Flexion Posteroanterior Radiographs Affect Both Enrollment for and Outcomes After Injection Therapy for Knee Osteoarthritis

Background: Knee injection therapy is less effective for severe osteoarthritis (OA), specifically Kellgren-Lawrence (KL) grade 4. Patient selection for knee injection trials has historically been based on extension anteroposterior (AP) radiographic evaluation; however, emerging evidence suggests that KL grading using a flexion posteroanterior (PA) radiograph more accurately and reproducibly predicts disease severity. The impact of radiographic view on patient selection and outcome after knee injection therapy remains unknown.

Hypothesis: A 45° flexion PA radiograph will reveal more advanced knee OA in certain patients. These patients will report worse pre- and postinjection outcomes.

Study design: Cohort study; Level of evidence, 3.

Methods: Four raters independently graded extension AP and flexion PA radiographs from 91 patients previously enrolled in a knee injection trial. Patients determined to have KL grade 4 OA by any rater on extension AP radiographs were excluded. Among included patients, those upgraded to KL grade 4 on flexion PA radiographs by at least 2 raters constituted group 2, while all remaining patients constituted group 1. Demographic data and patient-reported outcome scores before injection and at 6 weeks, 3 months, 6 months, and 12 months postinjection were compared between groups.

Results: Overall, 64 patients met the inclusion criteria, of which 19 patients (30%) constituted group 2. Compared with group 1, patients in group 2 were older (58.7 vs 52.3 years, P = .02), had worse visual analog scale pain scores before (6.6 vs 5.3, P = .03) and 6 months after injection (5.3 vs 3.5, P = .01), had less improvement in both Lysholm (8.5 vs 20.5, P = .02) and Short Form-12 physical component (-2.2 vs 1.7, P = .03) scores from preinjection to 6 months postinjection, and had less improvement in both Lysholm (1.6 vs 13.1, P = .03) and Knee injury and Osteoarthritis Outcome Score sport subscale (-2.1 vs 16, P = .01) scores from preinjection to 12 months postinjection.

Conclusion: One in 3 patients considered to have mild to moderate knee OA on extension AP radiography is upgraded to severe knee OA (KL grade 4) on flexion PA radiography. These patients report worse preinjection outcomes, worse pain scores at short-term follow-up, and decreased improvement in knee function scores between 6 months and 1 year postinjection.