New methods of sterilization have been introduced, but questions remain about the actual risk of infection and disease transmission
The use of allograft tissues is increasingly popular in the United States, with widespread use among orthopaedic surgeons—particularly in knee surgery. In 2005, more than 60,000 allografts were used in knee surgeries by members of the American Orthopaedic Society for Sports Medicine (AOSSM).
As the use of allograft tissue increases (Table 1), the safety of allografts remains an issue of paramount importance. In 2005, the U.S. Food and Drug Administration (FDA) finalized its requirements for current good tissue practice and has mandated new rules regarding the “manufacture” of allograft tissue.
Yet, many orthopaedic surgeons still have serious concerns about the safety of nonsterilized allografts. Although most believe that sterilized allografts are safe, they also believe that sterilization negatively affects tissue biology.
Disease transmission from musculoskeletal allograft tissue
Historically, the transmission of bacterial or viral infection from musculoskeletal tissue has been a rare event, but the potential for disease transmission remains a major concern for clinicians and patients. In fact, safety concerns are one of the factors limiting the use of allograft tissue in orthopaedic surgery.
Although several cases of viral infection—specifically human immunodeficiency virus (HIV), viral hepatitis, and human T-lymphotropic virus (HTLV)—have been reported, these transmissions occurred before the guidelines for donor screening for viruses and bacteria were implemented and before the availability of currently validated serologic tests.
Recent media reports about procedural irregularities at specific tissue banks, however, have intensified this concern. Increased federal oversight of tissue banks and improved donor screening techniques, including nucleic acid testing (NAT), should address these concerns.
How is disease transmitted?
Disease transmission occurs in two principal ways—either through an infected donor or during tissue procurement, processing, or packaging. Transmission of an infectious agent through an infected donor results from contamination from infectious bacteria in the blood either as the result of prolonged tissue recovery time, occult perimortem infection, or a screening failure.
The primary focus of every tissue bank is to ensure the safety of allograft tissue and minimize the chance for infection. Therefore, tissue banks follow several steps to ensure tissue safety. Each bank’s medical director has the critical role of overseeing quality control at each step.
Tissue procurement and preparation can be divided into tissue/donor screening and tissue processing. To optimize the safety of donor selection, a detailed screening process takes place, which includes the following steps:
- Medical and/or social risk assessment
- Medical history from hospitals, clinics, and private doctors’ offices
- Blood donor histories
- Review of autopsy reports
In addition, allograft tissue cultures are used to detect bacteria and fungi after aseptic tissue processing. Studies have shown, however, that the rates of sensitivity for these cultures are only 78 percent to 92 percent. Therefore, culturing alone is not a conclusive method for ensuring allograft sterility.
Tissue processors must adhere to procedures that minimize donor infection—including recovering tissues from donors within a safe period and testing to assess bioburden from the donor. The bioburden is the level of surface contamination detected (cultured) when the donor tissue is received. If the bioburden level is too high, the donor tissue may be rejected. Failure to adhere to established standards for tissue recovery time may result in the procurement of tissues from a contaminated donor, thereby increasing the risk for infection in tissue recipients.
Allograft tissue processing
Allograft tissues are generally harvested and processed aseptically; although it may not prevent contamination, aseptic processing does restrict or minimize contamination to the allograft tissue from the environment, processing personnel, and equipment. Tissue processing is a critical component for ensuring allograft safety—a difficult task given the complex structure of musculoskeletal tissue.
To minimize the possibility of contaminating the tissue, standard sterile techniques, including zone-specific prepping and draping, are used. The procurement technique requires the use of gowns, gloves, and sterile instruments; operating room technicians assist in the procedure.
Tissue recovery takes place at funeral homes, morgues, and medical examiners’ offices, but most allograft tissue is aseptically processed in operating room suites or dedicated tissue recovery suites.
Tissues are cultured, wrapped, labeled, sealed, and shipped in specified, qualified containers at wet ice temperatures.
Tissues should not be considered “sterile”
Despite these safeguards, aseptically processed tissues should not be considered sterile. The healthcare provider may contaminate the tissue during processing and transferring. Organisms in the donor’s gastrointestinal or respiratory tracts, which are not actively reduced by antibiotic solutions, may also be sources of contamination.
