Knee Arthrokinematics: Understanding Joint Movement, Mechanics, and Clinical Significance

What are the Arthrokinematics of the knee?

Arthrokinematics refers to the subtle, imperceptible movements (rolls, slides, and spins) that occur between joint surfaces, essential for full and pain-free range of motion, optimal load distribution, and joint health within the knee joint.

Understanding Arthrokinematics: A Foundation

To truly comprehend how our joints function, we must differentiate between two crucial types of movement:

• Osteokinematics: These are the gross, visible movements of bones relative to each other, often described in terms of degrees of freedom (e.g., flexion, extension, abduction, adduction, rotation). For the knee, these are primarily flexion and extension, with a small degree of rotation.
• Arthrokinematics: These are the specific, involuntary movements occurring at the joint surfaces themselves, which accompany osteokinematic motion. They are typically described as:
  • Roll: A new point on one joint surface contacts a new point on the opposing joint surface (like a tire rolling on the road).
  • Slide (or Glide): A single point on one joint surface contacts multiple points on the opposing joint surface (like a car skidding on ice).
  • Spin: A single point on one joint surface rotates on a single point on the opposing joint surface (like a top spinning in place).

These arthrokinematic movements are critical because they ensure that the joint surfaces remain congruent, distribute forces evenly, and prevent impingement or excessive compression during movement. Without proper arthrokinematics, joint motion would be limited, painful, and prone to degenerative changes.

Anatomy of the Knee Joint: A Quick Review

The knee is a complex modified hinge joint, primarily formed by the articulation of three bones:

• Femur: The thigh bone, with its two large, rounded condyles (medial and lateral) forming the superior part of the joint.
• Tibia: The shin bone, with its relatively flatter tibial plateau, which serves as the inferior articulating surface.
• Patella: The kneecap, a sesamoid bone embedded within the quadriceps tendon, which articulates with the trochlear groove on the anterior aspect of the femur.

Within the joint, key structures facilitate movement and stability:

• Articular Cartilage: Covers the ends of the femur and tibia, providing a smooth, low-friction surface.
• Menisci: Two C-shaped fibrocartilaginous pads (medial and lateral) that sit between the femoral condyles and the tibial plateau. They deepen the tibial plateau, improve congruence, absorb shock, and aid in load distribution.
• Ligaments: Provide stability. Crucial ones include the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL).

Understanding these anatomical relationships is vital for appreciating how the arthrokinematic movements unfold. The shape of the femoral condyles (convex) and the tibial plateau (relatively concave) dictates the specific roll and slide patterns according to the Convex-Concave Rule.

Arthrokinematics of Knee Flexion and Extension

The specific arthrokinematic movements of the knee differ significantly depending on whether the movement occurs in an open kinematic chain (OKC) or a closed kinematic chain (CKC).

Open Kinematic Chain (OKC) Movements

In OKC movements (e.g., knee extension while sitting, leg curls), the distal segment (tibia/fibula) is free to move, and the proximal segment (femur) is relatively fixed.

• Convex-Concave Rule for OKC: When a concave surface (tibial plateau) moves on a convex surface (femoral condyles), the roll and slide occur in the same direction.

  • Knee Flexion (e.g., bringing heel towards glutes):
    • The tibia (concave) rolls posteriorly on the femoral condyles.
    • Simultaneously, the tibia slides posteriorly on the femoral condyles.
  • Knee Extension (e.g., straightening leg from bent position):
    • The tibia (concave) rolls anteriorly on the femoral condyles.
    • Simultaneously, the tibia slides anteriorly on the femoral condyles.

Closed Kinematic Chain (CKC) Movements

In CKC movements (e.g., squat, lunge, standing up), the distal segment (tibia/foot) is fixed or bears weight, and the proximal segment (femur/trunk) moves.

• Convex-Concave Rule for CKC: When a convex surface (femoral condyles) moves on a concave surface (tibial plateau), the roll and slide occur in opposite directions.

  • Knee Flexion (e.g., descending into a squat):
    • The femoral condyles (convex) roll posteriorly on the tibial plateau.
    • Simultaneously, the femoral condyles slide anteriorly on the tibial plateau to prevent the femur from rolling off the back of the tibia.
  • Knee Extension (e.g., standing up from a squat):
    • The femoral condyles (convex) roll anteriorly on the tibial plateau.
    • Simultaneously, the femoral condyles slide posteriorly on the tibial plateau to prevent the femur from rolling off the front of the tibia.

The Screw-Home Mechanism of the Knee

The "screw-home mechanism" is a crucial involuntary rotation (spin) that occurs during the final degrees of knee extension (approximately the last 20 degrees) and is essential for locking the knee in full extension, providing stability for standing without constant muscle activity.

• During Open Kinematic Chain Extension: As the tibia extends, it externally rotates on the femur during the last 20 degrees to achieve full extension.
• During Closed Kinematic Chain Extension: As the femur extends (e.g., standing up), it internally rotates on the tibia during the last 20 degrees to achieve full extension.

To unlock the knee from full extension and initiate flexion, the popliteus muscle plays a vital role. It internally rotates the tibia (in OKC) or externally rotates the femur (in CKC), effectively "unscrewing" the joint and allowing for flexion to begin.

Patellofemoral Arthrokinematics

While often overshadowed by tibiofemoral movements, the articulation between the patella and the femoral trochlear groove also involves critical arthrokinematics.

• During Knee Flexion: The patella glides inferiorly (downwards) within the trochlear groove.
• During Knee Extension: The patella glides superiorly (upwards) within the trochlear groove.

Proper patellar tracking (the precise path of the patella within the groove) is vital for distributing forces across the joint, optimizing the leverage of the quadriceps muscle, and preventing conditions like patellofemoral pain syndrome.

Clinical Significance and Applications

A thorough understanding of knee arthrokinematics is not merely academic; it has profound implications for exercise science, rehabilitation, and injury prevention:

• Injury Prevention: Abnormal arthrokinematics (e.g., excessive glide, restricted roll) can lead to increased stress on ligaments, cartilage, and menisci, contributing to conditions like osteoarthritis, patellofemoral pain, or meniscal tears. Identifying these dysfunctions is key to prevention.
• Rehabilitation: Physical therapists and trainers use this knowledge to design specific exercises that promote correct joint mechanics. For instance, knowing when to use OKC vs. CKC exercises can be crucial for safely progressing a patient after knee surgery or injury, ensuring appropriate loading and movement patterns. Joint mobilization techniques are directly aimed at restoring normal arthrokinematic movements.
• Exercise Prescription: Understanding how the joint surfaces move helps optimize exercise technique. For example, in a squat, ensuring proper femoral glide helps protect the ACL and distribute forces effectively.
• Manual Therapy: Practitioners utilize specific joint mobilization techniques (e.g., glides) to restore lost arthrokinematic motion, which can improve range of motion and reduce pain.

Conclusion

The arthrokinematics of the knee joint, encompassing the intricate rolls, slides, and spins between the femoral, tibial, and patellar surfaces, are fundamental to its complex function. These subtle movements, dictated by the joint's anatomy and the Convex-Concave Rule, ensure smooth, efficient, and stable motion across the full range of flexion and extension. For fitness professionals, rehabilitation specialists, and anyone interested in optimal human movement, grasping these principles is essential for designing effective training programs, preventing injuries, and facilitating recovery, ultimately promoting long-term knee health and performance.