How does someone get a bone bruise without an injury? That's what my doctor thinks I have going on in my knee. But although I play a lot of sports, I haven't been knocked down or injured that I know of.

Bone bruising can occur with both contact and noncontact activities.

Bone bruises of the knee in athletes affect the subchondral bone. This is the first layer of bone underneath the cartilage of the knee. The most common injury associated with bone bruising is a rupture of the anterior cruciate ligament (ACL).

You've probably heard of ACL injuries. The ACL is one of two ligaments that criss-cross each other inside the knee. An injury severe enough to pull the ligament off the bone where it attaches can also cause bone bruising. In fact, 80 per cent of all patients who suffer an ACL rupture also have evidence of bone bruising on MRIs.

Without the ACL to hold the tibia (lower leg bone) from sliding too far under the femur (thigh bone) the impact of the injury, the loss of the ligament, and the movement of bone-on-bone leaves the site of the rupture bruised. In fact, there's even a telltale sign on the bone called the footprint that shows where the impact left the bruise.

Studies have shown that the severity of bone bruising is a direct result of the energy of the injury. Contact injuries (the athlete is hit by another player or falls and makes contact with the ground) have more energy behind them than noncontact injuries (the foot is planted on the ground and the player makes a sudden change in direction).

It's possible that you sustained a noncontact injury. Sudden shifts in direction as just described is the most likely mechanism. You may not have collided with another player but it is possible that you fell on your knee at some point during practice or play and sustained a bone bruise.

Hyperextension injuries of the knee or overload along the side of the knee are other ways bone bruising can occur. You may not be able to discover (or remember) the exact event that led to this injury. MRIs and other imaging studies will help detect other associated injuries that might be present. These injuries may help explain what happened.

Reference: 

Joseph P. DeAngelis, MD, and Kurt P. Spindler, MD. Traumatic Bone Bruises in the Athlete's Knee. In Sports Health. September/October 2010. Vol. 2. No. 5. Pp. 398-402.

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