Infrapatellar contracture syndrome, or patella infera, is a more advanced form of arthrofibrosis that can occur either in an isolated form - meaning that it affects just the patellofemoral joint – or in a more extensive form in that it also involves the entire knee joint, where it affects flexion and extension of the knee as well.
For the purposes of the discussion I will just limit the discussion to the type of scarring that affects the patellofemoral joint. Where this scarring usually occurs most commonly is in the medial and lateral gutters and directly behind the patellar tendon, scarring the patellar tendon down to the front of the tibia.
Some patients with this problem will essentially have normal flexion and extension of their knee but they will have increased contact pressures under the patella, or kneecap, and this will subsequently lead to pain and problems with functional activities – particularly things that involve weight-bearing, such as going up and down stairs, or hill climbing – things of that nature. When it becomes more severe, it actually creates a shortening, or contracture of the patellar tendon itself, and that becomes visible on X-rays – you can actually measure that shortening if you compare it from one side to the other, or if you compare it to what a normal relationship of the patellar tendon to the length of the patella would be - and that is known as patella infera or patella baja. That is a more advanced phase of this process.
So early on it just starts as scarring behind the kneecap and around the kneecap, which limits mobility of the kneecap and increases the joint contact pressures, and in the more advanced forms it actually creates shortening of the patellar tendon.