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ACL Graft Choices

Introduction

There are a number of choices available to the orthopedic surgeon in determining which graft is best for a patient who's undergoing ACL reconstructive surgery in their knee. Except where noted in the discussions below, they all have excellent clinical results in terms of providing a 90-95% "Good" to "Excellent" outcome and return to pre-injury level of sports and activities. Each of the available ACL graft tissue choices requires their own unique harvesting technique. Furthermore, there are usually different methods used for fixing the grafts in the bone tunnels depending on the characteristics and properties of the tissue selected. Because of these differences in graft techniques (yet apparently similar outcomes), ACL graft choice is frequently made by the surgeon based on his or her own experience and comfort level with the chosen technique.

This review of graft choices is presented to give the patient an understanding of some of the pro's and con's of using of these various tissues for ACL reconstruction. This topic is an extremely controversial one among sports medicine orthopaedists who are doing regular arthroscopic reconstructive knee surgery. It's important to realize that research has yet to demonstrate a "best" or perfect ACL graft. All choices have their disadvantages. Furthermore, every knee is unique and there are clearly instances where the same graft isn't best for two different patients. On the other hand, it certainly is appropriate for you to understand why your surgeon is recommending the graft that they are for your ACL reconstruction and you need to feel comfortable with that choice. After all, it's your knee and you want and deserve the best outcome possible!

Lastly, arthroscopic ACL reconstruction is a technically demanding and challenging technique. Should you as the patient have a bias towards a certain graft type it is more important that you have a surgeon who is experienced performing that procedure rather than trying to pressure your doctor into using a graft with which they're not experienced.

Patellar Tendon Graft

The patellar tendon bone-tendon-bone (BTB) graft has been the "gold standard" graft choice for ACL reconstructions since it was popularized in the mid-1980's. It has been used extensively by surgeons since that time and still remains the graft of choice for a high number of orthopaedists who perform this surgery regularly. The patellar tendon BTB graft has consistently demonstrated excellent surgical outcomes with a 90-95% success rate in terms of returning to pre-injury level of sports.

A patellar tendon graft is harvested through a 3-4" long vertical incision based just along the medial (inside) border of the tendon (ACL Reconstruction Technique using Patellar Tendon). A few surgeons harvest through two shorter horizontal incisions centered over each end of the tendon. The middle third of the tendon 10-11 mm wide is then removed longitudinally along with 2-2.5 cm long bone blocks in continuity at each end of the graft from the tibial tubercle and the outer surface of the patella respectively. This yields a composite bone-tendon-bone graft that has very strong insertion points of the tendon soft tissue into bone. The tensile strength of this graft has been measured by Noyes (1984) to be about 2950 Newtons to failure, versus the strength of an intact ACL at 2160 N.

Patellar tendon ACL graft

One of the advantages of this construct is that because the bone-tendon interface is quite strong, the surgeon only has to fix the block of bone in the bone tunnel rather than trying to fix the soft tissue itself. A headless screw is simply inserted next to the bone plug (think of it as a square peg in a round hole) to interference fit and lock the bone in place. The patellar tendon fibers are thereby immediately secured and are stable enough to begin motion and weight bearing when tolerated. The ends of the graft heal bone-to-bone in around 6-8 weeks, which appears to be quicker than the healing process for soft tissue-to-bone. Interference screws are now available in a bioresorbable material that actually dissolves within the bone over 2 to 3 years.

Patellar tendon ACL graft fixation
What happens to the remaining patellar tendon after a third of it has been removed? At OA we conducted a study with the Radiology Department at Maine Medical Center in which we serially MRI scanned our ACL reconstructed patients beginning six weeks after surgery, looking in part at the size and density of the residual patellar tendon. We found that over the course of three to four months after surgery the tendon regenerates or "grows back". Initially it seems to overgrow into a thick, large tendon that then remodels back to a more normal contour by 12-18 months postoperatively. Surgeons have even been able to re-harvest another patellar tendon graft from the original tendon once enough time has passed for tendon reconstitution (although there's now evidence that this repaired tissue may not be as strong as normal patellar tendon tissue). Hence patellar tendon ruptures at the donor site are unlikely after the first few months post-op. Patellar tendon ruptures can and do occur however during the initial 6-8 weeks after surgery if the remaining tendon is stressed too hard.

