The anterior cruciate ligament (ACL) is the most frequently disrupted ligament in the knee. It helps prevent the lower let bone (tibia) from moving forward and also helps prevent the knee from rotating abnormally. This ligament is injured in approximately 1 in 3500 people and has an incidence of more than 95,000 new injuries in the United States per year.
The ACL is a ligament which means it connects a bone to another bone. It consists of 2 bundles (antero-medial [AM] and postero-lateral [PL]) that run from the center of the knee from the lateral femur to the central tibia. The ligament is a collection of "fassicles" or bands and it has an "anchor" shaped insertion on the tibia and femur. The two bundles have separate and unique functions. The AM bundle prevents the tibia from coming forward with the knee in flexion, while the PL bundle prevents the tibia from coming forward and rotating when the knee is near extension. These reciprocal functions help the ACL provide knee stability throughout the knee range of motion.
The ACL is most frequently injured with a non-contact mechanism. This means that contrary to the old descriptions of an athlete getting hit from the side of the leg, most ACL injuries occur without the athlete getting struck by another individual. It appears from recent studies that most of these injuries occur with the knee near extension (straight) and somehow the athlete becomes off balance during a high speed maneuver (running, jumping). The result is a sudden force that tears the ACL. Once the ACL is torn, the athlete's knee feels unstable and he/she cannot continue the sporting event.
Without knowing any other information, if the athlete describes hearing a "pop" along with immediate swelling, there is a 70 percent chance that the individual has torn his/her ACL. The athlete will usually not be terribly uncomfortable immediately, but will develop significant pain and swelling over the next few hours. In addition to the ACL injury, the lateral meniscus also may be injured in the acute setting. There also may be soreness along the lateral (outside) knee after an ACL injury as a result of a lateral "bone bruise," meaning that the outside bones sustain direct contact and may be sore from this injury. These "bone bruises" usually resolve on their own over a period of weeks to months. Other ligament injuries (medial collateral ligament / MCL) or medial meniscus injury may also occur but are less common.
Your orthopaedic surgeon will probably examine the injured athlete's knee and depending upon his/her ability to obtain a good examine, may prescribe an ACL brace, physical therapy and a MRI to further evaluate the knee for other injuries. The purpose of the brace is to protect the knee during this initial phase after injury and the physical therapy is to allow the athlete to reduce his/her swelling and regain his/her normal motion prior to surgery. The time required to regain normal motion varies from athlete to athlete, but can be as short as 1 week and as long as a month. Once the athlete's knee motion and swelling are close to normal, your surgeon will discuss with you the surgical options for surgical reconstruction of the ACL.
The ACL does not heal on its own, so historically, attempts at repair of the ACL have been unsuccessful. Therefore, to regain function of the ACL and the knee, the ACL must be "reconstructed". Reconstruction means that some other material (tendon, ligament) must be used to replace the native ACL. Current materials used for this purpose are either autograft (material harvested from the patient) or allograft (cadaver material that has been harvested from someone who has died and the material tested for disease and then cleared for use). Current graft sources are the following: 1) Autograft bone-patellar tendon-bone; 2) Autograft hamstrings; 3) Autograft quadriceps tendon + patella; 4) Allograft tibialis anterior tendon; 5) Allograft Achilles tendon; 6) Allograft bone-patellar tendon-bone. Currently, the surgeon drills a tunnel in the tibia and a separate tunnel in the femur and the graft is pulled through both tunnels and secured with a screw or button.
Research over the last several years has determined that the ACL has 2 bundles that perform separate functions within the knee. In Europe and the Far East, surgeons have begun performing a double bundle ACL reconstruction whereby 2 tunnels are drilled in the tibia and 2 tunnels are drilled in the femur. Two grafts are then placed in the tunnels to better represent the native ACL. This technique is technically more demanding then the single bundle surgery and is only performed at a few select centers in the United States. Dr. Pearsall within the University of South Alabama Department of Sports Medicine has been performing this technique for nearly 2 years with excellent results. The double bundle ACL reconstruction is usually reserved for high demand athletes. The recovery is no longer then after a conventional single bundle technique.
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