Background/Purpose : While knee pain is a major complaint from subjects with knee osteoarthritis (OA), most epidemiologic studies have found only weak to moderate associations between knee radiographic OA (ROA) and presence of knee pain. These findings may be partly due to confounding by variation of pain sensitivity and tolerance between subjects.  In addition, few studies have prospectively assessed the relation of severity of ROA to the occurrence of knee pain.  To avoid across person differences in pain reporting, we performed a within-person knee-matched cohort study to examine the relation of Kellgren/Lawrence (KL) grade and joint space narrowing (JSN) to the risk of developing knee pain among participants of the Osteoarthritis Initiative (OAI).      

Methods : The OAI is a multi-center longitudinal study focusing primarily on risk factors for the onset and progression of knee OA. Subjects aged between 45-79 years were recruited at four centers across the US. At baseline and yearly follow-up visits knee-specific pain was assessed, including a question about presence of knee pain, aching or stiffness in more than half of the past 30 days (“frequent knee pain”).  KL grade (0-4) and JSN (0-3 using the OARSI atlas) were scored on PA view knee radiographs by experienced readers blinded to the time sequence. Included were subjects who had no frequent knee pain in either knee and had unequal KL grade at the 12-month visit in their two knees. They were considered as having incident frequent knee pain if it occurred in any of the later annual follow-up visits. Within each subject we compared risk of incident frequent knee pain in the knee with higher KL grade vs. that in the contralateral knee with lower KL grade. The two knees within a subject formed a matched set. We examined the association between KL grade and incident frequent knee pain using a Cox proportional hazards regression model adjusting for history of knee injury. We took the same approach to assess the relation of maximal JSN score to risk of the incident frequent knee pain.     

Results: Included were 1093 subjects who had no frequent knee pain and whose KL grades differed between two knees (mean age: 63.0,  52.4% women), and 712 subjects who had no frequent knee pain and whose JSN score differed between two knees (mean age: 63.8, 52.4% women) at 12-month visit. Higher KL grade was associated with an increased risk of incident frequent knee pain. Compared with knees with KL grade 0, the risk ratios of incident frequent knee pain were 1.2, 1.4, 2.3, and 3.3 for each increased grade of KL grade, respectively (p for trend <0.001). Similar association was observed for JSN (Table).

Conclusion: The radiographic severity of OA is strongly associated with an increased risk of frequent knee pain.  Contrary to the so-called structure symptom discordance, there is a dose-responsive relationship between structure and symptoms when between-person confounding is appropriately accounted for.