Thinning anterior ligament... how can a i restore it?

Can't: By exercising your are not directly restoring the thinned acl but improving the conditioning and strength of the lower extremity. This will reduce the likelihood of knee injuries and subsequent acl injury. When exercising focus on core/hip/knee/leg/ankle. If acl is torn its preferred that you fix it. If unsure from MRI then a scope could be considered. Discuss with your orthopedist.