➢ Patients undergoing anterior cruciate ligament (ACL) reconstruction with irradiated allograft tissue (1.5 to 2.5 Mrad) demonstrate increased postoperative objective laxity measurements and increased failure rates when compared with those who received non-irradiated allograft tissue.
➢ There are no comparative clinical studies on autograft or fresh frozen non-irradiated allograft versus low-level irradiated allografts (0.5 to 1.0 Mrad). This level of irradiation is currently used by many United States tissue banks.
➢ The risk of disease transmission from allograft sources is quite low, especially with enhanced donor screening practices, nucleic acid testing for human immunodeficiency virus (HIV) and hepatitis C virus (HCV), and improvements in tissue recovery, processing, and preservation. Patients should be made aware that when soft-tissue allografts are used, the risk of bacterial transmission is greater than the risk of viral transmission.
➢ When choosing allograft tissue as a graft source for ACL reconstruction, surgeons should attempt to use non-irradiated allografts from a trusted and reliable tissue bank that is accredited by the American Association of Tissue Banks (AATB).
➢ Determining whether allograft tissue has been irradiated can be challenging, as labeling and packaging practices vary among tissue banks. Orthopaedic surgeons should familiarize themselves with their local tissue bank(s) and designated company representative.
The use of allograft tissue is common in anterior cruciate ligament (ACL) reconstruction. Data from a large (11,050-patient) United States ACL registry show that allograft tissue is being used in more than 40% of primary ACL reconstructions and in more than 75% of all revision ACL reconstructions1. The benefits of allograft usage include the avoidance of donor-site morbidity, shorter operative time, and decreased surgical site pain. The major risk with allograft use is the potential for disease transmission to the recipient. Gamma irradiation is a common method of graft sterilization, which helps lessen this risk. At the same time, irradiation can decrease the graft integrity and biomechanical strength of allograft tissue, leading to increased laxity and failure rates as compared with those associated with autograft tissue2-9. The ideal solution for minimizing disease transmission while maintaining native graft strength has yet to be determined. Therefore, in order for orthopaedic surgeons to optimize patient outcomes in ACL reconstruction, they should be informed with regard to the impact of irradiation on allograft tissue.
Allograft Safety and Pathogen Susceptibility to Irradiation
The main concern with allograft tissue among both patients and providers is disease transmission. Over the past several decades, U.S. tissue banking practices have come under tighter regulations. These include stricter donor screening, nucleic acid testing for human immunodeficiency virus (HIV) and hepatitis C virus (HCV), and enhanced tissue recovery and processing techniques10. These quality improvements stem from oversight by the U.S. Food and Drug Administration (FDA) and the American Association of Tissue Banks (AATB), which is the main accrediting agency for tissue banks in the United States. As of 2012, there were 800 FDA-registered tissue facilities in the United States, with 128 having been accredited by the AATB. More than 90% of allograft tissue comes from these AATB-accredited facilities10.
With regard to the potential infection risk from musculoskeletal allografts, bacterial transmission accounts for the majority of cases. In 2002, the Centers for Disease Control and Prevention (CDC) reported that from the beginning of 1995 to March 2002 there was a total of twenty-six bacterial infections arising from contaminated allograft tissue11. The most common organisms were Clostridium species, and eight of the clostridial infections occurred in ACL reconstructions. It should be noted that none of these allografts had undergone sterilization procedures prior to implantation. In the six years following that CDC report, there were only two additional cases of bacterial transmission following ACL allograft reconstructions (one case of Streptococcus pyogenes in 2003 and one case of Chryseobacterium meningosepticum in 2006). In contrast to bacterial transmission, allograft-associated infections caused by viruses are much less common. With current screening practices, the risk of HIV transmission from an allograft has been estimated to be approximately 1 in 1.6 million12. There have been two previous reports of HCV and one report of hepatitis B virus (HBV) transmission from musculoskeletal allografts13-15.
