Anterior Cruciate Ligament Injuries

Who. ACL reconstruction runs at about 46 per 100,000 per year in German registry data, with a peak age of 15-25 years. In the same sport, females have a 2-8x higher injury rate than males; the factors are a narrower notch, hormonal influences and neuromuscular patterns, and prevention programmes reduce the risk by 50%. The high-risk sports are soccer, basketball and skiing.

What goes with it. Meniscal tears accompany 50% or more of acute injuries, MCL injury goes with a contact mechanism, and lateral meniscus root tears also occur. The Segond fracture on the radiograph and the bone-bruise pattern on MRI are described under Investigations.

Anatomy. The ACL arises from the posteromedial aspect of the lateral femoral condyle within the intercondylar notch and inserts on the anterior intercondylar area of the tibia, anterolateral to the medial tibial spine. It is 30-35mm long and 10-12mm wide. Its blood supply is the middle genicular artery, and that poor vascularity limits healing.

The two bundles. The anteromedial (AM) bundle is tight in flexion and is the main restraint to anterior translation; the posterolateral (PL) bundle is tight in extension and is the main restraint to rotation. The two work together, and losing the ligament leads to rotational instability, the pivot shift phenomenon.

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Function. The ACL is the primary restraint to anterior tibial translation, providing 85% of that restraint at 30 degrees of flexion. It also resists internal rotation and hyperextension.

Mechanism. 70% of injuries are non-contact: cutting or pivoting on a planted foot with the knee near extension and a valgus collapse. The other 30% are contact injuries, a direct blow producing valgus or hyperextension. Peak strain on the ligament comes near full extension with valgus and internal rotation, and it takes more than 2000N to fail it.

Injury grade. Most presentations are Grade III, a complete tear.

Pivot shift grade. The higher the grade, the greater the instability and the worse the functional outcome without surgery.

• Grade 0: negative, no subluxation
• Grade 1 (glide): subtle sliding
• Grade 2 (clunk): obvious subluxation and reduction
• Grade 3 (gross): transient locking with gross subluxation

Chronicity. An acute injury is within 3 weeks, with a haemarthrosis and limited range of motion. Subacute is 3 weeks to 3 months, when examination is better. Chronic is beyond 3 months, with functional instability and possible meniscal damage.

History. The story is a non-contact pivot, a pop, and immediate swelling from a haemarthrosis, followed by giving way with pivoting. Ask about the level of sport, which determines treatment, and about previous injuries, including the contralateral ACL.

Examination. The Lachman test is the most sensitive (85-95%); the anterior drawer is less sensitive; the pivot shift is the most specific. Look for an effusion, a haemarthrosis in the acute knee, and always examine for the associated injuries: the MCL, the menisci and the posterolateral corner.

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The pivot shift. It is best elicited with the patient relaxed, under anaesthesia, and it demonstrates the functional rotational instability that causes giving way.

Differential Diagnosis of the Acute Haemarthrotic Knee

A tense haemarthrosis after a twisting injury is ACL rupture until proven otherwise, but patellar dislocation and peripheral meniscal tears are the commonest mimics. Always exclude a spontaneously reduced knee dislocation when more than one ligament is involved.

Radiographs. A weight-bearing AP for alignment and fractures, and a lateral for the tibial plateau. Three findings should prompt further evaluation with MRI:

• A Segond fracture
• A tibial spine fracture, common in children
• A deep lateral notch sign, greater than 1.5mm

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MRI. The direct signs are ACL fibre discontinuity, abnormal signal and an empty notch. The indirect sign is the bone-bruise pattern on the posterolateral tibial plateau and the lateral femoral condyle, the kissing contusion. MRI also shows the associated injuries: meniscal tears and the MCL.

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Instrumented laxity. The KT-1000 arthrometer quantifies anterior translation side to side and is useful for research and outcomes:

• Less than 3mm difference: normal
• 3-5mm: suggestive
• Greater than 5mm: highly suggestive of a complete tear

Initial management. RICE, aspiration for comfort, a brace, and physiotherapy for range of motion.

The decision. Surgery versus non-operative treatment turns on age, activity, sporting demands, instability and associated injuries. Non-operative rehabilitation suits the low-demand, older patient who is willing to avoid pivoting and has no giving way. Reconstruction is for the young active patient, pivoting sports, recurrent instability, or a repairable meniscus.

