Knee Joint: Understanding the Patellofemoral Plane Joint

What is the plane joint in the knee?

While the knee is primarily classified as a modified hinge joint, its complexity includes a distinct articulation that functions as a plane (or gliding) joint: the patellofemoral joint, which involves the kneecap (patella) and the thigh bone (femur).

Understanding Joint Classification

To understand the knee's specific joint types, it's essential to first grasp the broader classification of joints. Joints are the points where two or more bones meet, and their structure dictates the type and range of motion they permit. Synovial joints, which include the knee, are characterized by a joint capsule, synovial fluid, and articular cartilage, allowing for significant movement. Within synovial joints, there are several sub-types based on their shape and movement capabilities:

• Hinge Joints: Allow motion primarily in one plane (e.g., elbow, main knee joint).
• Pivot Joints: Allow rotation around an axis (e.g., atlas and axis in the neck).
• Ball-and-Socket Joints: Offer the widest range of motion in multiple planes (e.g., hip, shoulder).
• Condyloid Joints: Allow movement in two planes, but no rotation (e.g., wrist).
• Saddle Joints: Similar to condyloid but with a unique saddle shape, allowing more specific movements (e.g., thumb).
• Plane (Gliding) Joints: Characterized by flat or slightly curved surfaces that slide over each other, allowing for limited, non-axial gliding movements.

The Knee: A Complex Joint

The knee is not a single, simple joint but rather a complex structure comprising two primary articulations within a single joint capsule:

• The Tibiofemoral Joint: This is the main articulation between the thigh bone (femur) and the shin bone (tibia). While often referred to as a hinge joint due to its primary movements of flexion (bending) and extension (straightening), it is more accurately described as a modified hinge joint. This modification allows for a small degree of internal and external rotation when the knee is flexed, crucial for activities like pivoting and locomotion.
• The Patellofemoral Joint: This articulation occurs between the posterior surface of the kneecap (patella) and the trochlear groove on the anterior (front) surface of the femur. It is this specific articulation that functions as a plane, or gliding, joint.

The Patellofemoral Joint: The Knee's Plane Joint

The patellofemoral joint perfectly exemplifies a plane (gliding) joint due to its unique anatomical structure and primary function:

• Anatomy: The patella, a sesamoid bone embedded within the quadriceps tendon, articulates with the femoral trochlea, a groove at the end of the femur.
• Function as a Plane Joint: During knee flexion and extension, the patella glides superiorly (upward) and inferiorly (downward) within the trochlear groove. This gliding motion is the defining characteristic of a plane joint. There is minimal, if any, rotation or angulation at this joint; its movement is predominantly translational.
• Key Role: The patellofemoral joint serves several critical biomechanical functions:
  • Mechanical Advantage: It acts as a fulcrum, increasing the lever arm of the quadriceps muscle, thereby enhancing its mechanical advantage in extending the knee. Without the patella, significantly more quadriceps force would be required for the same movement.
  • Protection: It provides a protective shield for the anterior aspect of the tibiofemoral joint.
  • Force Distribution: It helps distribute compressive forces across the femorotibial joint.

Biomechanics and Clinical Significance

The precise gliding motion of the patella is vital for healthy knee function. Any disruption to this smooth motion can lead to significant problems:

• Patellar Tracking: The patella must track centrally within the femoral trochlea during movement. Imbalances in the quadriceps muscles (e.g., vastus medialis obliquus weakness), tightness in lateral structures, or anatomical variations can cause the patella to track improperly, leading to pain and dysfunction.
• Patellofemoral Pain Syndrome (PFPS): A common condition characterized by pain around or behind the kneecap, often exacerbated by activities like squatting, stair climbing, or prolonged sitting. It's frequently linked to abnormal patellar tracking and excessive stress on the patellofemoral joint.
• Osteoarthritis: Over time, abnormal loading or repetitive stress on the patellofemoral joint can lead to the degeneration of articular cartilage, resulting in patellofemoral osteoarthritis.

Distinguishing from the Tibiofemoral Joint

It is crucial to differentiate between the two main articulations of the knee. While the tibiofemoral joint is the primary load-bearing and movement-generating joint (flexion, extension, and limited rotation), the patellofemoral joint is the specific plane joint of the knee, facilitating the gliding motion of the kneecap. Both are integral to the knee's overall function, but they serve distinct biomechanical roles and have different primary movement patterns.

Implications for Training and Health

Understanding the dual nature of the knee, particularly the plane joint mechanics of the patellofemoral articulation, has significant implications for exercise and rehabilitation:

• Targeted Strengthening: Exercise programs should focus on balanced strengthening of the quadriceps, hamstrings, and hip abductors/external rotators to promote optimal patellar tracking. For instance, exercises like squats and lunges should be performed with proper form to ensure the kneecap tracks correctly.
• Mobility and Flexibility: Maintaining flexibility in the quadriceps, hamstrings, and IT band is crucial to prevent undue tension that can pull the patella out of alignment.
• Load Management: For individuals with patellofemoral issues, modifying exercise loads and ranges of motion (e.g., avoiding deep squats initially) can help manage symptoms and promote healing.
• Proprioception and Stability: Training balance and proprioception can improve neuromuscular control around the knee, further supporting optimal patellar mechanics.

By appreciating the intricate design and specific functions of both the tibiofemoral and patellofemoral joints, fitness professionals and enthusiasts can develop more effective, safer, and targeted training strategies for knee health and performance.