Allograft Tissue Processing, Sterilization, and Impact of Gamma Irradiation
There are multiple ways by which tissue banks prepare allograft tissue for eventual implantation. So-called aseptic processing generally refers to the tissue harvest process whereby the allograft tissue is obtained, often in an operative suite. The tissue then undergoes a series of antibiotic soaks to reduce possible graft contamination that may have occurred during the harvest process. The next step includes bacterial and fungal cultures of the harvested tissues16. Because cultures are only 78% to 92% accurate, a negative culture does not necessarily indicate that the allograft tissue is sterile16,17. The FDA considers a sterility assurance level (SAL) of 10−3 as acceptable for implantable medical devices, including soft-tissue allografts, which means there is a one-in-1000 chance that a nonviral viable organism may survive on a biologic product18. There are numerous mechanisms by which allograft tissue can be sterilized, all of which may be detrimental to graft structural integrity. These include steam, ethylene oxide, vaporous hydrogen peroxide, chemical processes, and gamma irradiation. Gamma irradiation is the most common mechanism utilized today. Gamma irradiation is produced from the self-decay of radionuclide elements, most commonly cobalt-60 or cesium-137. Ionizing radiation produced by this self-decay process causes breakdown of DNA chains, therefore inhibiting microbial division and leading to death of the microorganism. In terms of pathogen susceptibility, bacteria and fungi are much more radiosensitive than viruses are. Non-spore-forming bacteria, yeasts, and molds are susceptible to low doses such as 0.5 to 1.0 Mrad, whereas spore-forming bacteria such as Clostridium species may require up to 2.0 Mrad of irradiation. Viruses such as HIV may require upwards of 4 to 5 Mrad to be inactivated19-21.
Although gamma irradiation is useful in tissue sterilization, it has been shown to decrease the biomechanical strength of grafts and delay allograft healing in ACL reconstruction2,19. The mechanism by which ionizing radiation impacts tissue morphology is by causing scission and structural breakdown of collagen. Haut and Powlison22 noticed an induced crimp pattern in tendon collagen after it was exposed to 2.0 Mrad of gamma irradiation. Curran et al.2 performed a controlled laboratory study evaluating the effects of cyclic loading on the mechanical properties and failure of paired bone-patellar tendon-bone (BPTB) allografts, with and without low-dose irradiation of 2 Mrad. They found that after 1000 cycles, BPTB allografts that had been exposed to 2 Mrad of gamma irradiation elongated 27% more than non-irradiated BPTB grafts did. In addition, failure load was only 80% of that seen in non-irradiated allografts. They concluded that surgeons should consider the use of non-irradiated allografts for ACL reconstruction. Fideler et al.19 demonstrated similar findings in BPTB allografts, showing a significant (p < 0.01) reduction in maximum force, strain energy, and modulus following 2.0 Mrad of irradiation. A dose-dependent decrease in biomechanical properties was found in the groups exposed to 3.0 and 4.0 Mrad of irradiation. Yanke et al.23 performed a recent cadaveric study examining the biomechanical effects of low-dose irradiation on human BPTB allografts. They showed that irradiation of 1.0 to 1.2 Mrad decreased graft stiffness by 20% but did not alter cyclic elongation, creep strain, or load to failure. The clinical relevance of these findings is uncertain at this time. Further clinical studies on low-dose irradiation of soft-tissue allografts are needed, as this level (1.0 to 1.2 Mrad) has been used by tissue banks.
Outcomes of Irradiated and Non-Irradiated Allograft Tissue in ACL Reconstruction
Numerous clinical studies have demonstrated the deleterious effects of irradiation on ACL allografts with regard to failure rates. Borchers et al.6 performed a case-control study and found that, following ACL reconstruction with irradiated tibialis tendon allograft, the odds of failure were 5.6 times greater than those for patients who received autograft tissue. In addition, there was a multiplicative interaction between higher postoperative activity level of patients and ACL reconstruction with use of allograft. When comparing allograft and autograft patients with the highest activity levels following ACL reconstruction, allograft patients had fourteen times the odds of having graft failure. A 2007 meta-analysis by Prodromos et al.5 included all English-language clinical series that used allograft for ACL reconstruction. They found that irradiated allografts had an abnormal stability rate that was 2.5 times higher than that of non-irradiated allografts.
