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Updated: Jun 10 2021

Knee Biomechanics

3.4

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Images
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https://upload.orthobullets.com/topic/9065/images/lateral retinaculum.jpg
  • Introduction
    • The knee is comprised of 2 joints
      • tibiofemoral joint
      • patellofemoral joint
  • Patellofemoral Articulation
    • Function
      • transmits tensile forces generated by the quadriceps to the patellar tendon
      • increases lever arm of the extensor mechanism
        • patellectomy decreases extension force by 30%
    • Biomechanics
      • patellofemoral joint reaction force
        • up to 7x body weight with squatting
        • 2-3x body weight when descending stairs
    • Motion
      • "sliding" articulation
        • patella moves 7cm caudally during full flexion
      • maximum contact between femur and patella is at 45 degrees of flexion
    • Stability
      • passive restraints to lateral subluxation
        • medial patellofemoral ligament
          • primary passive restraint to lateral translation in 20 degrees of flexion
          • 60% of total restraining force
        • medial patellomeniscal ligament
          • 13% of total restraining force
        • medial retinaculum
          • 10% of total restraining force
      • dynamic restraint
        • quadriceps muscles
      • Q angle
        • definition
          • line drawn from the anterior superior iliac spine --> middle of patella --> tibial tuberosity
        • normal Q angle
          • in extension
            • males
              • 13 degrees
            • females
              • 18 degrees
          • in flexion
            • 8 degrees
        • pathology
  • Tibiofemoral Articulation
    • Function
      • transmission of body weight from femur to tibia
    • Biomechanics
      • tibiofemoral joint reaction force
        • 3x body weight with walking
        • 4x body weight with climbing
    • Motion in sagittal plane
      • range of motion
        • 3 degrees of hyperextension to 155 degrees of flexion
        • thigh-calf contact is usually the limiting factor to full flexion
        • normal gait requires ROM from 0 to 70 degrees
    • Rotation
      • instant center of rotation
        • definition
          • point at which the joint surfaces are in direct contact
        • relevance
          • posterior rollback
            • as the knee flexes, the instant center of rotation on the femur moves posteriorly
            • allows for increased knee flexion by avoiding impingement
      • "screw home" mechanism
        • definition
          • tibial externally rotates 5 degrees in the last 15 degrees of extension
        • cause
          • medial tibial plateau articular surface is longer than lateral tibial plateau
        • relevance
          • "locks" knee decreasing the work performed by the quadriceps while standing
    • Stability
      • varus stress
        • lateral collateral ligament
      • valgus stress
        • superficial portion of medial collateral ligament
      • anterior translation
        • anterior cruciate ligament
          • attachments
            • origin
              • semicircular area on the posteromedial aspect of lateral femoral condyle
            • insertion
              • just anterior to and between the intercondylar eminences of the tibia
          • components
            • anteromedial bundle
              • tight in flexion
            • posterolateral bundle
              • tight in extension
          • function
            • primary static restraint to anterior translation
            • also plays a roll in axial rotation
      • posterior translation
        • posterior cruciate ligament
          • attachments
            • origin
              • anterolateral medial femoral condyle
            • insertion
              • tibial sulcus below articular surface
          • components
            • anterolateral
              • tight in flexion
            • posteromedial
              • tight in extension
          • function
            • primary static restraint to posterior translation
      • external rotation
        • posterolateral corner is the primary stabilizer of external tibial rotation
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