Did your vet grade his knees? Luxating patellas are graded from 1-4 with 4 being the worst. 1-2 usually don't need surgery, 3-4 do. If not surgically corrected, the joint will continue to deteriorate and arthritis will set in. Dogs who have luxating patellas are more likely to tear their ACL as you found out.
Here is some information:
Luxating Patella: A Knee Problem in Dogs