Hyperextension injury only refers to the mechanism of injury. (The knee bent past full normal extension.) When it is listed as hyperextension as per the witnessed event, it is not the diagnosis, nor does it define or limit which anatomic/medical diagnosis will
show on the MRI. Bruising is a common result of hyperextension with or without ligamentous injury.
Excerpts from RadioGraphics (a radiology medical journal) in an article titled "Mechanism-based Pattern Approach to Classification of Complex Injuries of the Knee Depicted at MR Imaging"
Enjoy
Hyperextension injuries, by virtue of the greater forces exerted on the extended or "locked" knee, produce more pronounced bone injury patterns, often with frank fractures.
Hyperextension injuries are characterized by broad areas of contiguous bone bruising at the anterior aspect of the knee.