ACL Injury — Workup & Reconstruction

The anterior cruciate ligament (ACL) is the primary restraint to anterior tibial translation and an important secondary restraint to internal tibial rotation. ACL tears are among the most common and significant sports injuries, with approximately 200,000 reconstructions performed annually in the USA. The decision to reconstruct, the timing, the choice of graft, and the rehabilitation protocol are all areas of active evidence development and surgical debate.

• ACL anatomy: two functional bundles — anteromedial (AM) bundle: taut in flexion, primary restraint to anterior translation; posterolateral (PL) bundle: taut in extension, primary restraint to rotational instability; the ACL runs from the posterior aspect of the medial wall of the lateral femoral condyle to the tibial eminence between the meniscal horns
• Incidence: approximately 70,000–200,000 ACL tears per year in the USA; peak incidence in athletes aged 15–25; female athletes at 2–8× higher risk than males in same sport; most common mechanisms — non-contact (cutting, pivoting, landing) accounting for approximately 70% of tears
• Female predisposition to ACL injury: multifactorial — hormonal effects on ligament laxity, wider Q-angle, narrower intercondylar notch, different neuromuscular recruitment patterns (quadriceps-dominant landing), smaller ACL cross-sectional area, and genu valgum; prevention programs (FIFA 11+, ACL injury prevention protocols) reduce incidence by approximately 50%

• History: non-contact deceleration, cutting, or landing mechanism; loud "pop" heard or felt; immediate swelling (haemarthrosis within 2 hours); inability to continue play; giving way on pivoting activities
• Lachman test: most sensitive test for ACL tear — knee at 20–30° flexion; stabilise distal femur; translate tibia anteriorly; positive = anterior translation >5 mm or soft endpoint; sensitivity 85–98%, specificity 94–99%; more sensitive than anterior drawer in acute setting due to reduced hamstring guarding
• Anterior drawer test: knee at 90° flexion; anterior tibial translation; less sensitive than Lachman in acute setting (hamstring guarding); sensitivity 41–93%
• Pivot shift test: most specific test for functional ACL instability; knee extended and internally rotated with valgus stress; positive = palpable/visible clunk as the tibia reduces from subluxed position at approximately 20–30° flexion; sensitivity 18–82% (reduced in awake patients due to guarding — best under anaesthesia), specificity 95–99%; correlates best with functional instability and return-to-sport outcomes
• Associated injuries: medial meniscus tear (most common — "unhappy triad": ACL + medial meniscus + MCL); lateral meniscus (more common in acute ACL); bone bruising (posterolateral tibial plateau and anterior lateral femoral condyle — classic pattern on MRI)
• Segond fracture: lateral capsular avulsion fracture from the anterolateral tibial rim; seen on AP knee X-ray; pathognomonic of ACL tear; represents avulsion of the anterolateral ligament (ALL) or iliotibial band attachment

• Knee radiographs (AP, lateral, skyline): assess for fractures (Segond, tibial spine avulsion, Pellegrini-Stieda in MCL), arthritic change, and bony alignment; mandatory in all knee injuries
• MRI knee: gold standard for soft tissue assessment; sensitivity for ACL tear approximately 87–95%, specificity 96–99%; identifies bone bruising (posterolateral tibial plateau + anterolateral femoral condyle = classic ACL injury pattern), meniscal tears, collateral ligament injuries, and chondral damage; useful for surgical planning
• KT-1000 arthrometer: objective measurement of anterior tibial translation; side-to-side difference >3 mm = significant; >5 mm = ACL deficient; used for pre- and post-operative assessment and in research
• EUA (examination under anaesthesia): allows accurate pivot shift grading (grade 0–3 under anaesthesia) prior to arthroscopic surgery; important for research and outcome prediction

• Non-operative management is appropriate for: older sedentary patients, patients with low demand activities, those unwilling to undergo surgery, partial ACL tears with minimal instability, and patients who adapt successfully to the ACL-deficient knee
• KANON trial (Frobell et al. 2013): RCT comparing early ACLR vs structured rehabilitation ± delayed ACLR; at 5 years, no significant difference in knee function scores (KOOS) between early reconstruction and rehabilitation-first strategy; approximately 50% of the rehabilitation group crossed over to surgery; conclusion: structured rehab is a valid first-line strategy for active patients — surgery is not mandatory for all ACL tears
• Structured rehabilitation: focuses on quadriceps and hamstring strength, proprioception, neuromuscular training; ACL-deficient patients with good functional stability ("copers") may succeed without surgery
• Contraindications to non-operative management: high demand athlete, significant rotational instability, combined ligamentous injury, meniscal tear requiring surgical management, young active patient wishing to return to pivoting sport

• Graft ligamentisation: the transplanted graft undergoes a process of "ligamentisation" — avascular necrosis of the graft initially, followed by revascularisation and remodelling; the graft is weakest at approximately 6–12 weeks (avascular phase); ligamentisation takes 12–24 months to approach native ACL properties; this biological process dictates rehabilitation pacing, not the structural appearance on MRI

