He had two years of problems and 3 surgeries to remove scar tissue. This type of problem is more common in reconstructions that are done soon after an injury. Straightening the knee is very important to walking. There have been people that cannot walk properly without pain on every step because they cannot extent their knee. Reconstructed knees may end up with more pain than prior to the surgery.
People especially have problems when kneeling on the floor.
The original or subsequent injury did damage to other parts of the knee. The surgery may be successful in regaining stability but there may be other problems if the cartilage has been damaged. People do not recover to the point they have a fully functional knee because of insufficient rehab. The knee may continue to swell with activity.