occur when the ligament is completely torn into two pieces, and the knee joint is no longer stable. this is the most common type of acl injury. around half of acl injuries occur in conjunction with injury to other structures in the knee, including the other ligaments, menisci, or cartilage on the surface of the bones. a specific pattern of injury called the "unhappy triad" (also known as the "terrible triad," or "o'donoghue's triad") involves injury to the acl, mcl, and medial meniscus, and occurs when a lateral force is applied to the knee while the foot is fixed on the ground. prevention interest in reducing non-contact acl injury has been intense. the international olympic committee, after a comprehensive review of preventive strategies, has stated that injury prevention programs have a measurable effect on reducing injuries. these programs are especially important in female athletes who bear higher incidence of acl injury than male athletes, and also in children and adolescents who are at high risk for a second acl tear. researchers have found that female athletes often land with the knees relatively straight and collapsing inwards towards each other, with most of their bodyweight on a single foot and their upper body tilting to one side; these four factors put excessive strain on the ligaments on the knee and thus increase the likelihood of acl tear. there is evidence that engaging in neuromuscular training (nmt), which focus on hamstring strengthening, balance, and overall stability to reduce risk of injury by enhancing movement patterns during high risk movements. such programs are beneficial for all athletes, particularly adolescent female athletes. injury prevention programs (ipps), are reliable in reducing the risk factors of acl inquiries, referring to dominance theories. the ligament dominance theory reduced peak knee abduction moment but should be more focused on prioritizing individualized, task-specific exercises focusing on an athlete's risk profile. it is more beneficial than a generic program. there is an increase in hip and knee flexion angles, such as plyometrics and jump-landing tasks, which reduces the risk of quadriceps dominance. however, there were no changes found for peak vgrf (vertical ground reaction force), which measures for "softer" landings. unfortunately, there was no conclusive data on how ipps reduces the risk associated with leg dominance theory. treatment treatment for acl tears is important to: reduce abnormal knee movements and improve knee function build trust and confidence to use the knee normally again prevent further injury to the knee and reduce the risk of osteoarthritis optimise long-term quality of life following the injury nonsurgical nonsurgical treatment for acl rupture involves progressive, structured rehabilitation that aims to restore muscle strength, dynamic knee control and psychological confidence. a living systematic review with meta-analysis, most recently updated in 2022, showed on the basis of three randomised controlled trials, that primary rehabilitation with optional surgical reconstruction results in similar outcomes as early surgical reconstruction. for some, the acl may heal without surgery during the rehabilitation process (i.e. the torn pieces re-unite to form a functional ligament). the purpose of exercise treatment is to restore the normal functioning of the muscular and balance system around the knee. research has demonstrated that by training the muscles around the knee appropriately through exercise treatment, the body can 'learn' to control the knee again, and despite extra movement inside the knee, the knee can feel strong and able to withstand force. typically, this approach involves visiting a physical therapist or sports medicine professional soon after injury to oversee an intensive, structured program of exercises. other treatments may be used initially such as hands-on therapies in order to reduce pain. the physiotherapist will act as a coach through rehabilitation, usually by setting goals for recovery and giving feedback on progress. estimated non-surgical recovery timeframe is 3–6 months and depends on the extent of the original injury, pre-existing fitness and commitment to the rehabilitation and sporting goals. some may not be satisfied with the outcome of non-surgical management and may opt for surgery later. surgery main article: anterior cruciate ligament reconstruction acl reconstruction surgery involves replacing the torn acl with a "graft," which is a tendon taken from another source. grafts can be taken from the patellar tendon, hamstring tendon, quadriceps tendon from either the person undergoing the procedure ("autograft") or a cadaver ("allograft"). the graft serves as scaffolding upon which new ligament tissue will grow. of the three different kinds of autografts quadriceps tendon grafts have shown to produce less pain at the site of the harvest when compared to patellar tendon and hamstring tendon grafts. quadriceps tendon grafts have also been shown to produce better results when it comes to knee stability and function. the surgery is done with an arthroscope or tiny camera inserted inside the knee, with additional small incisions made around the knee to insert surgical instruments. this method is less invasive and is proven to result in less pain from surgery, less time in the hospital, and quicker recovery times than "open" surgery (in which a long incision is made down the front of the knee and the joint is opened and exposed). young athletes or anyone opting for acl surgery should consider delaying their surgery and completing a 4-6 week prehabilitation program. although there is no consensus on what rehab should consist of, some of the basic parameters include restoring range of motion, decreasing swelling, and ensuring there is adequate quadriceps strength. patients that received a 4-6 week prehab program had better outcomes in the acute phases of surgery, while the outcomes 3-6+months out are still