50 AAOS Now May 2011 Research and Quality Research and Quality May 2011 AAOS Now 51

tion of tissue allografts. Grafts such as tendons or pieces of bone for skeletal reconstruction, which are used in structural situations, have to be processed in a manner that does not signi� cantly alter their inherent mechanical proper-ties. These grafts are cleaned and processed using low-dose irradia-tion, a chemical technique, or both to achieve “sterility.”

Allografts may also be processed aseptically without any second-ary attempts at sterilization. These grafts must be rigorously con-trolled for bacterial contamination and are usually stored in a frozen state. Freezing an allograft has little impact on the mechanical properties of the tissue and will diminish its immunogenicity (Table 1). Freezing will affect the viability of articular cartilage un-less some form of cryopreservation is employed.

Tissue can also be freeze dried. Freeze drying further diminishes the immunogenicity of the graft and may be helpful in eliminating the bacterial contamination (Ta-ble 2). It does however, have some negative effect on the mechanical strength of the grafts, especially if rehydration is not done properly in the operating room. Gamma radiation is effective in killing bac-teria, fungi, spores, and, to a lesser degree, viruses. Depending on the dose, however, gamma radiation can weaken the graft. Doses below 1.5 mrad do not seem to adversely affect the tissue strength.

“Nonstructural” products, which do not require mechanical properties, include demineralized

or “� ller” products to encourage new bone growth. An assay of the bioactivity of demineralized bone matrix may be available and should be considered when choos-ing an inductive product. These products can be processed either sterile or aseptically. Gamma irra-diation and excessive heat (> 60 degrees C) is known to dam-age bone proteins. Freezing or freeze drying does not appear to af-fect the healing properties of bone.

ConclusionIn considering the use of allograft tissue for orthopaedic reconstruc-tion procedures, the surgeon must understand both the source and the processing techniques used to pro-duce the graft. Only then can he or she take advantage of this valuable material to rebuild and replace skeletal structures biologically.

NOW

References and resources for this article can be found in the on-line version, available at www.aaosnow.org

Steven Gitelis, MD, and Ross Wilkins, MD, are members of the AAOS Biological Implants Committee. Dr. Gitelis can be reached at sgitelis@rush.edu; Dr. Wilkins can be reached at DrRMW@aol.com

Disclosure information: Dr. Gitelis—Wright Medical Tech-nology, Inc.; Dr. Wilkins—Wright Medical Technology, Inc., allosource, AATB

Table 1: Mechanical effect of processing on bone

Preservation process Compression Torsion Bending

Deep freezing 120% No change 90%

Freeze-drying 120% 39% 75%

Radiation (6 MRad) 80% 65% 70%

Note: Measures are compared to control (unprocessed allograft bone)

Table 2: Processing effects on bone strength, viability, and immunity

Technique Immunogenicity Mechanical Cartilage Properties Viability

Fresh Strong Normal strength Totally viable

Frozen Partial Normal strength Partially viable

Freeze-drying None Partially weakened Dead

Various preservation processes have different effects on the mechanical prop-erties of bone compared to unprocessed controls, as shown.

Allograft processing techniques may affect bone strength (mechanical prop-erties), immunogenicity, and cartilage viability, as shown.

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