Radiographic patellofemoral differences between end-stage knee rheumatoid arthritis and matched knee osteoarthritis...

To compare radiographic patellofemoral phenotypes between end-stage knee rheumatoid arthritis (KRA) and matched knee osteoarthritis (KOA), assess disease type by coronal alignment interactions, and characterize within-KRA variation by alignment and patella baja.

This retrospective cross-sectional study included 669 KRA and 669 KOA knees undergoing total knee arthroplasty from 2001 to 2024. After 1:1 propensity score matching for age, sex, and body mass index, 542 matched pairs were analyzed. Patellofemoral parameters were measured on standardized lateral and skyline radiographs. Paired tests, generalized estimating equations, nonparametric tests, and adjusted regression were used.

Patella baja was more prevalent in KRA than KOA (matched-pair odds ratio, 7.03; 95% confidence interval, 5.00-9.87), and Wiberg type III radiographic appearance was more frequent (39.5% vs. 11.3%; P < 0.001). KRA also showed lower Insall-Salvati index and higher lateral patellar displacement rate. All eight KRA-KOA differences remained significant in alignment-adjusted and Larsen grade 4-5-restricted sensitivity analyses (all P < 0.001). Significant disease type-by-alignment interactions were observed for Wiberg score, lateral patellar displacement rate, lateral patellar tilt angle, lateral patellar angle, and congruence angle. Differences in displacement, tilt, and congruence were greatest in valgus knees. Within KRA, Wiberg type III was more frequent in varus and valgus than neutral knees and increased across Larsen grades; within-alignment associations with patella baja were limited and non-uniform.

End-stage KRA showed a distinct radiographic patellofemoral phenotype compared with matched KOA, with several between-disease differences varying by coronal alignment.