Patellar Tendon Graft Disadvantages

The "gold standard" graft isn't perfect, however. There may be more pain associated with this donor site than from any of the other graft choices, particularly compared to the hamstring tendons. As a result there is sometimes a greater initial atrophy or wasting response of the quadriceps muscle compared to say either a hamstring or cadaver allograft. This can require more prolonged physical therapy to recover from and could possibly delay the initial return to sports.

The incision (scar) is bigger, and almost all patients end up with a permanent loss of sensation 2-3" in size just lateral to the incision. There is a risk of patellar tendon ruptures, as well as fracturing the patella both intraoperatively as well as postoperatively, although bone grafting the defect in the patella at the time of surgery has reduced the incidence of the latter. Patients who kneel a lot for a living are often unhappy with the patellar tenderness and sensitivity that can occur at the incision site and should probably consider an alternative graft choice.

One of the bigger issues with patellar tendon grafts that recently has a number of orthopaedists switching to alternative grafts is the incidence of anterior knee pain when patients try to resume athletic activities. Specifically there are some studies5 showing an increased rate of patellofemoral pain and/or tendonitis of the patellar tendon with stairs, jumping, skiing and other such activities 6-12 months out from surgery. Ultimately these are often treatable with continued strengthening, rest from sports, and time, but these symptoms can delay the expected time of return to sports. Freedman, et. al.5 found a higher incidence (17.4%) of long term anterior knee pain in a meta-analysis study comparing patients who had patellar tendon grafts compared to hamstring grafts (11.5% pain). Other review studies15, however, have failed to show a significant difference in anterior pain using different ACL graft types.

Some patients may not be candidates for patellar tendon ACL grafts. These include petite individuals with narrow patellar tendons. Patients who have a history of patellar tendonitis, chronic patellofemoral pain or arthritis of the patellofemoral joint may also have less problems after surgery if they consider an alternative ACL graft choice. And as mentioned, workers who kneel frequently should avoid having a patellar tendon harvested because of tenderness and pain at the donor site.

In summary, the patellar tendon BTB graft is a safe and effective option for ACL reconstruction. It has a consistently successful clinical track record at all levels of athletic activity with excellent outcomes and reproducible results. Its major disadvantages are primarily increased tenderness kneeling on the donor incision site, and the possible risk of problems with patellar and patellar tendon pain upon initial return to sports.

Hamstring Tendon Graft

There are actually several variations on hamstring tendon grafts in terms of the actual tissue used. At OA we are currently using a doubled (i.e. folded over) combined semitendinosus tendon and gracilis tendon graft (DSTG) because this provides the strongest tensile strength for a hamstring construct (4500 Newtons to failure [Hamner, 1995]; versus 1800-2196 N for a typical native ACL). This is sometimes referred to as a "quad" graft because there are actually four apparent strands to the final construct. Further use of the term "hamstring tendon graft" in this review refers to this DSTG specifically.

Assembled ACL hamstring graft

Hamstring ACL graaft with screwpost fixation
There has been a surge of interest in the use of hamstring tendon grafts due in part to improvements in graft fixation techniques. In the past, fixation of the hamstring tendons was performed by placing large sutures in the ends of the graft and tying these around a screw post outside of the bone tunnels.

This method of fixation required immobilization and a slower initial rehabilitation to prevent stressing the sutures while the tendons healed to the bone tunnels. Furthermore surgeons were concerned that the sutures at each end of the graft added to the length and elasticity of the whole unit thereby creating a "bungee cord" effect with a loss of graft stiffness. This could interfere with the ultimate healing of the graft soft tissue to its surrounding bone tunnel, or could result in a graft that was too loose due to its "give".

Recently some new hamstring fixation techniques have evolved to match and even exceed the initial pullout strength of patellar tendon bone fixed with interference screw. Interference screws with special blunt threads designed not to cut the hamstring tendons are now able to fix the tendon within the bone tunnel similar to the patellar tendon bone fixation.

