Overview & Epidemiology
Anterior cruciate ligament (ACL) injuries are among the most common and most significant sports injuries, with an estimated incidence of 200,000–250,000 per year in the United States alone. They predominantly affect young active adults (peak incidence 15–25 years) participating in pivoting sports (football, rugby, basketball, netball, skiing). Females have a 2–8 times higher risk of ACL injury than males in the same sports — attributed to a combination of anatomical (wider Q angle, narrower intercondylar notch), hormonal (oestrogen effects on ligament laxity), and neuromuscular (delayed hamstring activation, greater valgus collapse) factors. The ACL is the primary restraint against anterior tibial translation and internal rotation of the tibia relative to the femur; its loss creates characteristic rotatory instability that impairs sports participation and accelerates secondary meniscal and cartilage injury.
- ACL anatomy: the ACL runs from the posterior medial wall of the lateral femoral condyle (the femoral footprint is an oval at the 10-o`clock position on the lateral condyle wall — 15–18 mm long, 10–11 mm wide) to the anterior tibial spine (the tibial footprint is a wider oval, 28 mm long, 11 mm wide, anterior to and between the tibial eminences); the ACL consists of two functional bundles — the anteromedial (AM) bundle (primary restraint to anterior translation in all angles of flexion; taut in flexion) and the posterolateral (PL) bundle (primary restraint to rotational instability; taut in extension); both bundles work together to provide combined anterior and rotational stability
- Mechanism: non-contact mechanism (70–80% of ACL tears) — a deceleration, cutting, or landing manoeuvre with the knee near full extension in valgus (the `position of no return`); the foot is planted while the body rotates; valgus collapse + internal rotation = the highest risk position; contact mechanism (20–30%) — direct blow to the lateral knee (valgus stress); the unhappy triad (O`Donoghue) = ACL + MCL + medial meniscus (historically called `medial meniscus` but modern series show the LATERAL meniscus is more commonly torn acutely)
Clinical Diagnosis
| Test | Technique | Positive Finding | Sensitivity / Specificity |
|---|---|---|---|
| Lachman test | The MOST SENSITIVE clinical test for ACL tear; patient supine, knee at 20–30° flexion; examiner stabilises the distal femur with one hand and applies a firm anterior force on the proximal tibia with the other; assess: (1) degree of translation (graded 1+ = 0–5 mm, 2+ = 5–10 mm, 3+ = >10 mm); (2) endpoint quality (hard vs soft/absent) | Increased anterior tibial translation compared to the contralateral knee; soft or absent endpoint (in contrast to the firm endpoint of a normal or isolated MCL-injured knee); the endpoint quality is often more diagnostically significant than the absolute translation measurement | Sensitivity ~86%, specificity ~91% — the gold standard clinical test; the most reliable test in the acute setting (less hamstring guarding than the anterior drawer); in acute haemarthrosis + positive Lachman = ACL tear until proven otherwise |
| Pivot shift test | The MOST SPECIFIC test for ACL insufficiency; patient supine, relaxed; examiner holds the ankle in internal rotation and applies valgus stress while progressively flexing the knee from full extension; at approximately 20–30° of flexion the anterolaterally subluxed tibia suddenly reduces with a visible and palpable `clunk`; graded: 0 (negative — no shift), 1+ (glide — subtle), 2+ (clunk — definite reduction), 3+ (gross — the tibia `jumps` and catches) | A visible and palpable `clunk` or reduction as the knee is brought from extension into flexion; the IT band goes from an extensor to flexor role at ~20–30° of flexion, reducing the anterolaterally subluxed tibia | Sensitivity ~48% (awake patients guard), ~93% (under anaesthesia); specificity ~98%; the pivot shift is specific but insensitive when the patient is awake due to guarding; the grade of pivot shift correlates with functional disability and guides the decision to add a lateral extra-articular tenodesis (LET) — a higher-grade pivot shift = stronger indication for combined ACL + LET |
| Anterior drawer test | Knee at 90° flexion, foot stabilised; examiner pulls the tibia anteriorly; less sensitive than Lachman at 90° because the hamstrings are placed at a mechanical advantage at this angle (they tighten and reduce the anterior drawer); useful if Lachman cannot be performed | Increased anterior translation vs contralateral; soft endpoint | Sensitivity ~55–72%, specificity ~62–89%; lower sensitivity than Lachman at 90° due to hamstring guarding; the classic test but superseded by Lachman as the gold standard |
