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Information

 Bone and Soft-Tissue Disorders

  1. COMPLETE TEAR
    • failure to identify ligament
    • amorphous area of high SI on T1WI + T2WI with inability to define ligamentous fibers
    • focal discrete complete disruption of all visible fibers
  2. PARTIAL / INTRASUBSTANCE TEAR
    • abnormal SI within substance of ligament with some intact + some discontinuous fibers

Anterior Cruciate Ligament Injury (ACL Tear)  !!navigator!!

Frequency: in up to 69% of all patients undergoing arthroscopy; in up to 72% of acutely injured knees with hemarthrosis

Mechanism: twisting, valgus impaction + internal rotation, hyperextension of knee with foot planted (football) / lower leg forcibly externally rotated during knee flexion (fall backwards while skiing)

  • pivot shift test (82–90% sensitive) = examiner applies valgus stress on internally rotated leg while flexing the knee; induced anterolateral rotary subluxation reduces spontaneously at 40° flexion with an audible “pop”
  • “anterior drawer” sign (22–80% sensitive) = proximal tibia displaces anteriorly with the knee flexed at 60°–90°
  • Lachman test (77–99% sensitive) = same as “anterior drawer” sign with knee flexed at 10°–20°

Location: midsubstance of ligament / near femoral attachment (in adults) / avulsion of anterior intercondylar eminence or tibial spines (in children)

◊If the ACL appears intact in one of the sagittal oblique sequences discordant findings in other sequences can be disregarded!

Site: intrasubstance tear near insertion of femoral condyle (frequently); bone avulsion (rarely)

  • loss of fiber continuity + abnormal fiber orientation on PD image
  • T2-hyperintense signal (= focal fluid collection / soft-tissue edema) replacing the tendon substance in acute tear
  • pseudomass (hematoma + torn fibers) in intercondylar notch near femoral attachment
  • concavity of anterior margin of ligament
  • nondisplaced avulsion fracture of tibial eminence in children (coronal T1WI)

Secondary signs (low sensitivity, high specificity):

  • anterior translation of tibia (= “anterior drawer” sign) by >5 mm with respect to femur measured at midsagittal plane of lateral femoral condyle
  • “uncovering” of lateral meniscus = posterior displacement of posterior horn of lateral meniscus >3.5 mm behind tibial plateau
  • bowed PCL ← increased laxity = angle between proximal + distal limbs of PCL <105°

Associated signs:

  • for anterolateral rotary instability (football, skiing):
    • bone bruise in lateral compartment (posterolateral tibia + terminal sulcus of lateral femoral condyle) in 40–90% on fat-suppressed T2WI
      • ACL intact in 28% of adolescents with bone bruise
    • low-signal–intensity line surrounded by region of high-signal–intensity marrow edema in posterior aspect of lateral tibial plateau (= occult fracture) on STIR image
  • for hyperextension injury:
    • bone contusion in anterior tibial plateau + femoral condyles
  • varus stress with external rotation:
    • avulsion of joint capsule from lateral tibial rim (Segond fracture)
    • deepening of lateral femoral sulcus >1.5 mm ← osteochondral impaction injury when femur strikes posterior tibial plateau

False-positive Dx:

  1. slice thickness / interslice gap too great
  2. adjacent fluid / synovial proliferation
  3. cruciate ganglion / synovial cyst

Associated injuries: meniscal tear (lateral >medial) in 65%

Rx:

  1. conservative: strengthening of quadriceps muscle + brace for activities
  2. arthroscopic reconstruction with autograft (patellar tendon / combined semitendinosus and gracilis tendon) or allograft (cadaveric patellar / Achilles tendon)

Subacute ACL Tear

Definition: few weeks after injury

  • fibers better defined as hemorrhage + edema subside
  • change in fiber contour + angle of residual fragments

Chronic ACL Tear

Definition: months to years after injury

  • bridging fibrous scar within intercondylar notch (simulating an intact ligament with its low SI)
  • disorganized scar tissue instead of linear parallel fibers
  • major distal ACL fragment assumes a more horizontal orientation (= less steep than the roof of the intercondylar notch or Blumensaat line)
  • ACL may fuse to posterior cruciate ligament
  • complete absence of ligament

Partial ACL Tear (15%)

  • Extremely difficult to diagnose! 40–50% of partial tears are missed on MR!
  • positive Lachman test (in 12–30%)
  • MR primary signs positive for injury (in 33–43%)

Posterior Cruciate Ligament Injury (PCL)  !!navigator!!

Prevalence: 2–23% of all knee injuries

  • midsubstance of PCL most frequently involved (best seen on sagittal images)
  • bone avulsion from posterior tibial insertion (<10%), best seen on lateral plain film

Mechanism:

  1. Direct blow to proximal anterior tibia with knee flexed (dashboard injury)
    • midsubstance PCL tear
    • injury to posterior joint capsule
    • bone contusion at anterior tibial plateau + femoral condyles farther posteriorly
  2. Hyperextension of knee
    • avulsion of tibial attachment of PCL (with preservation of PCL substance)
    • ± ACL rupture
    • bone contusion in anterior tibial plateau + anterior aspect of femoral condyles
  3. Severe ab- / adduction + rotational forces
    • + injury to collateral ligaments

Associated with: coexistent ligamentous injury in 70%

  • joint effusion 64–65%
  • bone marrow injury 35–36%
  • medial meniscal tear 32–35%
  • lateral meniscal tear 28–30%
  • anterior cruciate ligament 27–38%
  • medial collateral ligament 20–23%
  • lateral collateral ligament 6–7%

◊A PCL injury is isolated in only 30%!

  • posterior tibial laxity
  • difficult to evaluate arthroscopically unless ACL torn

 Outline