Statistics

• In US 100,000-200,000 ACL injuries each year
• 1 out of 10 female athletes has an ACL tear
• 3.7 X more common in females
• Very common to have associated injuries
  • Meniscal tears
  • Other ligaments
  • Osteochondral fractures

Anatomy

• Innervated by Tibial Nerve
• Blood supplied by the middle genicular artery
• made up of multiple collagen fascicles
• Average length is from 25 to 35 mm
• Average width is 10-11mm
• Arises posteromedial corner of medial aspect of lateral condyle in the intercondylar notch
• Tibial attachment is in a fossa in front of and lateral to anterior spine

Two Bundles

• Anteromedial
  • Smaller
  • Tight in flexion
  • Prevents anterior translation
• Posterolateral
  • Bulkier
  • Tight in extension
  • Prevents rotational instability

Purpose

• Prevent anterior and rotatory instability in knee
• Part of “screw home” mechanism
• Proprioception
• The ligaments of the knee make sure that the weight that is transmitted through the knee joint is centered within the joint minimizing the amount of wear and tear on the cartilage inside the knee.

Biomechanics

• Ultimate tensile load 2160 N
• Stiffness 242 N
• Un-sectioned ACL permits 5-8mm ant. translation at 30 degrees of flexion
• Sectioned ACL 9-10 mm ant. translation but more important no or soft endpoint

Mechanism of injury

Non Contact Pivot..Internal Rotation

• Athlete runs and cuts to the same side
• Internal rotation of the tibia

Non Contact Pivot..External Rotation

• Athlete plants foot and rotates to the opposite site
• What is reproduced in pivot shift

Hyperextension

External Rotation Valgus Stress