Infection and Instability Increasing the Risk of Patella Baja and Pseudo-Patella Baja after Revision Total Knee Arthroplasty

BACKGROUND: Patella baja with patellar tendon shortening due to traumatic or ischemic injury is a widely known complication after primary total knee arthroplasty (TKA). Pseudo-patella baja may arise from the elevation of the joint line after excessive distal femoral resection. The maintenance of original patellar height is important in revision TKA because postoperative patella baja and pseudo-patella baja can cause inferior biomechanical and clinical results. We investigated the incidence and risk factors of patella baja and pseudo-patella baja after revision TKA. METHODS: We retrospectively reviewed data for 180 revision TKAs. Patella baja was defined as a truly low-lying patella with an Insall-Salvati ratio (ISR) of < 0.8 and a Blackburne-Peel ratio (BPR) of < 0.54. Pseudo-patella baja was defined as a relatively low-lying patella compared to the joint line within the normal range of ISR and with a BPR of < 0.54. Clinically, the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and range of motion (ROM) were evaluated. Risk factors increasing the incidence of patella baja and pseudo-patella baja after revision TKA were evaluated using multiple regression analysis. RESULTS: Before revision TKA, 169 knees did not exhibit patella baja or pseudo-patella baja, while 9 knees showed patella baja and 2 knees exhibited pseudo-patella baja. At 2 years after revision TKAs, 25 knees (13.9%) showed patella baja and 23 knees (12.8%) exhibited pseudo-patella baja. Despite no differences in the postoperative WOMAC score between groups with and without patella baja and pseudo-patella baja, the postoperative ROM was significantly smaller in the group with patella baja (113.3°) or pseudo patella baja (110.5°) than in the normal group (122.0°). Infection as the cause of revision TKA increased the risk of patella baja (odds ratio, 10.958; p < 0.001), and instability increased the risk of pseudo-patella baja (odds ratio, 11.480; p < 0.001). CONCLUSIONS: Infection and instability resulted in increases in the incidence of patella baja and pseudo-patella baja after revision TKA. Information about the risk factors of patella baja and pseudo-patella baja will help TKA surgeons plan the height of the patella after revision TKA and improve clinical outcomes.