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

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Abstract
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 multipleitem 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 and 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.