Problems at the donor site are common after removing the central third of the patellar tendon and using it as a graft for the torn ACL. The knee isn't always able to accept a full weight load and patients often report knee pain, especially during walking.
There may be a couple reasons why this happens. Doctors think nerves in the area are injured when removing the donor tissue. The nerve damage is less when a different technique is used to take the tissue out.
Other studies show the donor site never returns to normal. The tissue fills in the hole, but it still remains thin up to six years later. The kind of problem you're having has led researchers to look for other donor sites. Sometimes the hamstring tendon is used. Graft tissue from someone beside the patient is also a possibility.