Interference screw fixation of ACL hamstring graft
These too are available in bioresorbable forms. One of the screw types even comes impregnated with hydroxyapetite (HA), a bone growth stimulant, to encourage both resorption of the screw as well as stimulate bone growth into the graft. Fixation of the graft within the bone tunnel itself effectively shortens the functional length of the soft tissue graft, thereby reducing the "bungee effect". Some surgeons will further buttress the fixation by tying the ends of the graft with sutures over a screw post, button, or staple.


Bioresorbable hydroxyapetite-impregnated screw

Another new modification for hamstring femoral end fixation is the Closed Loop Endobutton. This device acts similar to a wall anchor in that it is pushed through a small hole in the bone and then deploys as a "T" to lock and prevent the graft from pulling out.Although we are no longer using this fixation technique routinely, we still use the endobutton for some situations, particularly in some revision (re-tear) operations.

Endobutton
A patented technique has created a knotless "rope" loop that the hamstring tendons are passed through and which provides an extremely stiff and strong fixation. A recent study by Brown, et. al. (1999) found the endobutton pull-out strength averaged 1345 Newtons versus 710 Newtons to failure for a patellar bone-tendon with interference screw.

Endobutton ACL fixation in position

Hamstring grafts require a smaller incision and are usually less painful to harvest. Thus the initial postoperative period is often easier and more comfortable with this graft choice. Similarly, because there is no violation of the patellar tendon, there seem to be fewer problems with knee pain during the first few months that a patient is allowed to return to sports. The hamstring incision is away from the patella so patients are usually comfortable kneeling after their reconstruction.

Because the quadriceps extensor mechanism isn't violated with a hamstring harvest there is often less initial quadriceps atrophy (wasting or muscle loss). With a quicker return of knee quadriceps strength some surgeons are allowing their fully recovered patients to return to sports a month or two earlier than they might for a patellar tendon BTB graft. This rationale has been supported by some clinical studies (Shelbourne, 1999) demonstrating no increased rate of ACL graft re-tears in a limited number of patients returning to sports 3-4 months. However there have not been any scientific studies examining the tensile strength to failure of a human ACL graft at three months after implantation that would support this approach. Certainly the animal research done on patellar tendon tensile strength in rhesus monkeys suggested that the graft was actually weakest at 3 months out before maturing at 6 months post surgery.

In younger patients who have torn their ACL's but still have open growth plates, the hamstring tendon graft is a good choice. This is because there are no graft bone ends that could bridge the growth plates and lead to premature closure or arrest of the bone. Premature growth plate closure can result in angular deformities or shortening of the limb.

Hamstring Graft Disadvantages

Once again there is no such thing as the perfect graft. Although the fixation techniques are quite good, hamstring tendon grafts still ultimately rely on soft tissue-to-bone healing. This biologic process is currently being studied intensely to learn what creates the optimal healing environment for these tendons within a bony tunnel. Factors such as the minimal length of the tendon-bone interface, the role of pre-impacting the bone tunnels through dilation techniques, bone tunnel diameters, and the effect of early joint mobilization are all being evaluated.

What is clearly known now, however, is that soft tissue-to-bone healing occurs at a slower rate than bone-to-bone healing. A number of surgeons are concerned that this fact is being ignored when patients are placed into an accelerated ACL rehab protocol without allowing extra time for the graft ends to begin to heal to the bone tunnels.

These fears may be justified. A phenomenon known as tunnel widening is sometimes seen as early as 3 months after hamstring graft ACL reconstruction. This is a condition where the x-ray views of the reconstructed knee show that the original borders of the bone tunnels in the femur and/or the tibia expand out or enlarge. Although this process is by no means unique to hamstring tendon grafts, there may be a higher incidence with these grafts. The biologic process(es) leading to tunnel widening aren't understood yet but the suspicion is that there may be some form of an inflammatory reaction occurring at the soft tissue-bone interface. The concern is that early motion and rehab may be triggering this phenomenon due to micro-motion of graft ends within the bone tunnels.

