Medial Patellofemoral Ligament (MPFL): Mechanism of Injury, Causes, and Management

What is the mechanism of injury for medial patellofemoral ligament?

The primary mechanism of injury for the medial patellofemoral ligament (MPFL) is a lateral patellar dislocation, typically resulting from a combination of knee valgus, external tibial rotation, and strong quadriceps contraction, which places excessive tensile stress on the ligament, leading to tearing.

Understanding the Medial Patellofemoral Ligament (MPFL)

The medial patellofemoral ligament (MPFL) is a crucial soft tissue structure located on the inner (medial) side of the knee. It plays a pivotal role in patellar stability, acting as the primary restraint against lateral displacement of the kneecap (patella).

• Anatomy and Location: The MPFL originates from the medial femoral epicondyle, specifically between the adductor tubercle and the medial epicondyle, and inserts onto the superomedial aspect of the patella. It is a thin, flat ligament often described as a thickening of the joint capsule, lying deep to the vastus medialis obliquus (VMO) muscle fibers.
• Primary Function: Its anatomical orientation allows it to resist lateral forces on the patella, particularly when the knee is in extension or early flexion (0-30 degrees). It contributes approximately 50-60% of the total medial restraining force against lateral patellar translation, making it indispensable for maintaining the patella's proper alignment within the trochlear groove of the femur.

The Primary Mechanism of MPFL Injury: Lateral Patellar Dislocation

The overwhelming majority of MPFL injuries occur during an acute lateral patellar dislocation. This event describes the patella completely exiting the trochlear groove and moving to the outside (lateral side) of the knee.

• The Event: A lateral patellar dislocation is typically a non-contact injury, often precipitated by a sudden change in direction, an awkward landing, or a direct blow to the patella. The mechanics involve a complex interplay of forces that collectively overwhelm the MPFL's capacity to resist lateral translation.
• Specific Forces and Biomechanics:
  • Knee Valgus: An inward collapse of the knee, placing the tibia in an abducted position relative to the femur. This creates a "knock-knee" posture.
  • External Tibial Rotation: The lower leg rotates outwards relative to the thigh.
  • Quadriceps Contraction: A strong, often eccentric, contraction of the quadriceps muscle, particularly the rectus femoris, which pulls the patella superiorly and laterally.
  • Combined Effect: The combination of knee valgus and external tibial rotation while the quadriceps is contracting creates a powerful lateral vector on the patella. As the patella is forced out of the trochlear groove, the MPFL is put under extreme tension. This excessive tensile load, exceeding the ligament's elastic limits, results in its tearing or avulsion. The injury typically occurs as the knee moves from an extended position into slight flexion (around 20-30 degrees), where the MPFL is the primary stabilizer.

Contributing Factors and Risk Factors

While an acute traumatic event is the direct cause, several underlying anatomical and biomechanical factors can predispose an individual to MPFL injury and recurrent dislocations.

• Anatomical Predispositions:
  • Trochlear Dysplasia: A shallow or flattened trochlear groove, which normally cradles the patella, reduces bony stability.
  • Patella Alta: A high-riding patella, meaning it sits higher than normal, making it less engaged in the trochlear groove, especially in early flexion.
  • Genu Valgum: A "knock-knee" alignment, which naturally increases the lateral pull on the patella.
  • Increased Q-Angle: A larger angle between the quadriceps tendon and the patellar tendon, indicating a greater lateral pull on the patella.
• Muscular Imbalances:
  • Vastus Medialis Obliquus (VMO) Weakness: The VMO is a part of the quadriceps that pulls the patella medially. Weakness or delayed activation can contribute to lateral instability.
  • Tight Lateral Retinaculum/Iliotibial Band (ITB): Tightness in these structures can increase the lateral pull on the patella.
• Ligamentous Laxity: Generalized joint hypermobility or inherent laxity in the knee ligaments can reduce overall stability.
• Biomechanical Factors:
  • Foot Pronation: Excessive inward rolling of the foot can contribute to internal tibial rotation and knee valgus.
  • Poor Landing Mechanics: Landing with knees valgus or excessive external rotation during sports can increase risk.

Spectrum of MPFL Injury

MPFL injuries can range in severity, from partial tears to complete ruptures, and can also involve bony avulsions.

• Partial Tear: Some fibers of the MPFL are torn, but the ligament remains largely intact. This may result in pain and swelling but less instability.
• Complete Tear: The MPFL is fully ruptured, leading to significant instability and often accompanied by a history of patellar dislocation.
• Avulsion Fracture: Instead of tearing through the ligament fibers, the MPFL pulls a fragment of bone from its attachment site (either the patella or the femur). This is common, particularly on the femoral side, and can be seen on imaging.

Clinical Implications and Management Considerations

Understanding the mechanism of MPFL injury is crucial for accurate diagnosis and effective management.

• Diagnosis: Clinical examination, including assessment of patellar stability and apprehension, is key. Imaging, such as X-rays (to assess for patella alta, trochlear dysplasia, and avulsion fractures) and MRI (to visualize the MPFL, cartilage damage, and bone bruising), confirms the diagnosis and extent of injury.
• Treatment Principles: Management depends on the severity of the injury, the presence of predisposing factors, and the patient's activity level.
  • Conservative Management: For first-time dislocations without significant predisposing factors, non-surgical approaches involving immobilization, pain management, and a structured physical therapy program focusing on quadriceps strengthening (especially VMO), hip abductor strength, and proprioception are often employed.
  • Surgical Intervention: Recurrent dislocations, significant anatomical abnormalities (e.g., severe trochlear dysplasia, patella alta), or specific types of MPFL tears (e.g., femoral avulsion) may warrant surgical repair or reconstruction of the MPFL, sometimes combined with other procedures to address underlying issues.
• Rehabilitation Focus: Post-injury rehabilitation aims to restore knee function, strengthen surrounding musculature, improve proprioception, and correct biomechanical deficiencies to prevent recurrence.

Prevention Strategies

While not all patellar dislocations can be prevented, addressing modifiable risk factors can reduce the likelihood of MPFL injury.

• Targeted Strengthening: Focusing on the VMO, hip abductors, and core musculature to improve dynamic knee stability.
• Proprioceptive Training: Balance and agility exercises to enhance neuromuscular control and reaction time.
• Biomechanical Assessment and Correction: Identifying and addressing issues like excessive foot pronation or poor landing mechanics through orthotics, footwear, or movement pattern retraining.
• Sport-Specific Drills: Incorporating drills that emphasize proper jumping, landing, cutting, and pivoting techniques to mitigate injury risk in athletic populations.