 months, 22% reported best possible pain score and 17% reported best possible quality of life score (Table ).
Ceiling and floor effects of the KOOS and WOMAC. Percentage of patients reporting best possible score (ceiling effect)/ worst possible score (floor effect).
Comparison of the KOOS to the WOMAC
All WOMAC subscales and the corresponding KOOS subscales Pain, Symptoms and ADL were rated as extremely or very important by over 90% of the patients (Table ). 91% of the patients rated the KOOS subscale knee-related quality of life as extremely or very important, indicating items such as awareness, life style modifications, and confidence being just as important as questions related to pain, other symptoms or functions related to activities of daily living. The WOMAC does not assess this dimension of disease. The KOOS subscale Sport and Recreation Function was considered as extremely or very important by 51%, indicating functions such as squatting, running, jumping, turning/twisting and kneeling being of great importance to every second patient undergoing total knee replacement. These functions are not assessed by the WOMAC.
The KOOS subscale knee-related QOL had the highest effect size of all subscales of the KOOS and the WOMAC (Table ). The relative efficiency when comparing corresponding subscales of the KOOS and the WOMAC (pain vs. pain, symptoms vs. stiffness) at the 6 and 12 month follow-up ranged from 1.0 to 1.04 indicating corresponding subscales of both measures being equally responsive. The KOOS subscale ADL is equivalent to the WOMAC subscale Function. No comparison to the KOOS subscales Sport and Recreation Function and knee-related Quality of Life were made since the WOMAC does not assess corresponding constructs.
The ceiling effects at 6 and 12 months of the WOMAC subscales Pain and Stiffness were higher than for the corresponding subscales of the KOOS, indicating KOOS having a better ability than WOMAC to detect future improvement post-operatively (Table ).