The clinical significance of tunnel widening also isn't clear. There doesn't yet seem to be any correlation of increased laxity or outright ACL graft failure in patients observed to have tunnel widening however there are a number of ongoing studies evaluating this. Tunnel enlargement does have implications though for patients unfortunate enough to have re-torn their ACL's and who require a repeat (revision) ACL reconstruction. In this case if the old tunnels have widened too much and interfere with ideal graft placement they sometimes have to be primarily filled with bone graft in a separate procedure prior to performing the actual revision reconstruction.

Another disadvantage of hamstring tendon grafts is that harvesting them is a technically demanding procedure that requires considerable surgical experience. Pitfalls such as transecting (cutting in half) a tendon or injuring nerves or ligaments in the area of dissection are possible during the stripping process. There is also a different technique for tensioning the hamstring tendon in the knee once the femoral end has been secured. The graft needs to be pre-tensioned and it's important that each of the four graft ends be individually tensioned during the tibial fixation for best results.

Most surgeons feel that unlike the patellar tendon, the hamstring tendons do not "grow back" after graft harvest. There are, however, limited case reports of regenerated "semitendinosus tendons" found on re-exploration of the hamstring donor site years after reconstruction. One Italian study6 claimed to find 3 such patients in their 2-year follow-ups as verified by exploration & biopsy. Statistically significant prospective or definitive studies on this issue are still pending, however. It is probably best to assume that there will be a permanent loss of hamstring strength which, although not drastic, can typically measure about 10% loss on the average after recovery. This could be a concern for athletes who regularly perform hamstring-specific sports such as running backwards, e.g. a defensive backfield football player.

Other contraindications for a hamstring tendon ACL graft include prior hamstring harvest or surgery in the area of the hamstrings such as a prior MCL repair or reconstruction (Ligaments of the Knee). Patients who demonstrate diffuse multiligamentous laxity - very flexible joints throughout ("double jointed") - may do better with the bony fixation of a patellar tendon graft. Lastly, patients with a history of a significant recurrent hamstring tear(s) should probably use an alternative graft choice.

Which Graft is Better - Patellar tendon or Hamstring?

Studies evaluating the results of hamstring tendon versus patellar tendon grafts in ACL reconstructions for the most part indicate comparable results in terms of successfully stabilizing the knee. Both grafts seem to provide excellent results both functionally as well as by clinical and instrumented ligament exam. There are some studies that suggest that patellar tendon grafts give a tighter result to instrumented measurement (KT 1000 testing), but there hasn't been a significant correlation with this measured difference and any functional significance. That is, patients have had similar results in terms of achieving a useful, stable knee using either graft material. And there may even be a down-side to this tightness - a greater risk of a too tight, or "over-captured" knee that is stiff or suffering from extensor mechanism pain. Too tight a graft may also lead to degenerative arthritis in the long term.

To date there have been a number of prospective and retrospective studies completed comparing patellar tendon bone-tendon-bone grafts to four-strand (DSTG) hamstring grafts7-15. Five of these have found similar laxity values and functional results between the two types of graft tissues7,10,12,13,15 while three8,9,11 found statistically tighter instrument measured values with the patellar tendon graft but this did not correlate with functionally different outcome. At this point, there doesn't appear to be any strong evidence to suggest that one of these graft choices is "better" than the other and most knee specialists consider them equivalent grafts. The bigger issue for a patient remains which tissue is best donated, i.e. which graft has the least implications for that individual in terms of having it removed in the first place. And with that goal in mind, other, less frequently used, graft options are reviewed below.


Quadriceps Tendon Graft

Another graft option for ACL reconstruction is the quadriceps tendon. This technique has been popularized by Dr. John Fulkerson in recent years and consists of harvesting a strip of the end of the quadriceps tendon along with a block of bone off the top surface of the patella. This yields a tendon graft with bone on one end (similar to a patellar tendon graft) and soft tissue on the other.

Quadriceps tendon ACL graft
The incision for harvesting a quad tendon graft is made just above the kneecap and is sometimes placed transversely, i.e. oriented horizontally across the knee as opposed to vertically. Thus it is a smaller incision than that used for a patellar tendon although a separate small incision needs to be placed down on the tibia for the tibial tunnel drill hole. The discomfort associated with obtaining a quadriceps tendon seems to fall somewhere in between that of patellar tendon and hamstring tendon harvesting. Numbness at the quad tendon incision is rare.