Several nonrandomized studies have compared the outcomes of irradiated and non-irradiated allografts in ACL reconstruction. Guo et al.7 performed a retrospective case-control study in patients who underwent single-bundle BPTB ACL reconstruction with either autograft, fresh-frozen allograft, or gamma-irradiated allograft (1.5 to 2.5 Mrad). There was a much higher prevalence of side-to-side difference (>5 mm on the KT-1000 test) as well as positive pivot-shift test results in patients who received gamma-irradiated grafts as compared with patients who received autograft or fresh frozen non-irradiated allograft. There were six total failures in the gamma-irradiated group and none in the other two groups. Rappé et al.3 performed a retrospective case-control study of a total of ninety patients who were undergoing allograft ACL reconstruction with either non-irradiated Achilles allograft or irradiated Achilles allograft. The average irradiation dose was 2.0 to 2.5 Mrad. Seventy-five patients were available for the minimum six-month follow-up, at which time there was a significant difference in the catastrophic failure rate, which was 2.4% (one of forty-two) in the non-irradiated group and 33% (eleven of thirty-three) in the irradiated group (Tables I and II). This led the authors to discontinue use of irradiated Achilles allografts in ACL reconstructive surgery. In one of the lone contrasting studies, Rihn et al.24 performed a retrospective case-control study of patients undergoing ACL reconstruction with use of BPTB autograft (n = 63) or irradiated BPTB allograft (n = 39). All allografts were from a single tissue bank and underwent 2.5 Mrad of irradiation. The average follow-up was 4.2 years. Postoperatively, there were no differences in overall International Knee Documentation Committee (IKDC) scores between the two groups and, after adjustment for age, the maximal manual KT-1000 side-to-side difference was not significant. There were no traumatic failures in the allograft group (zero of thirty-nine) and only one in the autograft group (one of sixty-three). Interestingly, the average patient age in the allograft group was forty-four years old, while in the autograft group it was 25.3 years old. In their final analysis, the authors attempted to statistically adjust for age as a confounding variable.
Sun et al. performed two prospective randomized studies on the use of irradiated versus non-irradiated allograft tissue in ACL reconstruction25,26. In their study published in 200925, a total of 102 patients undergoing arthroscopic ACL reconstruction were randomized into three groups: BPTB autograft, non-irradiated BPTB allograft, and irradiated BPTB allograft. All irradiated BPTB allografts were sterilized with 2.5 Mrad of irradiation prior to distribution. Ninety-nine patients were available for follow-up at an average of thirty-one months, at which time the irradiated allograft group had significantly (p < 0.05) worse outcomes during Lachman, anterior drawer, pivot-shift, and KT-2000 testing as compared with the outcomes in the autograft and non-irradiated allograft groups. During KT-2000 testing, just 31% (ten of thirty-two patients) in the irradiated allograft group had a side-to-side difference of <3 mm, compared with 88% (twenty-nine of thirty-three patients) in the autograft group and 85% (twenty-nine of thirty-four patients) in the non-irradiated allograft group. The failure rate, as determined by a grade-II Lachman score postoperatively, was 34% (eleven of thirty-two) in the irradiated allograft group, which was significantly (p < 0.05) higher than that seen in the autograft group (6%, or two of thirty-three) and the non-irradiated allograft group (8.8%, or three of thirty-four). The subjective IKDC score, Cincinnati knee score, Lysholm score, and Tegner activity score showed no significant differences among the three groups. However, there was a trend toward decreased function and activity level in the irradiated allograft group. Of note, one patient in the non-irradiated allograft group developed a late infection that resolved with antibiotic treatment only. No late infections occurred in the irradiated allograft or autograft groups. On the basis of these results, the authors discontinued use of irradiated BPTB allografts in ACL surgery and recommended against the use of gamma irradiation as a secondary sterilization measure.
