With Gronk's and Dennis Seidenberg's recent anterior cruciate ligament (ACL) injuries, many people have asked about ACL tears and the constructs, details, techniques and recovery involved in ACL injuries. It is a topic that I covered a while back in a previous article in a very basic nature, but has recently resurfaced with these major injuries. For injury mechanisms, basic repair techniques, and average recovery, check out the previous article. This piece will focus on graft selection and the differences between the allograft and autograft.
Tearing an ACL is never something that an active person never wants to experience. It commonly requires surgical intervention and an in depth recovery period. Familiar to anybody who's torn an ACL or may know someone who's torn one, part of the surgical intervention involves the reconstruction of the torn ligament with the use of an allograft or autograft.
While the selection of reconstructive construct (autograft versus allograft) has been examined and debated upon for the past two decades, there is a lot of data describing the pros and cons of each construct and defining some of the patient criteria used for selection.
So to start, it's important to quickly go through major the differences between an autograft and allograft as a refresher and preclude to a discussion on graft selection.
As most are aware autograft, in the simplest of definitions is a replacement tendon that is harvested from one of the patient’s own tendonous/ligamentous structures, whereas the allograft is a cadaveric or synthetic tendonous structure used for replacing the damaged/ruptured ACL.
As far as graft choices go, the “Gold standard” that was popularized in the 80’s is the Patellar Tendon, or Bone-Tendon-Bone (BTB) graft which is when the patellar tendon/part of it is harvested so that there is bone on each side of the graft (from the femur/thigh bone more proximally and the tibia/shin bone, more distally). This technique has had a lot of success with some studies showing 90-95% of patients experiencing an “excellent surgical outcome.” The theory behind such a graft choice is in the fact that many surgeons use a tunneling technique to fix the graft in place. Having bone on each side of the graft allows for better integration in the bone and quicker healing of the graft into the bony structures.
This naturally leads a patient to ask how this kind of graft is harvested, especially if they are familiar with the fact that the patellar tendon is the ligament that connects the patella, which is the kneecap, to the tibia (shin). The graft is taken from a 3-4” long incision that is located on the inner (medial) side of the knee cap (patella) spanning downward to the area just below it where the tendon is located. Some guys will harvest it in a smaller horizontal incision as well. A special tendon harvesting instrument is used and pushed up in the longitudinal axis of the tendon to take the middle 1/3rd of the tendon that’s about 10mm wide with around 2-3mm worth of bone on each side, which allows it to have very strong insertion points in the tunnel. In fact, there was a study that was performed in the mid 80’s that showed that the tensile strength of the BTB graft was actually stronger than that of the native ACL.
Essentially this construct allows not only for us to easily fix the graft in place, but it makes it easier in the sense that we are not trying to fix soft tissue to bone, rather we can fix bone to bone using specially made “interference” tunnel screws that are bioabsorbable to hold the graft in place or a tight roping technique to pull the tendon into place.
As for your patellar tendon, it was shown that it actually regenerates at about 6-8mo post-operatively back to a thick, strong tendon in a year to about a year and a half. Most of the tendon ruptures that have been reported have been in the first year of post-operatively before the tendon (the patellar tendon is actually a ligament, but it’s name is a misnomer) has reached it’s fully healed tensile strength.
So this sounds like a perfect technique, but it turns out there are some pitfalls to it. The patellar tendon harvesting, usually causes more chronic/long-term anterior/front knee pain because of the nature of the technique (17.4% of patients report long-term pain vs. about 11% who have other techniques). This has been a drawback for people who commonly engage in jumping activities or activities such as skiing, snowboarding, and surfing which tend to put a bit of stress across the anterior portion of the knee. Furthermore, there’s a risk of fracturing the patella or rupturing the tendon during harvest, which has caused some surgeons to back away from it.
Aesthetically, the incision is larger with a patellar tendon graft and one of the cutaneous nerves is usually sacrificed causing loss of skin sensation along the skin surrounding the incision.
The other really common site for allograft is the hamstring. There are actually several techniques for using the hamstring tendon grafting, but most commonly used is the semitendinousus tendon or the gracilis tendon, which are muscles that can be found in the posterior compartment, or back of the leg. The most common procedure using hamstring tendons involves both the semitendinosus and gracilis doubled (folded over) to increase the strength. The construct was shown to almost double the tensile strength of the replacement ACL compared to that of a native, untorn ACL as displayed during biomechanical studies.
