Anterior cruciate ligament (ACL) repairs are often done using a piece of the patellar tendon in front of the knee as a graft to replace the torn tendon. After removing this piece of tendon and stitching the tendon back together, the surgeon makes sure the patellar tendon isn't too tight.
If it's too tight it pulls the kneecap (patella) down. When the patella is down too far, the condition is called patella baja. The opposite (patella up too high) is called patella alta.