Quad tendon grafts have a thicker cross sectional area than a corresponding patellar tendon graft with about the same tensile strength (2352 N to failure; Staubli, 1996). Because the bone block comes from the upper pole of the patella it is possible to obtain a graft even if there has been prior removal of a patellar tendon bone-tendon-bone graft. Thus quad tendon grafts are a good alternative choice for revision ACL surgeries as well. Patients who have quad tendon grafts don't usually get patellar tendonitis symptoms upon returning to sports and often are able to kneel without too much discomfort.

Quadriceps tendon graft fixation in the bone tunnels utilizes the techniques employed for both patellar tendons and hamstrings. Typically the bone end of the quad tendon is fixed in the femur with an interference screw. Any of the variety of methods used for hamstring graft fixation can then be used to secure the tibial end of the quadriceps tendon graft.

Quadriceps Tendon Graft Disadvantages

Basically the donor site risks and behaviors are similar to those of the patellar tendon graft. The quad tendon donor site does, to some extent, reconstitute after harvesting. Challenges of fixation are similar to the other graft choices.

Allografts

All of our discussion of ACL graft choices up to this point has been about various autografts, i.e. tissues donated from the patient's own body. Another alternative available however is to use tissue from a cadaver that is called an allograft. Patellar tendon, hamstring tendon, and even Achilles tendon allografts can be used as ACL graft tissues and are inserted and fixed with the same techniques that are used for autografts.

The advantages of using cadaver graft tissue are obvious; no risks, pain, or scars from the donor site. Surgical time is quicker and because there is considerably less discomfort postoperatively, the incidence of joint stiffness and atrophy of the quadriceps muscle is significantly reduced.

Allografts are a good choice when there are limitations in a patient's own tissue availability. Complicated multiple ligament reconstructions needing several grafts (simultaneous ACL and PCL reconstructions after a dislocated knee, for example) routinely require the use of allograft tissue in addition to an autograft. Revision (repeat) ACL reconstructions where an autograft has already been harvested are also an indication for using a cadaver grafts. Most surgeons do not routinely recommend using an allograft for a primary, first time ACL reconstruction, however there are occasions (e.g. an older, medium-demand athlete) when this can be considered as a reasonable option.

Allograft Disadvantages

The biggest concern with using allografts is the risk of contracting a serious infection from the cadaveric tissue. Hepatitis and HIV can be transmitted through these tissues with potentially fatal outcomes. Bacterial infections are also a possibility and although not usually life threatening, can result in loss of the graft and cause subsequent arthritis.

The dilemma with allografts is that they can't be 100% sterilized without altering or even destroying the tensile strength of the graft tissue. Imagine what happens to any food that is pressure-cooked at temperatures over 270° F under pressure for 10 minutes and you'll understand what happens to a patellar tendon graft sterilized in an autoclave. Similarly, radiating grafts with high enough doses to kill viruses (over 2.5 Mrads is required to neutralize HIV) has been shown to alter the collagen tissue and reduce the graft's tensile strength.

Currently the preferred allograft treatment technique is a fresh frozen graft; the tissue is harvested, cleaned and then frozen in liquid nitrogen. The cadaver is screened extensively with hepatitis and HIV testing as well as a life style analysis to identify any high-risk behavior for these illnesses. Blood tests for HIV, however, are not infallible because they can lag 6 months between the time of infection and the conversion to a positive test. Nevertheless, the process is fairly safe and the published rate of contracting HIV from these tissue allografts is between 1 in 1.2 to 2 million. There are some graft procurement companies who are able to do actual direct HIV viral testing on their tissues which lowers the risks even more. And several companies have developed proprietary cleaning techniques that they claim can guarantee sterility of their graft tissues. Some grafts are also treated with low dose irradiation (1-2 Mrads) in a compromise attempt to provide some degree of sterilization without damaging the tissue characteristics. Unfortunately there are some studies indicating that ACL reconstructions using these tissues may stretch out over time so non-irradiated grafts would be the ideal structural choice if infection were not a concern.