Investigations
- MRI: the investigation of choice for confirming ACL tear and assessing associated injuries; sensitivity ~94–98%, specificity ~96–100%; MRI findings of ACL tear: (1) complete absence of the normal linear ACL fibres (direct sign); (2) abnormal signal (oedema/haemorrhage) within the torn ACL; (3) secondary signs — bone bruising (kissing contusions) at the lateral femoral condyle and posterolateral tibial plateau (the `footprint` of the pivot shift mechanism — present in ~60% of acute ACL tears; pathognomonic of the ACL injury mechanism); (4) Segond fracture — a lateral tibial rim avulsion fracture from the anterolateral ligament/lateral capsule; pathognomonic of ACL tear (95% have concurrent ACL tear); (5) anterior tibial translation >7 mm relative to the posterior femoral condyle line on sagittal MRI
- Arthrometer (KT-1000/KT-2000): a mechanical device that quantifies anterior tibial translation in millimetres; placed over the anterior knee and calibrated force applied; measures side-to-side difference in anterior translation; a difference of >3 mm between knees = significant ACL insufficiency; used in research and for objective assessment of laxity pre- and post-reconstruction
Graft Selection for ACL Reconstruction
| Graft Type | Source | Strength | Advantages | Disadvantages | Best Indication |
|---|---|---|---|---|---|
| Hamstring tendon (ST/G — semitendinosus ± gracilis) | Ipsilateral semitendinosus (and gracilis if 4-strand graft — quadrupled ST/G); harvested through a small anteromedial incision | 4-strand quadrupled ST/G: 4,090 N ultimate tensile load (stronger than native ACL ~2,160 N); best mechanical properties of autograft options | Excellent tensile strength; small harvest incision; less anterior knee pain than BPTB; no patellar fracture risk; preserves extensor mechanism | Slower biological incorporation (tendon-bone healing slower than bone-bone); risk of harvesting the saphenous nerve branch; loss of hamstring strength (~10–15% deficit initially — recovers at 2 years); cannot use if hamstring is injured; tunnel aperture fixation relies on interference screw purchase in soft tissue | Young active patients; athletes who need to kneel (BPTB causes anterior knee pain with kneeling); patients with patellar tendinopathy; female athletes (lower re-tear rates than BPTB in some studies) |
| Bone-patellar tendon-bone (BPTB) | Central third of patellar tendon with bone plugs from the patella and tibial tuberosity; harvested through a midline incision over the patellar tendon | 2,900 N; bone-to-bone healing (fastest biological integration — bone heals in bone tunnel within 6 weeks) | Gold standard graft — most data; bone-to-bone healing; rigid fixation; preferred for revision surgery; ideal for patients who cannot tolerate hamstring weakness | Anterior knee pain (especially with kneeling — `kneeler`s pain`); patellar fracture risk (<1%); patellar tendon rupture risk (<0.1%); donor site morbidity; harvest extends the incision; quadriceps weakness post-op; not ideal for patients who kneel for work/religion | Revision ACL reconstruction; contact athletes (high re-tear risk who need strongest graft); patients with hamstring injury; older high-demand athletes |
| Quadriceps tendon (QT) | Quadriceps tendon (partial or full thickness) ± patellar bone plug; harvested from the proximal patella region | Similar to BPTB; larger cross-sectional area than BPTB or ST; ~2,352 N | Larger graft diameter than hamstring; bone plug available if needed; less donor site morbidity than BPTB; growing evidence base; increasingly popular | Less historical data than BPTB or HT; quadriceps deficit at the donor site; technically more demanding harvest | Alternative autograft when HT and BPTB are unavailable (e.g. prior harvest); revision surgery; gaining popularity as primary graft choice |
| Allograft | Cadaveric graft (BPTB, Achilles tendon, tibialis anterior, hamstring); irradiated or fresh-frozen | Variable (fresh-frozen allografts have similar initial strength; irradiated grafts are significantly weaker) | No donor site morbidity; shorter operative time; any size available; useful in revision or multi-ligament reconstruction | Higher re-tear rates in young active patients (<25 years) — multiple studies show 3–5× higher failure rate vs autograft in young athletes; slower incorporation; disease transmission risk (extremely low with modern processing); inappropriate for primary ACL in young active patients | Older patients (>40 years) with lower activity demands; multi-ligament reconstruction; revision surgery where autograft sites are exhausted |
