Acl injury

Anatomy

  • Anteromedial and Posterolateral bundles
    • Named by their insertion on the tibia.

AM Bundle

  • Single bundle reconstruction traditionally based on the AM bundle.
  • Origin: Proximal and posterior on the medial wall of the lateral femoral condyle (LFC).
  • Insertion: Anterior to the posterior medial (PM) bundle.
  • Tight in flexion.

PL Bundle

  • Origin: Distal to the AM bundle.
  • Insertion: Posterior to the AM bundle.
  • Tight in extension.

Remember AMPLE
- Anteromedial tight in flexion, Posterolateral tight in extension.

Blood Supply

  • Middle geniculate artery branches and fat pad vessels.

Nerve Innervation

  • Posterior articular nerve (tibial nerve).
  • Ligament also has mechanoreceptors on its surface, providing inherent proprioception.

Biomechanics

  • Primary restraint to anterior tibial translation.
  • PM bundle prevents hyperextension.
  • Least tension on ACL at 30-40 degrees flexion (good position for examination – Lachman’s test).
  • Secondary restraint to:
    • Varus & Valgus.
    • Internal rotation in particular.
  • Mechanoreceptors provide proprioceptive role.

Mechanism of Injury

  • Typically a non-contact jumping, twisting, or deceleration injury.

History

  • Typical history and haemarthrosis have a sensitivity of 70% for ACL tear.

Examination

  • Immediate haemarthrosis.
  • Lachman test has 95% sensitivity and specificity.
  • Pivot shift test is similar but requires a relaxed patient.

Investigation

  • MRI.

Concurrent Injuries

  • Meniscal Injuries:
    • Acutely, lateral meniscus is most common.
    • In ACL-deficient knee, medial meniscus (MM) is more common due to anchoring to the plateau.
  • Kissing Lesion:
    • Anterior LFC and posterior-lateral lateral plateau.
  • Osteochondral Damage.
  • Classic Terrible Triad: ACL, MCL, and MM (O’Donoghue).
    • Actually, ACL & LM is more common.

Predisposing Factors

  • High-risk sports.
  • Reduced notch width.
  • ACL rupture is 4x more common in women – multifactorial:
    • Women have narrower notches.
    • Hyperlaxity; hormonal factors.
    • Lower limb mechanical differences in alignment.

Natural History

  • Sport in ACL deficiency causes further chondral & meniscal damage, likely leading to accelerated arthritis.
  • Not proven that ACL reconstruction alters the natural history of arthrosis.
    • At the time of rupture, release of cytokines and pro-inflammatory factors begins the degenerative process.

Management

Goals of Treatment: 1. Provide a stable asymptomatic knee. 2. Facilitate return to sports. 3. Prevent accelerated arthritis and other knee injuries.

  • Goal 1 is essential, Goal 2 is possible, Goal 3 is questionable.
  • Treatment should be individualized.

Non-operative

  • Good option if the patient is willing to make lifestyle changes.
  • If the patient is low demand to begin with.
  • Patient over 40 years should be considered carefully.
  • If significant degenerative changes in the knee exist.
  • Aggressive physiotherapy focusing on quads, hamstrings, and proprioception.
  • Modified sports.
  • Knee brace use is not proven to be beneficial.

Primary Repair

  • Not an option, and it has failed in the past.
  • ACL becomes coated in myofibroblasts, which prevent healing.

ACL Reconstruction Timing

  • Generally, wait until full range of motion (ROM) is achieved, especially extension, to minimize the risk of arthrofibrosis.
  • Some proponents perform immediate reconstruction for elite athletes, requiring extensive supervised physiotherapy and very motivated patients.

Graft Selection

  • Autograft, Allograft, or Synthetic.

Autograft

  • Advantages: Low morbidity, low immune reaction, cost-effective.

  • Revascularization and incorporation of the graft are advantages.

  • Autograft strength initially deteriorates; thus, the chosen graft must be stronger than the native ACL:

    • Quad Hamstring: 4500 N.
    • BPTB: 2500 N.
    • Native ACL: 2000-2500 N.

  • BPTB Graft:

    • Bony fixation with slightly lower re-rupture rate.
    • May cause patellofemoral osteoarthritis and anterior knee pain.

  • Hamstrings:

    • Single strand hamstrings alone are weaker than ACL.
    • Hamstrings folded to give a 4-strand graft have double the tensile strength of native ACL.
    • Slight 10% reduction in hamstring strength, but not clinically noticeable.

  • BPTB in elite sprinters is recommended as hamstrings are vital.

  • Failure rates: Comparable between hamstrings and BPTB.

Allograft

  • Options: Achilles, BPTB, quads tendon.

  • Advantages: Faster immediate recovery (no graft site morbidity), no strength reduction initially, better cosmesis.

  • Disadvantages: More expensive, infection and immune risk.

  • Freeze-dried allograft has similar properties to autograft.

  • Irradiated allograft loses its mechanical properties (no longer used).

Single or Double Bundle

  • Double bundle is more anatomic but has no proven clinical benefit.
  • Double bundle is technically more demanding.

Rehabilitation

  • Avoid braces if possible.
  • Focus on closed chain quads and hamstrings exercises.
  • Gradually build up exercises and return to full sports at 9 months.
  • Hamstrings are important for limiting anterior translation.
  • Proprioceptive training.

Surgical Principles

Tunnel Placement

  • Position of the femoral tunnel is more important.

  • Influenced by tibial tunnel position if using a trans-tibial technique.

  • Anterior placement of the tibial tunnel should be avoided.

  • Angle of the femoral tunnel should be 60 degrees to the joint line, aiming for the 10:30 or 1:30 clock position.

    • This avoids a vertical and anterior graft.
    • Anterior graft placement leads to impingement on PCL and pain, limiting range of motion in extension.

  • Notchplasty may be performed to visualize the wall of the lateral femoral condyle (LFC) and clear the resident’s ridge.

  • Tibial Tunnel: Lateral edge should be aligned with the lateral tibial spine, starting just anterior to the MCL insertion.

Graft Fixation

  • Options: Interference screw, cortical or cross pins.
    • Interference screws are easy but may widen tunnels and be proud.
    • Cortical screws (e.g., endobuttons) provide excellent results.
    • Cross pins are advantageous as they secure the graft closer to the joint, preventing windscreen-wiping.

Complications

  • Main technical issue is anterior tunnel placement, leading to impingement, pain, tightness, and decreased flexion.
  • Cyclops lesion (scar tissue from remnant ACL).
  • Graft impingement in the notch.
  • Arthrofibrosis and global stiffness occur if surgery is done too early before achieving full ROM.
  • Re-rupture rate: 2-5%.
  • Infection and thromboembolism: <1%.
  • Anterior knee pain is more common with BPTB.

Results of ACL Reconstruction

  • Re-rupture rate: 2-5%.
  • 80-90% have <3mm laxity on KT-1000.
  • 80-90% return to sport with good to excellent results.

Revision ACL Reconstruction

Principles

  • The most important determinant of a good result is understanding the cause of failure.
  • Graft selection: more often use allografts to avoid other leg graft morbidity and provide more graft length.
  • Revision is often done in two stages.
  • Bone grafting may be needed to restore bone stock if tunnels are widened.

Results

  • Revision ACL reconstruction has less favorable outcomes than primary ACL reconstruction.
Back to top