Biomechanics of the Knee Joint

Introduction

• Combination of joints: Tibiofemoral and patellofemoral articulation
• Focus on the tibiofemoral articulation first

Alignment of the Joint

Mechanical and Anatomical Axes

• Femur:
  • Mechanical: Center of femoral head to center of distal articulating surface
  • Anatomical: Through the mid-diaphysis to center of distal articulating surface
• Tibia:
  • Mechanical and anatomical axes start from the center of the tibial plateau to center of the ankle joint, aligned parallel
• Lower Limb: Mechanical axis from center of femoral head to center of ankle joint
  • Axis passing through/medial to center of knee joint is normal
  • Valgus deformity: Knee joint drastically medial to axis
  • Varus deformity: Knee joint lateral to axis
• Angle Alignments:
  • Femur: 9-degree valgus angle to vertical axis
  • Tibia: 3-degree varus angle
  • Resultant: 6-degree valgus at knee joint
  • Total knee replacement: Femoral component placed at a 6-degree valgus angle

Mechanisms of Motion

Screw-home Mechanism (Locking/Unlocking)

• Knee Extension: External rotation of tibia
• Knee Flexion: Internal rotation of tibia
• Anatomical Differences:
  • Medial tibial plateau longer than lateral
  • Medial femoral condyle is larger and extends distally
• Function: Decreases work performed by quadriceps during standing
• Kinetic Chain Mechanisms:
  • Open (Sitting): External rotation of tibia on extension
  • Closed (Standing): Medial rotation of femur on extension

Rollback Phenomena

• During Flexion: Contact point shifts posteriorly
• Controlled by PCL: Posterior Cruciate Ligament
• Purpose: Allows hyperflexion without impingement, improves quadriceps function

Stability Factors of the Tibiofemoral Component

• Valgus/Varus Prevention: Medial and lateral collateral ligaments
• Anterior Translation Prevention: Anterior cruciate ligament
• Posterior Translation Prevention: Posterior cruciate ligament

Patellofemoral Component

Anatomy of the Patella

• Type: Sesamoid bone
• Structure:
  • Superior/Anterior surface: Rough
  • Base: Quadriceps attachment
  • Apex: Patellar ligament attachment
  • Posterior surface: Larger lateral facet, smaller medial facet
• Function: Increases lever arm of the knee joint
  • Removal decreases extension strength

Patellofemoral Joint Articulation

• Sliding Articulation: Articulates differently with flexion/extension
• Maximum Contact: 45 degrees of flexion
• Primary Function: Improve efficiency of quadriceps contraction

Q Angle

• Definition: Angle between extension of anatomical axis of femur and line from patella to tibial tubercle
• Typical Angles: 13 degrees in males, 18 degrees in females
• Implications: Larger Q angle can cause excessive lateral pull on patella

Articulation in Various Degrees of Motion

• 0 Degrees Flexion: Patella at proximal femur
• 30 Degrees Flexion: Articulating surface changes more distally
• 120 Degrees Flexion: Superior part of the patella articulating with femoral condyles

Stability Factors of the Patellofemoral Component

• Medial Patellofemoral Ligament (MPFL): Prevents lateral displacement of the patella