Patella baja and total knee arthroplasty (TKA): etiology, diagnosis, and management.

Patella baja, that can be divided into congenital, acquired, or a combination of the two, is commonly encountered in total knee arthroplasty (TKA). Congenital patella baja refers to a patella distal in relationship to the femoral trochlea and present since an early age. Acquired patella baja may occur secondary to distal positioning of the patella relative to the femoral trochlea or shortening of the patellar tendon, as a result of trauma or surgery. Patella baja also can occur postoperatively as a result of scarring and shortening of the patellar tendon, scarring of the patellar tendon to the anterior aspect of the tibia, or both. Another cause of acquired patella baja seen commonly in TKA is elevation of the joint line, referred to as pseudo-patella baja. The patella remains in a normal position relative to the femoral trochlea; however, the distance between the patella and tibia is narrowed. Pseudo-patella baja can be a result of tibial or femoral over-resection, which necessitates a large polyethylene insert. Alterations of the patello-tibial distance can occur during TKA by excessive soft-tissue release that requires elevation of the joint to regain stability and placement of the patellar polyethylene component distally on the patella. Prevention is the easiest way to avoid potential problems with patella baja during TKA; however, the surgeon is often confronted with this situation during total knee revisions. Failure to address patella baja can lead to decreased range of motion (ROM), a decreased lever arm, extensor lag, impingement of the patella against the tibial polyethylene or tibial plate, anterior knee pain, increased energy expenditure, and rupture of the patellar or quadriceps tendons. Treatment of patella baja first depends on determining the cause and distinguishing between patella baja and pseudo-patella baja. Five different methods to measure patella baja are reviewed and include: (1) Blumensaat's line, (2) Insall-Salvati ratio, (3) Modified Insall-Salvati ratio, (4) Blackburne-Peel, and (5) Caton-Deschamps. Corrective measures include reestablishing the joint line by use of distal femoral augments, tibial tubercle osteotomy with proximal displacement, lengthening of the patellar tendon, shaving of the anterior portion of the tibial polyethylene, and placement of the patellar implant in a cephalad position.