The patella bone, or knee cap, is the third bone in the knee joint. is called a sesamoid bone. It forms within the tendon at a young age. The patella bone sits within the groove called the trochlear groove. This groove sits on the end of the femur bone. With patellar instability, the knee cap bone slides out of that groove causing subluxation or dislocation.
Patellar Instability can be classified into three types: acute, chronic, and habitual. It also can have multiple contributions including trauma, ligamentous laxity, bone dysplasia and mal-alignment, and occasionally complications from a lateral release surgery.
Patellar instability is most commonly due to a sudden injury. From twisting of the knee or a direct blow to the knee, the kneecap is able to jump out of the groove causing a dislocation. When it almost dislocates is called subluxation. Typically, the kneecap will go back into place with just extension or straightening of the leg. This injury alone typically can be treated without surgery. In some occasions a piece of cartilage or bone is broken off of the group or kneecap or The kneecap continues to be unstable in which case surgery may be necessary.
Chronic instability is usually a result of multiple contributing issues which include:
- Ligamentous laxity: the ligaments that help keep the kneecap in place include the medial patellofemoral ligament or MPFL. Some patients have disordered collagen tissue that makes these ligaments less strong than normal causing issues with joint stability. This is common in the kneecap joint.
- Trochlear dysplasia: the trochlea is the groove in the femur. If this does not form correctly at a young age instead of a Groove, you will have a flat area. This is inherently unstable and can be a cause of patellar instability and pain.
- Malalignment: The Q angle is the measurement of the quadriceps tendon to the tibial tubercle. In some cases, patients may be knocked kneed. In other cases, there may be an excessive twist of the femur or tibia. This is called the miserable malalignment syndrome which includes femoral anteversion genu valgum (knock-kneed) and external tibial torsion or twisting. These three bony changes create the most risk for patellar instability. In all these cases the mechanics of the knee joint make it inherently unstable causing patellar subluxation or dislocation. In cases of malalignment and patellar instability, soft tissue repair may not be sufficient and may require bony correction.
- Iatrogenic: a procedure known as Lateral Release was very commonly performed in the mid-2000s. Unfortunately, this procedure did not help many people and instead the kneecap joint iwas at risk for medial instability which is otherwise very rare. To correct this type of instability patients often need a lateral patellar tibial ligament reconstruction.
- Patella Alta: this is translated as high patella. This is a condition in which the kneecap rests too far away from the femoral groove and therefore has more distance to travel before engaging in the groove. This traveling distance puts the kneecap at risk for being dislocated.
In the acute knee dislocation, the knee cap looks particularly deformed. Once the knee cap has been reduced the knee may look swollen.
Many patients can describe the history of numerous instability events. It is important to understand the cause of these instability whether it’s from simple activities such as getting out of a chair or more related with athletic activities such as sports. The frequency of these dislocations is also important in decision-making of treatment.
Dr. Faucett perform a physical exam of the knee. Important things he will evaluate include the knee range of motion, alignment, strength, patellar tracking and patellar mobility.
X-rays will be obtained to rule out patella fracture and dislocation. Other x-ray views to evaluate for osteoarthritis, trochlear dysplasia and overall knee alignment are also important.
MRI: An MRI is important to Highway both the status of the cartilage as well as the status of the MPFL and position of a kneecap. It can also be helpful to look for loose bodies of cartilage and or bone in the knee. some measurements of dysplasia can also be obtained from the MRI
CT scan: a CT scan is very helpful to look at the Bony features around the knee including the rotational alignment, the overall alignment and femoral dysplasia. In some cases a dynamic CT scan can be performed to evaluate patellar tracking.
A first-time dislocation is often treated with non-operative treatment. These ligaments often heal on their own. A need for surgery after an acute dislocation would include a large loose body that needs to be retrieved.
In most cases 60 to 80% of people will never have another dislocation after the first one.
Treatment includes traditional physical therapy, which is used to strengthen the muscles around the knee and hip. Having strong hip abductors (gluteal muscles) helps control the knee rotation when running and landing. The vastus medialis oblique (VMO) muscle is part of the larger quadriceps muscle also plays an important role in dynamic stability of the knee cap.
Braces and Tape
Bracing and taping of the kneecap can help temporarily while rehabilitating the knee. They are not a very effective long term strategy.
The goal surgery would be to correct the stability of the knee as well as any underlying cartilage damage or bone irregularity that may have occurred or put the kneecap at risk for instability.
- MPFL reconstruction: using either the patient’s own tendon or tissue from the tissue bank, the torn ligament would be reconstructed to improve patellar tracking and stability. This is done through a few small incisions to secure the ligament to the inner aspect of the kneecap back down to the femur bone.
- Cartilage treatment: in many cases the cartilage on the patella is damaged during these dislocation events in some cases cartilage treatment may be required.
- Tibial tubercle osteotomy: In cases where there is a bony issue causing patellar instability a tibial tubercle osteotomy can be performed to correct the alignment of the extensor mechanism. a
- Trochleoplasty: In cases of trochlear dysplasia, the trochlea can be reshaped to allow a deeper groove and reduce patellar instability.
Patients will be placed in a brace for 6 weeks and be instructed to work on knee range of motion. They will be able to weight bear partially. It will take 2 months to return to normal walking and six months of physical therapy to return to sport.
- Pain: in some cases patients may have continued pain in the knee from patellofemoral pain syndrome or cartilage damage.
- Instability: despite efforts to correct a ligament and bony issues the kneecap may still be unstable.
- Arthritis: because of the nature of the injury to the cartilage arthritis may continue causing pain and swelling in the knee.