Mucoid Degeneration of the ACL on MRI can look similar to a tear. The cause is unknown but thought to be secondary to repetitive mictotrauma with micro tears and subsequent mucoid degeneration. This is what it looks like and how to differentiate it from a tear.

Image Above: Hyperintensity and expansion of the ACL. ACL fibres are still seen in normal alignment within the hyperintensity.

Image Above: Blue arrow ACL tear. ACL is hyperintense but no normal fibres seen. Yellow Arrow mucoid degeneration. ACL hyperintense and expanded and normal fibres see within it. Blue arrow cystic change which can be seen with mucoid degeneration.

Image Above: ACL mucoid degeneration associated with bone marrow oedema ( blue arrow) and extra osseous cyst ( yellow arrow).

Image Above: Mild mucoid degeneration ACL associated with intraosseous cyst formation ( blue arrow).