8 The Need for Structural Allograft Biomechanical Guidelines Satoshi Kawaguchi, MD Robert A. Hart, MD Abstract Because of their osteoconductive properties, structural bone allografts retain a theoretic advantage in biologic performance compared with artifi cial interbody fusion devices and endoprostheses. Current regulations have addressed the risks of disease transmission and tissue contamination, but comparatively few guidelines exist regarding donor eligibility and bone processing issues with a potential effect on the mechanical integrity of structural allograft bone. The lack of guidelines appears to have led to variation among allograft providers in terms of processing and donor screening regarding issues with recognized mechanical effects. Given the relative lack of data on which to base reasonable screening standards, a basic biomechanical evaluation was performed on one source of structural bone allograft, the femoral ring. Of the tested parameters, the minimum and maximum cortical wall thicknesses of femoral ring allograft were most strongly correlated with the axial compressive load to failure of the graft, suggesting that cortical wall thickness may be a useful screening tool for compressive resistance expected from fresh cortical bone allograft. Development of further biomechanical and clinical data to direct standard development appears warranted. Instr Course Lect 2015;64:87–93. Surgical implantation of structural al- form with limited anatomic modifi - by the US FDA as well as through vol- lograft bone continues to increase de- cations, modern tissue processing in- untary participation with the American spite advances in modern alternatives cludes preparations of amalgams of Association of Tissue Banks (AATB). to allograft, including spine interbody allograft bone tissue of specifi c shapes Guidelines for allograft bone products fusion devices and peripheral joint en- and sizes to suit specifi c surgical needs. have primarily focused on avoiding doprostheses. Although allograft bone Oversight of the allograft processing the transmission of neoplastic and in- historically has been prepared in bulk and delivery industry has been managed fectious disease. Few guidelines exist regarding donor eligibility and bone Dr. Hart or an immediate family member has received royalties from DePuy and SeaSpine; is a member of a speakers’ processing methods with an emphasis bureau or has made paid presentations on behalf of DePuy and Medtronic; serves as a paid consultant to or is an employee of DePuy and Medtronic; has stock or stock options held in Spine Connect; has received research or institutional support on the mechanical integrity of struc- from DePuy; and serves as a board member, owner, offi cer, or committee member of the American Academy of Orthopaedic tural allograft bone, thus raising con- Surgeons, the American Orthopaedic Association, the Cervical Spine Research Society, the International Spine Study cern for uniformity and reliability in the Group, the Lumbar Spine Research Society, the North American Spine Society, the Oregon Association of Orthopaedic Surgeons, and the Scoliosis Research Society. Neither Dr. Kawaguchi nor any immediate family member has received any- biomechanical performance of struc- thing of value from or has stock or stock options held in a commercial company or institution related directly or indirectly tural bone allografts. to the subject of this chapter. © 2015 AAOS Instructional Course Lectures, Volume 64 87 Orthopaedic Medicine and Practice spinal fusion is a rare event.2 The in- cidence of graft fracture is reportedly zero to 2.7% following multilevel cer- vical corpectomy and fi bular allograft fusion.12-15 Bone resorption resulting from creeping substitution16-18 and im- mune response17,19 has been implicated in some cases. Despite such low incidence of al- lograft bone fracture after spine sur- gery, this chapter’s authors experienced two consecutive cases within 3 months of delayed fractures of anterior fi bular strut allografts following combined three-level cervical corpectomy and posterior instrumented fusion20 (Fig- Figure 1 Graph showing the increase in structural allograft bone transplant procedures performed in the United States per year from 1990 to 2004 (left ures 2 and 3). The allografts used for y-axis) and the increase in tissue donors per year from 1994 to 2006 (right these patients had been harvested from y-axis). (Adapted with permission from Jurgensmeier D, Hart R: Variability the same donor, a 69-year-old woman. in tissue bank practices regarding donor and tissue screening of structural allograft bone. Spine [Phila Pa 1976] 2010;35[15]:E702-E707.) The occurrence of this rare complica- tion in two patients who shared the This chapter’s authors discuss