Knee Injury

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

• 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)

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