Kneecap (Patellar) Instability: Overview

Kneecap (Patellar) Instability: Overview

Having an unstable kneecap or patella can be an ongoing problem that keeps someone from doing their day-to-day activities. A dislocating kneecap, also called patellar instability, is when the kneecap slides out of the groove at the end of the thigh bone, causing pain and difficulty moving the knee. Before we get into how and why this happens, let us take a look at the anatomy of the knee.

Here is a right leg, viewing from the front. The main muscle group that pulls on the kneecap is the quadriceps on the front of the thigh, which has four muscles in it: the rectus femoris, vastus medialis, vastus lateralis and vastus intermedius. The patellar tendon connects the kneecap to the front of the shin. Through the kneecap, the quadriceps muscle and the patellar tendon pull on the shin bone to straighten the knee. Underneath the quad muscle, is the thigh bone thighbone, or femur, and here is the shin bone or tibia. At the end of the femur, the bone gets wider, forming the medial condyle on the inside and the lateral condyle on the outside. In between these two condyles is the trochlear groove, where the kneecap sits and glides up and down as the knee bends and straightens. These condyles rest on the tibial plateau, the top of the shinbone.

Looking at the end of the thighbone, the lateral condyle is taller. This extra bump of bone is built in to try and help keep the kneecap from sliding to the outside. In a knee with normal anatomy, the back side of the kneecap and the groove where it sits have the same shape. Speaker1: However, in some patients, those cartilage surfaces are much more flat, putting the kneecap at risk for sliding out of place. When the kneecap slides all the way out, it is called a dislocation. Sometimes, the kneecap pops back in on its own, but some dislocations need a health care provider to relocate it. There are patients who only have a partial dislocation where the kneecap does not completely come out, but it goes far enough to cause pain. This is called a subluxation.

Whether it is a dislocation or a subluxation, the ligament that is damaged is the medial patellofemoral ligament or MPFL. This ligament keeps the kneecap from sliding out during regular activity, but a severe fall or twist to the knee can stretch this ligament or even tear it. The two main reasons that the MPFL gets injured and a kneecap can slide out of the groove are either from an acute, one-time injury or if a patient's anatomy puts the kneecap at risk. For example, twisting a knee while playing sports or simply suffering from a fall at home can cause the kneecap to dislocate. These would be considered one-time, acute injuries.

However, if someone's kneecap dislocates over and over, it might be from the shape of the bones in their knee or if the alignment in their leg is a little off. Depending on the shape of the groove, some patients are at a higher risk of a kneecap dislocation than others. Also, the alignment of the knee can have an effect. If line A is in line with the quad muscle and line B is in line with the patellar tendon, then the angle between them is called the quadriceps angle or Q angle. Speaker1: Females typically have a larger Q angle than males do. There is evidence suggesting that this plays a role in a patient's risk of dislocating their kneecap.

If someone does have a one-time dislocation or subluxation, or especially repeated dislocations, it is a good idea to have an orthopedic provider examine the knee. Patients will likely have pain around their kneecap and will usually tell the provider that it feels like the kneecap will pop out at times. Some patients can also get swelling around the knee and may need crutches to walk. The provider will check the position of the kneecap and see if it feels loose or is not gliding where it should be. X-rays will be taken to see the position of the kneecap in relation to the thigh bone and shin bone, making sure it is not sitting too high or too low and that it is sitting in the center of the groove.

If the provider suspects a kneecap dislocation, an MRI will be ordered to take a look at the soft tissues in the knee and the amount of swelling that is in the joint. Someone that dislocated their kneecap from a one-time injury may be treated differently than someone who gets them over and over. All of these factors should be discussed with the orthopedic provider to determine the best course of treatment for each individual patient.