The patella is held in its normal location by a combination of ligaments (ropes between the bones) and muscles. Normally ligaments prevent the patella from completely coming out of its groove (the groove is also called the trochlea). Muscle tension also helps to hold the patella in place and cause it to glide or "track" normally. When the patella slips out of the groove it usually does so in early flexion just as it has entered the groove. Patellar dislocations can happen with a twisting injury to the knee with the involved leg planted on the ground such as when dancing or during athletic activities. It can less often happen from a direct injury to the knee. Individuals with hypermobility (that is "loose joints") are at increased risk for patellar instability. Some patients also have a congenitally (naturally occurring) shallow trochlear groove that makes patellar dislocation more likely. And sometimes, the tendency for patellar dislocation runs in families.
In order for the patella to actually dislocate, ligaments must be torn and often there is some degree of muscle injury as well. Often when the patella dislocates it spontaneously pops back but sometimes the patient or a medical provider has to put it back in place. At the time of dislocation sometimes bone and /or cartilage damage occurs to the patella or to the side of the trochlear groove. Often this is a bone bruise for which no specific treatment is necessary or possible but sometimes the injury is severe enough that there is a piece of bone or cartilage broken loose.
Treatment of acute dislocation
The goal of treatment is to have a stable patella (one that does not have a tendency to come out of the groove again) and to restore normal strength and function. In order to reach this goal, there must be adequate healing of the ligaments and muscles and thorough rehabilitation including strengthening and flexibility. Often the assistance of a physical therapist is helpful at this stage-of recovery.
According the medical literature, treatment of a first time patellar dislocation without surgery results is a knee that is satisfactory to the patient in about 2/3 of patients. Non-operative treatment generally includes immobilization initially to allow for swelling to decrease and initial healing to begin followed by a period of gradual increase in activity and rehabilitation. Recommendations vary somewhat between doctors, but I believe that a period of immobilization and protection of the involved ligaments and muscles is important for the first two weeks after injury. Recovery from the initial injury to full activity has averaged three months in some studies. Often a type of knee sleeve is prescribed to help hold the patella while rehab progresses.
When does patellar instability require surgery?
If the initial injury produces a loose piece of bone and cartilage then treatment will usually include surgery to replace and repair the fracture or if that is not possible due to the size nature and location of the fragment to remove it. The larger the fragment and the more bone that is part of the fragment, the more likely repair will be needed.
Acute repair of patellar dislocation
Sometimes surgery is also considered to perform repair of the torn ligaments after an initial dislocation. The idea here is to repair what is torn initially (usually what is called the medial patellofemoral ligament and the associated medial retinaculum and sometimes a portion of the vastus medialis obliquus muscle) with the expectation that this will improve the known 2/3 success rate with non-operative treatment as noted above. In fact, this approach is successful in reducing the rate of recurrent instability to about 10% or less in different studies but it does not assure pain-free return to function for all. It is not a perfect operation but it does decrease the rate of recurrence. In addition the choice of surgery after first time dislocation makes other complications such as infection and joint stiffness more likely that without surgery. Recovery from acute repair of a patellar dislocation typically takes about a minimum of three months to full athletic activity. The situation in which acute repair is most often considered is when the patient desires the most certain and fastest return to athletic activity, understanding the other risks involved. Sometimes this makes sense for an athlete looking forward to an upcoming season or in the case of a worker who wants to do all they can to prevent instability after return to active activity.
So what happens if non-operative treatment fails?
Treatment without surgery
If surgery is not done in a case of recurrent patellar instability, the best that one can do to prevent joint damage is to strengthen the muscles as much as possible around the knee and hip and wear a knee sleeve designed to restore support to the patella during all activities that might be provocative to the knee. Sometimes this is sufficient to produce satisfactory function and is well worth a try before considering major knee surgery.
If non-operative treatment fails and the patella continues to slip out of the groove with desired athletic activities, then surgery can be done to repair or to reconstruct the ligaments that did not heal well enough to stabilize the patella. If the remaining ligament can be repaired and retensioned sometimes that is enough, but if the ligament has been torn for a long time or is not very strong, reconstruction is often preferred. Reconstruction involves taking a tendon graft from another part of the leg (usually a hamstring tendon) or a tissue bank tendon and attaching it to the patella and the femur at the correct locations to recreate the "rope between the bones" that was originally torn. The is called a medial patellofemoral ligament reconstruction (or "MPFL reconstruction"). Less commonly surgery may be suggested to realign the bones to restore a more normal alignment or a deeper groove. The success rate for surgery in the chronic situation is also about 90 percent successful in eliminating the instability symptoms. (So if the non-operative treatment doesn't work, there is a good backup plan available.) The degree of pain that remains afterwards is often minimal but can depend on the degree of damage already done to the bone surfaces during prior episodes of instability. For this reason, it is wise to limit as much as possible the number of dislocations that occur once it becomes clear that the instability has recurred. Each dislocation increases the chance of a potentially catastrophic bone and cartilage injury to the patella or to the groove. Lateral release, which involves cutting loose the lateral side of the patella has been used for patellar instability but the track record of that operation for patellar instability is not so good in long term reports so I believe it is rarely if ever indicated for patellar instability symptoms.
We believe that understanding your problem is a key to your successfully choosing the option that makes the most sense for you as an individual. Individual cases may require slightly different approaches than those outlined above and this sheet is not intended to provide specific medical advice for individuals, but rather general information on this topic. If you have questions, please ask.