Reconstruction, not a Repair:
Contrary to what you might think, the ACL is not repaired, it has to be constructed with new tissue. To understand how the graft works, you have to understand the basics of the surgery. Above you can see the path of the normal ACL prior to any tear. (Below) When an ACL reconstruction is performed a drill hole (tunnel) is made in the tibia and another is made in the femur to replicate the path of the native ACL. Next the new graft is fed through the tunnel and anchored in on both ends to stabilize its position.
Front View
Rear View
I have covered several aspects of ACL tears including diagnosis, physical exam and MRI in detail on The Injury Source. However, this post is going to take a deeper look into the actual structure (graft) that is used in reconstructing a torn ACL. It is extremely important because graft failure typically occurs in younger patients with increased activity level. Therefore, the graft choice is the one aspect you can modify to reduce risk in active patients who wish to return to competitive sports.
What are the Graft Options?
Every patient is different, so we are just going to look at the most common options for young, active patients with closed growth plates. Typically there are 3 main options of tissues that can be used. The most common option is to use the hamstring tendon from the posterior knee. A second option is to use a portion of the patella tendon from the front of the knee. The third option is to use a portion of the quadriceps tendon. There are pros, cons and expert rationale for each of these choice which we will explore in the post. It is generally accepted that tissue from the patient (autograft) has better outcomes than tissue from another individual (allograft) in young patients.
Hamstring Graft
When using the hamstring tendon, the semitendinosus muscle tendon is typically used to reconstruct the new ACL. The tendon is harvested usually from the same knee that the ACL tear occurred, but can be taken from the opposite knee if needed. One negative aspect of using the hamstring tendon is the size of the tendon can be unpredictable. It is ideal to obtain the thickest, most robust graft to reduce the likelihood of re-tear. However, in some cases the thickness of the tendon is less than ideal. Additionally, even though the tendons regrows, hamstring strength can be significantly affected. This argument is a controversial aspect of using the hamstring as the graft, because research has shown that hamstring strength is critical to preventing re-injury. The upside of using the hamstring tendons is that there typically is less post-operative pain and the rehab tends to progress a bit more quickly in the early stages. On the other hand, newer data is suggesting a slightly increased re-tear rate compared to some of the other options available.
Bone-Patella Tendon-Bone Graft
A second option is harvesting the patella tendon. However, when this is performed, a portion of bone is taken out with the graft from the both the tibia and the patella (indicated by white squares). Therefore, you have a graft with a small piece of bone on each end, giving the name to the graft bone-tendon-bone (BTB). It is felt that the graft can heal better after reconstruction when you have bone fixated to bone rather than soft tissue being anchored directly to bone with screws. A few negative aspects of the this option is that many patients will complain of chronic knee pain in the front of the knee where the bony fixation occurred. Just as in the hamstring option, there is also a degree of unpredictability in size of the graft. The surgeon can usually get the width that they want, but the depth (thickness) may be limited.
Quadriceps Tendon Graft
A third option, the quadriceps tendon, is becoming more popular. Unfortunately, there is not nearly as many high quality studies evaluating the quadriceps tendon as there are looking at hamstring and BTB. There are advantages to choosing the quadriceps tendon. The first is that the tendon is quite robust which leads to a more predictable size of the graft in both the width and the depth. Patients also do not typically complain of the chronic anterior knee pain that is more commonly seen with BTB. Lastly, one end of the graft is harvested with a small piece of bone from the patella (white square) so you can get a partial benefit of the bone to bone fixation that is not seen with hamstring tendons. A downside of using the quadriceps tendon is that there typically is more post-operative pain and the rehab tends to be more difficult in the early stages.
This is just a simplified analysis of the ACL graft, you definitely would want to discuss the options with the surgeon as each case is different and the surgeon proficiency with each technique may differ.