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Lateral patellar instability is almost always due to a patellar (kneecap) dislocation. In this circumstance, an athlete or patient will experience their kneecap slipping out of the lateral (outside) aspect of their knee. It almost always occurs with the knee straight or at shallow degrees of knee flexion rather than with the knee bent. When the patella dislocates, it tears the structures on the inside of the knee, with the medial patellofemoral ligament (MPFL) most commonly torn.
There are many factors to evaluate in a patient’s prognosis when a lateral patellar dislocation exists. The cause of lateral patellar instability is very important to thoroughly evaluate, and each patient should be evaluated in terms of history, physical exam, X-rays, and MRI scans to determine the best treatment for them.
In the majority of circumstances, we treat these without surgery and attempt rehabilitation. Patients who have a normal patellar height and when injured, do not knock off any pieces of articular cartilage or bone, have a fairly good prognosis and have a low risk of recurrent patellar instability. However, in patients that are young and have dislocated their kneecaps previously, or in patients with conditions called patella alta, or trochlear dysplasia, there is a much higher risk of recurrent lateral patellar instability. However, in most of these patients, we still would recommend a formal non-operative rehabilitation program to assess their prognosis.
Patients who have recurrent instability may need surgery to restore both the medial stabilizing ligaments of the knee and any other associated pathology. This can include a medial patellofemoral ligament reconstruction, a lateral retinacular lengthening, a tibial tubercle osteotomy, and/or a trochleoplasty.
Patients who undergo a MPFL reconstruction must be non-weightbearing for 6 weeks after surgery. Knee motion is limited to 90 degrees for the first two weeks after surgery and then full motion is allowed. At six weeks after surgery, weight bearing is commenced and patients may wean off crutches when they can ambulate without a limp. The use of a stationary bike is also initiated at 6 weeks after surgery and endurance and agility exercises are initiated at 3 months after surgery. Most patients without any arthritis in their kneecap can return to full sporting activities at between 5-6 months after surgery.
The medial patellofemoral ligament is the ligament that courses from the femur to approximately the midportion of the patella. It is the most important stabilizer of the kneecap towards slipping to the outside or causing a lateral patellar subluxation and dislocation. For patients that do have a lateral patellar dislocation, the MPFL, by definition, has to be torn at some location.
A repair of the MPFL would involve putting stitches in or a suture anchor to repair a torn MPFL. This is usually performed after an acute injury rather than a chronic injury. The results of MPFL repairs with chronic injuries are not good, so reconstructions would be indicated in those circumstances.
The success rate for MPFL repairs after an acute injury is not well defined, but we know that the success rate after chronic injury is not good. There is maybe between a 50% and 60% success rate for MPFL repair in a chronic situation. Thus, a reconstruction would be indicated within these circumstances due to the 80% to 90% success rate with reconstructions over repairs.
The MPFL is an extraarticular structure. It is not located within the knee joint lining. Therefore, an attempted arthroscopic MPFL repair would most likely result in tightening of the capsule of the joint, which would have a lower chance of working compared to addressing a torn MPFL with an open surgery.
The surgical outcomes across the literature are better for reconstructions over repairs. This is because with reconstructions, one can start early motion, stability is present right away after surgery, and MPFL repairs may be more tenuous and require longer periods of immobilization which can result in a stiff knee. In addition, when a structure is stretched out/torn and sutured, the chance of success is much lower than if it is completely reconstructed. MPFL repairs may be better when there is a very localized area of the tear of this structure at the time of the dislocation, especially in younger patients.
Because the MPFL is not located in the joint, an open surgery is the best way to access a torn MPFL.
In most circumstances, the surgeon can decide at the time of surgery what a “safe zone” range of motion is after an MPFL repair. Therefore, motion can usually be started right after surgery within the safe zone of motion and slowly increased over time. In general, we keep patients nonweightbearing or limited weightbearing for the first 6 weeks after an MPFL repair to give the repair the best chance to heal without stretching out.
Because most patients will be nonweightbearing to limited weightbearing, the use of a knee immobilizer with the knee in full extension would be the primary means of treating a MPFL repair. For those patients who may have a very stout MPFL, possibly with a piece of bone, with a repair they may be able to wean out of an immobilizer as soon as they have good quad control and possibly using a patellar stabilization brace in these circumstances may be indicated. This would need to be a possibility based upon an individual case.
The main indications for an acute MPFL repair would be in a patient who has an osteochondral fracture of the kneecap that needs to be repaired, which would need to be done via open surgery, or in a patient who has an acute patellar dislocation and the patella stays subluxed outside the joint rather than being back in the normal position after this injury. One would need to assess the potential for success with a repair versus proceeding directly to a reconstruction depending upon the patient’s amount of stretching within the MPFL and also the underlying bony geometry. Patients who have significant trochlear dysplasia would probably better benefit from a reconstruction compared to a repair so that early motion can be performed with the minimal risk of having the MPFL repair stretch out.
