Kneecap (Patellar) Instability: Overview

Kneecap (Patellar) Instability: Overview

An unstable kneecap, also called patellar instability, can significantly interfere with daily activities. This condition occurs when the kneecap slides out of the groove at the end of the thigh bone, causing pain and difficulty moving the knee. To understand why this happens, it is helpful to first review the anatomy of the knee.

Looking at a right leg from the front, the main muscle group acting on the kneecap is the quadriceps on the front of the thigh. This muscle group includes the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius. The quadriceps connect to the kneecap and continue as the patellar tendon, which attaches the kneecap to the front of the shinbone. Together, these structures allow the knee to straighten.

Beneath the quadriceps lies the femur, or thigh bone, and below it is the tibia, or shinbone. At the lower end of the femur are two rounded areas called the medial and lateral condyles. Between them is the trochlear groove, where the kneecap sits and glides as the knee bends and straightens. These condyles rest on the tibial plateau, the top surface of the shinbone.

The lateral femoral condyle is normally taller, which helps prevent the kneecap from sliding outward. In a knee with normal anatomy, the underside of the kneecap matches the shape of the groove. In some patients, however, these cartilage surfaces are flatter, increasing the risk that the kneecap can slide partially or completely out of place.

When the kneecap moves completely out of the groove, it is called a dislocation. Sometimes it returns to position on its own, while other times it requires reduction by a health care provider. Partial displacement, where the kneecap shifts enough to cause pain but does not fully dislocate, is called a subluxation.

In both dislocations and subluxations, the medial patellofemoral ligament, or MPFL, is injured. This ligament stabilizes the kneecap during normal activity and can be stretched or torn during a fall, twist, or traumatic injury. MPFL injury may occur after a single acute event or develop over time in patients whose anatomy places them at higher risk.

Recurrent kneecap dislocations are often related to bone shape or alignment issues. One contributing factor is the quadriceps angle, or Q angle, which is formed between the line of pull of the quadriceps muscle and the patellar tendon. A larger Q angle increases lateral force on the kneecap, and females typically have a larger Q angle than males, which may contribute to increased instability risk.

Patients with kneecap instability commonly report pain around the kneecap and a sensation that it may slip out of place. Swelling may be present, and some patients require crutches for walking. During examination, the orthopedic provider evaluates kneecap alignment, stability, and tracking through the groove.

X-rays are obtained to assess kneecap position relative to the femur and tibia and to ensure the kneecap is centered within the groove. If instability is suspected, an MRI is ordered to evaluate the soft tissues, including the MPFL, and to assess joint swelling or associated injury.

Treatment decisions depend on whether the instability resulted from a one-time injury or from repeated dislocations, as well as the patient’s anatomy and symptoms. These factors should be discussed with an orthopedic provider to determine the most appropriate treatment plan for each individual.