ACL Tear - ProScan Education

Anterior Cruciate Ligament: Injury on MRI

Direct

Indirect

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

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

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

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

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.