Biomechanics of the Knee Joint

Introduction

• Continuation from previous lecture on hip joint biomechanics.
• Knee joint consists of two articulations:
  • Tibiofemoral articulation
  • Patellofemoral articulation

Tibiofemoral Articulation

Alignment of the Joint

• Mechanical and anatomical axes of bones and limbs.
• *Femur:
  • Mechanical axis: Center of femoral head to the center of the distal articulating surface.
  • Anatomical axis: Through the myelodiphysis to the center of the distal articulating surface.
• Tibia:
  • Mechanical and anatomical axes: Center of tibial plateau to the center of the ankle joint, aligned parallel.
• Lower Limb:
  • Mechanical axis: From the center of the head of the femur to the center of the ankle joint.
  • Normal axis passes through or slightly medial to the knee joint.
  • *Deformities:
    • Valgus deformity: Knee joint drastically medial to the axis.
    • Varus deformity: Knee joint lateral to the axis.
• Femur mechanical axis forms a 9-degree valgus angle with the vertical axis.
• Tibia mechanical axis forms a 3-degree varus angle, resulting in a combined 6-degree valgus at the knee joint.
  • Total knee replacement femoral component placed at a 6-degree valgus.

Knee Joint Motion Mechanisms

• *Screw-Home Mechanism (Locking/unlocking mechanism):

  • External rotation of tibia on femur during terminal extension.
  • Internal rotation of tibia during flexion.
  • Anatomical differences:
    • Medial tibial plateau is longer than the lateral.
    • Medial femoral condyle is larger and extends distally.
  • Decreases work of quadriceps while standing.
  • Depends on kinetic chain mechanism (open vs closed).

• *Rollback Phenomena:

  • Helps in knee flexion.
  • Contact point shifts posteriorly during flexion.
  • Controlled by posterior cruciate ligament.
  • Usage in prosthetic design - enables hyperflexion without impingement.

Stability Factors

• *Ligaments:
  • Medial and lateral collateral ligaments (prevent varus and valgus).
  • Anterior cruciate ligament (prevents anterior translation).
  • Posterior cruciate ligament (prevents posterior translation).*

Patellofemoral Articulation

Anatomy of the Patella

• Sesamoid bone in quadriceps tendon.
• Rough superior (anterior) surface, a base, and an apex (attachment of patellar ligament).
• Posterior surface has two articulating facets (larger lateral facet, smaller medial facet).
• Increases lever arm of the body (extends lever arc motion).

Motion and Function

• *Contact during Flexion/Extension:
  • Maximum contact at 45 degrees flexion.
  • Allows efficient quadriceps contraction.
• *Keu Angle:
  • Angle between anatomical axis of femur and line from the base of patella to tibial tubercle.
  • Normal values: 13 degrees (males) and 18 degrees (females).
  • Abnormals (>20 degrees) create excessive lateral pull of patella.
• *Articulation:
  • Patella articulates differently at various degrees of knee flexion:
    • 0 degrees (full extension): Proximal extent of femur.
    • Progresses distally with increased flexion.
    • 120 degrees flexion: Superior part of patella articulates with femoral condyles.*

Stability Factors of Patellofemoral Joint

• Medial patellofemoral ligament (prevents lateral displacement due to KO angle).

Summary of the biomechanics of the knee joint focusing on tibiofemoral and patellofemoral articulations, motion mechanisms, and stability factors.