During the past 50 years, the use of human bone allograft has been increasing, with the most dramatic increases occurring during the past decade. An estimated 1 million allografts will be used in the United States in 2013. Because allografts are biologic materials that come from various sources, orthopaedic surgeons need to understand not only the origin and safety profile of the materials, but also the material and structural properties of bone allografts in making a decision about their use in reconstructive surgery.
Disease transmission, either viral or bacterial, is extremely rare with bone allografts. The last incidence of a fatality due to disease transmission from bone allograft occurred in 2001 and was due to a major deviance from standard industry practice. The incidence of viral transmission, such as hepatitis or HIV, is calculated to be less than 1 in 1.6 million, with no cases reported in several years. This safety profile compares very favorably to other biologic materials.
Additional processing and disinfection techniques can further reduce these risks and are commonly used. Because these processing methods may affect the biologic and structural properties of the bone, it is important for surgeons to understand their impact.
Screening the donor
When a tissue recovery facility is notified of a potential donor, it obtains and carefully evaluates the donor’s social and medical history to determine suitability. This detailed history record is provided by the donor’s next of kin or someone who is very familiar with the donor’s lifestyle and medical history. The evaluation consists of more than 50 specific questions and covers medical history, social behavior, environmental risks, and specific medical conditions such as autoimmune disease or cancer. The decision to accept or reject the donor is initially based on these responses and the cause or circumstances of death.
Many types of tissues can be recovered, including living cells, bone, soft tissues, and skin. All grafts undergo multiple culturing sessions from the time of recovery until final release. Depending on the results of those cultures, secondary decontamination and/or “sterilization” may be required.
Fresh articular cartilage may also be obtained. It is initially refrigerated, then placed in cell culture media to sustain cell viability while donor suitability is determined.
The culturing methods used for these tissues are quite sophisticated. Although swab-type cultures are obtained, it is recognized that these are generally unreliable, and more sophisticated techniques with tissue homogenization, ultrasound, and/or extraction may be used. These additional methods can detect the presence of even a few bacteria. As the tissue culture results are obtained, additional consideration of especially virulent organisms, such as Clostridia species, Enterococci, or fungi, is required. Any contaminated tissues must be discarded; disinfection and sterility procedures used for the donor should be reevaluated.
The serology tests required for evaluating donor eligibility are extensive and include screenings for HIV, hepatitis B and C, syphilis, and exposure to other diseases. With advancing science, the “window” for false-negative tests is in the 1–2 week range. These newly developed tests are generally only available in the United States and developed Western countries. Surgeons should be very wary of any tissue that is procured in Eastern European countries or underdeveloped countries; instances of inadequate serologic testing of those donors have been reported.
In the processing facility, tissues may be processed into many varied products depending on the results of the donor’s medical history, activity level, age, sex, size, and cultures, as well as on the intended use. For example, only cortical bone from a young healthy donor should be used in structural applications. Tissues that are at risk for bacterial contamination need to be disinfected or “sterilized.” The requirement for tissue sterility differs from that imposed for a medical device because typical sterilization processes (harsh chemicals and high-dose radiation) have very deleterious effects on tissue properties.
Many techniques can be used with tissues to achieve sterilization or disinfection. These include combinations of washing (with or without pressurization), centrifugation with various chemicals such as alcohol and detergent, and combining antibiotics. Low-dose radiation (less than 1.5 gy) has been shown to be an effective decontamination method when used with other wash techniques and does not appear to dramatically affect the strength or biology of the tissue. Regardless of the technique used, the objective is to remove all possible infectious organisms while maintaining the structural and biologic properties necessary for the intended use of the allograft.
After the grafts are processed and cleaned, they are packaged and stored. All of these steps are regulated by the AATB and FDA and require validation.
Once the tissues have gone through their processing and all of the information is accumulated, the donor tissues are again evaluated for final release. This final release is determined by the medical director of the processing facility. The medical director should not only be acutely aware of all of the screening processes and details of the donor, he or she should be familiar with the intended use of the graft itself.
Once allografts are released to the hospital setting, the Joint Commission and FDA require that all grafts be tracked, from the tissue processor to the user facility and ultimately to a specific patient.
Effects of allograft processing
Various processing methods are used in the production of tissue allografts, especially in bone allografts. Structurally sound bone can be used in grafts for rebuilding or replacing existing bone or in reconstructive procedures. Bone that is deemed to be less mechanically competent, such as from an elderly female, should be used to produce nonstructural grafts, such as cancellous chips or nonstructural cortical products.
Allografts can also be processed aseptically without any secondary attempts at sterilization or disinfection. These grafts must be free of bacterial contamination and are usually stored in a deep frozen state. Freezing an allograft bone has very little impact on the mechanical properties of the tissue, but will diminish its immunogenicity. Freezing will affect the viability of articular cartilage unless some sort of cryopreservation is employed.
Freeze drying is another method of preserving tissue. Freeze drying diminishes the immunogenicity of the tissue and dramatically decreases its mechanical strength. This is especially true if sufficient time is not allowed for rehydration prior to use in the operating room.
Gamma radiation is proficient in killing bacteria, spores, and, to some degree, viral particles. The dose of radiation is quite important. Doses less than 1.5 gy do not significantly weaken bone structure; doses greater than 2.5 gy significantly alter the mechanical properties of cortical bone.
Nonstructural products that do not require mechanical integrity, including demineralized or filler products to encourage bone growth, are often used in bone reconstruction. An assay of the bioactivity of demineralized bone matrix should be available for each donor, and these values should be considered when choosing an inductive product. Because these products can be processed in multiple ways, including chemical and radiation techniques, it is essential to measure the bioactivity of the end product, not the bone protein prior to processing.
When considering the use of a bone allograft product, the surgeon must be aware of its source, the screening methods used to deem donor eligibility, and the processing methods used on the tissue itself. These are key issues, not only for the intended use of the product but also for the ultimate safety of the patient recipient. AATB accreditation and FDA inspection are essential criteria for choosing a tissue processing facility and donating agencies.
For additional information and references, see the online version of this article, available at www.aaosnow.org
Ross M. Wilkins, MD, and Steven Gitelis, MD, are members of the AAOS Biological Implants Committee. Along with Robert Hart, MD, and Alan E. Gross, MD, FRCSC, Prof., they will be presenting a symposium on Translational Research in Orthopaedics: Structural Bone Allograft from Benchtop to Bedside during the 2013 AAOS Annual Meeting (March 19, 1:30 p.m.–3:30 p.m., McCormick Place, Room S105).
Disclosure information: Dr. Wilkins—Allosource, AATB. Dr. Gitelis—Wright Medical Technology, Inc. Dr. Hart—DePuy, A Johnson & Johnson Company; SeaSpine; Kyphon Inc.; Medtronic; Synthes; Eli Lilly; Spine Connect; Orthopaedic Research and Education Foundation (OREF); Cervical Spine Research Society (CSRS) Editorial Committee; Spine; The Spine Journal; AAOS; American Orthopaedic Association; CSRS; Lumbar Spine Research Society; North American Spine Society; Oregon Association of Orthopaedics; Scoliosis Research Society. Dr.Gross—Zimmer, Journal of Bone and Joint Surgery–American; Journal of Bone and Joint Surgery–British; Clinical Orthopaedics and Related Research; Journal of Arthroplasty, Canadian Orthopaedic Association; Knee Society; Hip Society.
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