The ACL resists anterior tibial translation during extension and provides rotational stability[6-8]. The anteromedial bundle is taut when the knee is flexed and the posterolateral bundle is taut when the knee is extended. The anteromedial bundle is longest in flexion and may be the primary component that resists anterior displacement of the tibia in flexion. The posterolateral bundle seems primarily to resist anterior tibial translation in extension and also contributes to rotatory stability of the knee joint being employed in the “screw home” phenomenon i.e. during terminal extension of the knee, the tibia externally rotates relative to the femur serving to “lock” the knee in extension. The anteromedial and posterolateral bundles stabilize the knee joint in response to anterior tibial loads and combined rotatory loads in a synergistic way.
ACL tears may be partial or complete. Partial tears can range from a minor tear involving just a few fibres to a high grade near-complete tear involving almost all of the ACL fibres. A partial tear can involve both or only a single bundle to varying degree. Sometimes plastic deformity of the ACL without fibre discontinuity can occur causing ACL insufficiency.
The mechanism of the ACL injury includes internal rotation of the tibia relative to the femur. This commonly occurs during falls while skiing, as well as in contact sports such as football. With valgus stress, the medial femorotibial joint compartment is distracted producing medial collateral injury and medial meniscal injury (O’Donoghue’s triad). Another mechanism of ACL injury is hyperextension such as occurs during jumping or high kick maneuvers and will lead to contra-coup bone contusion on the anterior tibia and femoral condyle. ACL tears resulting from hyperextension frequently occur without concomitant collateral ligament or meniscal injury. The third mechanism is external rotation of the tibia relative to the femur with varus stress leading to impaction and bone oedema medially and distraction laterally resulting in avulsion of the lateral tibial rim (Segond fracture) and tear of the lateral collateral ligament.
The majority of the ACL injuries can be diagnosed by history and clinical examination. The anterior drawer test, Lachman test and pivot shift test are the most commonly applied clinical tests to diagnose ACL tear though they do rely both on the experience of the clinician and the degree of patient cooperation. In chronic ACL insufficiency, the pivot shift test has reported high sensitivities for detecting the ACL injury ranging from 84% to 98.4%. The test’s specificity has been shown to vary more widely, with reported values from as low as 35% in the alert patient to as high as 98.4% in the anesthetized patient. Anterior drawer and Lachman tests have similar sensitivity but lower specificity. However, in acute injury, if the patient is in pain or swelling, the examination may be limited and the sensitivity and specificity of the clinical tests are limited. Association injury such as meniscal tear or chondral injury may also limit a full clinical examination. As a result, magnetic resonance imaging (MRI) is helpful in the assessment of suspected ACL injury.
Most ACL tears (approximately 80%) are complete, occurring around the middle one-third of the ACL (90%) or less frequently close to the femoral (7%) or tibial (3%) attachments. Less frequently (approximately 20%), ACL tears are incomplete with partial disruption of the ACL fibres. Partial tears may involve only one or both bundles to a varying degree though the anteromedial band does tend to be the more commonly affected. Imaging, and in particular MRI, is very helpful in the assessment of suspected ACL injury.
Magnetic resonance imaging
MR sequences applied for optimal visualization of the ACL are 2D fast spin echo sequences either with or without fat suppression. Different planes are used for anatomical correlation. In most centres, the sequences used to visualize ACL include Turbo spin echo (TSE) sagittal intermediate weighted sequence either with fat suppression and non-fat suppression, TSE coronal T2 weighted fat suppression sequence and TSE axial intermediate weighted with fat-suppression sequence. In our centre, the standard knee protocol comprises the following three sequences (Figure 4A-C): (1) Coronal T2 weighted fat suppressed sequence; (2) TSE sagittal intermediate weighted sequence; and (3) TSE axial intermediate weighted with fat-suppression sequence.
Figure 4 Normal anterior cruciate ligament.
