| Mechanism of Injury | ||
| Introduction Basic Science
|
Non Contact Pivot....Internal Rotation Ireland has reported that the non-contact mechanism is responsible for 80% of anterior cruciate ligament tears.
The athlete runs and cuts to the same side , the ACL is stressed by the internal rotation of the tibia, and a tear can result. Non Contact Pivot....External Rotation
The more common opposite non-contact injury occurs when the athlete plants the foot and rotates to the opposite side, such as illustrated by this badminton player. This is a common mechanism and is what is reproduced in the pivot shift test. The mechanism is illustrated in this video. The video shows the tibia subluxing forward, tearing the anterior cruciate ligament. The knee flexes further and you can see the tibia reduce. She then collapses because of the pain. The athlete, lands in the flexed position, the quads contract and the tibia is subluxed anterior. Then with further flexion, the tibia reduces. This is the same mechanism that the pivot shift test mimics. The Quads Active Mechanism The quadriceps contraction may be of important in the production of the ACL injury. It has been calculated by Barrett et al that an eccentric quadriceps contraction can generate up to 6,000N. You can observe this force in basketball, in the jump stop landing. Ireland feels that there is a 'position of no return' for ACL injury. The body position is:
This position is seen in the video of the badminton player. There must be some addition unrecognized factor in this mechanism. Many of these athletes say that they have done that particular move a thousand times before, but why did the ACL tear that time. Ireland calls this a "heart attack of the knee", caused by a mini-stroke in the brain. There must be some inco-ordination of the firing of the hamstrings. They would normally protect the tibia from subluxing forward. This mechanism of active quads contraction has been reproduced in the lab by Bartlett et al. In this video the cadaver knee is clamped, the quads mechanism is held with the dry ice clamp, and the force of pulling on the quads subluxes the tibia forward and ruptures the anterior cruciate ligament. The next sequence shows the ruptured anterior cruciate ligament. The last sequence is demonstrating a positive Lachman test on the specimen. In summary, the body and knee must be in the correct position, the quadriceps must contract strongly enough to subluxe the tibia, and the hamstrings fail to protect the anterior subluxation. Can this be prevented? Perhaps by neuromuscular, proprioceptive training, some injuries may be prevented. Contact This is the internal rotation skiing injury described by Dr. Bob Johnson from Vermont.( Ettlinger, CF: Johnson, RJ; Shealy J,D. A method to help reduce the risk of serious knee sprains incurred in Alpine skiing. Am J Sports Med 1995 Sept-Oct 23(5): 531-7 With permission Am J Sports Med )
External rotation and valgus stress tears the ACL.
This is shown in this image where the left skate blade is caught between the boards and the ice.
This football image also illustrates the same type of injury mechanism. Another mechanism has been described by Dr. Bob Johnson. When the tail of the ski hits a bump on the snow, the high ski boot levers the tibial forward and tears the anterior cruciate ligament.(With permission AJSM ) HyperFlexion or HyperExtension
These mechanisms are less common and often have associated ligament injuries such as the posterior cruciate ligament.
|
