Anatomy and Biomechanics
Bundle Anatomy
The ACL comprises two functional bundles:
| Bundle | Femoral Origin (clock position, right knee) | Tibial Insertion | Primary Function |
|---|---|---|---|
| Anteromedial (AMB) | ~10:30 (proximal/posterior femoral footprint) | Anteromedial tibial plateau | Resists anterior tibial translation, especially in mid-flexion |
| Posterolateral (PLB) | ~9:30 (distal/anterior femoral footprint) | Posterolateral tibial plateau | Resists anterior translation near extension; rotational stability |
The femoral footprint lies on the medial wall of the lateral femoral condyle, posterior to the lateral intercondylar ridge (formerly "resident's ridge") and separated from the PLB origin by the bifurcate ridge. The tibial footprint measures approximately 15-18 mm mediolaterally and 11-12 mm anteroposteriorly; a footprint width ≥14 mm is required to accommodate two separate tibial tunnels in DB reconstruction.
Biomechanical Principles
The ACL provides approximately 86% of total resistance to anterior tibial draw. The bundles are non-isometric:
- AMB: taut in flexion, relatively lax near full extension
- PLB: taut near extension, relatively lax in flexion
This reciprocal tension pattern underpins the rationale for DB reconstruction, a single graft cannot fully replicate the range-of-motion behaviour of both bundles simultaneously.
A vertically oriented femoral tunnel (produced by the constrained transtibial technique) resists anterior translation but is relatively ineffective against internal rotation and pivot shift. Lower, more anatomically placed horizontal femoral tunnels restore rotational stability and reduce pivot shift.
Graft mechanical properties:
| Graft | Ultimate Tensile Load (N) | Stiffness (N/mm) |
|---|---|---|
| Native ACL | 2160 | 242 |
| 10 mm BPTB autograft | 1784 | 210 |
| 4-strand hamstring (ST/G) autograft | 4090 | 776 |
Femoral Tunnel Drilling Techniques
| Technique | Description | Tunnel Orientation | Key Limitation |
|---|---|---|---|
| Transtibial (TT) | Femoral guide passed through tibial tunnel; position constrained by tibial trajectory | Typically vertical (~11-12 o'clock) | Residual vertical graft; residual rotational instability |
| Anteromedial portal (AMP) | Guide placed through accessory AM portal, independent of tibial tunnel | More horizontal (~10-10:30 o'clock) | Requires knee flexion ≥120°; risk of posterior wall blowout if inadequately flexed |
| Outside-in (retrograde) | Femoral tunnel drilled from lateral cortex inward | Anatomic horizontal trajectory | Technically demanding; specialised instruments required |
The AMP technique centres the femoral tunnel within the native footprint and produces a more horizontal graft that better restores rotational stability. When optimal anatomic positioning cannot be achieved transtibially, the AMP technique should be used. In experienced hands, an optimal femoral tunnel can often be achieved transtibially using a lateralised trajectory, which a cadaveric study (Rue et al.) demonstrated can recreate the femoral footprints of both the AMB and PLB from a single tibial tunnel.
Single-Bundle vs Double-Bundle Reconstruction
Single-Bundle (SB) Reconstruction
- Most widely performed technique globally; reported good-to-excellent results in 90-95% of patients
- Recreates the AMB using one femoral and one tibial tunnel
- With anatomic (AMP or outside-in) femoral tunnel placement, the more horizontal graft position improves rotational stability compared with traditional transtibial techniques
- Shorter operative time, less bone loss, and easier revision surgery than DB
Double-Bundle (DB) Reconstruction
Separately reconstructs AMB and PLB. Tunnel configurations include:
- Two femoral tunnels + one tibial tunnel (most common)
- Two femoral tunnels + two tibial tunnels (more anatomic; biomechanically superior near extension)
Biomechanical advantages:
- Better controls anterolateral rotatory instability and pivot shift
- DB reconstruction is significantly better than SB at limiting anterior translation of the lateral compartment during pivot shift manoeuvre (cadaveric data)
- Two femoral tunnels with two tibial tunnels may produce better biomechanical outcomes especially near extension compared with two femoral/one tibial tunnel configurations
- Meta-analyses demonstrate DB reconstruction produces marginally less anterior laxity (KT-1000) than SB
Limitations: More technically demanding, longer operative time, greater bone loss, more difficult revision surgery.
Single-Tunnel Double-Bundle (STDB) Technique
A hybrid approach using a single femoral and single tibial tunnel with hamstring graft positioned so that semitendinosus strands represent the AMB and gracilis strands represent the PLB. Separating implants (e.g. Intrafix sheath with keel, or AperFix device) maintain bundle separation within the tunnel. Femoral tunnel length must be ≥30-35 mm for the AperFix device. This avoids the bone loss and technical challenges of two-tunnel DB reconstruction while potentially conferring some biomechanical benefit of separate bundle reconstruction.
