Knee Cap Shifting: Causes, Symptoms, and Treatment

What Causes a Knee Cap to Shift?

Patellar shifting, ranging from subtle maltracking to complete dislocation, primarily results from a complex interplay of anatomical predispositions, muscle imbalances, ligamentous laxity, and acute traumatic events that compromise the inherent stability of the patellofemoral joint.

Understanding the Patella and Knee Joint

The knee joint is a complex hinge joint, and the patella, or kneecap, is a crucial component of its extensor mechanism. The patella is a sesamoid bone embedded within the quadriceps tendon, sitting in a groove on the end of the femur called the trochlear groove. As the knee bends and straightens, the patella glides smoothly within this groove.

Key structures maintaining patellar stability include:

• The Trochlear Groove: The depth and shape of this groove are primary determinants of patellar tracking.
• Quadriceps Muscles: The four muscles of the quadriceps (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis) converge to form the quadriceps tendon, which encases the patella and attaches to the tibia via the patellar ligament. The vastus medialis obliquus (VMO), the innermost part of the vastus medialis, plays a critical role in pulling the patella medially (inward) and preventing lateral (outward) displacement.
• Ligaments and Retinacula: The medial patellofemoral ligament (MPFL) is the primary passive restraint against lateral patellar displacement. The lateral retinaculum provides lateral stability.

What Does "Shifting" Mean? Defining Patellar Instability

When we talk about a knee cap "shifting," we are referring to various degrees of patellar instability:

• Patellar Maltracking: This is a subtle deviation where the patella does not glide optimally within the trochlear groove. It might not fully displace but can cause pain, grinding sensations, and anterior knee pain (patellofemoral pain syndrome).
• Patellar Subluxation: This involves a partial or incomplete displacement of the patella from the trochlear groove. The patella partially slips out, usually laterally, but spontaneously returns to its normal position without external intervention. This often feels like the knee "gives way."
• Patellar Dislocation: This is a complete displacement of the patella from the trochlear groove. The patella typically shifts completely to the outside (lateral aspect) of the knee, resulting in a visible deformity. Dislocation usually requires manual reduction (pushing it back into place) or can spontaneously reduce with significant force.

Common Causes of Patellar Instability

The causes of patellar shifting are often multifactorial, involving a combination of anatomical, muscular, and traumatic factors.

Anatomical Factors

These are structural variations that predispose an individual to patellar instability:

• Shallow Trochlear Groove (Trochlear Dysplasia): If the groove on the femur is too flat or shallow, it provides less of a bony constraint for the patella, making it easier for it to slide out.
• Patella Alta (High-Riding Patella): When the patella sits unusually high on the femur, it engages the trochlear groove later in knee flexion, making it more vulnerable to displacement in the early range of motion.
• Genu Valgum ("Knock-Knees"): This alignment increases the "Q-angle" (the angle between the quadriceps pull and the patellar tendon), leading to an increased lateral pull on the patella.
• External Tibial Torsion: An outward twist of the tibia relative to the femur can contribute to malalignment and lateral patellar stress.
• Increased Femoral Anteversion: An inward twist of the femur can also alter the Q-angle and patellar mechanics.

Muscular Imbalances

Dysfunction or imbalance in the muscles surrounding the knee and hip can significantly impact patellar tracking:

• Vastus Medialis Obliquus (VMO) Weakness/Inhibition: If the VMO is weak or not firing effectively, it cannot adequately counteract the lateral pull of the vastus lateralis and other lateral structures, allowing the patella to shift outward.
• Tight Lateral Structures: Overly tight vastus lateralis, iliotibial band (IT band), or lateral retinaculum can exert excessive lateral tension, pulling the patella out of its groove.
• General Quadriceps Weakness: Insufficient strength in the quadriceps can compromise the dynamic stability of the patella.
• Hip Abductor and External Rotator Weakness: Weakness in these hip muscles can lead to increased hip adduction and internal rotation during movement, causing the knee to collapse inward (dynamic valgus), which increases the Q-angle and lateral stress on the patella.

