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Bone Bruising Patterns Reveal Mechanism of ACL Injury

Injuries to the anterior cruciate ligament are common among young athletes. Most of these ruptures occur as a result of a noncontact event. Usually, the athlete is landing from a jump with the knee in just the right amount of torque to rupture the ligament. But whether from a noncontact or contact injury, the exact mechanism of injury remains unknown.

In this study, researchers used MRIs to identify patterns of bone bruising in athletes with ACL injuries. Studying the impact on bone at the time of injury was helpful. They compared the depth, location, and intensity of bone bruising with the amount of energy generated in the knee at the time of the injury.

They found that there was much more bruising in the bone of the noncontact group. The greater amount of bruising in this group points to a larger amount of energy and more damage done with a noncontact injury.

Most of the bone bruising associated with ACL ruptures in both groups occurred in the lateral compartment of the knee. The mechanism of noncontact injuries leading to ACL rupture was characterized as translation (movement) of the tibia(lower leg) on the femur (thigh bone). There was internal (inward) rotation of the tibia at the same time. Injury to the medial compartment was much more common with noncontact ACL injuries than anyone had known previously.

The mechanism for the contact group was more of a mixed picture. With most ACL contact injuries, there was a major valgus force. Valgus force refers to getting hit from the outside of the leg. The force translates through the knee to the inside of the leg.

The long-term significance of bone bruising is unknown. MRIs show that the cartilage cells called chondrocytes do undergo change. There is thinning of the cartilage surface in the area of the bone bruise. Laboratory analysis of the chondrocytes confirms damage to the cells. Further studies are needed to assess the long-term effects of bone bruising.

References: Darius G. Viskontas, MD, FRCSC, et al. Bone Bruises Associated with ACL Rupture. In The American Journal of Sports Medicine. May 2008. Vol. 36. No. 5. Pp. 927-933.

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