Patellar Subluxation: Understanding, Causes, Symptoms, Diagnosis, and Treatment

What is Subluxation of the Patella?

Patellar subluxation refers to a partial and temporary dislocation of the kneecap (patella) from its normal position within the trochlear groove of the thigh bone (femur), often causing pain, instability, and a sensation of the knee "giving way."

Understanding the Patella and Knee Joint

To comprehend patellar subluxation, it's essential to understand the basic anatomy and mechanics of the knee joint. The knee is a complex hinge joint formed by three bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). The patella is a sesamoid bone embedded within the quadriceps tendon, sitting in a groove at the end of the femur called the trochlear groove. Its primary role is to enhance the leverage of the quadriceps muscle, facilitating knee extension.

For proper function, the patella must track smoothly within the trochlear groove as the knee bends and straightens. This tracking is influenced by the shape of the trochlear groove, the alignment of the leg bones, and the balanced pull of the quadriceps muscles, particularly the vastus medialis obliquus (VMO) which helps pull the patella medially (inward), counteracting the lateral (outward) pull of the vastus lateralis and the iliotibial (IT) band.

What is Patellar Subluxation?

Patellar subluxation occurs when the patella partially slips out of its trochlear groove, usually shifting laterally (towards the outside of the knee). Unlike a complete patellar dislocation, where the kneecap fully moves out of the groove and typically requires manual reduction, a subluxation often spontaneously reduces (snaps back into place) on its own.

While it may reduce quickly, the event itself can cause significant pain, damage to the supporting ligaments (especially the medial patellofemoral ligament, MPFL), and cartilage within the joint. Subluxation can be a one-time acute event, but it frequently becomes recurrent, particularly if underlying anatomical or muscular imbalances are not addressed.

Common Causes and Risk Factors

Several factors can predispose an individual to patellar subluxation, ranging from anatomical variations to muscular imbalances and acute trauma.

• Anatomical Factors:

  • Shallow Trochlear Groove (Trochlear Dysplasia): If the groove at the end of the femur is too shallow or flat, it provides less stability for the patella.
  • Patella Alta (High-Riding Patella): When the patella sits higher than normal, it may not engage properly in the trochlear groove until the knee is significantly flexed, making it more prone to slipping out during initial flexion.
  • Genu Valgum (Knock-Knees): This alignment issue increases the "Q-angle" (quadriceps angle), which is the angle between the quadriceps tendon and the patellar tendon. A larger Q-angle increases the lateral pull on the patella.
  • Increased Tibial Torsion: An outward twist of the shin bone can also contribute to an increased Q-angle.
  • Ligamentous Laxity: Generalized joint hypermobility or specific laxity of the medial patellofemoral ligament (MPFL), a key stabilizer on the inner side of the knee, can make subluxation more likely.

• Muscular Imbalances:

  • Weak Vastus Medialis Obliquus (VMO): The VMO is crucial for pulling the patella medially. Weakness relative to the vastus lateralis can lead to an imbalanced pull, favoring lateral displacement.
  • Tight Lateral Retinaculum or IT Band: A tight band of connective tissue on the outer side of the knee can pull the patella laterally, contributing to poor tracking.
  • Weak Hip Abductors and External Rotators: Weakness in these hip muscles can lead to compensatory movements at the knee, increasing stress and maltracking.

• Trauma or Activity:

  • Direct Blow: A direct impact to the inside of the knee can force the patella laterally.
  • Sudden Twisting or Pivoting Motions: Activities common in sports like basketball, soccer, or dance, involving a sudden change in direction while the foot is planted, can cause the patella to sublux.
  • Repetitive Stress: While less common for acute subluxation, chronic poor tracking can contribute to instability over time.

Signs and Symptoms

The symptoms of patellar subluxation can vary depending on the severity and frequency of the event.

• Acute Pain: Often sharp and sudden, typically felt in the front of the knee, particularly on the outer side.
• Sensation of Instability or "Giving Way": The feeling that the knee is buckling or about to collapse.
• Swelling: Fluid accumulation around the knee joint, often occurring hours after the event.
• Clicking, Popping, or Grinding Sensation: May be heard or felt as the patella slips out and back in.
• Difficulty Bending or Straightening the Knee: Due to pain, swelling, or mechanical blockage.
• Visible Displacement: In some cases, the patella may appear to have shifted slightly to the side.
• Apprehension: A feeling of fear or reluctance to move the knee into positions that might cause subluxation.