Ligamentous Laxity

The passive restraints of the knee can be compromised:

• Medial Patellofemoral Ligament (MPFL) Insufficiency or Rupture: The MPFL is the primary ligamentous stabilizer preventing lateral patellar displacement. It is frequently torn during a first-time patellar dislocation, making subsequent dislocations more likely.
• Generalized Ligamentous Laxity: Some individuals naturally have more flexible joints due to genetic factors (e.g., Ehlers-Danlos syndrome), which can contribute to overall joint instability, including the patella.

Traumatic Events

Acute trauma is a common cause, especially for first-time dislocations:

• Direct Blow: A direct impact to the patella, particularly from the medial side, can force it laterally out of its groove.
• Twisting Injury: A sudden twisting motion of the knee while the foot is planted, especially with the knee slightly bent and a valgus (knock-knee) force, can cause the patella to dislocate. This is common in sports.

Risk Factors for Patellar Shifting

Several factors can increase an individual's risk of experiencing patellar shifting:

• Previous History: A history of patellar subluxation or dislocation is the strongest predictor of future episodes.
• Female Gender: Females tend to have a wider pelvis, which can increase the Q-angle, placing more lateral stress on the patella.
• Adolescence and Young Adulthood: This age group is particularly susceptible due to rapid growth, sports participation, and potentially developing anatomical structures.
• Participation in High-Risk Sports: Sports involving pivoting, jumping, cutting, or sudden changes in direction (e.g., basketball, soccer, gymnastics, dance) carry a higher risk.
• Genetic Predisposition: A family history of patellar instability or generalized joint laxity.

Recognizing the Signs and Symptoms

If your kneecap shifts, you may experience:

• Sudden, Sharp Pain: Often intense at the moment of displacement.
• Feeling of "Popping Out" or "Giving Way": A distinct sensation of the kneecap moving out of place.
• Visible Deformity: In the case of a full dislocation, the kneecap will be visibly displaced to the side of the knee.
• Swelling and Bruising: Develops rapidly due to tissue damage and bleeding within the joint.
• Inability to Straighten or Bend the Knee: Movement becomes painful and limited.
• Apprehension: A feeling of instability or fear that the kneecap will shift again, especially with certain movements.

Diagnosis and Treatment

Diagnosis typically involves a thorough clinical examination, including assessing knee alignment, muscle strength, and range of motion. Imaging studies such as X-rays (to check for bony abnormalities or fractures) and MRI (to evaluate soft tissue injuries like MPFL tears or cartilage damage) are often used.

Treatment varies based on the severity and underlying cause. Initial management for acute dislocations often involves RICE (Rest, Ice, Compression, Elevation) and reduction of the patella. Long-term management focuses on:

• Physical Therapy: Strengthening the VMO and hip abductor/external rotator muscles, stretching tight lateral structures, and improving proprioception and neuromuscular control.
• Bracing: May be used to provide support and limit range of motion during recovery.
• Surgery: In cases of recurrent dislocations, significant anatomical abnormalities, or persistent instability, surgical intervention (e.g., MPFL reconstruction, trochleoplasty, tibial tubercle transfer) may be considered.

Prevention and Management Strategies

While some anatomical factors are unchangeable, many causes of patellar shifting can be addressed through targeted strategies:

• Targeted Strengthening: Focus on strengthening the VMO, hip abductors, and external rotators to improve dynamic knee stability.
• Flexibility and Mobility: Maintain good flexibility in the hamstrings, quadriceps, and IT band to prevent excessive tension that can pull the patella laterally.
• Neuromuscular Control and Balance Training: Improve coordination and body awareness to enhance dynamic stability during movement, especially in sports-specific activities.
• Proper Biomechanics: Learn and practice correct movement patterns during exercise and sports to minimize stress on the knee joint.
• Appropriate Footwear: Wear supportive shoes that promote proper foot and ankle mechanics, as imbalances here can affect the kinetic chain up to the knee.

When to Seek Professional Help

If you experience a suspected patellar dislocation, severe pain, significant swelling, inability to bear weight, or persistent instability, it is crucial to seek immediate medical attention from a healthcare professional or an orthopedic specialist. Early diagnosis and appropriate management can help prevent further damage and reduce the risk of recurrent episodes.