Patella baja after total knee arthroplasty: is it really patella baja?

In the prosthetic knee, narrowing of the space between the patella and the tibial polyethylene can be due to distal positioning of the patella or to a proximal transfer of the joint line. In the former case, one is dealing with patella baja. In the latter case, the patella is in its normal position relative to the femur, a situation we call pseudo-patella baja. The therapeutic options for patella baja and pseudo-patella baja are different because the former is due to a short patellar tendon, and the latter is due to a raised joint line. Eversion of the extensor mechanism during revision surgery may be more challenging with true patella baja compared with pseudo-patella baja. It is important not to attempt to detect true patella baja with either the Blackburne-Peel or the Caton-Deschamps ratio in the setting of a prosthetic knee because these ratios are affected adversely by the position of the joint line.