Knee Joint: Functional Classification, Movements, and Biomechanics

What is the functional classification of the knee joint?

The knee joint is functionally classified as a diarthrotic (freely movable) synovial joint, specifically a modified hinge joint (ginglymus), which primarily allows for flexion and extension but also permits limited internal and external rotation when flexed.

Understanding Joint Classification

Joints, or articulations, are the points where two or more bones meet. Anatomists and kinesiologists classify joints based on their structure and their function. Functional classification categorizes joints according to the degree of movement they permit. There are three main functional classifications:

• Synarthrosis: Immovable joints (e.g., sutures of the skull).
• Amphiarthrosis: Slightly movable joints (e.g., pubic symphysis, intervertebral discs).
• Diarthrosis: Freely movable joints (e.g., shoulder, hip, knee).

The Knee Joint: A Diarthrosis (Synovial Joint)

The knee is a prime example of a diarthrotic joint, meaning it allows for a wide range of motion. All diarthrotic joints are also classified structurally as synovial joints. Key characteristics of synovial joints include:

• Articular Cartilage: Covers the ends of the bones, providing a smooth, low-friction surface.
• Joint Capsule: Encloses the joint, forming a sealed cavity.
• Synovial Fluid: A viscous fluid within the joint capsule that lubricates the joint, nourishes the cartilage, and absorbs shock.
• Synovial Membrane: Lines the inside of the joint capsule, producing synovial fluid.
• Ligaments: Strong fibrous bands that connect bones, reinforcing the joint capsule and preventing excessive movements.
• Menisci: C-shaped fibrocartilaginous pads (medial and lateral menisci in the knee) that improve congruence between the femoral condyles and tibial plateau, distribute forces, and aid in lubrication.

Functional Classification of the Knee: A Modified Hinge Joint (Ginglymus) with Rotational Capabilities

While often described simply as a hinge joint, the knee's functional classification is more accurately a modified hinge joint (ginglymus).

• Hinge Joint (Ginglymus): A true hinge joint, like the elbow (humeroulnar joint), primarily allows movement in one plane, similar to a door hinge. This movement is typically flexion and extension.
• Modified Hinge Joint: The knee fits this description because, in addition to its primary movements of flexion and extension, it also permits a crucial, albeit limited, amount of rotation, especially when the knee is flexed. This rotational capability is vital for activities like pivoting and is facilitated by the asymmetry of the femoral condyles and the design of the menisci. The screw-home mechanism is a prime example of this modification, where the tibia externally rotates on the femur during the final degrees of knee extension, locking the knee in place for stability during standing.

Primary Movements of the Knee

The knee joint facilitates the following movements:

• Flexion: Decreasing the angle between the thigh and lower leg, bringing the heel closer to the buttocks.
  • Primary Muscles: Hamstrings (biceps femoris, semitendinosus, semimembranosus), gastrocnemius, popliteus.
• Extension: Increasing the angle between the thigh and lower leg, straightening the leg.
  • Primary Muscles: Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius).
• Internal Rotation (Medial Rotation): The tibia rotates inward relative to the femur. This movement is significantly limited and primarily occurs when the knee is flexed (typically beyond 20-30 degrees of flexion).
  • Primary Muscles: Semimembranosus, semitendinosus, popliteus.
• External Rotation (Lateral Rotation): The tibia rotates outward relative to the femur. Like internal rotation, this movement is also limited and occurs predominantly when the knee is flexed.
  • Primary Muscles: Biceps femoris.

Degrees of Freedom

The number of degrees of freedom refers to the number of independent planes or axes in which a joint can move.

• The knee joint possesses two degrees of freedom:
  1. Movement around a mediolateral axis (for flexion and extension).
  2. Movement around a longitudinal axis (for internal and external rotation when the knee is flexed).

Biomechanical Significance and Stability

The knee's functional classification as a modified hinge joint underscores its critical role in locomotion, weight-bearing, and athletic performance. While it offers extensive mobility in the sagittal plane (flexion/extension), its limited rotational capabilities, particularly in extension, are essential for stability. This balance between mobility and stability is achieved through:

• Strong Ligamentous Support: Cruciate ligaments (ACL, PCL) and collateral ligaments (MCL, LCL) provide crucial anterior-posterior and medial-lateral stability, respectively, guiding movement and preventing excessive translation or rotation.
• Menisci: These cartilaginous structures deepen the articular surface of the tibia, improving the fit with the femoral condyles and contributing to load distribution and shock absorption.
• Powerful Musculature: The quadriceps and hamstrings provide dynamic stability and control over knee movements.

Conclusion

The knee joint, functionally classified as a modified hinge (ginglymus) synovial joint, is a marvel of biomechanical engineering. Its primary role in providing powerful flexion and extension, combined with crucial, albeit limited, rotational capabilities when flexed, allows for the complex movements required for walking, running, jumping, and pivoting. Understanding this functional classification is fundamental to appreciating the knee's mechanics, its susceptibility to injury, and the principles guiding its rehabilitation and training.