Lateral patellar dislocation is the most common type of dislocation because of the bony geometry of one’s knee. It is very rare for one to have medial patellar dislocation unless one has had a lateral release. Iatrogenic medial patellar dislocations are quite common and almost always occur after lateral releases.
There is a strong relationship between having recurrent lateral patellar dislocations and the amount of “flatness” of the trochlea, which is called trochlear dysplasia. Patients who have a normal V shape of the distal femur trochlear groove have a much higher rate of success with nonoperative treatment of lateral patellar dislocations compared to those who have a flatter trochlear groove. The recurrence rate of patients with a normal V-shaped trochlear groove can be 10% whereas the recurrence of lateral patellar dislocations with those who have a flatter groove can be 30% or more.
There are , very few indications that a lateral release should be performed for patellar instability. Unfortunately, the side effects from a lateral release can be significant, including medial patellar instability.
The main indications to proceed directly to surgery after a lateral patellar dislocation is if there is a significant cartilage and bone piece that has been knocked off the patella or the trochlea groove. In these circumstances, a surgery to restore the cartilage surface as well as to repair or reconstruct the torn ligaments would be indicated.
In most other cases, a trial of physical therapy to work on quieting the knee down after the injury and working on restoring strength would be indicated. For those patients who have a normal shape of their trochlear groove, this can be quite successful, whereas those with increasing amounts of flatness of the trochlear groove, called trochlear dysplasia, the recurrence rate is much higher. It is still worth a shot at trying to get the tissues to heal after a lateral patellar dislocation on those with some trochlear dysplasia, but one needs to keep in mind that the recurrence rate is higher in these circumstances.
The main x-rays that are obtained to assess the patellofemoral joint are an AP view, a standing lateral view with the knee bent, and a patella sunrise view, usually obtained in about 45 degrees of knee flexion to look at the axial alignment of the patella.
The best test to assess lateral patellar instability is the lateral patellar apprehension test. In this test, the knee is held over the side of the bed and the examiner gently pushes the patella laterally to see if the patient has a feeling that it is going to slip out. Some patients will have natural laxity and they won’t have any pain when pushing the patella out, whereas those that have had a previous dislocation or subluxation usually will feel apprehensive that the patella is about ready to dislocate again. This is a positive test.
The main findings that one can see on MRI after a lateral patellar dislocation include a tear of the medial patellofemoral ligament, possible bone bruising or cartilage damage to the medial facet of the patella, and a bone bruise or cartilage damage to the lateral aspect of the trochlear groove. The bone bruises occur from the patella getting stuck on the outside of the knee with the dislocation and then being pulled back into place.
The best way to reduce a dislocated patella is to have the athlete lie supine, hold the patella gently laterally while you straighten their knee out, and then try to slide it back in the position when the knee is out straight. This maneuver should minimize the risk of further cartilage injury and also cause the least pain to a patient. Patients who have a lateral patellar dislocation after a knee replacement may have a nonreducible patellar dislocation of their prosthesis that becomes entrapped in scar tissue or against the metal surface of the components.
The most common fractures that occur after a lateral patellar dislocation are a fracture of the cartilage surface and the supporting bone off the kneecap. These can be quite significant and often can involve a large slough of the important cartilage at this joint. Because the patellofemoral joint has the most force on it of any joint in the body, restoring this cartilage surface whenever possible would be indicated after a lateral patellar dislocation. Other fractures that can occur include the lateral aspect of the trochlear groove. Often, these are fractures of the very edge of the cartilage surface and may not need surgery if these pieces are essentially chipped off and not repairable. In cases where there is a large portion of the trochlear groove that comes off with an osteochondral fracture, these fragments also should be repaired. In our hands, we have often seen that these fractures involve 2-3 mm of the far lateral aspect of the trochlear groove and at the time of surgery there are multiple fragments so a repair is not possible.
It is generally believed that working on a core strengthening program of the entire lower extremity is essential to prevent recurrent lateral patellar dislocations. In particular, working on the quadriceps muscles is important. However, making sure one’s hip muscles are also strong is essential in making sure that one’s muscles are properly balanced with daily activities.
It is not uncommon for patients to have problems with medial patellar instability after a lateral release. Thus, if one does have an aggressive lateral release, the patella may start to dislocate towards the inside. For patients, especially those who have had lateral patellar instability previously, this may still feel like the patella is slipping towards the outside because it is hard to differentiate medial versus lateral patellar instability. Unfortunately, this has often led to some patients having recurrent lateral releases to “address the problem,” which often leads to more disability.
We have found that after a lateral release, it is important to see if the patella does sublux medially and if pushing the patella medially reproduces their symptoms. If it does, our usual program is to try reverse McConnell taping to pull the patella to the outside, which is different than usual McConnell taping which pulls the patella towards the inside. In this circumstance, the patella would be pulled laterally to see if the lateral release was the culprit and caused medial patellar instability.