Patellar Instability

The kneecap or patellar sits in a groove on the thigh bone and moves smoothly up and down as you bend or straighten your knee. Patellar instability occurs when the patella deviates from its position in the groove—a groove on the thigh bone that guides the patella’s movement during knee flexion and extension. Instability can range from a mild partial dislocation (subluxation), where the patella shifts but remains partially within the groove, to full dislocations, where the patella exits the groove entirely.

Patellar instability results from recurring subluxation or dislocation. It is common is some adolescent women and some adults. When the kneecap comes out of its track it can usually resume its normal position, but sometimes it requires an orthopedic surgeon to reposition the kneecap. Repeated painful kneecap dislocations cause chronic patellar instability which can lead to arthritis. Patellar instability is most common in people in their teens and twenties.

The symptoms of patellar instability can vary depending on the severity and frequency of the instability episodes. Common symptoms include:

1. Knee Pain: Pain is often localized to the front of the knee and can be exacerbated by activities like running, jumping, or climbing stairs.
2. Swelling: Swelling may occur, particularly after a traumatic dislocation or repeated episodes of instability.
3. A Sensation of Giving Way: Many patients describe a feeling that their knee is unstable or at risk of “giving out” during activity.
4. Reduced Range of Motion: Stiffness and difficulty fully bending or straightening the knee may be present.
5. Visual Deformity: In cases of dislocation, the patella may appear visibly out of place, often shifted laterally.
6. Catching or Locking: Some patients report mechanical symptoms like the knee catching or locking during movement.

Patellar instability arises from a combination of anatomical, biomechanical, and traumatic factors. Understanding the underlying cause is crucial for determining the appropriate course of treatment. Common causes include:

1. Trauma: A direct blow to the knee or a sudden twisting motion can force the patella out of the trochlear groove, often leading to an acute dislocation.
2. Sports: High impact sports like football and activities that require quick changes of direction as found in basketball and soccer can increase the risk of patellar instability.
3. Structural Abnormalities:
   • Trochlear Dysplasia: The groove on the upper leg bone is too shallow or flat which can allow the kneecap to fall out of its groove and cause instability.
   • Patella Alta: A medical term meaning the kneecap sits higher than normal in relation to the thigh bone increasing the risk of instability.
   • Lateralized Patella: When the kneecap drifts outward causing instability.
   • Misaligned Hips: Women are more likely to experience patellar instability because of their wider hips.
4. Ligament Laxity: Generalized hypermobility or weakness of the medial patellofemoral ligament (MPFL)—a key stabilizer of the patella—can contribute to instability.
5. Muscle Imbalances: Weakness in the quadriceps, or tightness in hip flexors can contribute to patellar instability.
6. Recurrent Instability: After an initial dislocation, the risk of recurrent instability increases, particularly in younger patients or those with inadequate healing of the MPFL.

Diagnosing patellar instability involves a combination of clinical evaluation, patient history, and imaging studies. A thorough and individualized approach ensures patients receive an accurate diagnosis and appropriate care.

1. Patient History: The diagnostic process begins with understanding the patient’s symptoms, the mechanism of injury, and the frequency of instability episodes. Patients often report a “popping” sensation during an episode, followed by pain and swelling.
2. Physical Examination: The physical exam assesses:
   • Patellar alignment and mobility.
   • Signs of ligamentous laxity or muscle weakness.
   • Pain or apprehension during specific tests, such as the patellar apprehension test, which evaluates the tendency of the patella to dislocate laterally.
3. Imaging Studies:
   • X-rays: Standard radiographs help assess patellar position, trochlear groove anatomy, and any associated fractures.
   • MRI: MRI is particularly useful for visualizing soft tissue structures, including the MPFL, and identifying injuries such as cartilage damage or bone bruising.
   • CT scans: In cases where bony abnormalities are suspected, CT imaging can provide detailed information about the trochlear groove and patellar tracking.
4. Functional Testing: Evaluation of gait and movement patterns can reveal underlying biomechanical contributors, such as muscle imbalances or improper joint mechanics.