Knee Injury

Basics

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)

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%

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

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