Chemical sterilization of allograft dermal tissues.

Common terminal sterilization methods are known to alter the natural structure and properties of soft tissues. One approach to providing safe grafts with preserved biological properties is the combination of a validated chemical sterilization process followed by an aseptic packaging process. This combination of processes is an accepted method for production of sterile healthcare products as described in ANSI/AAMI ST67:2011. This article describes the validation of the peracetic acid and ethanol-based (PAAE) chemical sterilization process for allograft dermal tissues at the Musculoskeletal Transplant Foundation (MTF, Edison, NJ). The sterilization capability of the PAAE solution used during routine production of aseptically processed dermal tissue forms was determined based on requirements of relevant ISO standards, ISO 14161:2009 and ISO 14937:2009. The resistance of spores of Bacillus subtilis, Clostridium sporogenes, Mycobacterium terrae, Pseudomonas aeruginosa, Enterococcus faecium, and Staphylococcus aureus to the chemical sterilization process employed by MTF was determined. Using a worst-case scenario testing strategy, the D value was calculated for the most resistant microorganism, Bacillus. The 12D time parameter determined the minimum time required to achieve a SAL of 10-6. Microbiological performance qualification demonstrated a complete kill of 106 spores at just a quarter of the full cycle time. The validation demonstrated that the PAAE sterilization process is robust, achieves sterilization of allograft dermal tissue to a SAL 10-6, and that in combination with aseptic processing secures the microbiological safety of allograft dermal tissue while avoiding structural and biochemical tissue damage previously observed with other sterilization methods such as ionizing irradiation.

Assessment of bioburden on human and animal tissues: part 2--results of testing of human tissue and qualification of a composite sample for routine bioburden determination.

A quantitative method was developed and validated to assess bioburden on tissue from human donors and to compare bioburden determination results to swab culture results from the same donor. An initial study with allograft tissue from 101 donors showed a wide range of bioburden levels; values from no colony-forming units (CFU) detected to >28,000 CFU were observed. Tissues from donors that had swab cultures negative for objectionable microorganisms generally had lower bioburden than tissues from donors where objectionable microorganisms were recovered by swab culturing. In a follow-up study with 1,445 donors, a wide range of bioburden levels was again observed on tissues from donors that were swab culture negative for objectionable microorganisms. Tissues from 885 (61%) of these donors had no recoverable bioburden (<2 CFU). Importantly, tissues from 560 (39%) of the donors had recoverable bioburden which ranged from 1 to >24,000 CFU. Identification of bioburden isolates showed a diversity of genera and species. In compliance with the recent revision of the American Association of Tissue Banks K2.210 Standard, the quantitative bioburden determination method was validated with a composite tissue sample that contains bone and soft tissue sections tested together in one extraction vessel. A recovery efficiency of 68% was validated and the composite sample was shown to be representative of all of the tissues recovered from a donor. The use of the composite sample in conjunction with the quantitative bioburden determination method will facilitate an accurate assessment of the numbers and types of contaminating microorganisms on allografts prior to disinfection/sterilization. This information will ensure that disinfection/sterilization processes are properly validated and the capability of the overall allograft process is understood on a donor by donor basis.

Assessment of bioburden on human and animal tissues: Part 1--results of method development and validation studies.

Recovered human and animal tissues are used extensively in surgery for wound repair and reconstruction. In preparation for the validation of chemical disinfection and radiation sterilization processes, studies were performed on the development and validation of quantitative bioburden recovery methods for human bone and soft tissue and also for porcine dermis. The use of a swab-based method was not considered due to the known poor efficiency of recovery for this technique. The "exhaustive extraction" and "inoculated product" approaches to validation of a bioburden recovery efficiency factor have inherent strengths and weaknesses; in this study, tissues were inoculated and also subjected to a series of extractions to determine if/when "exhaustion" occurred. Femoral and tibial shaft rings, iliac crest wedges, sections of Achilles tendon, a soft tissue composite sample, and porcine dermis, were inoculated at several sites with Bacillus atrophaeus spores, and then subjected to either shaking by hand, mechanical shaking, or sonication plus mechanical shaking. Each of these methods of agitation were performed in combination with three rinse (extraction) fluids: phosphate buffer (Butterfield's buffer), phosphate buffer with 0.2% polysorbate 80 (a surfactant), and water with 1% peptone and 1% polysorbate 80 (Fluid D). The highest recovery efficiencies were observed with sonication plus mechanical shaking; of the three extraction media, Fluid D gave the highest first-rinse recovery efficiency (65%) and Butterfield's buffer gave the lowest (39%). Each of the three recovery methods, however appeared to reach "exhaustion", a subsequent rinse giving less than 10% of the recovery found in the first rinse. The results demonstrated the importance of performing bioburden method development and validation studies. The method validation strategy described here, using a combination of tissue inoculation and repetitive extraction, showed the superiority of sonication plus mechanical shaking using Fluid D as the rinse medium. In addition, the use of only the exhaustive extraction approach could have resulted in the development of a methodology that consistently underestimated the bioburden present on/in recovered tissue.