Volunteer, 24-year-old man with no history of injury and clinical instability. Sagittal intermediate-weighted magnetic resonance (MR) of the knee. A: Demonstrates a normal anterior cruciate ligament (ACL) which is characterized by a taut, continuous, low signal intensity fibres extending from the tibial plateau anteriorly to the medial aspect of the lateral femoral condyle. The more anterior portion of ACL is the anteromedial (AM) bundle (black arrows) while the more posterior portion is posterolateral (PL) bundle (white arrows). They cannot be well delineated from each other on this sagittal image. The PL bundle shows higher signal intensity than AM bundle. Coronal T2-weighted fat suppression MR knee image; B: The mid and distal ACL in the intercondylar fossa. The fibres are running superiorly and laterally within the intercondylar fossa from tibial attachment to the lateral femoral condyle (LFC). The more medial portion is the AM bundle (white short arrow) while the lateral portion is the PL bundle (white long arrow). Axial intermediate-weighted fat suppression MR image; C: Normal mid substance of ACL (curved white arrow). The ACL is elliptical in appearance because it is running obliquely to the scan plane. The two bundles cannot be differentiated from each other. *: Posterior cruciate ligament.
Additional sequences comprise oblique views, flexion views, T1-weighted sequences and small FOV or small coil images when necessary. T1-weighted sequences are useful for suspected fracture or characterizing loose bodies within the knee as any osseous fragments may contain a central marrow component. Small field of view sequences with the small coil placed directly over the area of interest are helpful at delineating peripheral pathology around the knee. Oblique views are helpful in determining the presence, severity and location of ACL tears and will be discussed later.
MR images of the knees in flexion can provide more space around the ACL within the intercondylar area, helping to decrease volume-averaging artifact and thereby allowing better visualization of the femoral end of the ligament[23,24]. Recently 3D fast spin echo imaging with or without suppression has been shown to have the same diagnostic accuracy as 2D sequences. This can decrease volume averaging artifacts and shorten overall MR examination time.
The normal ACL should have a taut, low to intermediate signal intensity with continuous fibres in all planes and sequences. It courses parallel or steeper than the intercondylar line. The PL bundle usually has higher signal intensity than the AM bundle.
MRI is highly accurate at diagnosing ACL tears with accuracy, sensitivity and specificity of more than 90%[26-28].
Diagnosis of ACL tear on MR images is usually based on direct signs[26,28,29].
The primary sign of ACL tear is fibre discontinuity (Figure 5A). The oblique sagittal plane is the most helpful in diagnosis supported by coronal and axial imaging. The empty notch sign on coronal imaging is a frequent finding in complete ACL tear. In acute or subacute injury, thickening and oedema of the ACL is found characterized by increased signal intensity on T2 or intermediate-weighted sequences (Figure 5B). In chronic case the fibres can be completely absorbed (Figure 5C) or the residual ACL stump can become adherent to the synovial envelope covering the posterior cruciate ligament. As the orientation of the ACL makes visualization of the entire ACL in one plane difficult, some authors advocate use of oblique planes either parallel or perpendicular to the ACL to increase ligament and tear conspicuity. Oblique coronal and sagittal views parallel to the ACL have been advocated and found to be effective in improving visualization of the ACL[32-34] (Figure 6A and B). We have initiated oblique axial imaging of the ACL and found it to very useful in allowing much clearer delineation of the two ACL bundles, and determining the presence, the location of the normal individual bundle anatomy and possibly individual bundle involvement of partial tears (Figure 7A and B). A clear potential benefit of imaging in an oblique axial plane is that it allows the ACL to be visualized on 11-15 contiguous images rather than on 2-3 contiguous images as with oblique sagittal or coronal imaging.
Figure 5 Primary signs of anterior cruciate ligament tear.
Sagittal intermediate-weighted images of three different patients showing different patterns of anterior cruciate ligament (ACL) tear. A: Typical appearance of ACL tear at the mid-substance with fibres discontinuity of ACL (arrowheads). Residual stumps on femoral (asterisk) and tibial sides (white arrow) are lax, thickened and increased in signal intensity; B: Chronic ACL tear with absence of normal ACL fibres compatible with complete resorption of fibres. PCL (Curved black arrow); C: Acute high grade intrasubstance tear as characterized by thickening and oedematous change of ACL fibres which show increased signal intensity (white arrows). The fibres are still in continuity suggestive of partial ACL tear.
Figure 6 High resolution imaging normal anterior cruciate ligament in oblique coronal and oblique sagittal planes.
Volunteer of a 31-year-old man with no history of injury and clinical instability. Note that the AM bundle (white long arrow) and PL bundle (white short arrow) can be well depicted on the oblique coronal image at the tibial attachment but not at the femoral attachment and not by the oblique sagittal image.