In the study by Sun et al.26 that was published in 2012, seventy-eight patients were prospectively randomized to irradiated or non-irradiated hamstring tendon allograft for ACL reconstruction. Irradiated allograft tissue was again sterilized with 2.5 Mrad of irradiation prior to distribution. Nine patients were lost to follow-up, leaving a total of sixty-nine patients for evaluation at an average follow-up time of 42.5 months. Patients underwent functional testing, clinical knee laxity measurements, and patient-oriented outcome scoring (Cincinnati knee score, IKDC subjective knee score, Tegner activity score, and modified Lysholm knee score). Significant (p < 0.05) differences were found in Lachman testing, anterior drawer testing, and pivot-shift testing scores between the two groups. A total of 84% (thirty-two of thirty-eight patients) in the non-irradiated allograft group and just 32% (ten of thirty-one patients) in the irradiated allograft group had a KT-2000 side-to-side difference of <3 mm. Despite this finding, the overall IKDC, Tegner activity27, vertical jump, and one-legged hop testing scores were not significantly different between the groups. In the non-irradiated allograft group, there was one case of superficial wound infection that resolved with antibiotic treatment. There were no cases of deep infection in either group. The authors concluded that despite similar functional outcomes between the groups, the inferior knee stability rate found in the irradiated hamstring tendon allograft group has led them to avoid the use of irradiated hamstring tendon allograft for ACL reconstruction.
Outcomes of Non-Irradiated Allograft and Autograft Tissue in ACL Reconstruction
Lamblin et al.28 recently performed a systematic review of eleven total studies (Level I to Level III) that compared the outcomes of ACL reconstruction with use of fresh-frozen non-irradiated allograft or autograft tissue. A total of 533 allograft patients and 469 autograft patients were included. Seven of the studies used patellar tendon autografts, while four used hamstring autografts. No significant (p < 0.05) differences were detected between fresh frozen non-irradiated allografts and autografts with regard to Lysholm scores, IKDC scores, Lachman examinations, pivot-shift testing scores, or KT-1000 measurements. Overall failure rates, as defined by revision ACL reconstruction, 2 to 3+ pivot-shift, >10-mm laxity asymmetry on KT-1000 evaluation, or functional instability, were 3.6% (nineteen of 533) in the fresh frozen non-irradiated allograft group and 2.8% (thirteen of 469) in the autograft group (Tables III and IV). This difference was not significant. Average patient age in these studies, including one additional study29, was in the middle 20s to early 30s30-40, and therefore care should be taken if extrapolating the results of this systematic review to a younger, more active patient cohort. Carey et al.41 performed a systematic review that included nine autograft-versus-allograft comparative studies. Five of the studies included only non-irradiated allografts, three included a proportion of irradiated allografts, and one did not report whether irradiation was used in the sterilization process. The results showed no significant difference with regard to Lysholm scores and instrumented laxity measurements between the groups. Clinical failure was noted in 2.2% (five of 230) in the autograft group and 4.6% (eleven of 240) in the allograft group, which did not reach significance. Krych et al.42 performed a systematic review of six prospective nonrandomized cohort studies that compared patellar tendon autograft to patellar tendon allograft. When irradiated and chemically processed allografts were excluded, there was no significant difference in IKDC scores, Lachman and pivot-shift testing, graft rupture, or rate of reoperation.
There are many graft choices for ACL reconstruction. Allograft tissue is popular because it eliminates donor-site morbidity and decreases surgical site pain. Gamma irradiation is the most popular method of sterilization of soft-tissue allografts. High levels of irradiation are required to completely eliminate viral contamination but are detrimental to graft structural properties. Numerous studies have shown that graft irradiation of as little as 1.5 to 2.5 Mrad leads to increased postoperative laxity and increased clinical failure rates as compared with the laxity and failure rates seen in fresh-frozen non-irradiated tissue. When choosing allograft tissue as a graft source for ACL reconstruction, the best available current evidence (Table V) suggests that surgeons should use non-irradiated allografts from a trusted and reliable tissue bank. It is unclear whether low-level irradiated allografts (0.5 to 1.0 Mrad) result in improved biomechanical and clinical outcomes as compared with the outcomes achieved in their more highly irradiated counterparts.
Source of Funding: The authors received no funding for this work.
Investigation performed at the Sports Health and Performance Institute, The Ohio State University, Columbus, Ohio
Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. One or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.
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