• Tunnel placement: anatomical placement is the most important technical determinant of outcome; the femoral tunnel should be in the centre of the native ACL femoral footprint (anteromedial portal technique allows more anatomical placement than the transtibial technique, which places the femoral tunnel too vertically); vertical graft = controls anterior translation but not rotational instability = poor pivot shift control
• Timing of ACL reconstruction: reconstruction should be delayed until acute swelling and haemarthrosis have resolved and full ROM has returned; reconstruction in the acute phase (<1 week, with haemarthrosis and stiffness) significantly increases the risk of arthrofibrosis; generally recommend 3–6 weeks after injury before surgery, or when quadriceps function, ROM and swelling have recovered
• Lateral extra-articular tenodesis (LET) and anterolateral ligament (ALL) reconstruction: augmentation procedures addressing the anterolateral capsular structures; added to intra-articular ACLR in high-risk patients (young, high-demand, hyperlaxity, revision) — reduces residual rotational instability and re-rupture rate; STABILITY trial (Getgood et al. 2022) showed LET addition reduces re-rupture rate in young athletes
• Double-bundle ACLR: reconstructs both AM and PL bundles separately; theoretically superior rotational control; RCTs have not consistently shown superiority over single-bundle anatomical reconstruction; technically more demanding; not standard of care

• Time-based return to sport criteria are insufficient: criteria-based return to sport (strength symmetry, functional tests, psychological readiness) is superior to time-based alone; recommendations: quadriceps limb symmetry index (LSI) ≥90%, hop test LSI ≥90%, psychological readiness (ACL-RSI score), completion of progressive running and agility programme
• Minimum time to return to pivoting sport: approximately 9–12 months — earlier return significantly increases re-rupture risk; each month of delay in return to sport beyond 9 months reduces re-rupture risk by approximately 51%
• Re-rupture rates: approximately 15–25% in young athletes returning to high-demand pivoting sport; higher in females; higher with allograft; lower with additional LET
• Contralateral ACL tear: the uninjured knee is also at significant risk — approximately 15–25% of young athletes sustain a contralateral ACL tear within 5 years of primary reconstruction; address neuromuscular risk factors bilaterally in rehabilitation

• Tibial spine avulsion fracture: ACL equivalent injury in skeletally immature children; the ACL avulses its tibial attachment rather than rupturing mid-substance (stronger physis than ligament); classified by Meyers-McKeever classification (Type I non-displaced to Type III completely displaced); non-displaced types managed in extension cast; displaced types require arthroscopic or open reduction and internal fixation (suture or screw); ACL integrity preserved if anatomical reduction achieved
• ACL reconstruction in the skeletally immature: physeal-sparing techniques to avoid iatrogenic growth disturbance; all-epiphyseal technique (femoral and tibial tunnels entirely within the epiphysis); transphyseal technique acceptable in patients within 1–2 years of skeletal maturity (smaller graft diameter, soft tissue graft only, careful tunnel positioning); untreated ACL tear in a child risks progressive meniscal and chondral damage from instability
• Revision ACL reconstruction: causes of failure — technical error (tunnel malposition most common), biological failure (graft necrosis), early return to sport, trauma; pre-operative CT and MRI assess tunnel position and size; staged revision (bone grafting of malpositioned tunnels first, then reconstruction) vs single-stage revision depending on tunnel expansion; use different graft type from primary (if BPTB failed, use hamstring or QT, or allograft)
• ACL and osteoarthritis: ACL-deficient knees develop OA at a higher rate than ACL-reconstructed knees; however, ACLR does not normalise OA risk to the non-injured population; the associated meniscal and chondral injuries at the time of ACL tear are the primary drivers of early OA; meniscal preservation and prompt reconstruction to prevent secondary meniscal injury are the most important OA-prevention strategies

• Lachman test: most sensitive (85–98%); knee 20–30°; anterior tibial translation; soft endpoint = positive
• Pivot shift: most specific (95–99%); correlates best with functional instability; best under anaesthesia
• Segond fracture: lateral capsular avulsion on AP X-ray; pathognomonic of ACL tear; represents ALL/ITB avulsion
• KANON trial: early ACLR vs rehab ± delayed ACLR — no difference at 5 years in KOOS; ~50% rehab group crossed over to surgery; rehab is a valid first-line strategy
• Graft ligamentisation: weakest at 6–12 weeks (avascular phase); ligamentisation takes 12–24 months; does NOT correlate with MRI appearance
• Tunnel placement: anatomical femoral footprint essential; anteromedial portal preferred over transtibial; vertical graft controls translation NOT rotation
• Timing: delay reconstruction until ROM and swelling resolved; acute reconstruction → arthrofibrosis risk
• LET/ALL augmentation: STABILITY trial — reduces re-rupture in young athletes; indicated in high-risk patients (young, revision, hyperlaxity)
• Return to sport: criteria-based not time-based; LSI ≥90% quad + hop; minimum 9–12 months; each extra month reduces re-rupture risk ~51%
• Tibial spine avulsion: ACL equivalent in children; Meyers-McKeever classification; fix displaced types arthroscopically