Lateral Extra-Articular Tenodesis (LET) — When to Add
- The STABILITY trial (Getgood et al., NEJM 2022): the landmark RCT that established the role of LET as an adjunct to ACL reconstruction; 618 young active patients with high-grade pivot shift (2+/3+) or revision ACL randomised to ACL reconstruction alone vs ACL + LET; at 2 years, the combined ACL + LET group had significantly lower ACL graft re-rupture rates (4.6% vs 9.8% — a 52% reduction in re-rupture risk); the LET group had better rotational stability (lower residual pivot shift grade) without significant increase in complications; conclusion: LET should be considered for young active patients with high-grade pivot shift, female athletes, and those returning to high-risk pivoting sports
- Indications for combined ACL + LET: high-grade pivot shift (2+/3+) pre-operatively; young active patients (<25 years) returning to cutting sports; revision ACL reconstruction; female athletes in high-risk sports; generalised ligamentous laxity (Beighton score >4)
- LET technique: a strip of the ITB is harvested from just proximal to Gerdy`s tubercle (the ITB tibial insertion is preserved); the strip is routed under the lateral collateral ligament and fixed to the lateral femoral condyle with a suture anchor or staple; acts as a static lateral restraint augmenting the anterolateral capsular complex and ALL; not the same as a lateral Lemaire reconstruction (which is entirely extra-articular)
Return to Sport Criteria
- Time alone is NOT sufficient: the traditional `9-month rule` for return to sport has been challenged by multiple studies showing that graft maturation (ligamentisation) takes 18–24 months; a minimum of 9 months post-operatively is recommended for high-level athletes; at 6 months the re-rupture rate for return to sport is approximately 4× higher than at 9 months; at 9 months it is still 2× higher than at 12 months; criteria-based return to sport is superior to time-based
- Criteria-based return to sport: (1) Limb Symmetry Index (LSI) >90% for quadriceps strength (isokinetic testing at 60°/s and 180°/s); (2) LSI >90% for hamstring strength; (3) single-leg hop tests (single hop, triple hop, cross-over hop, timed 6-metre hop) — all >90% symmetry; (4) psychological readiness — ACL-RSI (ACL Return to Sport after Injury) score >65 out of 100; (5) full range of motion; (6) no effusion; (7) successful completion of sport-specific agility training and plyometric programme; all criteria must be met before return to unrestricted pivoting sport
Exam Pearls
- Lachman test: most sensitive (86%) — knee at 20–30°; anterior force; soft endpoint = positive; anterior drawer at 90° is less sensitive (55%) due to hamstring guarding; pivot shift = most specific (98%) — clunk at 20–30° as knee flexed
- ACL bundles: AM (taut in flexion — primary anterior translation restraint) and PL (taut in extension — primary rotational restraint); both must be reconstructed or addressed for complete stability restoration
- Bone bruising: lateral femoral condyle + posterolateral tibial plateau = kissing contusions from the pivot shift mechanism; present in ~60% acute ACL tears on MRI; pathognomonic of ACL injury mechanism; Segond fracture (lateral tibial rim avulsion) = 95% associated with ACL tear
- Graft strength: quadrupled ST/G ~4,090 N (strongest); BPTB ~2,900 N; native ACL ~2,160 N; all autografts exceed native ACL strength; allograft (fresh-frozen) similar initial strength but higher re-tear rate in young athletes
- STABILITY trial: combined ACL + LET reduces re-rupture from 9.8% to 4.6% (52% reduction) in young high-risk patients; consider LET for: high-grade pivot shift, young active patients, female athletes, revision ACL, generalised laxity
- Allograft in young athletes: 3–5× higher re-tear rate than autograft in patients <25 years; DO NOT use allograft as primary graft in young active patients returning to pivoting sports; reserve for >40 years or multi-ligament reconstruction
- Return to sport: minimum 9 months; criteria-based (LSI >90% quad + hamstring; hop tests >90%; ACL-RSI score >65); time alone insufficient — re-rupture risk is 2× higher at 9 months vs 12 months