Although it would be convenient if allograft tissue could be sterilized just like other medical devices, harsh methods applied to metal and plastic cannot be applied to human cadaveric tissue without destroying the tissue’s mechanical and biologic properties.
To ensure sterility of musculoskeletal allografts, all bacteria, spores, fungi, and viruses must be eliminated. Furthermore, blood is a significant pathogen reservoir and is reported to be the primary source for endogenous contamination of musculoskeletal tissue.
Sterility is expressed as a mathematic probability of relative risk. According to the FDA, a sterility assurance level (SAL) of 10-3 means there is a 1 in 1,000 chance that a nonviral viable microbe exists in or on the implanted material. The Association for the Advancement of Medical Instrumentation (AAMI) states that an SAL of 10-6 (one in a million chance) in organisms is more desirable. The American Association of Tissue Banks (AATB) requires an SAL of 10-6 for tissue bank allografts.
The FDA does not dictate which specific sterilization process or technique should be used. In its Final Rule, however, the FDA asked each tissue banking establishment to reduce the risk of all communicable diseases with a verified or validated process, subject to short notice review by the agency. The FDA plans to make more unannounced tissue bank inspections and improve the monitoring of processing at each tissue bank.
Contamination of the graft can also occur during the final handling and packaging of tissue. Gamma irradiation is commonly used to terminally sterilize allograft tissue with lower doses of radiation. Many tissue banks label their packaged allografts as sterile. The FDA requires tissue banks to have validated justification for any sterility claims.
Packaging and storage of allograft tissue
Allograft tissue is kept frozen during shipping and delivery to the end user (eg, hospital or surgery center).
Freeze drying (lyophilization) is a preservation process that allows tissues to be stored at room temperature. Lyophilization freezes the tissue and reduces the water content to less than 6 percent of initial weight through a primary drying process (sublimation) and a secondary drying process (desorption). Although freeze-dried allografts (lyophilized grafts) are not commonly used for sports medicine applications in the United States, this process is commonly used with soft-tissue patches.
With improved donor screening techniques, such as nucleic acid testing (NAT), the current risk of transplanting tissue from an HIV-infected donor is reported to be between 1 in 1 million and 4 in 1 million.
Approximately 950,000 Americans are infected with HIV, and about one quarter of those infected are unaware of their disease status. Another 1.2 million Americans are infected with the hepatitis B virus, and 4.1 million are infected with hepatitis C. An additional 3,000 Americans have been infected with the West Nile virus. The actual number of potential donors who are infected with these viruses is currently unknown.
The National Nosocomial Infections Surveillance System of the Centers for Disease Control and Prevention (CDC) reports that postoperative infection rates range from 0.6 percent to 2 percent. The risk of bacterial infection from a screened and processed fresh platelet infusion is reported to be about 1 in 2,200.
According to a recent AATB survey covering data from 2003 to 2004, the current risk of an allograft infection to the average patient appears to be much less than the risk of infections surrounding the surgery itself. According to the report, there were 192 reports of suspected allograft-related infections in 2003-2004; 42 percent involved soft-tissue grafts and 37 percent involved bone grafts, with an overall incidence of 0.014 percent. Currently, better reporting of infections is actively under investigation to improve the accuracy of these numbers.
Questions concerning allograft tissue safety remain, such as the actual risk of infection and disease transmission following implantation. Appropriate donor screening, improved donor testing—including NAT—and adherence to good tissue procurement practices can decrease or eliminate disease transmission from the recipient, as well as infections that are the result of tissue processing.
In conclusion, the following final thoughts should be emphasized and remembered:
- Choose tissue provided by an AATB-member tissue bank.
- Be familiar with the different sterilization/processes used for allografts.
- Do not do routine culturing of allograft tissue in the operating room immediately prior to implantation. These cultures are documented to be inaccurate and may reflect the native airborne or backtable contamination.
C. Thomas Vangsness Jr., MD, is professor of orthopaedic surgery at Keck School of Medicine, University of Southern California, Los Angeles.