Incidence, predictors, and clinical impact of patella baja after primary total knee arthroplasty

Patella baja(PB) is a recognized radiographic finding after total knee arthroplasty (TKA), yet its incidence, predictors, and clinical relevance remain debated. Differentiating true from pseudo PB may provide additional radiographic insight into postoperative patellar inferiorization and may help clarify whether different mechanisms of patellar inferiorization translate into clinically meaningful outcomes. A total of 178 patients who underwent primary unilateral TKA were evaluated at a minimum follow-up of five years. Patellar height was assessed using the Insall–Salvati Index (ISI), modified ISI, Caton–Deschamps Index (CDI), and Blackburne–Peel Index (BBP). True PB was defined as ISI < 0.8, whereas pseudo PB was defined as CDI < 0.6 or BBP < 0.54 with ISI ≥ 0.8. Clinical outcomes included Visual Analog Scale (VAS), Knee Society Score (KSS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Knee Injury and Osteoarthritis Outcome Score (KOOS), Oxford Knee Score (OKS), and Forgotten Joint Score-12 (FJS-12). Multivariable logistic regression was used to identify independent predictors of PB, with model discrimination and calibration assessed. Postoperative PB was identified in 61 of 178 patients (34.3%), including 33 true and 28 pseudo cases. Among patients with paired radiographs (162 patients), 32.9% developed new-onset PB despite normal preoperative indices. Clinical outcomes did not differ significantly between patients with and without PB. Greater patellar inferiorization showed weak correlations with anterior knee pain and WOMAC scores. Lower preoperative ISI was independently associated with true PB, whereas lower preoperative CDI and BBP values were associated with pseudo PB. Younger age was independently associated with the development of any postoperative PB. PB was a common radiographic finding after primary TKA but was not associated with long-term functional outcomes when classified dichotomously. Preoperative patellar height indices were associated with the development of postoperative PB and may help identify patients at increased radiographic risk. Further studies incorporating direct joint-line measurements are needed to clarify the clinical significance of postoperative patellar inferiorization.