Introduction
Several knee-specific patient-reported outcomes (PROs) have been developed to capture current functional and/or symptom status of patients with various knee conditions [
6]. The International Knee Documentation Committee (IKDC) Subjective Knee Form, in particular, monitors symptoms and functional status (both in daily and sports activities) and has been extensively validated in patients with various knee pathologies [
13,
14] and meniscus injuries [
7]. This form has also been found to have equal or superior measurement properties to other similar measures of knee function in patients with complex knee disorders [
1], chondral defects [
11], meniscus injury (waiting list and post-surgery) [
23], ACL rupture and reconstruction [
24].
Translations of the IKDC whole form into other languages (
http://www.sportsmed.org/Research/IKDC_Forms/) as well as cross-cultural adaptations of the IKDC Subjective Knee Form in the Italian [
20], Dutch [
12], Thai [
17], Brazilian [
19], Chinese [
10], Korean [
15], Persian [
9] and Turkish [
3] languages have already been reported.
The purpose of this study was to provide a valid Greek version of the widely used IKDC Subjective Knee Form, to inform future knee-related outcome studies performed in Greek-speaking populations, and to provide a common PRO of knee functional status between populations with a different native language. A systematic cross-cultural adaptation process was followed [
2], and an evaluation of the internal consistency, between-day reliability, construct and content validity and responsiveness of this form was performed according to current recommendations of the minimum standards of testing the psychometric properties of PROs [
21].
Discussion
The process of cross-cultural adaptation of the IKDC-SKF in Greek followed the Guillemin criteria [
2] and was subsequently validated in a Greek-speaking population with various knee pathologies, demonstrating comparable measurement properties with the original scale [
13,
14] and those in several other languages (Table
3). Specifically, the test–retest reliability and agreement (
n = 35), the internal consistency, convergent-divergent validity and floor/ceiling effects (
n = 80), and the responsiveness (
n = 24) of the IKDC-SKF/GR were examined.
As can be seen in Table
3, for convergent validity, correlation with the PCS, PF and BP subscales of the SF-36 were similar to the original version, while correlation with the RP (
r = 0.68) and SF (
r = 0.60) subscales of the SF-36 were higher in our study, compared to the original scale (
r = 0.47 for both). Divergent validity of the IKDC/SKF-GR was confirmed to be similar to the original version, as correlations with the MCS, GH, VT, RE, MH subscales of the SF-36 were equally low. In addition, no floor or ceiling effects were recorded, which is a desired attribute of a questionnaire for scores not to be clustered at the top or lower end of a questionnaire [
21].
The relative reliability index ICC
2,1 = 0.95 was almost the same as in the original validation paper (ICC = 0.94) [
13], as well as in other validation studies. Additional information is contained in the SEM/SDC absolute agreement indices, expressing the degree to which scores are identical, in terms of the original measurement [
16]. In our study the SEM = 4.4 and SDC = 12.2 were slightly higher than the original validation of the IKDC/SKF, which reported an SDC of 9 points, with improvements (or deterioration) beyond this range considered as true change. The SDC levels in other validation studies ranged between 6.7 and 16.4 points. In the validation study of the Brazilian version of IKDC, the relevant SEM/SDC values were the lowest (SEM = 2.4/SDC = 6.7) [
19], and in the Turkish version the highest (SEM = 6.0/SDC = 16.4) [
3], while they were not reported in the other cross-cultural validation studies. In a study performed in patients with isolated meniscus injury the SEM/SDC were found to be similar to the original validation (SEM/SDC = 3.19/8.8) [
7]. In another study examining patients with focal articular cartilage defects, the SEM/SDC values were reported to be slightly better in the longer-term than in the shorter-term follow-up (SEM/SDC = 5.6/15.6 in 6 months vs SEM/SDC = 4.9/13.7 in 12 months) [
11].
Alternatively, values beyond the limits of agreement, as depicted in the Bland–Altman plots, can be considered as a meaningful change in IKDC scores, signifying an alteration in a patient’s symptomatology [
16]. In our study the mean difference between the two testing occasions was 1.59 (not statistically significant), and the LOA was between −10.50 and 13.68. The only other cross-cultural adaptation study reporting LOA values is the Brazilian validation study [
19], with a mean difference between testing occasions of only 0.50 and LOAs between −6.1 and 7.1.
The internal consistency of the IKDC/SKF-GR scale was found to be good (
α = 0.87) and at a slightly lower level than the original scale (
α = 0.92) [
13]. From previous similar studies, the internal consistency Chronbach
α index ranged between 0.77 and 0.97 [
3,
7,
9,
10,
12,
15,
17,
19,
20].
Responsiveness in the original validation of the IKDC in patients with a variety of knee conditions had an ES = 1.13 [
14] and in another study involving only patients with meniscus pathology ES = 2.11 [
7]. The responsiveness of the IKDC-SKF/GR was also found to be high (ES = 1.26) in our study, tested in a subsample of the whole population used (only in patients with meniscus pathology). Although the IKDC-SKF/GR change score value exceeded the reliability test–retest error (SDC and LOA), only indirect inferences can be made, as the test–retest data were derived from a different subsample, which included only 1 patient with meniscus pathology.
Finally, since the IKDC scale can account for differences across cultures, it may allow for combining and comparing data from populations of different language and cultural backgrounds. Such comparisons may also provide the possibility of studying differences in healthcare delivery and patient management between different countries.
In conclusion, the IKDC Subjective Form in Greek has demonstrated comparable psychometric properties to the original version, therefore the scale is recommended for further use in Greek-speaking patients with knee pathology.