Among 130 eligible participants whose MRI-lesion assessments were available, 60% were women, their mean age was 63.8 years, more than 80% of participants were White, and the mean BMI was 29.6 kg/m2. Among 260 knees from 130 eligible participants, the prevalence of Kellgren/Lawrence grade 0, 1, 2, 3 and 4 was 15.8%, 28.0%, 19.7%, 24.8% and 11.8%, respectively.
Patterns of co-existing MRI lesions
We identified five subgroups of knees based on the patterns of co-existing MRI lesions (Table 1): I (n=36, 13.9%), II (n=30, 11.5%), III (n=76, 29.2%), IV (n=65, 25.0%) and V (n=53, 20.4%) (Figure 2). The average posterior probability of membership was 1.00 for subgroups I, II and V, 0.93 for subgroup III and 0.92 for subgroup IV, respectively, suggesting that subgroup assignment was acceptably unambiguous.
The patterns of co-existing MRI lesions are depicted in Figure 2. In knees of subgroup I, there was no TFJ cartilage damage, whereas all knees of subgroup II had mild TFJ-CartM. All knees of subgroup III and IV had moderate TFJ-CartM, while the prevalence of severe TFJ-CartM was 100% in subgroup V. The prevalence and severity of MM and MExt also increased from subgroup I to subgroup V. Compared with subgroup III, subgroup IV was featured by a higher prevalence and greater severity of EFF, HFS and TFJ-BML. EFF, HFS and TFJ-BML often coexisted and their prevalence and severity were highly correlated. The prevalence and severity of PFJ-CartM and PFJ-BML was not correlated with that of TFJ-CartM. For example, although knees of subgroup V had greater TFJ-CartM than knees of subgroup IV, the prevalence and severity of PFJ-CartM and PFJ-BML was lower in subgroup V. Based on the differences in prevalence and severity of MRI lesions across subgroups, we labeled them as I: minimal lesions, II: mild lesions, III: moderate morphological lesions, IV: moderate multiple reactive lesions and V: severe lesions.
Subgroups of co-existing MRI lesions and knee pain at rest
The risk of having greater knee pain at rest increased from subgroup II to subgroup V compared with subgroup I (Table 2). The ORs and 95% confidence intervals (CI) of greater pain in bed were 1.0 (reference), 1.6 (0.3, 7.2), 2.2 (0.5, 9.5), 6.2 (1.3, 29.6) and 11.2 (2.1, 59.2) from subgroup I to subgroup V, respectively (P for test of homogeneity =0.056), indicating that risk of pain at rest varied among the different subgroups. Similar results were observed when knee pain at rest was measured using WOMAC subscale of pain that occurs when subjects were sitting/lying down. The corresponding ORs were 1.0 (reference), 1.0 (0.2, 5.6), 3.1 (0.7, 14.4), 14.4 (2.5, 81.7) and 28.9 (4.5, 184.0), respectively (P for test of homogeneity <0.001) (Table 2).
Subgroups of co-existing MRI lesions and knee pain on joint loading
Knees of subgroup IV and V had a higher risk of greater knee pain with walking compared with subgroup I (Table 3). The ORs of greater pain with walking were 1.0 (reference), 1.7 (0.5, 6.1), 0.7 (0.2, 2.3), 5.0 (1.4, 18.6) and 7.9 (2.0, 31.5) from subgroup I to subgroup V, respectively (P for test of homogeneity =0.001), suggesting that risk of pain with walk varied statistically significantly among five subgroups. Using WOMAC item for pain on stairs as one other measure of knee pain on joint loading, the corresponding ORs were 1.0 (reference), 0.9 (0.3, 3.3), 0.8 (0.3, 2.5), 2.4 (0.7, 7.4) and 6.6 (1.8, 23.9), respectively (P for test of homogeneity =0.003) (Table 3).