These days, most ACL procedures are performed using human tissue harvested from the patient or a donor.
Even in the case of a fresh autograft harvested from the patient at the time of surgery, the piece of tissue is not attached to any blood supply and not covered with the normal synovial cell lining of the natural ACL.
After an ACL autograft operation, the new 'ligament' is at its strongest on the first post-operative day. For the first few weeks, it progressively weakens as the cells are starved of their original blood supply. As a new blood supply grows in - and new cells replace the old - the ligament regains its strength and remodels itself.
Before the ligament can function like a normal ligament, it needs to grow a blood supply, a process known as 'revascularisation'.
The blood vessels grow in from the fat pad and the joint lining (synovium) together with synovial cells. These cells grow from above and below to form a richly vascular sheath on the outer surface of the graft, while the cells from the deeper part of the graft actually actually die off. Nutrients from the sheath diffuse into the deeper part of the graft and eventually it undergoes its own infiltration of blood vessels and other cells that will take over the role of ligament cells.
This process occurs from about day 10 to the first 6 weeks. Thus during this period of time the graft tissue needs protection to allow this process to occur.
Weeks 6-12 see any bone blocks (eg, in bone-patellar tendon-bone grafts) unite with the surrounding bone, and the graft and blood vessels grow into the graft itself.
At about 4 months, new collagen - the material that characterises ligaments - becomes laid down and the graft begins to remodel itself in response to the forces being applied through it.
From months 6 - 12 the structure begins to look internally like a real ligament, although it is likely to have only a little more than half of its original strength.
The natural ACL is complex in structure. I believe that even after the process of ligamentisation, an ACL ligament graft acts only as a gross check-rein for joint displacements, unlike the complex control offered by the fibres of a normal ligament.