Unlike organ transplants, allografts aren't usually at risk for tissue rejection by the host. This is because there's very little protein antigen in these washed grafts (the bone ends are completely cleansed of any marrow elements). The majority of the grafts are primarily made up of collagen, which has very low antigenicity. Laboratory studies have shown that there is universally a low grade immune reaction to insertion of these foreign tissues, but this doesn't appear to be clinically significant in terms of achieving a successful outcome. Bone tunnel widening is sometimes seen with the use of allografts, but similar to the case of hamstring grafts, doesn't seem to have any significance in terms of functional problems.

We are occasionally asked if a family member (usually a parent) can donate graft tissue to the ACL patient. Unfortunately, a fresh, non-prepared, un-cleaned graft would have to be tissue-typed (just as blood donations are) to be sure that the tissue match would be compatible. Most facilities are not set up for proper graft preparation. Furthermore, there are concerns that an older donor's graft tissues aren't strong enough to handle the loads and demands of say, a competitive 21-year-old athlete.

There are some early studies suggesting that allografts take longer to heal in the knee than comparable autograft tissue. At the same time the patient is recovering from the surgery quicker because of the reduced pain and morbidity of not having donated their own graft tissue. Typically allograft patients will feel like they're ready to get back into sports in just 3 or 4 months since their full strength and joint mobility are often achieved at that point. The combination of delayed allograft incorporation with an accelerated recovery can obviously spell disaster in terms of the ACL graft stretching or rupturing altogether. So patients with allografts must completely understand the healing process and comply with the temporary restrictions even though they may think their bodies are telling them it's OK to be doing more athletically. It takes a lot of mental discipline. And obviously it's a misconception that an athlete can return to sports earlier using an allograft in view of the above.

There have been some early concerns that in the long term, allograft tissues may begin to stretch. A number of studies have found measurable but not statistically or clinically significant (i.e. in day-today usage) increased "play" in the ACL allografts over 5-10 years. Harner & Fu4 reviewed an 8-14 year follow-up comparing 51 of their patellar tendon allograft patients with 51 autograft patients and found slightly less kneecap/patellar tendon pain in the allograft group, but increased instrumented measurement of the ACL graft by KT1000 testing. These differences were not statistically significant however. A follow up study by Harner looking at his allograft results 8-15 years after surgery failed to show any statistically significant difference in strength, function or ligament laxity compared to his autograft patients. Another study by Noyes,et.al.14, looked at 4 and 7 year follow-up of nearly 70 allograft patients and found no significant deterioration of the allografts over those time periods.

The last disadvantage of allografts relates to the practical issues of cost and availability. There has been a national shortage of patellar tendon allografts due to increasing demand combined a low supply of suitably qualified cadavers (there just aren't a lot of young, healthy cadaver donors with low-risk life styles). This shortage has been created in part by physicians who routinely use allografts as their first choice for ACL reconstruction grafts in spite of the fact that autograft tissues work wonderfully. Other cadaveric tissues such as hamstrings, Achilles tendons, and even anterior tibialis and posterior tibialis tendons (some of the ankle tendons) are often being used instead of patellar tendons due to this availability issue. Some surgeons simply don't have ready access to the facilities that procure and process allografts. And allografts are expensive, running anywhere from $2000 to $10,000 depending on the tissue type and your geographic location.

Synthetic (Prosthetic) grafts

The appeal of using a synthetic material to replace a torn ACL is undeniable. Perhaps some day the technology will exist to allow surgeons to simply open up a package to obtain a new ACL which we can then safely implant with none of the risks or morbidity associated with other graft choices.

That artificial ACL will then perfectly mimic all the characteristics of a normal ACL in terms of strength, compliance, elasticity, and durability without any side effects. No healing period would be needed once the patient had recovered full function from the surgery itself and patients could return to sports in half the recovery time.

