Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

A, B

C

Figure 1-36.  The biofidelic test setup replicated the clinically observed failure mode of lag screw migra- tion and cutout ( A ) in surrogate specimens ( B ), and cadaveric femoral head-neck specimens ( C ), shown in a cross-sectional view to illustration varus migration. (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.)

loading. It is important to note that a static and a dynamic loading pattern can lead to different failure mechanisms and strength results for the same construct subjected to the same loading mode. For example, static bending of a plate construct will typically lead to plate bending when tested in normal bone, or to diaphyseal fracture at the end screw when tested in osteo- porotic bone. Dynamic loading of the same construct may lead to plate fatigue fracture rather than plate bending when tested

in normal bone, or to failure of screw fixation when tested in osteoporotic bone. 88 For this reason, static testing should be reserved for stiffness testing only. Strength testing should be conducted under dynamic loading whenever possible to account for dynamic loading in vivo. Traditionally, dynamic loading has been used for wear and fatigue testing of arthroplasty implants by repeating a load pattern for millions of cycles to simulate multi-year loading

Figure 1-37.  Specimen constraints have a profound effect on construct behavior. A: Rigid constraint of both specimen ends prevents the construct from buckling and bending and will yield a high axial stiffness. B: Suspending the specimen ends between ball joints allows for plate bending due to the plate offset from the loading axis, yield- ing a lower axial stiffness and asymmetric gap motion.

A

B

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