Reliability and validity of the International Knee Documentation Committee (IKDC) Subjective Knee Form

Abstract

Background

Patient-oriented questionnaires are important measures of clinical outcomes in medical practice but require systematic testing of reliability and validity. The International Knee Documentation Committee (IKDC) Subjective Knee Form is a patient-oriented questionnaire that assesses symptoms and function in daily living activities. The purpose of this study was to validate the IKDC Subjective Knee Form in a large patient population with various knee disorders.

Methods

One thousand five hundred and thirty-four knee patients seen at a sports medicine clinic at a large medical center completed the IKDC Subjective Knee Form. Factor structure was determined by exploratory factor analysis with promax rotation, and internal consistencies of the identified subscales were calculated with Cronbach's alpha. Concurrent validity was assessed by correlating the IKDC Subjective Knee Form dimensions to the summary scales of the SF-12. Finally, item characteristics were analysed using graded response item response theory (G-IRT).

Results

Exploratory factor analyses yielded a two-factor solution, and the dimensions were termed as follows: symptom and knee articulation (SKA), and activity level (AL). Confirmatory factor analysis confirmed the selection of 15 items within the study. Both SKA and AL demonstrated good internal consistency (0.87 for SKA; 0.88 for AL). Both SKA and AL demonstrated statistically significant correlations to the SF-12 total score, more substantively to the physical component summary scale than to the mental component summary scale of the SF-12.

Discussion

G-IRT analyses revealed that nearly all questionnaire items demonstrated clear response patterns, with higher levels of the latent trait corresponding to more adaptive clinical endpoints concerning pain, symptoms, function, and sport activity.

Conclusions

The IKDC is a reliable and valid instrument worthy of consideration for use in a broad patient population.

Introduction

Conventional approaches to the measurement of outcomes in knee function have typically involved objectively defined parameters, such as radiological findings, strength, range of motion, and ligamentous laxity [1]. While these strategies retain their value, patient-oriented questionnaires increasingly have become recognized as important complementary measures of clinical outcomes. To ensure their utility, two objectives are essential. First, it must be feasible to compare outcomes concerning symptoms, function, and activity levels across diverse knee conditions and patient populations. Second, to ensure utility, patient-oriented questionnaires must be validated both systematically and extensively, including the testing of reliability, validity, and responsiveness.

Many different scoring systems for patient-focused instruments have been developed to assess knee disability and function before and after treatment [2], [3], [4], [5], [6], [7], [8]. One such instrument is the International Knee Documentation Committee (IKDC) Subjective Knee Form, an 18-item, region specific instrument designed to measure symptoms, function, and sports activity [9], [10]. The IKDC Subjective Knee Form was created from a committee of international knee experts from the American Orthopedic Society for Sports Medicine (AOSSM) and the European Society for Sports Traumatology, Knee Surgery and Arthroscopy (ESSKA). These groups formed the International Knee Documentation Committee (IKDC) with the goal of standardizing an international documentation system for assessing patient outcomes after knee surgery or treatment [10].

Although recently introduced, the IKDC Subjective Knee Form is gaining recognition in the literature [11], [12], [13], [14], [15], [16], [17]. The instrument contains 18 selected items designed to measure symptoms assess pain, stiffness, swelling, joint locking, and joint instability, while other items designed to measure knee function assess the ability to perform activities of daily living. Items purported to measure activity levels assess the respondent's ability to run, jump and land, stop and start quickly, ascend and descend stairs, stand, kneel on the front of the knee, squat, sit with the knee bent, and rise from a chair. Examples of questions include, “what is the highest level of activity that you can perform without significant knee pain?”, “what is the highest level of activity that you can participate in on a regular basis?”, and “how does your knee affect your ability to go up stairs?” Response types include 5-point Likert scales, 11-point Likert scales, and dichotomous “yes–no” responses. A copy of the IKDC Subjective Knee Form is available elsewhere [10].

The IKDC Subjective Knee Form has been validated in Dutch [14] and Italian [16] as well as English [10]. Previous research [10] has suggested a one-factor solution using polychoric correlations, with an internal consistency coefficient (Cronbach's alpha) of 0.92 and test–retest reliability of 0.95. Concurrent validity for the IKDC Subjective Knee Form was demonstrated, as it related significantly to the SF-36 [18] physical function subscales (r = 0.44–0.66), but not to emotional function subscales (r = 0.16–0.26). Partial credit item response theory (PC-IRT) analysis suggested that patients with lower levels of function appropriately scored selections of the lowest ordinal response categories [10]. Finally, analyses of differential item functioning, advocates that questions functioned similarly for men versus women, young versus old, and for those with different diagnoses [10].

The purpose of our study was to explore further the psychometrics of the IKDC Subjective Knee Form using graded response model IRT (G-IRT). G-IRT differs from the PC-IRT used initially by Irrgang and colleagues in that the model is specifically used when items have different numbers of response categories (as with the IKDC Subjective Knee Form) and when discrete items are examined independent of the sum of the exponentials of the instrument [19], [20]. Findings may broaden support of the use of the IKDC Subjective Knee Form and allow further examination of each individual item within the form for use in an item bank.