Comparative Clinical Evidence
| Parameter | SB | DB |
|---|---|---|
| Anterior laxity (KT-1000) | Reference | Marginally less (meta-analyses favour DB) |
| Pivot shift control | Good | Better (biomechanical and some RCT data) |
| Objective IKDC | Comparable | Comparable to slightly better in select RCTs |
| Patient-reported outcomes (IKDC subjective, Lysholm, KOOS) | No consistent difference | No consistent difference |
| Operative time / bone loss | Less | Greater |
| RCT evidence (3 RCTs, n=375) | No difference in clinical or subjective outcomes | No difference in clinical or subjective outcomes |
Key conclusion: DB reconstruction may offer biomechanical advantage, particularly for rotational stability, but consistent clinical superiority over anatomic SB reconstruction has not been demonstrated in RCTs. Three RCTs comprising 375 patients found no advantage to DB reconstruction in clinical or subjective outcomes.
Tunnel Anatomy and Positioning
Femoral Tunnel
| Landmark | Relevance |
|---|---|
| Lateral intercondylar ridge ("resident's ridge") | Anterior border of femoral ACL footprint |
| Bifurcate ridge | Separates AMB (proximal) from PLB (distal) femoral origins |
| Posterior cartilage margin | Minimum 1-2 mm posterior wall required |
| Medial wall of lateral femoral condyle | Site of tunnel drilling |
- AMB femoral tunnel: ~10:30 position (right knee); proximal and posterior within footprint
- PLB femoral tunnel: ~9:30 position (right knee); distal and anterior; near posterior cartilage margin, higher risk of posterior wall fracture
- Knee must be flexed ≥120° when drilling via AMP to achieve adequate posterior wall thickness
- Tunnel length ≥25 mm required for adequate intra-tunnel graft contact
- SB tunnel: typically 8-10 mm (match graft diameter); DB tunnels: AMB ~7-8 mm, PLB ~5-7 mm
Tibial Tunnel
| Landmark | Relevance |
|---|---|
| Medial tibial spine | Medial border of footprint |
| Anterior horn of lateral meniscus | Posterior border guides entry point; avoid posterior encroachment |
| PCL | Intra-articular exit should be ≥7 mm anterior to PCL |
| Intercondylar eminence | Central reference |
- Extra-articular entry: ~3 cm medial to tibial tuberosity, ~1 cm proximal to joint line, directed posteromedially
- Intra-articular exit: centred in ACL tibial footprint, 7 mm anterior to PCL, knee at 90° flexion
- Tunnel angle: typically 55-65° from horizontal; a more lateral trajectory lateralises the femoral tunnel when using the transtibial technique
- DB tibial tunnels: AMB exits anteromedially, PLB exits posterolaterally within the footprint; ≥14 mm tibial footprint required to prevent tunnel confluence
Critical positioning errors:
| Error | Consequence |
|---|---|
| Too anterior | Roof impingement in extension |
| Too posterior | PCL or posterior wall impingement |
| Too medial / vertical femoral tunnel | Residual rotational instability, persistent pivot shift |
| Inadequate posterior wall (AMP) | Posterior wall blowout, loss of fixation |
Indications and Contraindications
Indications for Operative Reconstruction
- Symptomatic instability in active individuals (cutting, pivoting, jumping sports)
- Failed non-operative management with persistent functional instability
- Combined ligament or meniscal injury requiring joint stability for repair healing
- Manual labour or occupationally demanding activities
Double-Bundle Specific Indications
- Tibial footprint ≥14 mm
- High-grade preoperative pivot shift
- Persistent rotational instability after prior SB reconstruction (selected revision cases)
- Associated meniscal injury where rotational stability is critical to repair healing
Contraindications to Double-Bundle Reconstruction
| Absolute | Relative |
|---|---|
| Tibial footprint <14 mm | Narrow notch |
| Advanced articular degeneration (Outerbridge grade ≥3) | Open physes |
| Active infection | Multiligament injury (SB preferred) |
| Patient non-compliance | Severe bone bruising of lateral femoral condyle |
Graft Selection
| Graft | Advantages | Disadvantages |
|---|---|---|
| BPTB autograft | Bone-to-bone healing (faster incorporation), lower revision rate, established long-term data | Anterior knee pain, extensor mechanism morbidity |
| 4-strand hamstring (ST/G) autograft | High tensile load, low donor-site morbidity, cosmesis | Soft tissue-to-bone healing (slower); ~2× revision rate vs BPTB at 4 years (AOANJRR) |
| Quadriceps tendon autograft | Large cross-section, suitable for revision, minimal anterior knee morbidity | Fewer long-term data |
| Allograft | No donor-site morbidity | Higher failure rate in young/active patients; primarily for revision or multiligament setting |
AOANJRR data: Hamstring autograft has approximately twice the revision rate compared with BPTB autograft at 4-year follow-up; young age (particularly <20 years) is the dominant risk factor for revision across all graft types.