Unfortunately, medicine isn't there yet. Several synthetic ligaments have come and gone but none have met the qualifications needed for a lasting ACL substitute. In the last decade we saw the Gore-Tex graft which was a knitted cable with eyelets on each end. The cable was placed through drill holes similar to the current ACL reconstruction techniques and the end eyelets were then fixed to the bone outside of the tunnels with screws.

Although these grafts allowed rapid return to sports, they were too rigid and gradually began to fragment due to repeated cycling of the knee and probably some chafing at the edges of the bone tunnels. This led to particles of Gore-Tex shedding and distributing throughout the joint, even occasionally spreading into the lymphatic system. The joint became perpetually swollen and eventually most of these grafts failed and had to be removed.

Other materials have been tried such as Dacron and carbon fiber, all with similar results. More recently the Kennedy Ligament Augmentation Device (LAD) was used in an attempt to make a tissue graft more durable. This was essentially a fiber strip looking very much like a white shoelace that was inserted and fixed on top of the ACL tissue graft to further protect the reconstruction. But instead of augmenting the graft, it seems that the device only shielded the graft from normal stress loads that were needed to help the graft heal and mature correctly.


Eventually the LAD would fail because it had different stretch characteristics than the autograft, and the underlying tissue graft was then abruptly exposed to high tensile stresses to which it hadn't yet adapted. The result then was often stretching or failure of the biologic graft itself. These devices are no longer used for ACL reconstructions.

There is still much interest in synthetic grafts and research continues to try and create the perfect ACL replacement. The requirements of a prosthetic ligament are extensive though. It must be strong but have just the right stiffness to match the compliance of a normal ACL. It must have the durability to withstand high tensile loads for millions of cycles without wear. And it must be perfectly tolerable to the host without bone, joint, or systemic reaction. All in all, this is a daunting task.

Several synthetic ligaments are currently being used in Europe and time will tell if they meet these lofty goals. A newer synthetic augmentation graft, Artelon® - a series of woven polymer fibers with similar mechanical properties to ligaments, is under study. This material is biocompatible and apparently serves as a scaffold to allow native tissue to gradually grow in over 3-4 years. It slowly weakens over time which gives the new biologic tissue a progressive load stimulus that allows it to adapt and strengthen to its ultimate maturity. There are currently, however, no prosthetic ligaments in the U.S. approved by the FDA for ACL reconstruction surgery.

Other Graft Possibilities

There is some very interesting scientific work ongoing, looking at a number of unconventional concepts and products that could successfully bring other ACL graft options to the forefront. They include:

Xenografts

Non-human animal tissues (xenografts) have been tried as ligament graft tissues in the late 80's and 90's, all with dismal outcomes due to the lack of successful tissue incorporation into the human host. A new process of cleaning and preparing pig patellar tendon BTB grafts has been developed that may remove the immune system's rejection response to these tissues. Human trials are currently underway.

Bioengineering and Gene Manipulation

A number of efforts are looking at bioengineering ACL's by culturing progenitor ligament cells on a matrix scaffold. Critical to its success is mechanically loading and stimulating the cells to mature in the correct orientation that would mimic a normal ACL's cellular geometry. Several research projects are reportedly experimenting with biomaterials that would serve as a suitable scaffold material for this process. Silk is one the materials currently being studied as it seems to be well tolerated by the body, is strong, and it promotes a healthy in-growth response.

Gene therapy will undoubtedly be a tool of the future as well. Numerous factors can be manipulated genetically to promote ligament collagen and blood vessel growth, as well as to turn off factors known to produce scar tissue. Whether these synthesized ACL's are manufactured in the laboratory, or by injection therapy into the patient's knee, there certainly are many exciting and potentially successful possibilities that may some day become routine methods of treatment for the ACL deficient knee. Only time will tell!

References

1. Hamner, D. L.; Brown, C .H.; Steiner, M. E.; Hecker, A. T. and Hayes, W. C.: Hamstring tendon grafts for reconstruction of the anterior cruciate ligament: biomechanical evaluation of the use of multiple strands and tensioning techniques. J. Bone and Joint Surg., 81-A: 549-557, 1999.

