What's all that noise? The effect of co-morbidity on health outcome questionnaire results after knee arthroplasty

Background We modified the Charnley Classification for hips to facilitate its use with knee arthroplasty patients and investigated what affect the different classes of co-morbidity had on the results of a spectrum of outcome questionnaires. Patients and methods 3600 patients from the Swedish Knee Arthroplasty Registry were surveyed by post with a variety of questionnaires ranging from multiple-item general health, to a single-item knee arthroplasty specific questionnaire. All patients also completed a co-morbidity questionnaire, from which a modified Charnley Classification was generated for each patient. We then investigated the correlation and relationship between the results of the questionnaires band the different classes of co-morbidity. Results The results of the questionnaires tested varied significantly by Charnley Class, regardless of the specificity of the questionnaire used. Interpretation We suggest that co-morbidity should be taken into account in outcome studies utilizing general health or disease/site specific questionnaires.