Clinical History: A 40 year-old female presents after a skiing injury. Proton-density weighted fat-suppressed sagittal (A) and axial (B) images are provided. What are the findings” What is your diagnosis?
Subtle focal hyperintensity in the substance of the proximal anterior cruciate ligament (arrow) and a slightly wavy contour of its anterior margin (arrowhead) are demonstrated on (A), a fat-suppressed proton-density sagittal image.
The fat-suppressed proton-density axial image (B) demonstrates focal fluid signal along the anterior margin of the anterior cruciate ligament (arrow) in the intercondylar notch. Avulsive marrow edema (asterisk) is present in the lateral femoral condyle deep to the femoral origin of the anterior cruciate ligament. Posterior fibers of the anterior cruciate ligament remain attached to the femur (arrowhead). Edema and hemorrhage obscure the medial collateral ligament (short arrows).
Partial tear of the proximal anteromedial band of the anterior cruciate ligament and tear of the medial collateral ligament.
Partial tears of the anterior cruciate ligament (ACL) are common. The prognosis of a partial ACL tear is controversial and is dependent on the extent of the partial tear and associated meniscal, ligamentous, and osteochondral injuries. Small tears involving less than 25% of the ACL cross-section have a favorable prognosis of healing while maintaining stability of the knee. Tears involving 50-75% of the ACL demonstrate a significant probability of progressing to a complete tear.1
Biomechanics of the ACL
The anterior cruciate ligament is composed of densely organized fibrous collagenous connective tissue that attaches the femur to the tibia. The ACL is most commonly composed of two bands: the anteromedial and posterolateral. The anteromedial band is the stronger of the two and is taut in flexion while the posterolateral band is lax. In extension, the posterolateral band becomes taut and the anteromedial band is lax (B and C). This reciprocal tension pattern in flexion and extension allows the ACL to remain under tension and contribute to knee stability throughout the normal range of motion. Both bands of the ACL restrain anterior tibial translation depending on the position of the knee. In flexion, the anteromedial bundle is the primary restraint to anterior tibial translation. Deficiency of the anteromedial bundle is tested for by the anterior drawer test, which is performed at 90 degrees of flexion. With the knee in or near extension, the posterolateral bundle serves as the primary restraint to anterior tibial translation, thus posterolateral band deficiencies are more commonly detected by the Lachman test and pivot shift test. In addition, the posterolateral bundle appears to be a crucial component in maintaining rotational stability with the knee near extension.2
This 3-D graphic representation of the knee in extension with the medial femoral condyle removed depicts laxity of the anteromedial bundle (red) and a taut posterolateral band (blue).
In flexion, the anteromedial band (red) becomes taut and the posterolateral band (blue) becomes lax.
Mechanism of Injury
The anteromedial band of the ACL is most commonly injured in partial ACL tears. Injury mechanisms are the same for partial tear and complete tear. The most common contact-related injury is the “clipping” type injury sustained in football. The knee is flexed, and with the foot planted, a valgus force is applied to the knee. Hyperextension injuries with or without varus can result in ACL disruption. Non-contact related injuries are increasingly recognized and are common in athletes involved in rapid deceleration with cutting, pivoting, or jumping.
The accepted physical examination findings of a partial ACL tear include:
- History of injury to the ACL
- Positive Lachman or anterior drawer test with a firm endpoint
- Negative pivot shift test
- KT-1000 side to side difference of less than 5 mm.
However the accurate detection of partial ACL tears by physical exam is limited by several factors. The ACL may be difficult to evaluate in large patients. Patient guarding and soft tissue swelling in the acute setting, as well as strong secondary muscle restraints, may limit the sensitivity of the physical examination in detecting partial ACL tear.1 In addition cadaver studies have proven that a functionally significant partial tear involving the anteromedial band may be missed by physical exam and arthrometric testing.3
A fat-suppressed proton-density weighted sagittal image demonstrates the normal straight anterior margin of the ACL correlating with the anteromedial band. Note the normal striated appearance of the ACL.
The normal ACL is ovoid and of low signal on axial images at the mid-intercondylar level demonstrated on this axial T2-weighted FSE image. The regions of the anteromedial band (red) and the posterolateral band (blue) are indicated by the superimposed oval.
The ACL is normally seen as a longitudinally striated band, which parallels the intercondylar roof (E) on slightly obliqued sagittal images. Close inspection of the axial (F) and coronal views is also necessary for a thorough evaluation of the ACL. Thin T2-weighted sagittal images angled along the anticipated course of the ACL have become an accepted method for evaluating the ACL. This frequently enables a definitive diagnosis of a normal or torn ACL tear. However, we have found that coronal oblique T2-weighted images (G, H, I) are more easily performed and more reliable for depicting the ACL, particularly when differentiating partial or complete tears from volume-averaged fluid in the intercondylar notch.
A fat-suppressed proton-density sagittal image demonstrates a poorly defined ACL with intrasubstance increased signal (arrows). The yellow lines indicate the area covered and the angle used in obtaining coronal oblique images to better demonstrate the ACL.
This T2-weighted coronal oblique view of the same knee as in illustration (G) demonstrates a normal appearance of the ACL (arrows).
In another patient, the T2-weighted coronal oblique image depicts a high-grade partial tear of the proximal ACL. The anteromedial bundle fibers are completely disrupted at the femoral origin (arrowhead) and the posterolateral band fibers are lax (arrow).
