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Anterior Cruciate Ligament

Anatomy

  • Completely intra-articular, extra-synovial ligament, measuring 33 X 11 mm
  • Maximum tensile strength 1700 N
  • Strain rate plays a role in the location of ligament failure
    • Midsubstance tears occur at higher rates
    • Bone ligament complex tears occur at lower rates
  • Tibial attachment is on the non articular surface of the tibial plateau, extending from just behind the medial and lateral spines forward for about 3 cm
  • Femoral attachment is on the lateral and posterior portion of the medial femoral condyle in the intercondylar notch
  • Composed of 2 fascicles : anteromedial and posterolateral
    • Anteromedial fibers are tight in flexion and limit anterior translation of tibia on femur
    • Posterolateral fibers are tight in extension and limit anterior translation and ER
  • Some fibres of the ligament are tight in all positions of the knee
  • Anterior fibres are the most isometric, with the postero-lateral fibres relaxing somewhat in flexion
  • No fibres are truly isometric, but the antero-medial fibres have the least variation in length (1.5 mm)
  • Most of the fibres of the ACL are taught in extension
  • Functions
    • Primary function is to prevent anterior displacement of the tibia on the femur
    • Secondary function
      • Restraint to internal rotation
      • Restraint to varus and valgus stresses
    • Screw home motion occurs around ACL axis
    • Contains mechano receptors with proprioceptive role

Pathology

  • Rupture occurs through one of 3 mechanisms :
    • Flexion, Abduction and IR of the femur on the tibia
    • Hyperextension
    • Deceleration
  • Unhappy triad of O'Donaghue : MCL, ACL and medial meniscus tear
  • As MCL attaches to the meniscus disruptive forces that pull on the MCL may disrupt the medial meniscus and if strain sufficient will go on to rupture the ACL

Clinically

  • 70% of these injuries are sustained during sporting activities, mainly in non contact sports involving jumping or cutting
  • A History of cutting / pivoting knee injury plus a popping sound has 75% accuracy
  • Hemarhtrosis, effusion, giving way
  • Positive Lachman, Anterior drawer, Pivot shift tests
  • >5 mm total displacement or >3 mm difference compared to normal side is diagnostic
  • Associated meniscal injuries:
    • Acute ACL tears:
      • lateral meniscus tear: 57%
      • medial meniscus tear: 36%
    • Chronic ACL deficiency:
      • medial meniscus tears are most common
  • Usually, it is not possible to assess the extent of disability imposed by an isolated ACL rupture for a few months, until near normal ROM and muscle force are regained

Treatment

  • Conservative treatment
    • Indications
      • Low-demand, middle age patient
      • Old patients
    • Modalities
      • RICE
      • Weight bearing as tolerated
      • Physiotherapy
        • Restoration of ROM
        • Muscle strengthening
    • Results
      • Satisfactory results in 85 - 90% of low risk patients
      • Unsatisfactory results in 60 - 85% of high risk patients
  • Direct Repair
    • Screw fixation or pull out sutures for avulsion fractures
    • Not recommended for mid-substance tears, due to poor healing potential
  • Reconstruction
    • Indications
      • Less than 35 years of age and closed growth plates
      • Moderate to severe instability
      • Participates in high risk sports or work
    • Both bone-tendon-bone (BTB) and hamstring grafts have been used successfully
    • Satisfactory results in 85 - 95% of reconstructed patients
  • Associated injuries
    • Meniscus injuries : treat at the same time as ACL reconstruction
    • MCL injuries : best managed if allowed to heal conservatively before ACL reconstruction
    • PLC injuries : repair at the same time as ACL reconstruction

ACL injury in the skeletally immature

  • Conservative treatment and muscle exercises
  • Avoid any procedure involving bony tunnelling until skeletal maturity
  • If operation is indicated, use extra-articular reconstruction, e.g. McIntosh repair
  • McIntosh procedure: Iliotibial band mobilised proximally and re-routed beneath the LCL, around the intermuscular septum and back beneath the LCL to be reattached to Gerdy's tubercle

Rehabilitation

  • Post operation CPM 0 - 90o
  • Mobilise in a brace when gains leg control and allow 0 - 90o ROM for 3 weeks
  • ROM exercises with muscle strengthening plus unrestrained mobilisation after 3 weeks
  • Recommence activity on the basis of muscle tone, power and control 
  • Resume sport activities when reaches 90% of normal power

Complications

  • Infra-patellar contracture syndrome (IPCS)
    • A result of inadequate post-op ROM
    • Failure to achieve full ROM 2 weeks post-op increases the chance of IPCS
    • Management
      • <6 weeks : aggressive physiotherapy
      • >6 weeks : MUGA
  • Premature graft failure
  • Decreased ROM
  • Patellar pain / fracture (BTB grafts)

References

  • Pitman M.I., Luks H.J. : Knee ligament injury. In Spivak JM, Di Cesare PE, Feldman DS : Orthopedics; A Study Guide; 1999
  • Baumfeld J.A., Hart J.A. and Miller M.D : Sport medicine. In Mark D. Miller : Review of Orthopedics; 2008


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