Kneecap Dislocation Overview
A kneecap dislocation is a fairly common injury, and most often happens to athletes in the teenage years. It is also a fairly common skiing injury so we see them quite frequently in Vail.
The kneecap is a free-floating bone (which we call a sesamoid bone) located between one’s quadriceps tendon and the patellar tendon. It has the thickest cartilage in the body and this is important because it allows us to have a mechanical advantage to pull the tendons away from the end of the thigh bone for maximum thigh strength and efficiency. When one does lose their kneecap due to a fracture, the overall strength for straightening their knee is reduced by about 40%.
Almost all kneecap dislocations which occur after an injury are to the outside of one’s knee. Ultimately, the way that one is built can make a big difference in how stable one’s kneecap is. There is a groove on the end of the thigh bone, called the trochlea groove, in which the kneecap rests. If one has a shallow groove, then one has a much higher risk of having the kneecap both slip out and also having it recurrently slip out after one has an injury. In addition, if the tendon that holds one’s kneecap to the shin bone (the patellar tendon) is long, the kneecap can sit up high above the groove and not be protected by one’s bony geometry and it also has a higher risk of slipping out. Finally, if one’s thigh bone or shin bone is twisted inwards or outwards, the kneecap is also at a disadvantage to having it slip out.
When one does dislocate their patella to the outside, the soft tissues on the inside of the knee have to be torn. The main structure which holds the kneecap in place when the knee is out straight is the medial patellofemoral ligament, also called the MPFL. When the MPFL is torn in patients who have fairly normal bony geometry, it often can heal without the need for surgery. This can happen in over 90% of the time. However, when one has a shallow trochlear groove, or when the kneecap is riding up high, the chance of success with a nonoperative program may be 50% or lower.
How to Diagnose Kneecap Dislocation
Therefore, one of the most important things when one sustains a kneecap dislocation is to have both a comprehensive physical exam and also a set of x-rays to determine if one’s kneecap sits up high or if the trochlea groove is shallow. Having the kneecap dislocate out again is worrisome because every time it slips in and out, it could knock some cartilage off the kneecap or the end of the thigh bone which in effect is an area of arthritis. We do not have a cure for arthritis so this needs to be carefully evaluated to minimize it from happening due to recurrent injuries.
How to Treat Patella Dislocation
I cannot emphasize enough the importance of a well-guided physical therapy program for kneecap related problems. This is especially true for somebody who has dislocated their kneecap, or have a partial dislocation called a subluxation. Because the kneecap sits within the tendons which help one to straighten out one’s knee, a special therapy program with an emphasis on thigh (quadriceps) strengthening is essential. In addition, it is well recognized that many causes of kneecap problems are related to weak hip muscles, so a concurrent hip muscle strengthening program should be a part of all kneecap related therapy programs.
If one does dislocate their kneecap and does not have any pieces of cartilage knocked off which have to be replaced, the general feeling is that these patients should try a program of rehabilitation. If their kneecap keeps slipping out, then one may have to go to surgery sooner rather than later. The use of kneecap braces which hold the kneecap in position may be indicated for these patients, but ensuring one has a proper hip and knee core strengthening program is essential to determine if a nonoperative treatment program will be successful overall.
The treatment of kneecap dislocations is complicated. A thorough physical exam and the combined use of x-rays, CT scans and MRIs is usually indicated. It is well recognized that repairs of the MPFL do not do well and that the MPFL should be reconstructed. However, only reconstructing the MPFL may result in the kneecap dislocating again if one has a high riding kneecap or if one has a very shallow trochlea groove. Therefore, a thorough evaluation must be performed, with both CT and MRI scans, to determine if the bump of bone where the patella tendon attaches should be moved, called a tibial tubercle osteotomy, or if the end of the thigh bone has to have its entry point deepened, called a trochleoplasty. A perfectly well-done MPFL reconstruction does not good if one’s bony geometry is out of whack.
Kneecap problems can be particularly disabling. Starting off with a well-directed physical therapy regimen is a must. For those cases in which the therapy regimen does not work to restore stability, or one’s bony geometry is off such that the kneecap keeps slipping, one should consider surgery which addresses all the fine points noted above to maximize one’s ability to
