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Knee Biomechanics

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Topic updated on 07/31/16 10:49pm
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
      • lateral 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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Qbank (2 Questions)

TAG
(OBQ12.115) Which of the following best describes normal tibio-femoral joint kinematics ? Topic Review Topic

1. The femur undergoes internal rotation with knee flexion
2. The lateral femoral condyle remains stationary on the lateral tibia plateau during knee flexion from 0 to 120 degrees
3. The tibia undergoes internal rotation with knee flexion
4. The medial femoral condyle moves posteriorly on the medial tibial plateau during knee flexion from 0 to 120 degrees
5. Beyond 120 degrees of flexion only the lateral femoral condyle participates in femoral rollback

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TAG
(OBQ06.32) Which of the following motions shows the greatest difference between a normal and ACL deficient knee? Topic Review Topic

1. Posterior femoral translation at 30° flexion
2. Posterior femoral translation at 60° flexion
3. Axial rotation in full extension
4. Axial rotation at 50° flexion
5. Varus angulation at 30 ° flexion

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