Knee Anatomy: Structures Behind the Patella and Common Issues

What is behind the knee cap?

Behind the knee cap, or patella, lies the crucial articulation point where the patella glides within a groove on the thigh bone (femur), supported by cartilage, synovial fluid, and a protective fat pad, forming the essential patellofemoral joint.

Understanding the Knee Joint: A Brief Overview

The knee is one of the largest and most complex joints in the human body, primarily functioning as a hinge joint that allows for flexion and extension, with a small degree of rotation. It's formed by the articulation of three bones: the thigh bone (femur), the shin bone (tibia), and the knee cap (patella). The patella, a sesamoid bone embedded within the quadriceps tendon, serves as a crucial mechanical pulley, enhancing the leverage of the quadriceps muscles and increasing the force they can generate for activities like walking, running, and jumping. Its position and movement are critical for efficient knee function.

Key Structures Directly Behind the Patella

When we refer to what is "behind" the patella, we are primarily discussing the components of the patellofemoral joint and the immediate structures that facilitate its smooth operation.

• Femoral Trochlear Groove: This is the specific groove on the anterior (front) aspect of the distal femur. The patella is designed to glide smoothly within this concave depression as the knee bends and straightens. The shape and depth of this groove are critical for patellar tracking.
• Articular Cartilage: Both the posterior (back) surface of the patella and the surface of the femoral trochlear groove are covered with a layer of hyaline cartilage. This incredibly smooth, resilient tissue reduces friction between the bones during movement, allowing for effortless gliding, and acts as a shock absorber. Damage to this cartilage (e.g., chondromalacia patellae) can lead to pain and impaired function.
• Synovial Fluid and Capsule: The entire knee joint, including the patellofemoral compartment, is encased within a synovial capsule. This capsule contains synovial fluid, a viscous, egg-white-like substance that lubricates the articular surfaces, nourishes the cartilage, and further reduces friction during movement.
• Infrapatellar Fat Pad (Hoffa's Fat Pad): Located beneath the patella and behind the patellar tendon, this is a soft, highly innervated tissue. It acts as a cushion, filling the space within the joint and protecting underlying structures. Inflammation or impingement of the infrapatellar fat pad (Hoffa's syndrome) can be a source of anterior knee pain.

Deeper Structures of the Knee (Posterior to the Patellar Region)

While not directly behind the patella in the immediate sense, several other vital structures lie deeper within the knee joint, posterior to the patellofemoral complex. These are fundamental to overall knee stability and function.

• Cruciate Ligaments: These are two strong, fibrous bands located deep within the knee joint, crossing each other in an "X" shape.
  • Anterior Cruciate Ligament (ACL): Prevents the tibia from sliding too far forward relative to the femur and limits rotational movements.
  • Posterior Cruciate Ligament (PCL): Prevents the tibia from sliding too far backward relative to the femur.
• Menisci: These are two C-shaped wedges of fibrocartilage (medial and lateral menisci) that sit on top of the tibia. They act as shock absorbers, distribute weight across the joint, and improve the congruency (fit) between the rounded femoral condyles and the relatively flat tibial plateau.
• Popliteus Muscle and Tendon: A small, deep muscle located at the back of the knee. Its primary role is to "unlock" the knee from full extension by internally rotating the tibia, allowing for knee flexion to begin. Its tendon runs posterior to the lateral meniscus.
• Bursae: Small, fluid-filled sacs that reduce friction between bones, tendons, and muscles. Several bursae are located around the knee, including the prepatellar bursa (in front of the patella), infrapatellar bursa (below the patella), and pes anserine bursa (on the medial side below the knee).
• Neurovascular Structures: Essential blood vessels and nerves pass through the popliteal fossa (the space behind the knee).
  • Popliteal Artery and Vein: Major blood vessels supplying and draining the lower leg.
  • Tibial Nerve and Common Fibular Nerve: Branches of the sciatic nerve that innervate muscles and provide sensation to the lower leg and foot.

The Patellofemoral Joint: Function and Common Issues

The intricate interplay of the structures behind the patella is vital for normal knee function. The patella's primary role is to optimize the leverage of the quadriceps, allowing for powerful extension of the knee.

Common Issues Related to Structures Behind the Patella:

• Patellofemoral Pain Syndrome (PFPS): Often referred to as "runner's knee" or "anterior knee pain," this condition is characterized by pain around or behind the patella, particularly during activities that load the knee joint, such as squatting, stair climbing, or prolonged sitting with bent knees. It's often attributed to issues with patellar tracking, muscle imbalances (e.g., weak vastus medialis obliquus, tight IT band), or overuse.
• Chondromalacia Patellae: This refers to the softening and breakdown of the articular cartilage on the underside of the patella. While often associated with PFPS, it specifically describes the state of the cartilage itself and can range from minor softening to severe erosion.
• Patellar Instability/Dislocation: If the patella repeatedly slips out of the trochlear groove, it indicates patellar instability. This can be due to anatomical variations (e.g., a shallow trochlear groove), ligamentous laxity, or muscle imbalances.
• Infrapatellar Fat Pad Impingement (Hoffa's Syndrome): As mentioned, inflammation or compression of the fat pad can cause pain and tenderness directly behind or below the patella.

Importance for Movement and Health

Understanding the structures behind the knee cap is paramount for anyone involved in fitness, sports, or rehabilitative exercise. Proper alignment, muscular balance, and controlled movement patterns are essential to maintain the health of the articular cartilage, prevent excessive stress on the patellofemoral joint, and avoid conditions like PFPS. For fitness enthusiasts and trainers, this knowledge informs:

• Exercise Selection: Choosing exercises that promote healthy patellar tracking and minimize undue stress.
• Technique Correction: Identifying and correcting movement patterns that contribute to patellar issues.
• Injury Prevention: Implementing strategies to strengthen supporting musculature and improve flexibility.
• Rehabilitation: Guiding individuals through recovery from knee injuries by targeting specific anatomical structures.

By appreciating the complex anatomy behind the knee cap, we can better understand the mechanics of knee movement and implement effective strategies for pain prevention, performance enhancement, and long-term joint health.