Introduction / Definition

The anterior cruciate ligament (ACL) is the primary static restraint preventing anterior tibial translation and secondary restraint resisting internal tibial rotation and hyperextension. Rupture of the ACL compromises native knee kinematics, precipitating functional instability, secondary meniscal tears, and accelerated degenerative osteoarthritis.

Epidemiology

• Incidence: Estimated at 30–60 per 100,000 person-years, with peak incidence occurring in individuals aged 15–25 years participating in pivoting and cutting sports (e.g., football, rugby, netball, skiing).
• Sex Predisposition: Female athletes demonstrate a 2- to 8-fold higher risk per athletic exposure compared to male counterparts due to:
  • Biomechanical factors: Increased dynamic knee valgus, decreased hamstring-to-quadriceps co-contraction ratio, stiffer landing profiles.
  • Anatomical factors: Narrower intercondylar notch width index, increased posterior tibial slope, joint hyperlaxity.
  • Hormonal influences: Oestrogen and relaxin receptor interactions modulating ligament tensile strength.

Relevant Anatomy

Bundles & Kinematics

The ACL originates from the posteromedial aspect of the lateral femoral condyle within the intercondylar notch and inserts onto the tibial plateau anterior and lateral to the anterior tibial spine:

• Anteromedial (AM) Bundle: Tightens in flexion, primary restraint against anterior tibial translation.
• Posterolateral (PL) Bundle: Tightens in extension, primary restraint against rotatory subluxation and internal rotation.

Blood Supply & Innervation

• Vascularity: Predominantly supplied by the middle genicular artery, with minor contributions from the inferior medial and lateral genicular arteries. Synovial sheaths carry vessels that provide poor intrinsic repair capacity within intra-articular synovial fluid.
• Innervation: Posterior articular branches of the tibial nerve, containing mechanoreceptors (Ruffini, Pacinian, and Golgi-like corpuscles) critical for dynamic neuromuscular proprioception.

Classification

While complete versus partial disruptions are assessed arthroscopically and on magnetic resonance imaging (MRI):

• By Location: Proximal avulsion (Sherman Type I), midsubstance (most common), and distal detachment.
• Paediatric Avulsion Fractures: Classified by Meyers & McKeever:
  • Type I: Undisplaced anterior tibial spine avulsion.
  • Type II: Anteriorly elevated with intact posterior hinge.
  • Type III: Completely displaced/detached fragment (IIIA unrotated, IIIB rotated).
  • Type IV (Zaricznyj): Comminuted avulsion fracture.

Clinical Presentation & Examination

History

• Classic non-contact deceleration, cutting manoeuvre, or awkward landing.
• Sensation or audible "pop" (present in >70% of cases).
• Rapid-onset tense haemarthrosis (<2–4 hours).
• Inability to continue sporting play.

Physical Examination

• Lachman Test: Performed at 20°–30° flexion. The most sensitive and specific test for ACL disruption (>95% sensitivity); loss of a crisp end-point indicates tear.
• Pivot-Shift Test (Galway & MacIntosh): Performed with valgus stress, axial compression, and internal rotation moving from extension to flexion. Demonstrates sudden reduction of anteriorly subluxed lateral tibial plateau at 30°–40° flexion. Highly specific (>98%) for functional rotatory instability; often only fully appreciated under anaesthesia.
• Anterior Drawer Test: Evaluated at 90° flexion. Less sensitive in acute injury due to hamstring guarding and posterior horn of the medial meniscus blocking translation.
• Secondary Injuries: Assess collateral ligaments, joint lines (menisci), and posterolateral corner (PLC).

Investigations

• Plain Radiographs (AP, Lateral, Skyline, Rosenberg 45° PA view):
  • Rule out avulsion fractures (tibial spine, Segond fracture—a cortical avulsion of the anterolateral ligament complex pathognomonic for ACL rupture).
  • Assess posterior tibial slope and identify osteochondral lesions or arthrosis.
• Magnetic Resonance Imaging (MRI):
  • Primary signs: Non-visualisation or discontinuity of fibres, hyperintensity within substance, horizontal orientation of distal ligament stump.
  • Secondary signs: Bone bruise pattern affecting the posterior lateral tibial plateau and mid-portion of the lateral femoral condyle (sulcus terminalis sign), uncovering of the posterior horn of the lateral meniscus, deep sulcus terminalis sign (>1.5 mm notch depth).
  • Evaluate meniscal tear configuration (e.g., ramp lesions, root tears) and concomitant collateral ligament pathology.