Figure 7 High resolution imaging anterior cruciate ligament in oblique axial plane.
A: Oblique axial image can clearly delineate the two bundles and assess each bundle separately; AM bundle (long white arrow) PL bundle (short white arrow). B: Partial tear of the anterior cruciate ligament. Oblique axial image at the femoral side shows thickening and hyperintense signal intensity of the AM bundle (black long arrow) while fibres are absent in the region of the PL bundle (black short arrow). Features are compatible with high grade partial AM bundle tear and complete PL tear which were confirmed in arthroscopy. LFC: Lateral femoral condyle; *: Posterior cruciate ligament; F: Fibular head.
Partial tears of the ACL are more difficult to diagnose than complete ACL tears. Partial tears are characterized by increased signal intensity and fiber laxity with increased concavity (or bowing) of the ACL (Figure 8). Continuous fibres are evident which suggest the tear is not complete. The sensitivity (40% to 75%) and specificity (51% to 89%) of MRI in the diagnosis of partial tears is poor[35,36] though this poor performance may be improved by the higher resolution afforded by 3T imaging. A recent study employing 3T MRI reported a sensitivity of 77% and specificity of 97% in detecting partial tears of the ACL. If more than 50% of the ACL fibres are torn this would be considered a high grade tear, a medium grade tear is 10%-50% of fibres torn, while a low grade tear is less than 10% of fibres torn. The Holy Grail, with respect to imaging of partial ACL tears, would be to have sufficient resolution to determine whether there was a low, medium or high grade tear in each particular ACL bundle.
Figure 8 Partial tear of the anterior cruciate ligament.
Sagittal intermediate-weighted image of partial anterior cruciate ligament (ACL) tear. The ACL appears lax, concave in appearance (white arrow) and increased in signal intensity. However, the fibres are still in continuity suggestive of partial ACL tear.
Potential pitfalls of MR in defining ACL tears are partial volume artifact and reparative fibrosis following an ACL injury bridging the residual ACL stump and adjacent structures, such as femoral notch, the posterior cruciate ligament (PCL) or the synovial envelope covering the PCL. This will help retain a near normal alignment of the ACL and may lead to an ACL appearing to be intact in the previous complete or partial tear. It is conceivable however that such a re-attached ACL may still remain functionally sound and this possibility should be consistently reported. Flexion imaging is particularly helpful in this situation as even 20 degrees of flexion helps change the orientation of central structures in the knee joint to such a degree that the ACL stump can be seen to be adherent to the PCL (Figure 9).
Figure 9 Magnetic resonance knee in partial flexion.
Volunteer, 31-year-old man with no history of injury and clinical instability. Sagittal intermediate-weighted magnetic resonance image in full extension. A: And 30 degree of knee flexion; B: Demonstrates the usefulness of knee flexion. When the knee is extended, sagittal image shows features suspicious of an anterior cruciate ligament (ACL) tear. When the knee is flexed, a gap (black arrow) is clearly present confirming the presence of an ACL tear.
Bone bruising is very common in ACL tears. McCauley et al suggested that the presence of bone bruising in the posterior aspect of the tibial plateau and posterior displacement of the posterior horn of the lateral meniscus are highly specific for a torn ACL. However, some authors found that secondary signs such as bone bruising do not help significantly in the diagnosis of ACL tears[37-39].
A bone bruise is commonly caused by internal rotation in valgus stress injury, where there is impaction of the posterior aspect of the tibial plateau against the mid or anterior portion of the femoral condyle (Figure 10). This abnormal medullary signal intensity is attributed to subcortical microfracture, oedema, or haemorrhage[28,40]. In the case of anteromedial mechanism of injury or direct compression injuries, the bone bruise pattern is different, such as kissing bone oedema lesions or involving the posteromedial tibial plateau.
Figure 10 Bone bruises.
A 26-year-old man suffered knee injury. Sagittal T2-weighted fat suppression magnetic resonance knee image shows that there are bone rises (asterisks) present in the mid-lateral femoral condyle and posterolateral tibial plateau which indicate that the mechanism of injury is internal rotation of the tibia in valgus stress injury. This pattern of bone bruise has a high association of anterior cruciate ligament complete tear which is present in this patient (not shown).
