There are a lot of knee injuries. You should have it checked out with a doctor. List of knee injuries are: sprains, strains etc
Anterior cruciate ligament injury
An injury to the anterior cruciate ligament can be a debilitating musculoskeletal injury to the knee, seen most often in athletes. Non-contact tears and ruptures are the most common causes of ACL injury. The anterior cruciate ligament (ACL) is an important ligament for proper movement. ACL injury more commonly causes knee instability that does injury to other knee ligaments. Injuries of the ACL range from mild such as small tears to severe when the ligament is completely torn.
There are many ways the ACL can be torn; the ligament tears because it is overstretched. The movements of the knee that can result in a tear is when the knee is straightened more than 10 degrees beyond its normal maximal position (hyperextension) - the lower leg is forced forward in relation to the upper leg. Tears in the anterior cruciate ligament often take place when the knee receives a direct impact from the front of the thigh while the leg is in a stable position, for example a standing football player is tackled sideways when his feet are firmly planted.
Orthopedic surgery or orthopedics (also spelled orthopaedic surgery and orthopaedics) is the branch of surgery concerned with conditions involving the musculoskeletal system. Orthopedic surgeons use both surgical and nonsurgical means to treat musculoskeletal trauma, sports injuries, degenerative diseases, infections, tumors, and congenital disorders.
Nicholas Andry coined the word "orthopaedics" in French as orthopédie, derived from the Greek words orthos ("correct", "straight") and paidion ("child"), when he published Orthopedie (translated as Orthopaedia: or the Art of Correcting and Preventing Deformities in Children) in 1741. The correction of spinal and bony deformities became the cornerstone of orthopedic practice.
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An unhappy triad (or terrible triad, "horrible triangle", O'Donoghue's triad or a "blown knee") is an injury to the anterior cruciate ligament, medial collateral ligament, and the meniscus. The triad refers to a complete or partial tear of the anterior cruciate ligament, medial collateral ligament, and the meniscus. Originally the "unhappy triad" included the medial meniscus and not the lateral meniscus. However, during the 1990s, analysis indicated that the 'classic' O'Donoghue triad is actually an unusual clinical entity among athletes with knee injuries. In this type of injury, acute tears of the medial meniscus always present with a concomitant lateral meniscus injury. However, the lateral meniscus tears are far more common than medial meniscus tears in sprains of the ACL.
In 1936 Cambell stated that an "impairment of the anterior crucial and medial ligaments is associated with injuries of the internal cartilage". In 1950, O'Donoghue described the unhappy triad as: (1) rupture of the medial collateral ligament, (2) damage to the medial meniscus and (3) rupture of the anterior cruciate ligament. O'Donoghue estimated the incidence rate in the traumatic sports knee to be 25%.