their fusion and larger endoprostheses for same donor suggested that the grafts clinical experience and review Ameri- limb and pelvic reconstruction, struc- may have been structurally inadequate can tissue bank practices with regard to tural bone allograft implantation has for the intended clinical use. At the screening donors and processing struc- continually increased in the United time, the tissue bank involved did not tural bone allograft. These issues are States since the late 1990s5 (Figure 1). use donor age or osteoporosis as ex- further evaluated with biomechanical Compared with artifi cial interbody clusion criteria for structural allograft data evaluating factors important to fusion devices and endoprostheses, donation. The occurrence of these frac- the mechanical performance of cortical structural bone allografts retain an ad- tures led this chapter’s authors to ques- femoral shaft allograft. Further efforts vantage in biologic performance because tion how allograft providers operate to assess the importance of these pa- of their osteoconductive properties. donor and tissue screening of structural rameters on clinical outcomes, as well as Also, they are cost effective and provide allograft bone, especially with respect the development and adoption of bio- easier radiologic assessment of bony fu- to variables that may infl uence mechan- mechanical performance standards for sion than do metal or plastic implants.6-8 ical strength of the graft. structural allograft, may be warranted. However, the mechanical performance of structural bone allografts may be a Current Regulation of Structural Bone Allograft disadvantage, made potentially worse Allograft Bone Screening in Orthopaedic Surgery by the negative effects of tissue process- The FDA began regulatory oversight of The use of structural bone allograft is ing and the less predictable effects of the recovery and processing of human a long-established surgical technique fatigue and postoperative remodeling cadaver tissues in 1993, after reports used for interbody spinal fusion1,2 as on the strength of the graft. Although of serious disease transmission from well as for reconstruction of defects fracture of structural allografts follow- allograft tissues.21-23 Since then, reg- of the long bones.3,4 Despite recent ing segmental grafting of various long ulation of the human allograft supply advances in modern alternatives to bone defects has been well docu mented, with respect to infection control and structural bone allografts, including with a re ported incidence of 14% to 29% disease transmission has continued to prosthetic interbody devices for spinal in recent literature,9-11 graft fracture after expand as additional pathogens have 88 © 2015 AAOS Instructional Course Lectures, Volume 64 The Need for Structural Allograft Biomechanical Guidelines Chapter 8 been identifi ed and effective screening AATB. The AATB was established in Current regulations have done much tests have been developed. 1976 and currently has 100 member to address risks of disease transmission In addition to the FDA oversight, the organizations, accounting for 90% of and tissue contamination. However, allograft tissue industry is self- regulated human allograft tissue used clinically comparatively few guidelines exist re- through voluntary membership in the in the United States. garding donor eligibility and bone pro- cessing issues with a potential effect on the mechanical integrity of structural allograft bone. Given this relative lack of guidelines from the FDA and the AATB, current practice with respect to these issues within the allograft in- dustry has not been known. Current Practices of Allograft Providers To assess current practices among allograft providers with regard to screening and processing and the struc- tural assessment of allograft bones, a questionnaire-based survey was de- veloped by this chapter’s authors.5 Figure 2 A, Lateral radiograph demonstrating apparent healing of the At the time of the survey, 45 AATB exterior graft at 6 months after surgery. B, Sagittal CT image of the cervical member tissue banks were involved to spine showing fracture of the allograft. (Adapted with permission from Jones some degree in procuring or process- J, Yoo J, Hart R: Delayed fracture of fi bular strut allograft following multilevel anterior cervical spine corpectomy and fusion. Spine [Phila Pa 1976] 31[17]: ing structural allograft bone. Of these, E595-E599.) 16 organizations participated in tissue Figure 3 A, Lateral radiograph demonstrating apparent healing of graft. B, Lateral radiograph of the cervical spine demonstrating fracture of the allograft and fracture of posterior instrumentation. C, Axial CT of the cervical spine demon- strating fracture of the allograft. (Adapted with