5 Knee Injury Misses on MRI

Knee injuries are common, and MRI is a highly accurate way to visualize the problem. But, there are five knee injuries that are frequently overlooked, particularly by inexperienced readers.

While catching these injuries might not change clinical management or lead to surgery, it can reduce patient symptoms, such as pain, tenderness, clicking, locking, and instability. Identifying them can also potentially help patient avoid further injury.

In an article published June 17 in the American Journal of Roentgenology, a team from The Chinese University of Hong Kong outlined what these commonly missed injuries are and what providers should look for to detect them.

Ramp Lesion

This injury is most common with young male patients who play contact sports. There are five types of lesions, and they typically involve:

• vertical longitudinal tears located at the peripheral meniscocapsular junction of the posterior horn medial meniscus
• the peripheral part of the posterior horn or attachments to the posterior horn, particularly the meniscocapsular and meniscotibial ligaments.

These lesions are best seen on sagittal proton density-weighted and T2-weighted fat-saturated MR sequences. To catch them, radiologists should become familiar with the normal appearance of the meniscocapsular junction of the posterior horn or the medial meniscus. When viewing these images look for:

• non-compact tissue applied to the posterior horn of the medial meniscus that comprises the meniscocapsular ligament and the meniscotibial ligament with intervening fatty issue
• thin vertical peripheral tears of the meniscus or small corner tears of the meniscocapsular and meniscotibial ligaments with fluid interposition

Meniscocapsular Tear

These tears can occur at the posterior horn medial meniscus, the medial meniscal body or posterior horn and lateral meniscus body, and they can lead to hypermobility in the knee that can cause clicking and locking, as well as further injury.

The superior and inferior popliteomeniscal fasciles (PMF) seen with these injuries are best seen on T2-weighted or proton density-weighted sagittal sequences, particularly a 45-degree oblique coronal proton density-weighted sequence.

To avoid overlooking these injuries:

• be familiar with the normal appearance of meniscocapsular attachments
• routinely inspect the peripheral attachments of menisci in all knee MRI exams
• look for PMF tears and posterior root ligament tears
• evaluate the posterior horn of the lateral meniscus
• exclude the abnormal far lateral extension of the meniscofemoral attachment

Meniscal Root Ligament Tear

These injuries mostly affect the posterior roots of the medial or lateral menisci, but they can also impact anterior roots. Posterior roots are more susceptible because they are relatively immobile and experience significant force during knee flexion.

There are five types of tears, and they are best seen on proton density-weighted and T2-weighted fat suppressed coronal and sagittal imaging. When reviewing images, do not mistake a fissured appearance of the posterior root of the medial meniscus as a tear.

To catch these tears:

• always inspect the root ligaments, following the meniscal tissue into its bony attachment
• do not mistake loose fibrotic or reparative-type T2-weighted or proton density-weighted hyperintense tissue at the attachment site as normal root ligament
• inspect posterior root of the lateral meniscus using oblique coronal imaging

In unclear circumstances, report findings as “possible root ligament tear.”

Posterior Capsular Ligament Tear

These injuries are difficult to diagnose because the ligament cannot be identified directly and capsular-ligamentous discontinuity is uncommon. They are best visualized on axial images at the level of the knee joint.

Although edema can occur with these ligaments in both non-trauma and trauma situations, the amount and location of swelling is used for diagnosis. It can indicate mild, moderate, and severe injury.

Partial Anterior Cruciate Ligament Tear

Accounting for approximately 30 percent of all ACL tears, these injuries are often overlooked because they are difficult to evaluate clinically and on routine arthroscopy.

Double oblique axial proton density-weighted sequences are helpful for evaluation. However, flexion sagittal views are more helpful to distinguish between partial and complete ACL tears.

When looking at an MRI image, suspect a partial tear when:

• ligament continuity is maintained
• ACL seems indistinct, attenuated, bowed, and more separate than usual

This most common injury pattern is:

• complete tear of one bundle, roughly affecting the anteromedial and posterolateral bundles equally
• partial tear of the other bundle