Dr. Pomeranz’s Ultimate Guide To:

Assessing Knee MRI

ACL (Anterior Cruciate Ligament)

The anterior cruciate ligament (ACL) originates from the medial aspect of the lateral femoral condyle, and inserts on the anterior tibial plateau 1cm posterior to its most anterior margin.



Anterior Cruciate Ligament: Injury on MRI


  • Atrophy
  • Abrupt caliber change
  • Focal angulation
  • Focal signal change
  • ACL sheath hematoma
  • Fraying and fibrillation of the ACL fibers (the “octopus” sign)
  • ACL discontinuity with retraction
  • Abnormal ACL course or direction (the “laying down” sign)


  • Posteromedial and posterolateral tibial contusions
  • Lateral femoral sulcus terminalis contusion
  • Buckling of the posterior cruciate ligament- a sign of ACL laxity, injury, or tear
  • Anterior tibial translation or luxation of the femur pos­teriorly in relation to the tibia5



Anterior Cruciate Ligament: Pearls

  • Approximately 12 mm in thickness [in the most frequently scanned position appears straight without angulation]
  • Intermediate to hypointense
  • Intermediate signal along the distal tibial end is not uncommon due to the presence of sheath thickening
  • Periligamentous tissues may be of intermediate signal, particularly if there is capsulosynovial thickening
  • Continuous and can be seen in at least two major bundles, anterolateral and posteromedial
  • Extends from posterosuperior and lateral to anteroinferior and medial
  • Can be found in the lateral aspect of the femoral tunnel
  • With knee flexion the ACL may appear lax or may not be visualized
  • Thinner, straighter, and less hypointense than the PCL
  • Extrasynovial intracapsular location
  • Two major fiber bundles, anteromedial and posterolateral
  • Normal width equals 10-13 mm
  • Length: 4 cm
  • Becomes buckled or poorly visualized with knee flexion or over-rotation
  • Tears are most often in the ACL midsubstance
  • Avulsion with fragments of bone usually occurs at the femoral attachment
  • Postoperative graft
    • Size approximates 12 mm
    • Tibia tunnel 2 cm anterior to posterior curiae tibia insertion
    • Femoral tunnel placed as far posterior and close to posterior cortex as possible
    • Graft may assume intermediate signal due to ingrowths of granulation tissue up to 6 months 5



Anterior Cruciate Ligament: Complete Tear

Direct Criteria

  • Complete midsubstance discontiguity
  • Abnormal cruciate course [ligament does not attach to its normal posterosuperior femoral site or anteroinferior tibial site]
  • Midsubstance intracapsular hematoma pseudomass
  • Cruciate ligament corrugation or buckling

Indirect Criteria

  • Buckling of the posterior cruciate ligament
  • Posterior femorotibial subluxation [MR drawer sign] 5



Anterior Cruciate Ligament: Incomplete Tear

  • ACL thinning or caliber change (diameter less than 10 mm)
  • Periligamentous hematoma pseudomass with intact fibers
  • Increased intraligamentous signal with residual intact fibers 5



Anterior Cruciate Ligament: Pitfalls

  • Volume averaging of the posterior femoral cortex may simulate ACL contiguity
  • The femoral end of the ACL may be poorly visualized in the sagittal projection due to imprecise angulation, and the coronal view should be examined
  • Bleeding into the sheath of the ACL without complete tear or severe capsulosynovial thickening may obscure its visualization on proton density or T1 imaging
  • The chronically injured or torn ACL may be focally angulated, but may remain contiguous due to the pres­ence of fibrosis and hemosiderin at the injury site
  • Avulsion of an epiphyseal or tibial spine cortical surface
    • May not be easily seen without plain films and may be missed
    • A large hemarthrosis is frequently present
    • This injury is most common in children and represents the avulsion type of ACL injury
  • A reverse pitfall is the presence of a high grade or complete ACL tear with a normal arthroscopy
    • We have seen this on several occasions, where the sheath of the ACL demonstrates no evidence of discontiguity, edema, hyperemia or bleed
    • It is only with careful probing that the orthopedic surgeon finds the true high grade ACL tear 5



Anterior Cruciate Ligament: References

  • Bonamo JJ, Saperstein AL. Contemporary MRI of the knee. In Fitzgerald SW, ed. MRI Clinics of N America. 2: 488, 1994.
  • Johnson, Clin Orthop 183: 122, 1982.
  • Martire JR, Levinsohn EM. Imaging of athletic injuries. New York, McGraw-Hill, Inc., 1992: 3.
  • O'Brien, JBJS 72A: 278, 1991.
  • Pomeranz SJ. Gamuts & Pearls in MRI & Orthopedics. Ohio, The Merten Company, 1997.
  • Shelboume, AJSM 18: 484, 1990.
  • Watanabe BM, Howell SM. Arthroscopic findings associated with roof impingement of an anterior cruciate ligament graft. Am J Sports Med 23: 616, 1995.
  • Wiener DF, Siliski JM. Distal femoral shaft fracture: a complication of endoscopic anterior cruciate ligament reconstruc­tion. Am J Sports Med 24: 244, 1996.