ACL primarily provides stability of the knee joint during anterior translation and rotational (pivot) movements thus its rupture or absence may lead to progressive damage to the nearby key structures such as meniscus and articular cartilage leading to early osteoarthritis.
As the knee continues to move in an abnormal fashion, the grinding movements of the femoral condyle due to absence of the protecting mechanism causes damage to the underlying meniscus, if this process continues for longer duration it ultimately leads to chondral damage and loose body formation.
Though there is rarely pain associated with ACL injury but the symptoms of instability persists, complete ACL tears have a very low potential to heal on its own and almost always require reconstruction.
On the other hand partial ACL tears do have potential to heal and are kept under observation and surgery is required later if instability develops, proper rest and physiotherapy is required to prevent stiffness.
Thus in an ACL deficient knee meniscus takes over the stability of the knee joint and repeated pivoting movements with unprotected weight bearing leads to progressive damage of both menisci followed by articular cartilage damage and finally leads to osteoarthritic changes in early age, once there is established osteoarthritic changes there are very limited treatment options for knee preservation and depending on the stage of osteoarthritis partial or total knee replacement is required, therefore early detection, intervention and rehabilitation is necessary in ACL injury.