Introduction:
Most commonly occurs in its mid substance. The next common site is near the femoral attachment site, and the least at the tibial attachment site.
Complete discontinuity of the fibers with alterations in both the signal intensity and morphology of the ligament.
High T2 signal intensity traverses the ligament. Appears indistinct and may demonstrate an abnormal slope.
Mechanism of injury:
Common in football players, basketball players, skiers, gymnasts
Can be contact (skiiers, footballer) or non-contact (basketballers, footballers)type
Most common type: forced vulgus in external rotation is associated with MCL and medial supporting structure injury. Common in skiing.
Second common: hyperextension injury associated with meniscal tear. Also PCl and posterior capsule injury
Third common: Blow to flexed knee with plantar flexion of ankle (turf injuries)
Others: External rotation, abduction and hyperextension. Forward displacement of tibia Internal rotation in full extension
O'Donoghue triad: ACL, MCL and MM seen in the above type of injury or in clip injury
May be associated with posterior horn of LM injury
May be associated with posterolateral instability
May be isolated in pivot deceleration injury
Associations:
Hemarthorsis 75% of acute
Meniscal tears- 50% of acute and 90% of chronic ACL injuries. LM common in acute and MM in achronic injuries
Erosions and chondral fractures of medial and lateral compartments in 25% of acute and 50% of chronic
O'Donoghue triad: ACL, MCL, MM
Shelbourne and Nitz traid: ACL, MCL, LL
Clinical signs:
Lachman's sign (anterior drawer in 15-30 degree flexion) positive in most
Pivot shift test with vulgus stress in flexion psotive in most
Anterior drawer sign needs disruption of medial capsule
Plain radiograph:
Segond fractue (lateral tibial rim), posterior fracture of lateral tibial plateau, osteochondral fracture of lateral femoral condyle are associated
MRI:
Discontinuity: sag and axial, sometimes in only one plane.
Deformity: angling in mid ACL, wavy pattern, lax contour, posterior bowing, concave anteriormargin, more horizontal orientation
Signal abnormality: commonly seen at femoral attachment. High signal on T2 and T2*. Changes less well seen on FSE or fat-sat SE. Edema around the region. Interstitial tear shows variable signal abnormality. Fluid within proximal fibres and between torn fibres best seen on axials.
Size: Widening of entire ligament in interstitial tear.
Failure of ACL to parallel Blumensaat's line (intercondylar roof line)
Empty lateral wall sign: absence of ACL in the lateral intercondylar notch. Seen commonly in chronic tear
Anterolateral instability: anterior displacement of tibia. The vertical drawn at tangent to posterolateral femoral condyle fails to intersect the vertical drawn tangent to posterolateral tibial plateau. Buckling of patellat tendon is another sign.
Bone contusions: lateral femoral condyle, posterolateral tibial plateau
Positive PCL sign = buckling of PCL: Line drawn along PCLfails to intersect medullary cavity within 5cm of diatl femur
Positive posterior femoral angle sign: Line drawn at 45 degrees from posterosuperior corner of Blumensaat's line fails to intersect flat portion of proximal tibial surface or fails to intersect a point within 5cm of its posterior margin
Avulsion of tibial intercondylar eminence: 5%. Marrow edema is usually minimal. Seen on T1, but missed on T2 FSE
Partial tears: Small focus of signal abnormality within the substance. Fat sat T2 are most sensitive. AMB more commonly involved. Rupture of AMB is functionally equivalent to complete tear.
Chronic tears: empty lateral wall sign. ACL adhered to PCL
Intercondylar notch ganglion cysts: can arise on both surfaces or within substance. Commonly located in mid or proximal ACL. May represent mucinous degeneration or synovial herniation. May be septated. usually uniform high signal on T2 and do not enhance.
Shear injury: linear fulid collection in Hoffa's fat - indirect sign
Accuracy of MRI:
95% accurate
100% specific for ACL injuries
Not accurate in differentiating partial and complete tears
Treatment:
MR evaluation of post-procedure ACL:
Prognosis:
ACL tears less than 25% favorable outcome
Ganglion cyst in ACL:
Mucinous degeneration of the connective tissue, may be due to herniation of synovial tissue through defect in joint capsule or tendon sheath
POST-OPERATIVE IMAGING:
Cyclops lesion:
About 5% following ACL reconstruction due to fubrous tissue deposition after uplifting of fibrocartilaginous tissue during drilling of tibia
Consists of central granulation tissue surrounded by dense fibrous tissue
May result in loss of full extension
Debrided surgically
MR:
Soft-tissue mass anteriorly or anterolaterally in intercondylar notch near tibial insertion of reconstructed ACL, int-to-low signal on all pulse sequences
References:
1. Stoller DW. MRI in orthopaedics & sports medicine; Second edition; 1997. Chapter 7: the knee. Lippincott Williams & Wilkins
2. Sheldon PJ et al. Imaging of Intraarticular Masses. RadioGraphics 2005;25:105-119
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