Patellar baja refers to the position of the patella or kneecap. In patellar baja, the kneecap rides down low over the femur (thighbone).
This can just be the way you were born. More often it occurs as a result of the patellar tendon shortening after surgery or injury. Either way, a patellar tendon that is too-short can pull the patella downwards.
A recent study of stiff knees after total knee replacement (TKR) identified patellar baja as a possible cause. Women were at higher risk of patellar baja. Younger age was also a factor but age was linked with joint stiffness after TKR, not the patellar baja.