What is the patella, why can it dislocate, and who’s at risk?

The patella, better known as the kneecap, is a small circular-triangular bone that articulates (i.e. forms a joint) with the femur (thigh bone) called the patellofemoral joint.  The patella travels within a groove on the femur called the trochlea.  The quadriceps tendon attaches to the top portion of the patella, while the patellar tendon originates from the bottom of the patella prior to attaching to the tibia (lower leg bone).  This confluence of the kneecap and tendons helps form the unit that extends your knee.  The patella’s role is to act as a fulcrum or pulley for the quadriceps as it contracts to extend the knee.  The patellofemoral joint therefore encounters a significant amount of force.  One way the patella protects itself from this force is through a thick cartilage covering on its undersurface.  In fact, the cartilage under the patella has the thickest layer of cartilage of any bone in the body.
 

The patella also needs to stabilize itself, and does so through multiple restraints. The primary ligamentous restraints are called the Medial Patellofemoral Ligament (MPFL) and the Lateral Patellofemoral ligament.  The MPFL prevents the patella from tracking towards the lateral (outside) aspect of the knee, the most common direction for the patella to dislocate.  The four quadriceps muscles provide the muscular restraints, with the most important being the vastus medialis obliquus (VMO).  The VMO provides a medial force during quadriceps contraction, helping to keep the patella centered within the trochlea.  Weakness of the VMO can cause the patella to track laterally (outwards).  The final restraint is the trochlea.  Some patients don’t have a well-developed trochlear (i.e. their groove is shallow or misshapen), a condition known as trochlear dysplasia, which is a significant risk factor for patellar instability and dislocation.

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Other risk factors for patella dislocation include patella alta, bony malalignment, and ligamentous laxity.  Patella alta, known as a high-riding patella, is a condition where the patella sits higher than normal.  This high-riding patella does not effectively engage the trochlea and can therefore slip out of the groove.  Bony malalignment (such as being knock kneed) is abnormal alignment between the bones of the knee that leads to a lateralizing (outward) force on the patella during knee extension.  Ligamentous laxity (i.e. loose ligaments) can result in MPFL being more loose than it should be.

 

Patellar dislocation can occur with or without trauma and most commonly occurs in adolescents, specifically females.  Approximately 49% of dislocators have a history of dislocation.  As previously alluded to, lateral dislocation is most common.  The patella, however, can dislocate medially, especially in patients with a history of a knee surgery called a Lateral Release.

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Symptoms:

Most patellar dislocations happen quickly, with the patella dislocating and spontaneously returning to its normal position.  This is especially true in the case of chronic dislocators, where spontaneous dislocations can frequently.  The patella, however, can sometimes remain dislocated and have to be manually reduced back to its normal position, in which case patients will typically fall and keep their knee in a flexed position, while the patella will be visibly disfigured.  Patients experience varying degrees of knee pain, swelling, and instability with patellar dislocations.
 

Diagnostic Testing:

An x-ray is usually ordered to evaluate for fracture, bony pathology, and patella alta.  An MRI may also be ordered to evaluate the MPFL ligament and assess for any injury to the patella cartilage and its underlying bone (i.e. osteochondral injury).  It’s important to realize that when the patella dislocates, a portion of the patella cartilage and underlying bone can be damaged, occasionally leading to a small piece of that cartilage and bone (i.e. a loose body) floating around within the knee joint. 

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Treatment Options:

Nonoperative treatment is theoretically appropriate for 1st time dislocators without any evidence of chondral damage or loose body within the knee joint.  These patients are usually placed into a brace that locks the knee in extension and allowed to weight bear as tolerated so long as they remain in the knee immobilizer when doing so.  Patients will also be expected to appropriately modify their activity.  Eventually, patients will start physical therapy and focus on strengthening the quadriceps muscles, particularly the VMO.  It’s very important to understand that 30-45% of patients will dislocate again following nonoperative management.  Risk factors for additional dislocations following nonoperative management are female gender, patella alta, adolescents, trochlear dysplasia, and bony malalignment.

 

Surgical Treatment:

It is often recommended to perform surgery even in first-time dislocators.  This is because there is significant risk of cartilage damage with each dislocation, and if cartilage damage does occur, the current treatment options often do not allow for return to normal healing,  Surgery is also recommended in 1st time dislocators who already have injury to the patella cartilage/bone or loose body within the joint.  Surgery is almost always recommended for recurrent dislocators.  The optimal procedure for stabilization of the patella is tailored to the patient’s specific anatomy and type of dislocation, and ranges from MPFL repair/reconstruction to bony realignment procedures.  Dr. Lee will discuss his surgical recommendation with you during your appointment.
 

Recovery Expectations:

Recovery from surgery depends on the type of surgery performed.  Most patients, however, can expect to wear a knee brace that locks in extension for a period of time ranging for about 4 weeks following surgery.  Physical therapy will eventually be started to improve knee range of motion and strength.  Physical therapy usually lasts 6-12 weeks. 

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Immediate Post-Operative Instructions

Please refer to the following pages for more information:

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• What should I expect after surgery?

• What food or supplements should I have after surgery?

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*It is important to note that all of the information above is not specific to anyone and is subject to change based on many different factors including but not limited to individual patient, diagnosis, and treatment specific variables. It is provided as an educational service and is not intended to serve as medical advice.  Anyone seeking specific orthopedic advice or assistance should consult Dr. Steven Lee or an orthopedic specialist of your choice.

 

*Dr. Steven Lee is a board certified orthopedic surgeon and is double fellowship trained in the areas of Hand and Upper Extremity Surgery, and Sports Medicine. He has offices in New York City, Scarsdale, and Westbury Long Island.