Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

• ACL Tear: Clinical and Radiological Diagnosis

  Introduction

  Anterior cruciate ligament (ACL) injury is one of the most common ligament injuries of the knee, particularly in athletes involved in pivoting sports.

  Early and accurate diagnosis is essential to:

  • Restore knee stability
  • Prevent secondary meniscal injury
  • Reduce long-term osteoarthritis risk
  • Guide appropriate treatment planning

  Diagnosis is based on a combination of:

  • Clinical examination
  • Radiographic evaluation
  • MRI findings

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  Anatomy and Function of the ACL

  The ACL is a:

  • Central intra-articular ligament of the knee

  Its primary functions are to prevent:

  • Anterior translation of the tibia
  • Rotational instability

  The ACL is also an important stabilizer during:

  • Pivoting movements
  • Deceleration activities
  • Cutting maneuvers

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  Mechanism of Injury

  Most ACL injuries occur through:

  • Non-contact pivoting mechanisms

  Common injury scenarios include:

  • Sudden deceleration
  • Twisting with the foot planted
  • Landing awkwardly after jumping
  • Rapid change in direction

  These mechanisms commonly produce:

  • Rotational valgus stress on the knee

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  Clinical Presentation

  Typical symptoms of ACL injury include:

  • Audible or felt “pop”
  • Immediate deep knee pain
  • Rapid swelling due to hemarthrosis
  • Instability or giving-way sensation

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  Hemarthrosis

  Rapid swelling occurs because of bleeding from:

  • Middle genicular artery injury

  Hemarthrosis developing within a few hours strongly suggests:

  • Intra-articular ligament injury

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  Clinical Examination

  Gait Pattern

  Patients may demonstrate:

  • Quadriceps avoidance gait

  This occurs because the patient attempts to minimize anterior tibial translation during walking.

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  Lachman Test

  Most Sensitive Clinical Test

  The Lachman test is considered the:

  • Most sensitive examination test for ACL injury

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  Technique

  • Knee flexed to approximately 20–30°
  • Examiner stabilizes femur
  • Tibia is pulled anteriorly

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  Positive Findings

  • Increased anterior tibial translation
  • Soft or absent endpoint

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  Radiological Evaluation

  Role of X-Rays

  Plain radiographs may identify:

  • Associated fractures
  • Avulsion injuries
  • Indirect signs of ACL rupture

  Although X-rays do not directly visualize the ACL, they are important in the initial evaluation.

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  X-Ray Findings Associated with ACL Injury

  ACL Avulsion Fracture

  The ACL may avulse from the:

  • Tibial eminence (tibial spine)

  This is more commonly seen in:

  • Children and adolescents

  The injury may be visualized on:

  • AP view
  • Lateral view

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  Tibial Spine Fracture

  Tibial spine fractures are considered the:

  • Pediatric equivalent of ACL rupture

  because the immature bone fails before the ligament itself.

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  Segond Fracture

  Definition

  A Segond fracture is:

  • A small avulsion fracture from the lateral tibial plateau

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  Clinical Significance

  It is strongly associated with:

  • ACL tears

  and should always prompt evaluation for associated ligament injury.

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  Lateral Femoral Notch Sign

  This refers to:

  • An impaction fracture or depression of the lateral femoral condyle

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  Diagnostic Criteria

  A sulcus depth greater than:

  • 2 mm

  is considered suggestive of ACL injury.

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  Arcuate Fracture

  Definition

  An arcuate fracture is:

  • An avulsion fracture of the fibular head

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  Clinical Importance

  It indicates possible injury to the:

  • Posterolateral corner (PLC)

  and is commonly associated with:

  • ACL injuries
  • PCL injuries

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  Important Clinical Pearl

  Failure to recognize associated posterolateral corner injury may lead to:

  • Failure of ligament reconstruction

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  MRI Evaluation

  Gold Standard Imaging

  MRI is considered the:

  • Gold standard investigation for ACL tears

  It provides excellent visualization of:

  • Ligament integrity
  • Bone bruising
  • Meniscal injury
  • Associated soft tissue pathology

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  Direct MRI Signs of ACL Tear

  Fiber Disruption

  The ACL fibers appear:

  • Discontinuous
  • Torn
  • Irregular

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  Non-Visualization of the ACL

  In complete tears:

  • ACL fibers may not be visualized

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  Wavy or Dangling Fibers

  Partial or proximal tears may show:

  • Retracted proximal fibers
  • Sagging distal fibers

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  Empty Notch Sign

  Definition

  Absence of ACL tissue within the:

  • Intercondylar notch

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  Significance

  Typically indicates:

  • Proximal femoral avulsion of the ACL

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  Indirect MRI Signs of ACL Tear

  Bone Bruise Pattern

  One of the most important indirect MRI findings is the classic:

  • Lateral compartment bone bruise pattern

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  Typical Locations

  Bone bruising commonly involves:

  • Mid lateral femoral condyle
  • Posterior lateral tibial plateau

  This pattern is highly suggestive of:

  • Pivot-shift injury mechanism

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  Anterior Tibial Translation

  MRI may demonstrate:

  • Anterior displacement of the tibia relative to the femur

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  PCL Buckling

  Secondary changes may include:

  • Buckling or increased curvature of the posterior cruciate ligament (PCL)

  This occurs due to altered knee mechanics after ACL rupture.

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  Associated Injuries

  Acute ACL Tears

  Acute ACL injuries are more commonly associated with:

  • Lateral meniscus tears

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  Chronic ACL Deficiency

  Chronic ACL instability more commonly leads to:

  • Medial meniscus tears

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  Biomechanical Explanation

  The posterior horn of the medial meniscus functions as a:

  • Secondary stabilizer of the knee

  In chronic ACL deficiency:

  • Increased stress is transferred to the medial meniscus
  • This predisposes it to tearing

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  Importance of Combined Clinical and Radiological Assessment

  MRI findings must always be correlated with:

  • Clinical examination
  • Mechanism of injury
  • Patient symptoms

  Not all MRI abnormalities are clinically significant.

  Similarly:

  • Clinical instability may exist despite subtle imaging findings

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  Key Clinical Pearls

  • Lachman test is the most sensitive clinical test for ACL injury.
  • Segond fracture is strongly associated with ACL rupture.
  • Tibial spine fracture is the pediatric equivalent of ACL tear.
  • Classic MRI bone bruise pattern involves the lateral compartment.
  • Acute ACL tears commonly involve the lateral meniscus.
  • Chronic ACL deficiency commonly leads to medial meniscus injury.
  • Posterolateral corner injuries must not be missed.

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  Final Take-Home Message

  ACL injury diagnosis requires a careful combination of:

  • Clinical examination
  • Plain radiography
  • MRI assessment

  Understanding both direct and indirect radiological signs is essential for:

  • Accurate diagnosis
  • Detection of associated injuries
  • Surgical planning

  Early recognition and treatment help restore knee stability, reduce secondary joint damage, and improve long-term outcomes.