Fixation
Femoral Fixation
| Device | Mechanism | Notes |
|---|---|---|
| Suspensory (Endobutton, TightRope) | Cortical fixation via adjustable loop | High ultimate strength; allows tunnel backfill |
| Aperture interference screw | Compresses graft at tunnel aperture | Metal, bioabsorbable, or PEEK (PEEK allows imaging of graft-tunnel interface) |
| Hybrid | Suspensory + aperture | Combines cortical strength with aperture biology |
Tibial Fixation
| Device | Mechanism | Notes |
|---|---|---|
| Interference screw | Wedge compression of graft in tunnel | Standard thread screws cause clockwise graft rotation, reverse-thread screw improves position for right-sided reconstructions |
| Staple / post-and-washer | Extra-cortical | Backup fixation; suitable for short graft stumps |
| Intrafix (sheath and screw) | Radial compression aperture fixation | Used in STDB; sheath keel maintains AMB/PLB separation |
Graft tensioning: AMB fixed with ~40 N tension at full extension (0°); PLB tensioned at 0-20° of flexion.
Complications
| Complication | Mechanism | Prevention |
|---|---|---|
| Tunnel malposition | Non-anatomic / vertical femoral tunnel | Independent femoral drilling (AMP/outside-in); confirm landmark identification arthroscopically |
| Posterior wall blowout | Insufficient posterior wall, AMP technique | Flex knee ≥120°; confirm ≥1-2 mm posterior wall |
| Roof impingement | Tibial tunnel too anterior | Ensure exit is posterior to Blumensaat line at 90° flexion |
| Graft failure / re-rupture | Premature return to sport; tunnel malposition; hamstring graft in young patient | Anatomic placement; criterion-based return-to-sport; graft selection counselling |
| Distal femur fracture | Weakened lateral cortex (DB or excessively long/large tunnel) | Adequate bone bridge between tunnels; avoid excessive tunnel diameter |
| Arthrofibrosis | Surgery in acute inflammatory phase; inadequate rehabilitation | Restore full ROM pre-operatively; delay surgery until inflammation settled |
| Hamstring donor weakness | Aggressive harvest | Rehabilitation; gracilis preservation in selected patients |
Paediatric Considerations
- ACL deficiency in skeletally immature patients carries high rates of secondary meniscal and chondral injury if untreated
- Challenge: restore stability while avoiding physeal damage
- Physeal-sparing techniques (all-epiphyseal, extra-articular iliotibial band reconstruction) preferred in Tanner 1-2
- Partial transphyseal or transphyseal techniques acceptable in Tanner 3-4 (near-closure physes) using soft-tissue grafts traversing the physis without hardware at the physis
- DB reconstruction generally deferred until skeletal maturity given greater bone resection and growth disturbance risk
- Young age (<20 years) is the single greatest risk factor for revision ACL reconstruction in AOANJRR data regardless of technique
Outcome Measures
| Measure | Domain |
|---|---|
| IKDC Subjective Knee Form | Symptoms, function, activity, most widely used ACL-specific PROM |
| IKDC Objective (grade A-D) | Physical examination: pivot shift, Lachman, arthrometer |
| Lysholm Knee Score | Symptoms and function |
| Tegner Activity Scale | Activity level pre/post-injury; return-to-sport benchmarking |
| KOOS | Pain, symptoms, ADL, sport, QOL; captures long-term osteoarthritis trajectory |
| ACL-RSI | Psychological readiness to return to sport |
Key Examination Points
- Bifurcate ridge and lateral intercondylar ridge are the essential arthroscopic landmarks for anatomic femoral tunnel placement, identify these confidently
- AMB origin ~10:30, PLB origin ~9:30 on the medial wall of the lateral femoral condyle (right knee)
- AMP technique: enables independent, horizontal femoral tunnel placement; requires knee flexion ≥120° to prevent posterior wall blowout
- Tibial footprint <14 mm: absolute contraindication to DB reconstruction
- Three RCTs (n=375) comparing SB vs DB found no difference in clinical or subjective outcomes, DB shows biomechanical but not consistent clinical superiority
- DB reconstruction is biomechanically superior for pivot shift and rotational stability, especially in knees with associated meniscal injury or high-grade preoperative pivot shift
- Two tibial tunnels (vs one tibial tunnel) in DB reconstruction may produce better biomechanical outcomes particularly near extension
- AOANJRR: hamstring autograft ~2× revision rate of BPTB at 4 years; young age is the dominant revision risk factor
- Standard interference screw on right-sided ACL reconstruction causes clockwise graft rotation and posterior final position, reverse-thread screw addresses this
- Tibial fixation with ~40 N tension at full extension for AMB; PLB tensioned at 0-20° flexion
Sources