Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

been for fractures of the tibia, 1,92,175,213 proximal humerus, 91 and distal femur. 35,177 Plate dynamization with active locking plates has been documented in a prospective observational study on humeral shaft fractures. 148 While early clinical results are encour- aging, randomized prospective studies will be required to quan- tify the effect of axial dynamization of plating constructs on the time to union and the nonunion rate.

BIOMECHANICAL EVALUATION OF FIXATION CONSTRUCTS

This section provides orthopedic surgeons with information about how to evaluate biomechanical studies of fracture fixation constructs. It may also serve as a starting point to design a bio- mechanical study in collaboration with a biomechanical engi- neer. Comprehensive and detailed guidance on study design is provided elsewhere. 152,197 This section will focus on basic concepts common to a wide range of biomechanical studies on fracture fixation. A thorough understanding of these principal concepts will be crucial to assess the clinical relevance and lim- itations of biomechanical studies.

Figure 1-29.  Screw dynamization using far cortical locking (FCL) screws 34 or dynamic locking screws (DLS) 75,91 with elastic screw shafts, and plate dynamization using an active locking plate with elastically suspended locking holes inside the plate. 209 (Reprinted with permis- sion from Claes L. Mechanobiology of fracture healing, part 1: princi- ples. Unfallchirurg . 2017;120(1):14–22.)

BENEFITS AND LIMITATIONS OF BIOMECHANICAL STUDIES

Because of inherent limitations of clinical studies, biomechan- ical studies form the foundation for the design, improvement, and evaluation of osteosynthesis strategies. Evaluating the

Figure 1-30.  Effect of dynamization strategies on the type and magnitude of interfragmentary motion for plating of a distal femur fracture. Bridge plating constructs with a short bridge span are very stiff and can suppress axial motion at the near cortex below the 0.2-mm threshold for stimulation of callus formation, regardless if locking or nonlocking screws are used. A long bridge span of a locked construct can lead to shear-dominant motion. Far cortical locking (FCL) screws and active locking plates can enhance axial motion without requiring a long bridge span, and without introducing shear-dominant motion. (Adapted with per- mission from Henschel J, Tsai S, Fitzpatrick DC, et al. Compari- son of 4 methods for dynamization of locking plates: differences in the amount and type of fracture motion. J Orthop Trauma . 2017;31(10):531–537.)

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