The surgeon is faced with several dilemmas while treating patients with deficient Anterior Cruciate Ligament (ACL). Once the decision to reconstruct is made the next critical decision is with regards to the graft choice. The factors considered by the surgeon include donor site morbidity, reported rates of graft failure, familiarity with the graft, surgical time, associated complications, ability to restore the patient’s activity to pre-injury level and cost-effectiveness. The choice of the graft is broadly between an autograft, allograft or a synthetic graft. The two most commonly used autografts are bone-patellar tendon-bone (BPTB) and hamstrings tendon. A recent Cochrane review on the outcomes of BPTB and hamstrings tendon was inconclusive [1]. There have been several studies comparing the outcomes of allografts with autografts [2-8]. The use of synthetic graft is becoming very less considering the associated complications [9-11]. The aim of this review is to understand the current concepts in graft choice for ACL reconstruction.

The two most common autografts used currently are BPTB and hamstrings tendon though previously Ilio-tibial band [12] has been used and some surgeons still use quadriceps tendon [13].

The demand for the use of allograft has been steadily increasing over the last decade and is expected to rise. This is because allografts are able to completely eliminate disadvantages associated with donor site morbidity. Eugene Bircher better known for his pioneering work on arthroscopy conveyed his experience with kangaroo tendon as an augment as well as a sole graft in 1929 and was followed by Micheli who published his results 4 years later [46, 47]. Kangaroo grafts however remained to be used rarely like other xenografts, however the use of human tissues such as tibialis anterior tendon, calcaneal tendons, patellar and Achilles tendons gained popularity [48]. However in the nineties the allograft became unpopular due to increased risk of viral disease transmission (e.g. HIV, Hepatitis C). Sterilisation methods prevalent during those days were known to affect the collagen structure and the mechanical properties of the graft [49]. However, with the advent of graft friendly sterilisation techniques Allografts have gained back some ground [50]. With proper aseptic tissue procurement, sterilisation and donor screening the disease transmission through allografts are on decline. The rate of HIV transmission from a properly screened allograft is about 1 in 1.5 million. In addition to screening the allografts can also be sterilised [51]. The techniques that are not currently used (high dose radiation and ethylene glycol sterilisation) could have resulted in a structurally weak graft tissue leading to poor functional outcomes. Some tissue banks now just rely on aseptic procurement of the grafts or sterilise with other proprietary chemical solutions. Hence it is important to know about the graft procurement and sterilisation technique when analysing a research study.

Other disadvantages with using an allograft include the immunogenic response of the host to the graft and delayed graft incorporation when compare to the autografts. In a histological study assessing 9 allografts retrieved during autopsy at 2 years showed poor vascularisation in the centre portion of the graft, which had remained acellular [52]. Thus unlike previous reports of good incorporation of allograft at 18 months [53] this study shows that the incorporation might take 3 years or more. However despite the above concerns allografts are still frequently used and it is important to analyse if the data supports the use of allogrfafts in ACL reconstruction.

Ho Noh et al. in a recently published randomised controlled trial comparing free Achilles tendon allograft to HT grafts in 65 patients followed up for 2 years. There found that short term outcomes including Lachmann test, IKDC score, Lysholm score and Tegner daily activities score, were comparable to the autogenous HT graft [54].

Foster et al. conducted a recent systematic review of level I and II studies to analyse the graft sources affecting the functional outcomes in ACL reconstruction [55]. Out of the 31 studies that were included, 27 studies with 2,184 patients examined autograft reconstructions and only 4 studies with 137 patients examining allograft reconstructions. They found that prospective data comparing allograft versus autograft were scarce and the studies had small populations. Despite this there was no difference in the outcomes for allografts compared to autografts in patients with upto 6 year follow up.

Victor et al. looked at the long term morbidity associated with BPTB allografts when compared to autogenous BPTB grafts [56]. They found no significant difference in the morbidity at 2 years however they found that 3 tendon ruptures out of the 25 patients in the allograft group. Because of this they hypothesised that some allografts might not revascularise and act as passive restraints until they fail from fatigue and abrasion. Kleipool et al. looked at 36 BPTB allograft patients versus 26 autograft patients with a mean of 46 months follow up [57]. They reported a trend of improved functional outcomes in the allograft group however they were not statistically significant. They also did not find any difference in graft integrity and overall morbidity. Edgar et al. evaluated the outcomes for hamstring allograft and autograft reconstruction and found similar laxity and functional scores in both the groups at 3 and 6 ears follow up [58].

There are not many studies comparing the outcomes between two allografts, which would help determine which the best available allograft was. Siebold et al. in a study comparing BPTB allograft (n=183) and Achilles tendon allografts (n=42) with an average follow up of 37.7 months showed comparable results with lower failure rate in Achilles tendon bone allograft.

