Link Between Flat Groove and Kneecap Dislocation

People who suffer one patellar (kneecap) dislocation after another search for ways to prevent this from happening. The obvious first question is: what is causing this to happen? Most often the patella pulls away from the knee in a lateral direction. Lateral means sideways in a direction away from the other knee.

To better understand how knee problems occur, it is important to understand some of the anatomy of the knee joint and how the parts of the knee work together to maintain normal function. The knee is the meeting place of two important bones in the leg, the femur (the thighbone) and the tibia (the shinbone). The patella is also made of bone and sits in front of the knee.

The patella is held in place by its shape and the supporting soft tissue structures such as muscle, tendon, cartilage and ligaments. As the knee moves, the patella glides up and down in front of the knee joint. There is a groove on the front of the femur (the trochlear groove) of the femur. The back of the patella has a corresponding V-shape that fits inside the groove and helps hold it in place.

Any changes in the shape of the bone, alignment, ligamentous laxity (looseness), muscle weakness, or other soft tissue problems can contribute to patellar instability. In this article, trochlear dysplasia is the focus as a possible cause of recurrent (repeated or chronic) patellar dislocation. Trochlear dysplasia refers to a groove that is shallow -- too shallow to hold the patella in place as it glides up and down.

A shallow groove by itself may not be enough to really result in chronic patellar instability. Experts think there are multiple factors contributing to the problem. Each factor must be addressed in treatment in order to keep the patella centered in the trochlear groove. Preventing further patellar dislocations requires evaluation of the bony, soft tissue, and alignment issues.

What's the first step in the process? The examiner will need to figure out if the knee is giving way because it hurts or if there is pain because the patella is unstable leading to dislocation. Interviewing the patient helps create a picture of what's happening and when it's occurring. The duration and severity of the problem will be revealed through this process.

Next, the examiner performs an evaluation looking at motion, strength, alignment, tissue integrity, ligamentous laxity, position of the patella, and so on. One of the most accurate tests for patellar instability is called the apprehension test.

In this test, the patient's patella is pushed to the side as the knee is bent. A positive response occurs if the patient's quadriceps muscle starts to contract during this movement or if the patient feels like the kneecap is going to pop off center and dislocate again. Patellar movement is also evaluated with the knee in full extension.

X-rays are next. X-rays help show any unusual patellar shapes that might be part of the problem. There are special views that can be taken to show the position of the patella in the trochlear groove, the depth of the groove, and how well the two bones match up. In some cases of patellar instability, the patella is riding up above the groove. This condition is called patella alta. This is one of the many alignment factors that can put the knee at risk for dislocation.

The radiologist also looks for the presence of the crossing sign on X-rays as an indicator of trochlear dysplasia. This sign is visible when looking at the knee from the side. It is an indicator of the depth of the trochlear groove.

Once all the information has been collected, the surgeon can begin to determine the best plan of action. Conservative care with a rehab program under the guidance of a physical therapist may be helpful in addressing some of the soft tissue and alignment factors. Surgery is more effective when trying to change the bony factors including trochlear dysplasia.

There are several different surgical approaches that can be taken. For example, the surgeon can perform a trochleoplasty. In this procedure, a piece of bone is removed from the trochlear groove and the area is deepened and reshaped. A different approach would be to use bone graft material to build up the lateral (outside) wall of the groove. Or the surgeon might change the rotational angle of the femur so the two bones (femur and patella) line up as they are supposed to.

The idea that trochlear dysplasia plays a significant role in patellar instability is fairly new. Which surgical technique to use has not been studied fully. The few studies that have been done report good results with trochleoplasty (no more dislocations) but knee pain and swelling continue. Arthritic changes in the knee joint are typical after any of these procedures.

Additional surgery to change the biomechanics of the knee may be needed. For example, the ligaments on either side of the knee (the medial and lateral patellofemoral ligaments) must each exert the right amount of tension at various points in the knee range-of-motion. If they do not, the patella can be pulled too far in one direction or the other and dislocate. Restoring normal constraints offered by these ligaments can also provide stability and offset a deficient trochlea.

The authors conclude by saying that repeated dislocations of the patella (kneecap) may be difficult to treat without surgery. Many factors are at play here but new insights suggesting a closer look at trochlear dysplasia as a key player have been reported. Anatomy, biomechanics, and causes of this condition are explored. Treatment as it has been developed so far is reviewed. Expect to see more on this topic in the coming years.

Reference: Matthew Bollier, MD, and John P. Fulkerson, MD. The Role of Trochlear Dysplasia in Patellofemoral Instability. In Journal of the American Academy of Orthopaedic Surgeons. January 2011. Vol. 19. No. 1. Pp. 8-16.

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