inconclusive the american academy of orthopedic surgeons has stated that there is moderate evidence to support the guideline that acl reconstruction should occur within five months of injury in order to improve a person's function and protect the knee from further injury; however, additional studies need to be done to determine the best time for surgery and to better understand the effect of timing on clinical outcomes. however, delaying acl reconstruction in pediatric and adolescent populations for more than 3 months has been shown to increase the risk or meniscus injuries significantly. there are over 100,000 acl reconstruction surgeries per year in the united states. over 95% of acl reconstructions are performed in the outpatient setting. the most common procedures performed during acl reconstruction are partial meniscectomy and chondroplasty. asymmetry in the repaired knee is a possibility and has been found to have a large effect between limbs for peak vertical ground reaction force, peak knee-extension moment, and loading rate during double-limb landings, as well as mean knee-extension moment and knee energy absorption during both double- and single-limb landings. analysis of joint symmetry along with movement patterns should be a part of return to sports criteria. rehabilitation the goals of rehabilitation following an acl injury are to regain knee strength and motion. if an individual with an acl injury undergoes surgery, the rehabilitation process will first focus on slowly increasing the range of motion of the joint, then on strengthening the surrounding muscles to protect the new ligament and stabilize the knee. finally, functional training specific to the activities required for certain sports is begun. delaying return to sport is recommended for at least a minimum of nine months as retear rates become 7x more likely for those returning prior to 9 months. additionally, it takes around 2 years for the acl to mature however it is unrealistic to expect athletes to wait two years to return to sports. another factor to consider is that 30% of retear rates occur within the first 30 athletic exposures and 50% within the first 72 athletic exposures. lastly, a patient reduces their likelihood of a retear with each month they delay return to sport after the 9 month mark. in the pediatric setting re-ruptures of the acl post surgically are prevalent, 94.6% of which require a revision surgery. without proper rehabilitation, growth or angular deformities can occur, also requiring a revision surgery. patients need to ensure their physical therapist is experienced with treating acl patients as many therapists can set their patients up for failure. more than half of physical therapists still utilize manual muscle testing techniques to measure leg strength for return to sports which is subjective and not reliable data. in addition there is no agreed upon criteria for return to sport however there are considerations a therapist should make before clearing their patient. patients should be put through battery of tests throughout their rehab to ensure their prepared for the demands of there sport. the tests should include a psychological component, plyometric testing, strength symmetry between both lower limbs, and different functional movement assessments that relate to the patients sport. prognosis the prognosis of acl injury is generally good, with many people regaining function of the injured leg within months. acl injury used to be a career-ending injury for competitive athletes; however, in recent years acl reconstruction surgery followed by physical therapy has allowed many athletes to return to their pre-injury level of performance. long term complications of acl injury include early onset arthritis of the knee and/or re-tearing the ligament. factors that increase risk of arthritis include severity of the initial injury, injury to other structures in the knee, and level of activity following treatment. not repairing tears to the acl can sometimes cause damage to the cartilage inside the knee because with the torn acl, the tibia and femur bone are more likely to rub against each other. nevertheless, acl tears alone increase inflammatory markers in the knee which can have influence on the development of osteoarthritis. unfortunately, young female athletes have a significant risk of re-tearing an acl graft, or tearing the acl on the other knee after their recovery. this risk has been recorded as being nearly 1 out of every 4 young athletes. therefore, athletes should be screened for any neuromuscular deficit (i.e. weakness greater in one leg than another, or incorrect landing form) before returning to sport. epidemiology there are around 200,000 acl tears each year in the united states. acl tears newly occur in about 69 per 100,000 per year with rates in males of 82 per 100,000 and females of 59 per 100,000. when breaking down rates based on age and sex, females between the ages of 14-18 had the highest rates of injury with 227.6 per 100,000. males between the ages of 19-24 had the highest rates of injury with 241 per 100,000. sports rates of re-rupture among college athletes were highest in male football players with 15 per 10,000, followed by female gymnasts with 8 per 10,000 and female soccer players with 5.2 per 10,000. high school athletes are at increased risk for acl tears when compared to non-athletes. among high school girls in the us, the sport with the highest risk of acl tear is soccer, followed by basketball and lacrosse. in the us women's basketball and soccer experience the most acl tears then all other sports. the highest risk sport for high school boys in the us was basketball, followed by lacrosse and soccer. in basketball, women are 5-8 times more likely to experience an acl tear than men. dogs cruciate ligament rupture is a common orthopedic disorder in dogs. a study of insurance data showed the majority of the breeds with increased risk of cruciate ligament rupture were large or giant.