Acute injury leading to an injury of the anterior cruciate ligament (ACL) are usually traumatic with a lot of force behind them. In fact, ACL tears alone are uncommon.
Most athletes or active adults who rupture the ACL also suffer other injuries at the same time. There may be other ligament sprains, tears of the mensiscus, and injuries to the joint cartilage. Bone bruises and bone fractures are less common but sometimes do occur.
A bone bruise is also known as a post-traumatic bone marrow lesion. The force of the trauma is great enough to actually leave its mark called a footprint on the bone. Evidence of a bone bruise is found on MRIs.
There's evidence that even if the cartilage doesn't appear damaged, when a bone bruise is seen, it's likely that the impact was enough to injure the joint cartilage, too.
Biologically, a bone bruise means the cells of the bone marrow are affected. There may be a disruption in their metabolic rate and function. Some cells may even die or die sooner than they would have if no injury had occurred.