As was stated before, there are many surgical techniques and approaches that can be used to place a hamstring tendon graft. Some include the use of tunnels and bioabsorbable screws, knotless rope/loops, tight ropes, or endobuttons. It’s usually the surgeons training location/comfortability that determines what construct he will use.
The tendon is harvested with the same kind of instrument as the patellar tendon graft and is taken from the posterior (back) side of the knee where the hamstring inserts. It is associated with less anterior (front) knee pain because the patellar tendon is not violated. There is also less quadriceps wasting because the extensor mechanism of the knee is left untouched. Quadriceps wasting is a commonly experienced post-operative problem that many patients will complain of. Essentially, it is observed that the quads will become weaker and atrophy and require physical therapy to help regain former levels of athletic training. With the direct anterior harvesting approach of the patellar tendon graft, the extensor mechanism is violated and will be associated with more wasting of the muscles in the post-op period. This is key for athletes because it allows them to return to play much quicker when they have a hamstring graft because there is less wasting. Furthermore, the biomechanical advantage of the increased tensile strength also may reduce post-op ruptures as well.
As with the patellar tendon grafting, this has it’s drawbacks as well. As was stated before it is believed that the patellar tendon graft insertion sites heal better because of the bone-bone healing process. With the hamstring grafting, there is soft-tissue to bone healing that must occur, which takes longer. This has some individuals concerned with the accelerated return to play protocols that some athletes may try to engage in. Tunnel widening is also more commonly seen with the hamstring graft and most often occurs in the first couple months or so post-op. It essentially means that when we take an xray post-op we can see the tunnel seemingly enlarged, which may loosen up the graft and prolong healing and increase chance of rupture. It is not unique to hamstring grafting, but has been postulated that it may be more common with this construct.
The other commonly used autograft is the quad tendon graft. This is essentially the more proximal tendon that attaches the patella to the quadriceps. It is thicker and wider, but is not commonly used in the primary procedure and is used more often for revision cases when the initial graft fails.
This obvious leads to the discussion of allografts and what that entails. Clearly, the advantage with the allograft is that there are no extra scars for harvesting of the tendonous graft. Part in parcel with this is the obvious fact that it is less intense surgically for the surgeon and therefore has a quicker operative time. In addition, it is associated with less pain due to the fact that we aren’t harvesting any tendons and there is no violation of the extensor mechanisms.
As for the disadvantages, the biggest concern that most providers will tell you about is the risk for contracting a blood-borne pathogen such as HIV, hepatitis and other diseases from the donor. This is relatively uncommon, but it is, nonetheless, a problem that has caused a few reported deaths in the literature over the past 20-25 years. This risk for disease contracture, though, has lead to the common practice of irradiating and attempting to sterilize the implants/grafts. Unfortunately biomechanic studies have shown that this irradiation and sterilization may significantly weaken the tensile strength of the graft. This is clearly a drawback, but a necessary evil. Our studies haven’t really indicated whether or not this increases the risk of rupture, but it definitely remains a concern of many surgeons. This is partially why many surgeons will not recommend an allograft for first time reconstruction.
Synthetic grafts were used at one point and are not commonly used anymore. They allow rapid return to sports and activity, but have failed in terms of how stiff they are. They are commonly associated with a lot of stiffness and tightness and are not really a great option. I wouldn’t recommend them at all. Furthermore, xenografts are from animal tissue. This is a fairly old concept in ACL repair and was used primarily in the early 80’s. It’s fallen out of use due to rejection of grafts and failure.
Essentially, many of the best sources for orthopaedic data agree that we cannot use the same construct on every patient. It is an individualized treatment plan that must be evaluated and considered for what is best for the individual.
Furthermore the data suggest that it depends more on what the surgeon is comfortable with as well because the constructs are generally quite similar and surgical technique and learning curve may be more important than the construct in terms of success.
Most will agree that the hamstring autograft is probably the best construct for a young athletic individual. Not only is its tensile strength higher than any of the other constructs, but there is a significant decrease in reported anterior knee pain post-operatively. With this being said, it is important to discuss your surgery with your surgeon and ask him/her any of the questions you may have with regards to their reasoning for selecting a particular construct.
As we advance with our knowledge, the field of arthroscopic ligamentous repair of the ACL is ever expanding and progressing. Each year we understand injury mechanisms better and further advance our data regarding outcomes, surgical techniques and constructs. Hopefully this article helped explain the basics between the allograft and autograft and some of the techniques we use to reconstruct ACLs.
Wishing the speediest of recoveries to both Gronk and Seidenberg! Let's go Pats and B's! Hopefully we can see another championship in Boston for 2014!
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