Interestingly in terms of this review the reduction in morbidity when using an allograft has not been supported. There is also a risk, although rare, of disease transmission in allografts. Despite these risks some investigators still believe in the role of allografts in ACL reconstruction.

To overcome the disadvantages associated with allografts while keeping the benefits such as lack of donor site morbidity and easier surgical technique, there was an enthusiasm towards synthetic grafts in the early nineties. In 1981, Dandy et al. first implanted a carbon fibre reinforced substitute for ACL through an arthroscopic method. However, after encouraging preliminary results there were reports of early ruptures due to poor resistance to torsion forces leading to carbon deposition in the liver and inflammatory synovitis in the knee [59].

In 1986 ligaments made by expanded polytetraflouroethylene (PTFE) (Gore-Tex) grafts were approved in the US for use in failed autologous grafts. These synthetic ligaments had a very high ultimate tensile strength and also stiffness. They had vey encouraging results initially [60] but later studies showed complications in 76% of the cases [61] with upto 29% graft failure rate [62], tunnel osteolysis [63] and deposits of PTFE particles in lymph nodes distant to the knee [64]. They were eventually withdrawn from the market in 1993.

The Dacron ligament came to be used for ACL reconstruction after having been used in the acromioclavicular joint injuries. This ligament is made of polyester and is designed to replace the ACL permanently. Despite initial encouraging results [65] a very high rupture rate ranging from 29% to 60 % were reported in the literature [66, 67]. While presenting the long term outcomes authors also noted that 83% of the patients had degenerative changes in the knee joint with only 14% of patients having an acceptable stability and functional outcome [66]. This product was later withdrawn from the market in 1994.

The Leeds - Keio (LK) ligament was developed in 1982 from collaboration between University of Leeds, UK and University of Keio, Japan. It is made by woven polyester fibers constituting a tubular bundle measuring 10 mm in diameter. The LK ligament is a “scaffold” type of prosthesis, as it acts as an inducer for tissue ingrowth; porous coating allows the induction of biological tissue and promotes the formation of a neo-ligament on the intra-articular portion. In a study of 152 patients with a follow-up of more than four years: 90.1% had a negative Lachman test and 82.2% a negative anterior drawer sign; no major complications were noticed. Second-look arthroscopies performed three and six months after surgery reported a good coverage of the implanted ligament; ruptures occurred only in 3.3% of cases [68]. Engstrom et al. in a prospective randomised controlled trial comparing LK ligament versus autologous BPTB grafts noticed increased anterior laxity and positive pivot shift test after an average follow up of 27 months [69].

After all the previous discouraging results from artificial ligaments used for ACL reconstruction there has been some encouraging reports regarding the use of Ligament Advanced Reinforcement System (LARS ligament). These ligaments are made of polyethylene terephtalate and their structure allows tissue ingrowth in the intra-articular part. Lavoie et al. presented their study of 47 patients with a follow up of 8 to 45 months with good results in subjective parameters and satisfying Tegner activity level. A subsequent study from the same group comparing the results to BPTB autografts showed comparable results with better subjective and objective findings for LARS ligament in the early years. The results with LARS ligament are promising however long term results are awaited.

Research in the field of artificial ligaments demonstrates that the ultimate characteristic required for these materials is biocompatibility. The mechanical properties should also be similar to the natural ligament. Despite several studies, every material has been found to have a drawback and the search for the ideal synthetic graft is ongoing.

Through this review it is clear that a perfect graft for ACL reconstruction continues to be a myth. The decision making for the surgeon with regards to the graft choice is dependent on the familiarity with the surgical technique. Several patient factors such as the nature of the job and religion should also be taken into consideration while arriving at a decision. There is still a lack of evidence to refute the claim that BPTB autologous grafts as the gold standard. The association of higher donor site morbidity i.e. anterior knee pain has also not been proven beyond doubt in several prospective randomised studies comparing the BPTB grafts with HT autografts, several allografts and even synthetic grafts. The lack of statistically significant results could be attributed to the low number of patients in the study groups. It was also noted during this review that there are no standard outcome measures being employed to evaluate the outcomes, which could lead to difficulty in generalising the results from a single unit. This review has not taken into consideration other factors playing a role in graft failure such as graft fixation method and also early rehabilitation process, which are also currently controversial.

Further research is necessary to standardise the processes involved in ACL reconstruction including the graft choice, graft fixation methods and rehabilitation protocols. There should also be a national registry such as the national joint registry, which could enable future monitoring of ACL reconstruction techniques and outcomes.