Diagnosis

A thorough medical evaluation is crucial for diagnosing patellar subluxation and identifying contributing factors.

• Physical Examination: A healthcare professional will assess the knee's range of motion, palpate for tenderness, check for swelling, and perform specific tests like the patellar apprehension test, where gentle pressure is applied to the patella to mimic a subluxation, eliciting apprehension from the patient. They will also evaluate leg alignment, muscle strength, and flexibility.
• Imaging Studies:
  • X-rays: Used to evaluate bone alignment, identify patella alta, and assess the depth of the trochlear groove (often specific views like a Merchant or Sunrise view are taken to look at the patella from above).
  • Magnetic Resonance Imaging (MRI): Provides detailed images of soft tissues, including ligaments (especially the MPFL), tendons, cartilage, and any bone bruising that may have occurred during the subluxation. It's excellent for assessing acute damage.
  • Computed Tomography (CT) Scan: Can provide precise measurements of bony anatomy, including trochlear dysplasia and tibial tuberosity-trochlear groove (TT-TG) distance, which helps assess alignment.

Treatment Approaches

Treatment for patellar subluxation typically begins with conservative measures, with surgery reserved for recurrent cases or significant structural damage.

• Conservative Management:

  • R.I.C.E. Protocol: Rest, Ice, Compression, and Elevation immediately after an acute episode to reduce pain and swelling.
  • Pain Management: Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) can help manage pain and inflammation.
  • Physical Therapy: This is the cornerstone of conservative treatment. A physical therapist will design a program focusing on:
    • Strengthening: Targeting the vastus medialis obliquus (VMO), hip abductors, hip external rotators, and core muscles to improve knee stability and patellar tracking.
    • Stretching: Addressing tightness in the hamstrings, quadriceps, IT band, and lateral retinaculum.
    • Proprioception and Balance Training: Exercises to improve the body's awareness of joint position and enhance neuromuscular control.
    • Taping or Bracing: McConnell taping techniques can temporarily improve patellar tracking, and specific knee braces can provide external support during recovery or activity.
    • Activity Modification: Temporarily avoiding activities that exacerbate symptoms.

• Surgical Intervention: For individuals with recurrent subluxations, significant ligamentous damage, or severe anatomical abnormalities that don't respond to conservative treatment, surgery may be considered.

  • Medial Patellofemoral Ligament (MPFL) Reconstruction: The MPFL is the primary soft tissue restraint against lateral patellar displacement. This procedure reconstructs the torn ligament, often using a hamstring graft.
  • Tibial Tubercle Osteotomy (TTO): Involves cutting and repositioning the bony attachment of the patellar tendon (tibial tubercle) to improve the pull on the patella and centralize its tracking.
  • Trochleoplasty: A procedure to deepen or reshape a shallow trochlear groove to provide a more stable track for the patella.

Prevention and Long-Term Management

Preventing recurrent subluxations and managing long-term knee health involves ongoing commitment to physical conditioning and awareness of body mechanics.

• Consistent Strength and Flexibility Training: Maintain a balanced strength program focusing on all quadriceps heads (especially VMO), glutes, hamstrings, and core. Incorporate regular stretching for flexibility.
• Proper Warm-up and Cool-down: Prepare muscles for activity and aid recovery.
• Gradual Progression: Avoid sudden increases in training intensity, volume, or new activities.
• Address Biomechanical Issues: Work with a physical therapist to identify and correct any underlying alignment or movement patterns that contribute to patellar instability.
• Appropriate Footwear and Technique: Ensure proper footwear for activities and focus on good form during exercise and sports.
• Adherence to Rehabilitation Protocols: If an injury occurs, diligently follow prescribed rehabilitation exercises to restore full strength and function.

When to Seek Medical Attention

If you experience symptoms of patellar subluxation, it's advisable to consult a healthcare professional, such as an orthopedic surgeon or sports medicine physician.

• Acute Injury: If you experience sudden, significant knee pain, swelling, or a sensation of your kneecap shifting.
• Recurrent Instability: If your kneecap frequently feels like it's giving way or partially dislocating.
• Inability to Bear Weight: If you cannot put weight on your affected leg.
• Persistent Symptoms: If pain, swelling, or instability continues despite initial rest and conservative measures.

Early diagnosis and appropriate management are key to preventing further damage and ensuring a return to activity with a stable and functional knee.