ACL injuries are usefully classified using the Sherman system.
Type I is where the ligament literally pulls right off the bone from where it takes its origin in the notch of the femur
Type II is when the tear is close to, but not quite at, the attachment point on the wall of the femur.
Type III is about a third of the way along the ligament
Type IV is a mid-substance tear, right in the middle of the ligament
Sherman MF, Lieber L, Bonamo JR, Podesta L, Reiter I. The long-term followup of primary anterior cruciate ligament repair. Defining a rationale for augmentation. Am J Sports Med. 1991 May-Jun;19(3):243-55.
Sherman MF, Bonamo JR. Primary repair of the anterior cruciate ligament. Clin Sports Med 1988; 7:739–750
Here is a reference for an interesting read about the history of cruciate ligament surgery:
Cruciate ligament surgery has a long and fascinating history, and the story continues to evolve.
In fact, in our own time the decade of the 1980s is sometimes referred to as 'the lost decade'. This is the decade when keyhole surgery (arthroscopy) was developed, and knee surgeons started to develop clever tools and new systems for reconstructing cruciate ligament tears via tiny keyhole incisions. However it coincided with a movement to use artificial materials, and surgeons argued a lot about the best position for the tunnels - and many of these operations ultimately failed.
Nowadays there is much more of a consensus, with most unstable ligament tears being reconstructed using hamstrings tendon.
Sherman, when he described his Sherman System of classification, was interested in the idea of repairing rather than reconstructing the torn cruciate ligament, a concept that is beginning to regain favour as better materials and techniques continue to evolve.
The ideal patient for a repair is one who has a femoral detachment with a good quality ACL remnant, and a fairly acute injury (less than 6 weeks or a healthy stump if stuck to the PCL).