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There is no medicine which can cure ACL ligament tear. Surgery is a must if it is a complete tear or unstable knee. It is usually done by arthroscope with small incisions
so entire knee is not opened, it is done using scopes which look inside the knee and repair the damaged ligament. Usually cast and bandage will suffice in incomplete tears which will heal in 6 weeks time if there is good condition for healing like immobility and good nutrition.