The patella (kneecap) is a bone that lies in between your quadriceps muscle above and patella tendon below. It moves through a groove in your femur. Ideally it should track centrally through the middle of your groove
Your patella is out of alignment such that is tracking on the outside half . This leads to increased pressure on the protective surface (articular cartilage ) which results in this cartilage breaking down so the knee swells as the body absorbs the loose cartilage fragments. This then exposes the bone which has many nerve endings causing the pain. A good analogy is that your car tyre is out of alignment such that you have worn through all the rubber on the outside of your tyre and you are down to your hubcabs
The X ray below shows the patella is out of alignment with bone on bone on the outside of the patella
An MRI scan shows this better with the articular cartilage gone on the outside of the patella and the bone swelling (white bone) seen from the increased pressure
There is a thickened lateral retinaculum seen on the MRI above which is holding down the outside of your patella. Via key hole surgery a specialized instrument divides the thickened lateral retinaculum this unloads the worn out area and transfers the load to the normal cartilage on the inside
This operation will not cure your patellofemoral arthritis however it is very likely to make your knee feel better but not normal. It is probable your knee will continue to be a bit sore and swollen particularly if you over do it. Although it takes about six weeks to recover from your keyhole operation it can take up to a year to get the full benefit from the surgery as it takes time for the damaged bone to recover.
Our research on this operation shows that two years after surgery we reduce the pain from 6/10 to 2.8/10. When asked how normal the knee feels before and after the surgery this increases from 3.3/10 to 6.4/10 and when asked how satisfied they were with their outcome it is 7.8/10
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Source: Understand.com