Knee Injury

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Basics

Description

  • Cruciate ligament injuries:
    • Anterior cruciate ligament (ACL):
      • From the posteromedial aspect of the lateral femoral condyle to the intraspinus area on the tibia
      • Prevents excessive anterior translation of the tibia, internal rotation of the tibia on the femur, or hyperextension of the knee
    • Posterior cruciate ligament (PCL):
      • Twice as strong and twice as thick as the normal ACL, less commonly injured
      • From anterolateral aspect of medial femoral condyle to the posterior tibia
  • Meniscal tears:
    • Medial meniscus injury most common:
      • More firmly attached to the joint capsule and less mobile than lateral meniscus
    • Tears are the result of tensile or compressive forces between the femoral and tibial condyles
    • Extension of meniscal tear may result in a free segment that may become displaced into the joint, resulting in a true locked joint
  • Medial collateral ligament (MCL):
    • From the posterior aspect of medial femoral condyle to the tibia, distal to joint
    • Often accompanied by other injury:
      • Hyperextension with external rotation (ACL/PCL injured first)
      • Anterior stress (ACL injured first)

Epidemiology

Incidence and Prevalence Estimates
  • ACL:
    • Most commonly injured knee ligament
    • 200,000 ACL injuries annually in the U.S.
    • 2/3 of all ACL injuries are noncontact
    • Female gender: 3 × greater risk
  • Associated injuries:
    • ∼50% ACL injuries are associated with meniscal tears
    • ACL injuries commonly have chondral and subchondral injuries
  • Meniscus:
    • Medial meniscus injury 10 × more common than lateral
    • True locked joint in only 30%

Etiology

  • Cruciate ligament injuries:
    • ACL: Often deceleration with flexion and rotation, or hyperextension:
      • Usually sports related, especially skiing and football
      • Plant-and-pivot or stop-and-jump mechanism
    • PCL:
      • “Dashboard injury”: Flexed knee with posteriorly directed force to the anterior proximal tibia (motor vehicle crash or direct trauma)
      • Fall on flexed knee
  • Meniscus injury:
    • Sudden rotary motion of knee associated with squatting, pivoting, turning, and bending
    • Common in sports with low-stance positions (wrestling/football) or kneeling position (carpet installers, plumbers)
  • MCL injuries:
    • Direct trauma to lateral knee
    • Most common: Valgus stress with external rotation on flexed knee:
      • From catching a ski tip
      • Side tackle (football)

Pediatric Considerations
  • The ACL is the most frequently injured knee ligament in children
  • Isolated MCL injury infrequent before growth plate closure (<14 yr old)

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Basics

Description

  • Cruciate ligament injuries:
    • Anterior cruciate ligament (ACL):
      • From the posteromedial aspect of the lateral femoral condyle to the intraspinus area on the tibia
      • Prevents excessive anterior translation of the tibia, internal rotation of the tibia on the femur, or hyperextension of the knee
    • Posterior cruciate ligament (PCL):
      • Twice as strong and twice as thick as the normal ACL, less commonly injured
      • From anterolateral aspect of medial femoral condyle to the posterior tibia
  • Meniscal tears:
    • Medial meniscus injury most common:
      • More firmly attached to the joint capsule and less mobile than lateral meniscus
    • Tears are the result of tensile or compressive forces between the femoral and tibial condyles
    • Extension of meniscal tear may result in a free segment that may become displaced into the joint, resulting in a true locked joint
  • Medial collateral ligament (MCL):
    • From the posterior aspect of medial femoral condyle to the tibia, distal to joint
    • Often accompanied by other injury:
      • Hyperextension with external rotation (ACL/PCL injured first)
      • Anterior stress (ACL injured first)

Epidemiology

Incidence and Prevalence Estimates
  • ACL:
    • Most commonly injured knee ligament
    • 200,000 ACL injuries annually in the U.S.
    • 2/3 of all ACL injuries are noncontact
    • Female gender: 3 × greater risk
  • Associated injuries:
    • ∼50% ACL injuries are associated with meniscal tears
    • ACL injuries commonly have chondral and subchondral injuries
  • Meniscus:
    • Medial meniscus injury 10 × more common than lateral
    • True locked joint in only 30%

Etiology

  • Cruciate ligament injuries:
    • ACL: Often deceleration with flexion and rotation, or hyperextension:
      • Usually sports related, especially skiing and football
      • Plant-and-pivot or stop-and-jump mechanism
    • PCL:
      • “Dashboard injury”: Flexed knee with posteriorly directed force to the anterior proximal tibia (motor vehicle crash or direct trauma)
      • Fall on flexed knee
  • Meniscus injury:
    • Sudden rotary motion of knee associated with squatting, pivoting, turning, and bending
    • Common in sports with low-stance positions (wrestling/football) or kneeling position (carpet installers, plumbers)
  • MCL injuries:
    • Direct trauma to lateral knee
    • Most common: Valgus stress with external rotation on flexed knee:
      • From catching a ski tip
      • Side tackle (football)

Pediatric Considerations
  • The ACL is the most frequently injured knee ligament in children
  • Isolated MCL injury infrequent before growth plate closure (<14 yr old)

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