One of the jobs of the anterior cruciate ligament (ACL) is to keep the lower leg bone (tibia) from sliding too far forward on the upper leg bone (femur). A weak, lax, or insufficient ACL means higher stress on the knee cartilage.
The medial meniscus is affected the most. This is the C-shaped piece of cartilage on the inner (medial) side of the knee joint. Repairing the ACL will unload the medial meniscus and make it less prone to further damage or degeneration.
Your surgeon will be able to guide you in making this decision based on your symptoms, the result of tests, and the peek he or she had inside the joint during the meniscal repair. The results of many studies suggest a better long-term result if the ligament is repaired either at the same time as the meniscal tear or soon after.