Clinical and radiological characteristics of novel subtypes of end-stage knee osteoarthritis based on joint space loss patterns in standing extended view and fixed flexion view

Background This study aimed to classify end-stage knee osteoarthritis (KOA) based on the pattern of joint space loss in standing extended view (SEV) and fixed flexion view (FFV) and to investigate clinical and radiological differences. Methods A total of 459 knees from 300 patients with Kellgren-Lawrence grade 4 KOA were retrospectively analyzed. The knees were divided into three groups based on the pattern of joint space loss in SEV and FFV: group 1 (all loss) with joint space loss in both SEV and FFV, group 2 (flexion loss) with joint space loss only in FFV, and group 3 (extension loss) with joint space loss only in SEV. The primary endpoints were clinical and radiological parameters, while the secondary endpoints included intraoperative measurements and the survival rate until total knee arthroplasty (TKA). Results A total of 459 knees from 300 patients were included. Among the participants, there were 77 men (25.7%) (average age of 72.21 ± 7.35 years), and 223 women (74.3%) (average age of 72.75 ± 6.56 years) ( p = 0.546). Compared to group 2, group 1 showed a larger hip-knee-ankle angle (9.8 ± 7.0° and 6.3 ± 5.0°, p < 0.001), higher VAS (6.3 ± 2.4 and 4.6 ± 2.5, p < 0.001), shorter time to surgery (7.1 ± 7.7 months and 11.0 ± 8.7 months, p < 0.001), smaller full flexion angle (114.3 ± 13.4° and 121.2 ± 11.9°, p = 0.001), and a higher total knee arthroplasty rate (76% and 57.2%, p < 0.001). Group 3 showed a larger flexion contracture angle compared to group 2 (10.00 ± 9.6° and 5.3 ± 5.4°, p = 0.032). The posterior tibial slope (PTS) was largest in group 2 (11.3 ± 3.3°), followed by group 1 (8.1 ± 3.3°), and smallest in group 3 (5.4 ± 2.7°) (both p < 0.001, respectively). There were no statistical differences in the intra-operative measurements. TKA was performed on 259 knees (64.3%), and the survival rates at 1 year were 48.1% for group 2, 29.2% for group 3, and 26.7% for group 1 (log-rank test, p < 0.001). Conclusions This study demonstrates that radiological and clinical differences exist within end-stage KOA based on joint space loss patterns. Additionally, our findings suggest that a larger PTS may be associated with less symptom severity in advanced KOA, contrary to its currently recognized negative effects. These findings may be beneficial for developing patient-specific treatment plans. Level of evidence Retrospective cohort study, Level III

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