Biomechanics of Knee

Biomechanics of Knee - Bones, Hinges, Lines

• Bones: Femur (condyles), Tibia (plateaus), Patella (leverage), Fibula (non-weight bearing).
• Joint Type: Ginglymus (hinge) primarily; flexion/extension. Modified hinge: some rotation.
• Key Lines/Axes:
  • Mechanical Axis: Hip to ankle center.
  • Anatomical Axis (Femur): ~5-7° valgus to mechanical.
  • Anatomical Axis (Tibia): Aligns with mechanical.
  • Transverse (Hinge) Axis: Flexion/extension.

⭐ Normal Q-angle: 13-18°. Formed by lines: ASIS to mid-patella & mid-patella to tibial tubercle. ↑Q-angle risks patellar maltracking.

Biomechanics of Knee - Motion & Magic Twist

• Primary Motions: Flexion-Extension (0° to 135-150°; transverse axis); Axial Rotation (max at 90° flexion, IR ~10°, ER ~20-30°; longitudinal axis).
• Degrees of Freedom (DOF): Primarily 2 (flexion/extension, int/ext rotation).
• Screw-Home Mechanism ("Magic Twist"):
  • Terminal 20-30° of extension: Tibia externally rotates on femur.
  • Locks knee, ↑ stability. Caused by larger medial femoral condyle.
  • Unlocking: Popliteus muscle initiates tibial internal rotation.

⭐ The screw-home mechanism is an involuntary, terminal rotation (tibial ER) during knee extension, critical for joint stability. Popliteus "unlocks" the knee by initiating tibial IR during early flexion.

Biomechanics of Knee - Superstars Support

• Menisci (Cartilaginous Cushions):
  • Functions: Load transmission (up to 50-70% in extension), shock absorption, joint stability, lubrication, proprioception.
  • Medial: C-shaped, less mobile. Lateral: O-shaped, more mobile.
• Ligaments (Stabilizing Ropes):
  • ACL (Anterior Guardian): Primary restraint to anterior tibial translation (ATT) & internal rotation.
  • PCL (Posterior Pillar): Primary restraint to posterior tibial translation (PTT). Strongest knee ligament.
  • MCL (Medial Shield): Primary restraint to valgus stress & external rotation.
  • LCL (Lateral Lock): Primary restraint to varus stress. Works with Posterolateral Corner (PLC).

⭐ The ACL provides approximately 85% of the total restraining force to anterior tibial translation at 30° and 90° of knee flexion.

Biomechanics of Knee - Kneecap Glide & Grind

• Patellar Tracking: Patella's path in trochlear groove. "J-sign": lateral glide in terminal extension.
• Q-Angle: Angle: ASIS-Mid Patella-Tibial Tubercle. Norm: Men ~13°, Women ~18°. ↑Q-angle → lateral maltracking.
• PFJ Reaction Force: ↑ with knee flexion; peaks 60-90° (e.g., squat).
• PFJ Contact Area: ↑ with flexion; shifts distal to proximal on patella.
• PFJ Stress: $Stress = Force / Area$. Lowest in full extension.
• Factors ↑ PFJ Stress: ↑Q-angle, VMO weakness, tight lateral retinaculum.

⭐ An increased Q-angle (>20°) significantly elevates risk for patellar instability and anterior knee pain.

Biomechanics of Knee - Loads & Locomotion

• Knee joint loads: Ground Reaction Force (GRF) is transmitted proximally. Muscle forces (Quadriceps, Hamstrings) & ligament tension counteract GRF, creating Joint Reaction Force (JRF).
• JRF (multiples of Body Weight, BW):
  • Walking: 2-4x BW
  • Stair Ascent: 3-5x BW
  • Stair Descent: 5-7x BW
  • Deep Squat: >7x BW
• Muscle activity (co-contraction) critical for stability, but increases JRF.
• Patellofemoral JRF (PFJRF) ↑ significantly with knee flexion & quadriceps activity.

⭐ Peak tibiofemoral JRF during level walking is approximately 2.5 to 3 times body weight, occurring at heel strike and late stance.

High‑Yield Points - ⚡ Biggest Takeaways

• Screw-home mechanism: Tibia externally rotates in terminal extension, locking the knee.
• Q-angle: Normal ~14° (males), ~17° (females); ↑ Q-angle linked to patellar maltracking.
• Menisci: Vital for load distribution, shock absorption, and joint congruity/stability.
• ACL: Primary restraint to anterior tibial translation; also checks hyperextension.
• PCL: Strongest ligament; primary restraint to posterior tibial translation.
• Patellofemoral forces: ↑ dramatically with knee flexion (e.g., stairs, squats).
• Instantaneous center of rotation (ICR): Moves posterosuperiorly during flexion.