Because of its ability to depict anatomy in multiple planes and its non-invasiveness, MRI offers distinct advantages over arthroscopy as a means of evaluating the ACL. MRI evaluation is effective in preventing unnecessary arthroscopy by assessing the severity of the ACL tear and coexisting injuries.4
A fat-suppressed proton-density sagittal view of an acute partial ACL tear depicts focal increased signal of the ACL (arrowheads) and a wavy contour of posterior fibers, which remain in continuity (arrow).
Direct signs of an acute partial ACL tear include focal angulation of the ACL and focal intrasubstance high signal intensity on T2-weighted images, while still visualizing intact ACL fibers (I, J). A chronic partial tear of the ACL may appear angulated and attenuated in caliber. Frequently there is overlap in the appearance of a partial tear and complete tear. In such cases, MRI, by evaluating the extent of ACL signal abnormality and the presence of coexisting injuries, is effective in stratifying ACL tears into low-risk partial ACL tears versus high-risk partial or complete ACL tears. High-grade partial tears are more commonly found with other major ligamentous (A, B), meniscal, and osteochondral injuries.4 Partial tears that demonstrate a narrowed transverse dimension of the ACL on axial images with an otherwise normal anteroposterior dimension and a normally oriented ACL correlate to a stable partial tear.5 Absence of the anteromedial or posterolateral bundle (K, L, M) correlates with an unstable high-grade partial tear. Partial tears associated with bone bruises are higher grade and at greater risk of progressing to complete tears.6 However, in the skeletally immature patient, typical pivot-shift bone bruises can occur without injury to the ACL.7
Fat-suppressed proton-density sagittal view demonstrates subtle laxity of the anteromedial portion of the ACL (arrow).
The next lateral slice for the patient in (H) demonstrates abnormal laxity of the posterolateral bundle fibers (arrow).
Fat-suppressed proton-density axial images of the patient in (K and L) confirm the posterolateral band tear (arrowhead). The anteromedial band is thickened and edematous (arrow). This represents a single bundle ACL, which is at high risk for functional ACL instability.
The indirect signs of a partial ACL tear are the same as for a complete ACL tear, but are typically more subtle. Indirect signs of an ACL injury include pivot-shift bone bruises and fractures of the lateral femoral condyle and posterolateral tibia, “contrecoup” bone bruises in the posteromedial tibial plateau near the semimembranosus insertion, anterior displacement of the tibia, Segond fracture, avulsion fracture of the fibula, lateral meniscal tears at the junction of the posterior meniscofemoral ligament (Wrisberg Rip Web Clinic October 2003), and buckling of the PCL.
The ultimate goal of the treatment of a patient with a complete or partial ACL tear is achieving functional stability of the knee and prevention of osteoarthritis. Not all ACL tears warrant surgical repair. Treatment is tailored to the complexity of the coexisting injuries, the future athletic demands of the patient, objective signs of instability by physical exam, and subjective symptoms of instability by the patient. In order to more closely reconstruct the functional biomechanics of the ACL, double bundle ligament reconstructions are being performed with increasing frequency. The single bundle technique, which has been widely used, is effective at reducing or eliminating anterior translation of the tibia by reproducing the function of the anteromedial bundle. The goal of the double ligament ACL reconstruction technique is not only to reproduce the anteromedial bundle function, but to also reproduce the posterolateral bundle function, thus improving rotational stability.8 Relatively early data suggests an improved outcome using this method.9
Partial tears of the ACL are a common injury. A significant percentage of partial tears will progress to a functionally complete ACL tear. MRI helps guide the treatment decision process by demonstrating the extent of the ACL injury and by demonstrating co-existing injuries of the ligaments, menisci, articular cartilage and capsule.
1 Noyes FR, Mooar LA, Moorman CT 3rd, McGinniss GH: Partial tears of the anterior cruciate ligament. Progression to complete ligament deficiency. J Bone Joint Surg Br 1989 Nov; 71(5): 825-33.
2 Dienst M, Burks RT, Greis PE. Anatomy and biomechanics of the anterior cruciate ligament. Orthop Clin North Am. 2002;33(4):605-20.
3 Lintner DM, Kamaric E, Moseley JB, Noble PC: Partial tears of the anterior cruciate ligament. Are they clinically detectable” Am J Sports Med 1995 Jan-Feb; 23(1): 111-8.
4 Vincken PW, ter Braak BP, van Erkell AR, et al: Effectiveness of MR imaging in selection of patients for arthroscopy of the knee. Radiology 2002 Jun; 223(3): 739-46.
5 Roychowdhury S, Fitzgerald SW, Sonin AH, et al: Using MR imaging to diagnose partial tears of the anterior cruciate ligament: value of axial images. AJR Am J Roentgenol 1997 Jun; 168(6): 1487-91.
6 Zeiss J, Paley K, Murray K, Saddemi SR: Comparison of bone contusion seen by MRI in partial and complete tears of the anterior cruciate ligament. J Comput Assist Tomogr 1995 Sep-Oct; 19(5): 773-6.
7 Snearly WN, Kaplan PA, Dussault RG: Lateral-compartment bone contusions in adolescents with intact anterior cruciate ligaments. Radiology 1996 Jan; 198(1): 205-8.
8 Yagi M, Wong EK, Kanamori A, Debski RE, Fu FH, Woo SL. Biomechanical analysis of an anatomic anterior cruciate ligament reconstruction. Am J Sports Med. 2002;30(5):660-666.
9 ACL Study Group 2004.
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