Bone bruising is a result of impaction at the time of injury. Particularly in young patients who most likely have more elasticity of their ACL fibres, it is distraction and impaction, particularly of the more central rather than posterior aspects of the condyles, that can occur even in the absence of an ACL tear. We would consider bone bruising to be a strong but not absolute indicator of ACL tear.
The deep lateral femoral notch sign, although uncommon, is quite specific for ACL tear and is due to impaction injury of the lateral femoral condyle onto the tibia (Figure 11). A notch depth of over 2 mm is diagnostic of ACL tear. Segond fracture is due to avulsion fracture of the iliotibial band, fibular collateral ligament and biceps femoris tendon and can be seen on MRI examination (Figure 3B). Bosch-Bock bump relates to a bone excrescence located 2-5 mm below the lateral articular margin of the tibia. This bump indicates a chronic tear of the ACL.
Figure 11 Patellar buckling sign and lateral femoral notch sign.
A 37-year-old man who suffered knee injury. Osteochondral injury and patellar buckling sign. Sagittal intermediate-weighted magnetic resonance image demonstrates a deep depression of the middle portion of the lateral femoral condyle (curved white arrow). Normal condylopatellar sulcus should be smaller than 1.5 mm. Notch depth between 1 and 2 mm is suggestive and over 2 mm is diagnostic of anterior cruciate ligament tear. Buckling of proximal patellar tendon (white arrow) also indicates the underlying anterior cruciate ligament tear.
Since the function of the ACL is to prevent anterior tibial translation in extension, ACL tear should increase the degree of anterior tibial translation during knee extension giving rise to “anterior tibial translocation”[41-43] (Figure 12). It is usually apparent on sagittal MR images at the mid-lateral femoral condyle. If there is ≥ 5 mm anterior translocator of the tibia relative to the femur, this would be indicative of ACL tear (sensitivity 86%, specificity 99%) while an anterior tibial translation > 7 mm is fully diagnostic of ACL tear.
Figure 12 Uncovered posterior horn lateral meniscus and anterior tibial translation.
A 32-year-old man who suffered knee injury. Sagittal intermediate-weighted magnetic resonance image of complete anterior cruciate ligament tear patient. The anterior displacement of the tibial translation is measured as the distance between two lines parallel to the picture frame (white lines). At the mid-sagittal plane of lateral femoral condyle, a line is drawn through the most posterior corner of the lateral tibial plateau, and the second line is tangent to the most posterior aspect of the lateral femoral condyle. Anterior tibial translation between 5 and 7 mm is suggestive and over 7 mm is diagnostic of anterior cruciate ligament tear. Note that the lateral meniscus also intersects the tangent to posterior margin of tibia and represents the uncovered posterior horn of lateral meniscus.
Femorotibial translation and rotation gives rise to a host of other signs which are all moderately suggestive of ACL injury such as buckling of the patellar tendon (Figure 11), buckling of the posterior cruciate ligament (Figure 13), a posterior PCL line, uncovered posterior horn of the medial or lateral meniscus (Figure 12) or visibility of the whole posterior cruciate ligament or lateral collateral ligament in one coronal image (Figure 14).
Figure 13 Posterior cruciate ligament buckling and Posterior cruciate ligament line sign.
Sagittal intermediate-weighted magnetic resonance knee image of a completely torn anterior cruciate ligament. A: Posterior cruciate ligament (PCL) buckling sign. The PCL is considered to be hyperbuckled if any portion of its posterosuperior border is concave (white arrow); B: The PCL line sign. A line tangential to the posterior margin of the PCL is drawn (black line) and if this tangent does not intersect the posterior cortex of the femur within 5cm of its distal end, the PCL line is considered to be positive.
Figure 14 Coronal whole posterior cruciate ligament and lateral collateral ligament sign.
Coronal T2-weighted fat suppression magnetic resonance knee images of two different patients with complete anterior cruciate ligament tear show that A: The entire posterior cruciate ligament (*); B: Entire lateral collateral ligament (white arrows) can be seen in a single coronal image.
Shearing fat pad injury is also associated with ACL tear and results in fracture of the infrapatellar fat pad (Figure 15).
Figure 15 Shearing of fat pad.
Sagittal T2-weighted fat suppression magnetic resonance image shows a fracture of Hoffa’s fat pad that has isolated a segment of fat (white arrows).