We identified 696 people from MOST and 336 people from Framingham as having knees discordant for presence of frequent knee pain. For these participants in MOST, 418 (60%) were female, the mean age was 62 (SD 8, range 50-79) years, and the mean body mass index was 31 (SD 6, range 18-56). In Framingham, 208 (62%) of these participants were female, the mean age was 68 (SD 10, range 49-93), and the mean body mass index was 29 (SD 5, range 17-58).
Tables 1 and 2 show the distribution of Kellgren and Lawrence grades for the paired knees within people who were discordant in their frequent knee pain status in the MOST and Framingham studies. Both tables show that knees with frequent pain were more likely to have higher Kellgren and Lawrence grades than were the contralateral knees without frequent pain (upper right hand parts of the tables). In MOST, knees with Kellgren and Lawrence grades 1, 2, 3, and 4 had 1.5 (95% confidence interval 0.9 to 2.3), 3.9 (2.4 to 6.5), 9.0 (5.2 to 15.6), and 151 (43 to 526) times higher odds of frequent knee pain, respectively, than knees with Kellgren and Lawrence grade 0 (P<0.001 for trend) (fig 1). As few participants had one knee with Kellgren and Lawrence grade 4 and the contralateral knee with grade 0, the confidence interval for this effect estimate was relatively wide. The corresponding odds ratios for Framingham participants were 1.2 (0.6 to 2.5), 3.1 (1.5 to 6.5), 15.1 (5.6 to 41.2), and 73 (16.2 to 331) (P<0.001 for trend). Furthermore, although both osteophytes and joint space narrowing were associated with presence of frequent knee pain in a dose-response manner, the magnitude of association with joint space narrowing was stronger than that for osteophytes (fig 2).
Table 1 Kellgren and Lawrence (KL) grade frequencies for pairs of knees discordant for frequent knee pain in MOST (n=696)
Table 2 Kellgren and Lawrence (KL) grade frequencies for pairs of knees discordant for frequent knee pain in Framingham (n=336)
Fig 1Associations of frequent knee pain with Kellgren and Lawrence (KL) grade among people with two knees discordant for frequent knee pain status. No of case knees (those with frequent knee pain) and control knees (those without frequent knee (more ...)
Fig 2Associations of maximal osteophyte (OST) and joint space narrowing (JSN) grades, mutually adjusted for one another, with frequent knee pain among people with two knees discordant for frequent knee pain status. Note that y axis is logarithmically (more ...)
The severity of radiographic knee osteoarthritis and individual radiographic features were also strongly associated with consistency of frequent knee pain. Compared with knees with a Kellgren and Lawrence grade of 0, the odds ratios for consistent frequent knee pain versus no frequent knee pain were 1.3, 5.5, 10.0, and 317 for knees with Kellgren and Lawrence grades of 1, 2, 3, and 4, respectively (P<0.001 for trend) (table 3). The odds of inconsistent frequent knee pain versus no frequent knee pain also increased as severity of radiographic osteoarthritis increased; the magnitude of association, however, was smaller than that for consistent frequent knee pain. Our findings on the relation of severity of radiographic osteoarthritis to consistent versus inconsistent frequent knee pain were similar.
Table 3 Associations of consistency of knee pain with Kellgren and Lawrence grade, maximal osteophyte grade, and maximal joint space narrowing grade among people with two knees discordant for consistency of knee pain in MOST
As shown in table 4, severity of radiographic osteoarthritis and severity of knee pain were also positively associated. Compared with knees with Kellgren and Lawrence grade 0, the odds ratio of severe to extreme pain versus no pain was 129 for knees with Kellgren and Lawrence grade 4 (P<0.001 for trend). We found similar results for the relation of severity of radiographic knee osteoarthritis to mild to moderate pain versus no pain (P<0.001 for trend) as well as to severe to extreme versus mild to moderate knee pain (P<0.001 for trend).
Table 4 Association of severity of knee pain with Kellgren and Lawrence grade among people with two knees discordant for knee pain severity in MOST
Furthermore, even among people in whom both knees were painful, when one knee had more severe pain than the other, as defined by differences in WOMAC pain scores (difference of ≥20% and absolute difference of ≥2 on a 0-20 scale), we saw similar associations. For example, increasing radiographic severity by Kellgren and Lawrence grade was associated with odds ratios of 1.0 (referent), 1.3, 2.4, 6.3, and 30.8 (P<0.001 for trend) for having more severe compared with less severe knee pain. We also found similar associations for the relation of osteophytes and joint space narrowing to severity of pain, although the magnitudes of effect for osteophyte grades were not as large as those for joint space narrowing grades (data not shown).