2. Brown, C. H.; Wilson, D. R.; Hecker, A. T. and Ferragamo, M.: Comparison of hamstring and patellar tendon femoral fixation: cyclic load. Awaiting publication, Am. J. Sports Med., Orthopedic Biomechanics Laboratory, Harvard Medical School, Boston, MA.

3. Noyes, F. R.; Butler, D. J.; Grood, E. S.; Zemicke, R. F. and Hefzy, M. S.: Biomechanical analysis of human ligament grafts used in knee ligament repairs and reconstructions. J. Bone and Joint Surg., 66-A: 344-352, 1984.

4. Westerheide, K.J.; Flumhe, D.J.; Francis, K.A.; Irrgang, J.J.; Fu, F.H. and Harner, C.D., (Pittsburgh, PA): Long term follow up of allograft versus autograft bone patellar tendon bone ACL reconstruction. Presented at the June, 2002 AOSSM Meeting, Orlando FL; awaiting publication.

5. Freedman, K.B.; D'Amato, M.J.; Nedeff, D.D.; Kaz, A.; Bach Jr., B.R., (Chicago, IL): Arthroscopic Anterior Cruciate Ligament Reconstruction: a meta-analysis comparing patellar tendon and hamstring autografts. Am J Sports Med. 2003 Jan-Feb;31(1):2-11

6. Ferretti, A.; Conteduca, F.; Morelli, F.; and Masi, V.: Regeneration of the semitendinosus tendon after its use in anterior cruciate ligament reconstruction. Am. J. Sports Medicine, Vol. 30, No. 2, 204-207, March-April, 2002.

7. Aglietti, P.; Buzzi, R.; Zaccherotti, G.; De Biase P.: Patellar tendon versus doubled semitendinosus and gracilis tendons for anterior cruciate ligament reconstruction. Am. J. Sports Medicine, Vol. 22, 211-218,1994.

8. Aglietti, P.; Zaccherotti, G.; Buzzi, R.; De Biase P.: A comparison between patellar tendon and doubled semitendinosus/gracilis tendon for anterior cruciate ligament reconstruction. A minimum five-year follow-up. J. Sports Traumatol. Rel. Res., 1997; 19: 57-68.

9. Corry, I.S.; Webb, J.M.; Clingeleffer, A.J.; Pinczewski, L.A: Arthroscopic reconstruction of the anterior cruciate ligament. A comparison of patellar tendon autograft and four-strand hamstring tendon autograft. Am. J. Sports Medicine, Vol. 27, 444-454,1999.

10. Marder, R.A.; Raskind, J.R.; Carroll, I.M.: Prospective evaluation of arthroscopically assisted anterior cruciate ligament reconstruction. Patellar tendon versus semitendinosus and gracilis tendons. Am. J. Sports Medicine, Vol. 19, 2478-484,1991.

11. Otero, A.L.; Hutcheson, L.: A comparison of the doubled semitendinosus/gracilis and central third patellar tendon autografts in arthroscopic anterior cruciate ligament reconstruction. Arthroscopy, Vol 9, 143-148, 1993.

12. Eriksson, K.; Anderberg, P.; Hamberg, P.; Lofgren, A.C.; Bredenberg, M.; Westman, I.; Wredmark, T.: A comparison of quadruple semitendinosus and patellar tendon grafts in reconstruction of the anterior cruciate ligament. J. Bone Joint Surg. Br., Vol 83, 348-354, 2001.

13. Beard, D.J.; Anderson, J.L.; Davies, S., Price, A.J.; Dodd, C.A.: Hamstring vs. patellar tendon for anterior cruciate ligament reconstruction: a randomised controlled trial. Knee, Vol. 8, 45-50, 2001.

14. Noyes, F.R.; Barber-Westin S.D.: Reconstruction of the anterior cruciate ligament with human allograft: Comparison of early and later results. J. Bone Joint Surg 1996;78A: 524-537.

15. Spindler, K.P.; Kuhn, J.E.; Freedman, K.B.; Harrell, F.E.; Dittus, R.S.: ACL Reconstruction Autograft Choice: Does it Really Matter?. Presented at the June, 2004 AOSSM Meeting, Quebec City, CA; awaiting publication.