Q: Is it possible to have a whiplash injury to the knee? I heard that term somewhere. What would that be?

A: You may be referring to a mechanism called contrecoup. It's a bit like a neck whiplash injury where the head is thrown forward, then backward, then forward again. The force of the accident is enough to accelerate the head in one direction. Then there is a rebound or movement in the opposite direction as the head whips back. The head movement decelerates (slows down) as the head comes back to a neutral position.

With anterior cruciate ligament (ACL) injuries of the knee there is a similar back and forth shifting as a result of the force placed on the knee. When the injury occurs, the foot is planted firmly on the ground while the person (often an athlete) is moving in a different direction. The intensity and speed of the event shifts the weight over the knee.

The femur (thigh bone) rotates and the resulting shear force ruptures the ACL. The contrecoup mechanism describes the motion in the knee as the joint complex shifts back to compensate for the first pivot-shift during the initial injury. Like a whiplash injury with three directions (forward-back-forward), there can be a side-to-side-to-side shift in the knee before movement stops. This would be called a coup-contrecoup shift.

The reason this mechanism is important is that it brings with it the potential for injury to other parts of the knee besides the ACL. For example, bone contusion has been documented with MRIs in association with a contrecoup shift.

A lateral force on the knee intense enough to rupture the ACL is often enough to damage other aspects of the knee as well. A recent study showed how bone contusions (bruising) occur much more often than ever realized. With minimal force, there may be no bone contusion. With moderate force, the lateral side of the joint is affected first. And with severe force, there is enough energy behind the event to bruise the bone on both sides (medial and lateral).

This makes sense if we remember the contrecoup mechanism. The initial injury damages one side of the joint. The knee shifts (rupturing the ACL) then shifts back (bruising the bone and tearing the medial collateral ligament). With enough force on the either side, the meniscus can be torn as well.

It's very similar to a whiplash injury of the neck but instead of forward and back acceleration and deceleration, it's more of a side-to-side shift in one direction then shift back followed by a final shift.

Reference: Kyoung Ho Yoon, MD, et al. Bone Contusion and Associated Meniscal and Medial Collateral Ligament Injury in Patients with Anterior Cruciate Ligament Rupture. In The Journal of Bone and Joint Surgery. August 17, 2011. Vol. 93-A. No. 16. Pp. 1510-1518.