What KANON adds. In selected acute tears, structured rehabilitation with optional delayed reconstruction is a valid alternative to immediate surgery: there was no difference in 2-year KOOS, and 61% of the optional-delayed group avoided surgery.

Graft selection. BTB autograft is the gold standard for the young athlete under 25 in contact or pivoting sports, and integrates fastest. Hamstring suits non-contact sports and the patient with a kneeling pain concern. Allograft is for the low-demand patient and is avoided under 25, where its failure rate is 4x higher. Patient factors and activity level guide the final choice; the table sets out the trade-offs.

Timing. Wait for full range of motion, extension above all, for the swelling to reduce and for good quadriceps control, and a minimum of 3 weeks from injury; early surgery risks arthrofibrosis. The optimal window is 4-12 weeks, once motion is restored. Bottoni's RCT of hamstring reconstruction found no difference in motion lost between surgery within 21 days and surgery beyond 6 weeks when both arms followed an extension-focused rehabilitation, which challenges a rigid mandatory delay, though it does not exclude arthrofibrosis when motion has not first been restored.

Femoral tunnel. Place it at the centre of the native ACL footprint: low and deep on the wall of the lateral femoral condyle, behind the resident ridge, which is the landmark, at about 10:30 in a right knee and 1:30 in a left. The anteromedial portal technique reaches this point. Anatomic placement is essential for restoring rotational stability.

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Tibial tunnel. Anterolateral to the medial tibial spine.

Graft preparation. BTB is the central 10mm of the patellar tendon with bone blocks of 9-10mm. Hamstring is gracilis and semitendinosus quadrupled, aiming for a diameter greater than 8mm: in hamstring-autograft cohorts, grafts of 8mm or less had higher early revision rates than grafts over 8mm, especially in patients under 20. Quadriceps is a central strip, with or without bone, of 10mm or more.

Fixation. On the femur, an interference screw for a bone block and suspensory fixation for soft tissue; on the tibia, the interference screw is the gold standard. Tension the graft at 20-30 degrees of flexion with a posterior drawer applied.

The anterolateral complex is a secondary restraint to internal rotation, the motion the pivot shift tests. It comprises the anterolateral ligament (ALL), running from the lateral femoral epicondyle to the anterolateral tibia between Gerdy's tubercle and the fibular head, together with the iliotibial band with its deep Kaplan fibres and the anterolateral capsule.

Lateral extra-articular tenodesis (LET) is an extra-articular augment: classically a strip of iliotibial band (modified Lemaire or MacIntosh technique) left attached distally at Gerdy's tubercle, routed deep to the LCL and fixed to the lateral femur. It reduces anterolateral rotatory instability and offloads the intra-articular graft. Anatomic ALL reconstruction with a free graft is an alternative.

Arthrofibrosis. The risk factors are early surgery and poor range of motion; prevention is the delay and restored motion described under Management. Treatment is aggressive physiotherapy and lysis.

Graft failure. The highest risk sits with allograft in the young, non-anatomic tunnels and return to sport before 9 months; BTB autograft, anatomic tunnels and delayed return to sport together minimise it. A failed graft is managed by revision reconstruction.

Anterior knee pain. Follows the BTB graft and usually settles.

Infection. A risk of any surgery, prevented by aseptic technique and managed with washout and antibiotics.

ACL reconstruction in a child with open physes is a distinct problem because tunnels can injure the growth plate. The distal femoral physis contributes the majority of lower-limb growth, so a misplaced tunnel, a bone block, or hardware across it risks growth arrest, limb-length discrepancy or angular deformity.

Assess skeletal maturity, not just age, by bone age and Tanner stage, and match the technique to it:

Do not simply wait for maturity in an unstable knee. A complete tear left unreconstructed in an active child leads to recurrent giving way and progressive, often irreparable, meniscal and chondral damage, so age-appropriate reconstruction is favoured over watchful waiting. The displaced tibial eminence (spine) avulsion is the paediatric bony equivalent of a mid-substance tear and is fixed, by arthroscopic suture or screw.

What predicts a good result. Anatomic reconstruction, an appropriate graft, compliant rehabilitation, adequate time and an isolated injury.

What predicts a poor one. Malposition, early return to sport, cartilage injury, meniscectomy and non-compliance.

The long term. 50% develop radiographic osteoarthritis by 10-15 years regardless of treatment, and more do if a meniscectomy was performed. The contralateral ACL carries a 10-25% lifetime risk, especially in young females.

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