Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

LOADING MODES All bones are exposed to complex physiologic loading that can be divided into three principal loading modes, namely axial loading, torsion, and bending. Testing implant performance individually for each of these principal loading modes sim- plifies load application and enables isolation of failure mech- anisms specific for each principal loading mode (Fig. 1-34). However, it also requires testing to be individually conducted in each loading mode. To reduce the number of loading sce- narios, testing is sometimes only performed in the dominant loading modes for the anatomic site being studied. For exam- ple, because the femur is loaded primarily in axial compression and bending, unicortical locked plating of femur fractures was initially tested in axial compression and bending only, which yielded encouraging results. 149 However, the clinically observed failure mechanism of screw pull-out was subsequently linked to poor fixation strength of unicortical locking screws in tor- sion. 88 Therefore, results obtained in one loading mode cannot be extrapolated to alternate loading modes. As an alternative to simplified testing under principal load- ing modes, implants may be tested under joint-specific, com- plex loading representative of the activities of daily living. Physiologic loading data for a range of activities have been obtained for the hip, knee, shoulder, and spine, using implants instrumented with load sensors and telemetry for wireless

Surrogate and cadaveric specimens may also be combined in the same study. 80 For example, surrogate specimens are used for relative comparison between implant types under various load- ing conditions. After time and cost-efficient testing over a wide parameter range using surrogate specimens, the key finding is validated by replication on a small number of paired cadaveric specimens. Two important conclusions that orthopedic sur- geons should be aware of are as follows: • The performance and failure mode of fracture fixation con- structs differs between strong bone and weak bone speci- mens. Therefore, results from strong bone specimens may not be extrapolated to the osteoporotic population. • Cadaveric and surrogate specimens only simulate the early postoperative phase and do not account for time-dependent changes of bone in vivo, such as gradual load sharing by callus formation, remodeling, or stress-shielding induced osteolysis. LOADING CONSIDERATIONS Three aspects of loading application must carefully be con- sidered when designing or evaluating a biomechanics study: the loading mode (simple or complex), the specimen con- straints (fixed or suspended), and the loading pattern (static or dynamic).

Figure 1-34.  Examples of isolated testing of a plating construct in the three principal loading modes. For axial testing, the specimen is fixed at the bottom, and loaded through a ball joint on top. Torsion is applied around the diaphyseal axis. Bending is shown for a four-point-bending setup, which induces a constant bending moment between points A and B, unlike three-point bending. The depicted specimen alignment and constraints are only examples and vary between studies. (Adapted from Fitzpatrick DC, Doornink J, Madey SM, et al. Relative 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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