Management

Non-Operative Management

• Indications: Low-demand, sedentary patients without episodes of subjective instability during daily activities, or those willing to modify pivoting sports activities. Landmark KANON trial (Frobell et al., 2010) demonstrated that structured rehabilitation with early optional delayed reconstruction yields comparable mid-term functional scores to immediate surgery in selected recreational populations.
• Protocol: Quadriceps and core strengthening, closed kinetic-chain exercises, neuromuscular proprioceptive rehabilitation (NMT), and functional derotation bracing.

Operative Management

Surgical intervention is indicated to restore rotatory and translational stability, enable return to multi-directional sports, and safeguard secondary stabilisers (menisci, chondral surfaces). Early reconstruction (<3 months) reduces secondary meniscal tears and irreparable cartilage deterioration according to AAOS guidelines.

Surgical Technique & Principles

• Graft Options:
  • Bone–Patellar Tendon–Bone (BPTB): Gold standard for high-level athletes; rapid bone-to-bone healing in tunnels (6 weeks). Disadvantage: anterior knee pain, kneeling discomfort, patella fracture risk.
  • Hamstrings (Semitendinosus ± Gracilis): Multi-strand construct; low kneeling morbidity, smaller incision. Disadvantage: tendon-to-bone healing via Sharpey fibres (8–12 weeks), hamstring flexor deficit, subtle graft elongation.
  • Quadriceps Tendon: High tensile load, versatile (with or without bone block), lower donor site morbidity.
  • Allografts: Avoid in young, active patients due to significantly higher re-rupture rates, delayed biological incorporation, and cost; reserved for multiligament reconstructions or complex revisions.
• Tunnel Positioning:
  • Femoral Tunnel: Anatomical footprint placement via an independent anteromedial (AM) portal or outside-in technique rather than traditional transtibial drilling (which risks a vertically oriented, non-anatomical graft causing persistent pivot-shift).
  • Tibial Tunnel: Centred on the native footprint, 7 mm anterior to PCL margin, guided by the anterior horn of lateral meniscus and tibial spines; avoid excessive anterior placement to prevent notch impingement in full extension.
• Lateral Extra-Articular Tenodesis (LET) / ALL Reconstruction:
  • The STABILITY trial (Getgood et al.) established that adding a modified Lemaire LET to hamstring autografts in high-risk young athletes (age <25, high-grade pivot shift, generalised laxity, pivoting sports) significantly reduces graft failure rates and improves stability.
• Paediatric Considerations:
  • In skeletally immature patients, physeal-sparing techniques (e.g., all-epiphyseal tunnels using fluoroscopy or iliotibial band over-the-top reconstructions) prevent growth disturbance and angular deformities.

Complications

• Arthrofibrosis / Loss of Extension: Avoid early surgery during the acute "hot" phase with poor range of motion or severe effusion. Preoperative rehabilitation addressing full hyperextension is vital.
• Graft Rupture: 5–15% risk in young athletes; associated with early return to sports (<9 months), non-anatomical tunnel placement, undersized grafts (<8 mm diameter), or unrecognised secondary pathologies (high posterior tibial slope >12°, PLC deficiency).
• Cyclops Lesion: Fibrovascular nodule anterior to the tibial tunnel causing painful terminal extension loss; managed with arthroscopic debridement.
• Infection: 0.5–1.5%; graft retention feasible with early arthroscopic lavage and targeted antibiotic therapy, especially with vancomycin graft pre-soaking.

Exam Pearls

• Lachman vs. Pivot-Shift: Lachman is the most sensitive; the pivot-shift is the most specific physical exam sign for complete ACL tears.
• Segond Fracture: Represents avulsion of the anterolateral complex off the proximal lateral tibia and signifies an ACL tear in >90% of presentations.
• Graft Choice in Young Pivoting Athletes: Autograft is strongly favoured over allograft in young athletic populations due to significantly higher allograft failure rates.
• Tunnel Malposition: The commonest cause of technical failure in primary ACL reconstruction is non-anatomical femoral tunnel placement (too anterior or vertical).
• Meniscal Preservation: Concomitant meniscal ramp lesions and root tears must be inspected and repaired at the time of reconstruction to protect the graft from excessive translation and rotational forces.