Patellofemoral Pain Syndrome: Key Structures, Muscular Influences, and Biomechanics

What structures are involved in patellofemoral pain syndrome?

Patellofemoral Pain Syndrome (PFPS) involves a complex interplay of the patella (kneecap), femur (thigh bone), surrounding soft tissues, and the muscles that control knee and hip motion, all contributing to painful tracking issues within the knee joint.

Understanding Patellofemoral Pain Syndrome (PFPS)

Patellofemoral Pain Syndrome, often referred to as "runner's knee" or "anterior knee pain," is a common overuse injury characterized by pain around or behind the kneecap. It arises from irritation of the tissues where the patella glides over the end of the femur during knee flexion and extension. While the exact cause is often multifactorial, it fundamentally stems from improper tracking of the patella within the trochlear groove of the femur, leading to increased stress and friction on the articular cartilage and surrounding soft tissues.

The Core Structures: The Patellofemoral Joint

The primary structures directly involved in PFPS are those that form the patellofemoral joint itself:

• Patella (Kneecap): This is a sesamoid bone embedded within the quadriceps tendon. Its primary role is to act as a fulcrum, increasing the mechanical advantage of the quadriceps muscle by extending the lever arm of the knee joint. In PFPS, the patella often tracks improperly, shifting laterally or tilting, which increases pressure on specific areas of its undersurface.
• Femur (Thigh Bone): Specifically, the distal (lower) end of the femur features a specialized groove known as the trochlear groove. The patella is designed to glide smoothly within this groove as the knee bends and straightens. Deviations in patellar tracking, often due to muscular imbalances or structural abnormalities, can cause the patella to rub against the edges of this groove, leading to pain and cartilage wear.
• Articular Cartilage: Both the undersurface of the patella and the surface of the trochlear groove are covered with a layer of smooth, slippery articular cartilage. This cartilage reduces friction and absorbs shock during movement. In PFPS, repetitive abnormal loading or friction can lead to softening (chondromalacia patellae) or erosion of this cartilage, contributing to pain.

Supporting Structures: Ligaments and Retinacula

These soft tissue structures help to stabilize the patella and guide its movement:

• Medial Patellofemoral Ligament (MPFL): This is the primary medial stabilizer of the patella, preventing excessive lateral displacement. Weakness or injury to the MPFL can contribute to patellar instability and lateral tracking.
• Lateral Patellar Retinaculum: A strong fibrous band on the lateral (outer) side of the patella that connects it to the iliotibial band and vastus lateralis muscle. Tightness in this retinaculum is a common finding in PFPS, pulling the patella laterally and increasing pressure on the lateral facet of the patella.
• Medial Patellar Retinaculum: A less robust fibrous band on the medial (inner) side. Weakness or laxity here, often in conjunction with lateral tightness, contributes to imbalanced patellar tracking.
• Patellar Ligament (Patellar Tendon): This strong ligament connects the inferior pole of the patella to the tibial tuberosity (a bony bump on the shin bone). While not directly involved in tracking, inflammation or tendinopathy in this structure can co-exist with PFPS or mimic its symptoms.

Muscular Influences: The Quadriceps Femoris and Beyond

Muscular strength, balance, and coordination play a crucial role in patellar tracking. Imbalances are a major contributor to PFPS.

• Quadriceps Femoris: This group of four muscles on the front of the thigh is essential for knee extension and patellar control.
  • Vastus Medialis Obliquus (VMO): The most medial and distal part of the vastus medialis, its fibers pull the patella medially. Weakness or delayed activation of the VMO relative to the vastus lateralis is a key factor in poor patellar tracking in PFPS.
  • Vastus Lateralis: The large muscle on the lateral side of the thigh. Overactivity or dominance of the vastus lateralis can pull the patella laterally, exacerbating tracking issues.
  • Rectus Femoris & Vastus Intermedius: These muscles contribute to overall quadriceps strength and knee extension, but their direct influence on patellar tracking is less specific than the VMO and vastus lateralis.
• Hip Musculature: The strength and control of the muscles surrounding the hip significantly influence the alignment and rotation of the femur, which in turn affects patellar tracking.
  • Gluteus Medius & Minimus: These hip abductors are critical for stabilizing the pelvis and preventing excessive hip adduction (knees knocking inwards) and internal femoral rotation during weight-bearing activities. Weakness here can lead to dynamic knee valgus, increasing stress on the patellofemoral joint.
  • Gluteus Maximus: A powerful hip extensor and external rotator. Weakness can contribute to internal femoral rotation and poor lower limb alignment.
  • Tensor Fasciae Latae (TFL) & Iliotibial Band (ITB): The TFL muscle connects to the ITB, a thick band of fascia running down the lateral thigh to the tibia. Tightness in the TFL/ITB can exert a strong lateral pull on the patella via the lateral patellar retinaculum, contributing to lateral tracking and increased lateral pressure.

Other Contributing Structures

While less commonly the primary source of pain than the joint itself or muscular imbalances, other structures can become irritated:

• Synovial Plica: These are normal folds in the joint lining (synovium) within the knee. If a plica becomes thickened or inflamed due to repetitive irritation, it can get pinched between the patella and femur, causing pain.
• Hoffa's Fat Pad (Infrapatellar Fat Pad): This is a cushion of fatty tissue located below the patella and behind the patellar ligament. It can become impinged or inflamed, especially with hyperextension or direct trauma, contributing to anterior knee pain.
• Nerves: Small sensory nerves around the patellofemoral joint can become irritated or compressed, transmitting pain signals.

Biomechanical Considerations and Interplay

PFPS is rarely due to a single structural issue but rather a combination of factors that disrupt the delicate biomechanics of the lower limb. The structures listed above are interconnected within the kinetic chain. For example, poor foot pronation (flat feet) can lead to internal tibial rotation, which in turn can cause internal femoral rotation if the hip stabilizers are weak. This altered femoral position directly impacts how the patella tracks in the trochlear groove, increasing stress and leading to pain.

Conclusion

Understanding the diverse array of structures involved in Patellofemoral Pain Syndrome—from the direct joint components to the intricate network of surrounding ligaments, retinacula, and especially the influential muscular groups of the thigh and hip—is crucial for effective assessment and rehabilitation. Addressing imbalances and dysfunctions across these interconnected structures, rather than focusing solely on the knee, is key to successful management and long-term relief from PFPS.