Anterior Cruciate Ligament (ACL) Rupture
Summary
The Anterior Cruciate Ligament (ACL) is the primary restraint to anterior tibial translation and rotational instability. Rupture typically occurs via a Non-Contact Pivot mechanism (Valgus + External Rotation) in sports. The patient often hears a "Pop" followed by rapid swelling (haemarthrosis). Diagnosis is clinical (Lachman Test is Gold Standard) confirmed by MRI. Management is stratified: Conservative (Physio) is appropriate for many ("Copers"), while Surgical Reconstruction is indicated for symptomatic instability ("Giving way") or high-demand athletes. [1,2,3]
Key Facts
- The "Pop": Experienced by >80% of patients.
- Rapid Swelling: Swelling within <4 hours indicates BLOOD (Haemarthrosis). In a traumatic knee, haemarthrosis = ACL tear (70%) or Patella dislocation or Fracture.
- Segond Fracture: An avulsion fracture of the anterolateral tibia (ALL insertion). It is Pathognomonic for an ACL tear.
Clinical Pearls
"Lachman beats Drawer": The Anterior Drawer test is weak (hamstring spasm hides the laxity). The Lachman test (at 20° flexion) negates the hamstrings and is far more sensitive.
"Pivot Shift is the Money Test": The Lachman tests anterior laxity (AP plane). The Pivot Shift tests ROTATIONAL instability. This "clunk" is what makes the knee give way on the pitch. It is difficult to perform in awake patients.
"Female Paradox": Female athletes are 4-6x more likely to tear their ACL due to wider pelvis (Q-angle), hormonal laxity (ovulation), and landing mechanics (valgus collapse).
Demographics
- Incidence: Common. 1 in 3000.
- Gender: Female > Male (in same sport).
- Age: 15-45 years (Active population).
Anatomy
- Origin: Lateral Femoral Condyle (Medial wall).
- Insertion: Anterior Tibial Plateau (Intercondylar eminence).
- Bundles:
- Anteromedial (AM): Tight in flexion (controls AP translation).
- Posterolateral (PL): Tight in extension (controls Rotation).
Mechanism
- Valgus + External Rotation: The foot is planted, the body twists away. The ACL winds around the PCL and snaps.
- Hyperextension: The "Boot top" fracture mechanism.
Symptoms
Signs
Imaging
- X-Ray Knee (AP/Lat):
- Rule out fracture.
- Segond Fracture: Avulsion of lateral tibial plateau.
- MRI Knee: Gold Standard.
- Sensitivity >95%.
- Look for "Bone Bruising" on Lateral Femoral Condyle and Posterolateral Tibia (where they kissed during the subluxation).
- Check for "Double Meniscus" sign (Bucket handle tear).
ACUTE KNEE INJURY
(Pop + Rapid Hemarthrosis)
↓
EXCLUDE FRACTURE / DISLOCATION
(X-ray / Vascular Status / MRI)
↓
ACL RUPTURE CONFIRMED
↓
LOCKING MENISCUS?
┌───────┴───────┐
YES NO
↓ ↓
URGENT SCOPE PRE-HABILITATION
(Repair Meniscus) (Physio: ROM + Quads)
↓
DEMAND / INSTABILITY?
┌────────┴────────┐
LOW (Coper) HIGH (Non-Coper)
↓ ↓
CONSERVATIVE SURGERY
(Physio Only) (Reconstruction)
1. Pre-habilitation (The "Quiet Knee")
- Mandatory. You cannot operate on a stiff/swollen knee (High risk of Arthrofibrosis).
- Goal: Full extension and settling of effusion (usually 4-6 weeks post-injury) before surgery.
2. Conservative (Non-Operative)
- Indication: Low demand patients, "Copers" (no instability in daily life), Arthritis present.
- Protocol: Quadriceps and Hamstring strengthening.
- KANON Trial: Showed that starting with physio and only operating if unstable gave similar results to early surgery.
3. Surgical: ACL Reconstruction
- Not "Repair": You can't sew the mop-ends together. You must replace it with a graft.
- Graft Options:
- Hamstring (Gracilis/Semitendinosus): Standard. Low donor morbidity.
- Bone-Patella-Tendon-Bone (BPTB): Gold standard for high-performance athletes. Faster healing (bone-to-bone). Risk of anterior knee pain.
- Quadriceps Tendon: Gaining popularity. Beefy graft.
- Allograft (Cadaver): Risk of re-rupture in young patients.
Cyclops Lesion
- A fibrous nodule forms anterior to the graft in the intercondylar notch.
- Result: Loss of Extension. The knee clicks/blocks in terminal extension.
- Treatment: Arthroscopic debridement.
Graft Rupture
- Incidence: 5-10%.
- Risk factors: Returning to sport too early (<9 months), Allograft, Vertical tunnel placement.
Infection
- Septic arthritis (rare <1%). Disaster for cartilage.
The KANON Trial (Frobell et al. NEJM 2010)
- Comparison: Early ACL reconstruction vs Rehab with delayed reconstruction if needed.
- Finding: No difference in PROMs (KOOS score) or OA rates at 2 or 5 years.
- Impact: Supports a trial of conservative management for many patients.
Return to Sport Criteria
- Not time-based alone (though usually >9 months).
- Criteria: >90% Limb Symmetry Index (LSI) on hop tests, Full ROM, No effusion, Psychological readiness.
What is the ACL?
It is a rope in the middle of your knee that stops the shin bone sliding forwards. You snapped it when you pivoted.
Do I need surgery?
Not necessarily. Many people can live normal lives without an ACL, provided they strengthen their muscles to compensate. We only operate if your knee keeps buckling ("giving way") or if you play high-level twisting sports (football/netball).
The Operation
We can't sew the old ligament back together. We take a spare tendon (usually from your hamstring) and drill tunnels in your bone to weave a new ligament in its place.
Recovery
It is a long road. 9-12 months before you play sport again. The graft is actually weakest at 3 months (when the body is remodeling it), so don't be fooled by feeling good early on.
- Frobell RB, et al. A randomized trial of treatment for acute anterior cruciate ligament tears (KANON). N Engl J Med. 2010.
- Shelbourne KD, et al. Arthrofibrosis in acute anterior cruciate ligament reconstruction. Am J Sports Med. 1991.
Q1: What is the "Segond Fracture"? A: An avulsion fracture of the anterolateral tibial plateau, at the insertion of the Anterolateral Ligament (ALL) / Capsule. It is considered pathognomonic for an ACL rupture (present in 75% of tears).
Q2: Compare Hamstring vs BPTB grafts. A: Hamstring: Less donor site pain, strong, cosmetically better. Slower integration (soft tissue into bone). BPTB: Bone plugs heal fast (6 weeks). Stiffer biomechanics. High risk of Anterior Knee Pain (Kneeling pain) and Patella Fracture.
Q3: Describe the Pivot Shift mechanism. A: It mimics the injury. Apply a Valgus force, Internally Rotate the foot, and Flex the knee. The subluxed tibia (due to no ACL) suddenly reduces (clunks) back into place at ~30 degrees of flexion as the IT Band switches from an extensor to a flexor.
(End of Topic)