Patella Baja Incidence, Risk Factors and Motion After Revision Total Knee Arthroplasty

Patellar height changes may have significant clinical implications, particularly in the setting of revision total knee arthroplasty (rTKA). True patella baja (PB) is due to patellar tendon shortening, whereas pseudo-PB (PPB) is distal displacement of the patella due to joint line elevation. Our purpose was to examine the incidence and risk factors associated with PB and PPB following rTKA.

A retrospective analysis of 415 rTKAs by a single surgeon between 2009 and 2019 was conducted. PB was defined by an Insall-Salvati ratio less than 0.8 and a Blackburne-Peel ratio less than 0.54. PPB was characterized by a relatively lower patella position compared to the joint line, but within the normal range of Insall-Salvati ratio (0.8-1.2) with a Blackburne-Peel ratio less than 0.54. Univariate and multiple regression analysis identified risk factors for PB and PPB.

After rTKA of 375 included knees, incidence of PB was 24% and PPB was 14%. Patients revised for infection were 9 times more likely to develop PB compared to other indications (P = .002). Number of prior surgeries and preoperative PB also increased the risk of postoperative PB (OR, 2.18; P = .013; OR, 319.90; P < .001). PB and PPB groups showed minimal change in range of motion from preoperative to final follow-up (-1.6 and 0.6°), whereas patients with normal patella height showed an average range of motion increase of 7.2°.

Identifying risk factors for PB and PPB such as preoperative patella height, number of previous surgeries, and revision indication of infection may help surgeons to address this difficult problem.