Development and evaluation of outcome measures in children with knee disorders

Abstract

Background and aim: The knee joint is one of the most common sites for injury in children. Severe knee injuries are often associated with long-term symptoms, functional impairment and disability. Today there is a lack of appropriate clinical outcome measures to evaluate knee function in children with knee disorder. As a result, instruments developed for adults are often used. Unfortunately, this may lead to inaccurate evaluation and selection of treatment. The overall aim of this thesis was to develop and evaluate outcome measures for use in children with knee disorders. In Study I, the aim was to evaluate the reliability of knee muscle strength measurements in healthy subjects using the Strength Measuring Chair (SMC) and to evaluate the agreement between the SMC and an Isokinetic Dynamometer (ID). The aims of Study II, were to evaluate the Single-limb mini squat test and the Quadriceps-angle (Q-angle), as discriminative tests of medio-lateral knee position, with respect to reliability and reference values. In Studies III and IV the aim was to evaluate the comprehensibility of the Knee Injury and Osteoarthritis Outcome Score (KOOS) when used in children, to suggest modification for a pediatric version (KOOS-Child), as well as to evaluate the psychometric properties of the KOOS-Child when used in children with knee disorders.

Patients and Methods: In Studies I and II, healthy children and adults were recruited and a test- retest design was used. In Study I, muscle strength tests were performed in 20 children and 23 adults during three sessions; two in the SMC and one in the ID. In Study II, 246 children were included and dynamic and static medio-lateral knee position was assessed by the Single-limb mini squat test and by the Q-angle respectively. In Study III and IV, children with various knee disorders were recruited. In Study III, cognitive interviews were conducted with 34 Swedish children to evaluate the comprehensibility of the KOOS when used in children. According to the findings the KOOS was modified and the KOOS-Child was developed. In Study IV, 115 children participated in three sessions to evaluate the psychometric properties of the KOOS-Child.

Results: In Study I, the SMC was found to reliably measure knee muscle strength in children and adults; however, a large disagreement was found between the instruments. In Study II, the reliability of the Single-limb mini squat test was determined moderate and a fair to moderate reliability of the Q-angle measurements was found. Q-angle reference values varied with age and sex, however the difference may not be clinically relevant. Findings from Studies III and IV, showed that the KOOS was not well understood by children, thus the KOOS-Child was developed. The KOOS-Child demonstrated good psychometric properties, i.e. it is valid, reliable and responsive to clinical change when used in children with knee disorders.

Conclusion: Outcome measures for the evaluation of different aspects of knee function, specifically developed for a pediatric population is very important and necessary. In the present thesis, existing measures originally designed for adults were evaluated for use in children and new outcome measures were developed. The studies have highlighted the importance of using instruments that are specifically designated for the study population when measuring knee muscle strength, and emphasized the difficulties encountered when comparing results from different strength measuring devices. Evaluation of medio-lateral knee position showed that the Single- limb mini squat test can be used in a pediatric population however the Q-angle needs further investigation before its use can be justified. A new patient-reported outcome measure, the KOOS-Child, was also developed to measure knee function and knee-related quality of life in children with various knee disorders. KOOS-Child is recommended to be used whenever studies intend to evaluate patient reported outcomes in children with knee disorders.