One of the complications I see occasionally with BTB (patellar tendon) grafts for ACL reconstruction is scarring of the patellar tendon and resulting patella baja. This can lead to quad weakness, anterior knee pain and limited knee range of motion. Furthermore, it is really hard to treat.
This small study showed very minimal lowering of the patella after BTB ACL surgery and did not correlate size of the graft harvested with post-operative x-ray changes. I think it would take thousands of patients to really assess this phenomena.
Read the study: Changes in patellar height after anatomical ACL reconstruction with BTB autograft with a focus on patellar tendon removal volume.