Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

Figure 1-32.  Test results of cadaveric specimens ( white lines ) illustrate dramatic difference in implant migration (varus rota- tion) and durability (cycles to failure) that do not closely cor- relate to bone density, stated in terms of T-scores. In contrast, results of synthetic surrogate specimens ( red line ) deliver a con- sistent, reproducible migration history. (Adapted from Ehmke LW, Fitzpatrick DC, Krieg JC, et al. Lag screws for hip fracture fixation: Evaluation of migration resistance under simulated walking. J Orthop Res . 2005;23(6):1329–1335. Copyright © 2005 Orthopaedic Research Society. With permission.) (Pacific Research Labs, Vashon Island, WA). These surrogate specimens typically consist of a glass–fiber-reinforced epoxy cortex and cancellous bone replicated by rigid polyurethane foam, designed to have structural properties in the physiologic range of healthy bone. Different from cadaveric bone, the vari- ability in stiffness and strength between surrogate specimens is within 2% to 10%. 109,201 Currently available bone surrogates possess mechanical properties adequate to evaluate the per- formance of implants in normal bone. 52,81,109 However, epoxy- based surrogates cannot replicate the complex density gradients and load-optimized architecture of the trabecular structure of cadaveric bone. Furthermore, these surrogates do not represent osteoporotic bone because the performance and failure mode of fracture fixation constructs differ between strong bone and weak bone specimens. 14,189,192 Testing fixation constructs in strong bone will typically induce implant breakage or bend- ing. 32 Testing in osteoporotic bone will typically cause fixation failure at the implant–bone interface (Fig. 1-33). 88 A validated osteoporotic bone surrogate has been developed only for the femoral diaphysis. 201

describes the unmet clinical challenge to be investigated, the sur- geon-scientist teamwill propose a specific performance criterion or a clinically relevant failure mode that will subsequently guide the experimental design and choice of outcome parameters. A thor- ough literature review is crucial to identify the appropriate choice of specimens, loading mode, loading pattern, instrumentation, and outcome parameters. After successful generation of bench-top results, further research by computational simulation using finite element modeling may be conducted to explore additional test and outcome parameters, as outlined at the end in this chapter. The following section describes key test and outcome parameters for biomechanical evaluation of osteosynthesis devices, and it summarizes common shortcomings that the sur- geon must recognize when interpreting results of biomechani- cal research studies. SPECIMEN SELECTION Implants are typically tested in either cadaveric specimens or surrogate specimens. Fresh frozen or nonembalmed cadaveric specimens realistically account for the complex structure and material properties of bone. However, cadaveric specimens can vary greatly in geometry and material properties, even when specimens of similar bone density are selected. Differences in specimen age, the degree of osteoporosis, cortex thickness, and overall size have been reported to cause up to a sevenfold variability in bone strength. 117 The resulting variability within the same test group can be even more pronounced. For exam- ple, when simulating varus migration and lag screw cut-out in cadaveric proximal femurs, one specimen with a low bone den- sity T-score of –3.9 failed by cut-out after 103 loading cycles, while another specimen with a similarly low T-score of –3.1 did not exhibit cut-out even after 20,000 cycles (Fig. 1-32). 80 Variability can be somewhat reduced by using paired speci- mens, whereby interventions are randomly assigned to either right or left specimens. While this necessarily confines testing to the comparison of a single independent parameter between two experimental groups, it provides the undisputed benefit of generating results using the most clinically realistic specimen. Alternatively, whole bone synthetic surrogates of the femur, tibia, humerus, radius, and ulna are commercially available

Figure 1-33.  Testing with strong bone surrogates induced implant failure: screw breakage under torsion and plate breakage under bending. Testing with osteoporotic specimens induced surrogate failure: torsion induced a spiral fracture, and bending induced a transverse fracture at the end screw. (Reprinted from Fitzpatrick DC, Doornink J, Madey SM, et al. Rela- tive stability of conventional and locked plating fixation in a model of the osteoporotic femoral diaphysis. Clin Biomech (Bristol, Avon) . 2009;24(2):203–209. Copyright © 2009 Elsevier. With permission.)

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