MR Imaging of ACL Injuries in Pediatric and Adolescent Patients

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Technical Considerations

MR imaging (MRI) is the main modality for visualization of the ACL. Injuries are shown best by a combination of high-resolution, high-signal-intensity images, and water-sensitive images. In the past, proton density (also called intermediate-weighted) images have provided high anatomic detail, and T2-weighted images and short Tau inversion recovery images have provided sensitivity to edema or fluid. Fat-saturated, intermediate-weighted sequences are increasingly used, because they have both high

ACL Injury

MRI is the preferred imaging modality for the diagnosis and assessment of ACL injuries. The overall sensitivity and specificity of MRI for the detection of ACL tears in children are 95% and 88%, respectively.23 The sensitivity and specificity for individual primary findings of ACL tears have been reported to be 94% and 96% for abnormal angle with the Blumensaat line; 79% and 88% for increased SI in the substance of the ligament; and 21% and 100% for fiber discontinuity.23

ACL injury patterns

Imaging of ACL Graft Reconstruction in Children

Evaluation of the ACL graft can be performed with different imaging modalities. Radiographs allow assessment of tunnel positioning and integrity of the hardware. On the frontal radiograph, the femoral tunnel should open above the lateral femoral condyle. The tunnel should be located between 10 and 11 o'clock on the right femur and 1 and 2 o'clock on the left femur. The graft should be angled less than 75° in the coronal plane to avoid graft laxity and maintain extension. On lateral radiographs,

Cartilage Degeneration After ACL Injuries

There are 2 main approaches to cartilage imaging: Morphologic and structural. In clinical practice, morphologic characterization of cartilage allows for identification of fractures, focal defects, and diffuse chondral processes, as well as anatomic evaluation after operative interventions. In children, imaging of cartilage should also allow for an adequate differentiation between zones of cartilage. Multiple sequences based on gradient recalled imaging and spin echo imaging can provide 3D maps

Knee Injuries Associated With ACL Tears

When imaging the ACL, it is important to search for associated injuries. Certain MRI findings suggest specific mechanisms of injury that point to associated lesions. Overall, there seems to be a lesser incidence of associated injuries in younger children, perhaps because their musculoskeletal system is lighter and more compliant.24

The ACL and the medial meniscus are both restrictors of anterior tibial translation; therefore, once an ACL injury is detected, it is important to search carefully

Summary

MRI is the preferred imaging modality for pediatric knee disorders such as tears of the ACL because it allows for accurate characterization of all the soft tissues in the joint without exposing the child to ionizing radiation. Imaging of the ACL is especially challenging and requires adequate understanding of the unique structural features of the pediatric skeleton, injury patterns as they relate to skeletal maturity and specific mechanisms of trauma.

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