Relationship Between Activity Level and Knee Function Is Influenced by Negative Affect in Patients Undergoing Cell Therapy for Articular Cartilage Defects in the Knee

BACKGROUND: Increased activity level is generally reported to be positively related to improved knee function after knee surgery. However, little research has been conducted into this relationship on an individual patient basis, or the influence of demographic and psychosocial factors such as patient affect—the subjective experience of emotion. HYPOTHESIS: The relationship between postoperative activity level and knee function will vary between patients and will be influenced by the patients’ affect and demographic characteristics. STUDY DESIGN: Cohort study; Level of evidence, 3. METHODS: Activity, knee function, demographic, and affect data were collected from patients enrolled in an ongoing trial for the treatment of articular cartilage lesions at preoperative and 2-, 12-, and 15-month postoperative points. Quantile mixed regression modeling was used to determine the patient-to-patient variation in activity level and knee function. Multiple linear regression and partial correlation analyses were performed to determine whether demographic characteristics and patient affect were associated with this variation. RESULTS: A total of 62 patients were included in the study (23 female; 39 male; mean age, 38.3 ± 9.5 years). We found substantial variation between patients in the relationship between activity level and knee function, with most patients (n = 56) demonstrating a positive relation (positive slope), but 6 patients demonstrating a negative relation (negative slope). A negative affect (NA) score was significantly correlated with the slope between activity level and knee function (r (S) = –0.30; P = .018) and was a significant individual predictor of knee function at 15 months postoperatively (coefficient = –3.5; P = .025). CONCLUSION: Our results suggest that the relationship between activity level and knee function varies between patients. The patients with a higher NA score were likely to report smaller improvements in knee function with increasing activity levels compared with those with a lower NA score.