Baseline pain characteristics

Clinical pain was assessed at baseline (prior to quarterly pain assessments), across several domains important to understanding chronic pain trajectories [45, 46]. These included: (1) knee pain frequency (“On average, how many days per week do you experience pain in your knee?”); (2) knee pain intensity (worst, average, current); (3) knee pain duration (“For how long have you been experiencing pain in your knee?”); and, (4) total number of painful body sites. Knee pain frequency of more than 5 days per week was classified as “experiencing knee pain on most days”. Knee pain intensity was assessed using three items from the Graded Chronic Pain Scale (GCPS) [47], specific to knee pain (i.e., worst and average pain over the past 6 months, and current pain), rated on a 0 (“no pain”) to 10 (“pain as bad as could be”) numeric rating scale (NRS). These ratings were averaged and multiplied by 10 to generate a characteristic knee pain intensity score [47]. The NRS has been shown to be a reliable (ICC = 0.95) and valid (r = 0.94, visual analog scale) measure of OA pain [48, 49]. Knee pain duration was categorized as: (1) less than 6 months; (2) 6 months to 1 year; (3) 1 to 3 years; (4) 3 to 5 years; or, (5) more than 5 years. The total number of painful body sites was calculated as the sum of self-reported painful body sites reported. Participants could indicate if pain was experienced on the left, right, or both sides across 14 body sites. Items were coded as follows: hands (right or left = 1, both = 2); arms (right or left = 1, both = 2); neck (right or left = 1, both = 2); shoulders (right or left = 1, both = 2); head/face/jaw (right or left = 1, both = 2); chest (right or left = 1, both = 2); stomach (right or left = 1, both = 2); pelvis (right or left = 1, both = 2); upper back (right or left = 1, both = 2); lower back (right or left = 1, both = 2); knees (right or left = 1, both = 2); legs (right or left = 1, both = 2); feet (right or left = 1, both = 2), and/or 1 ‘other’ body region (free response; right or left = 1, both = 2, and summed to produce a total score with a range from 0 to 28. These four variables (pain frequency, pain intensity, pain duration, and pain sites) were summarized using descriptive statistics and compared across identified pain trajectory subgroups [45, 46].

The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) [50], was administered at baseline to assess knee OA-related symptoms in the past 48 h. The three subscales of the WOMAC (i.e., pain, stiffness, and physical functioning) were summed to create a total symptom burden score ranging from 0 to 96 (i.e., WOMAC Global score), which was used for analysis. Higher scores indicate greater overall symptom burden. The WOMAC is a well-validated measure of pain and function in lower extremity OA [50, 51].