Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

Figure 1-28.  Quality of fracture healing outcome as a function of shear and axial stiffness of fracture fixation (for a 3-mm fracture gap). Ellipses demonstrate the range of stiffness values for different fixation constructs. (Reprinted by per- mission from Springer: Claes L. Mechanobiology of fracture heal- ing, part 2: relevance for internal fixation of fractures. Unfallchirurg . 2017;120(1):23–31. Copyright © 2016 Springer Medizin Verlag Berlin.)

may delay the healing process. For unreamed nails, low shear stiffness suggests a modest to long delay in bone healing. For reamed nails with a larger diameter and press-fit insertion, bone healing is expected to range between optimal healing conditions and some delay. Plates typically have a high shear and axial stiff- ness, which is predicted to induce only minor callus formation and which may also delay the fracture healing process. IMPROVEMENT OF FRACTURE FIXATION Unreamed intramedullary interlocking nails are typically too flex- ible, specifically in the shear direction. The largest nail diameter possible can increase the shear stiffness 214 and thus can improve the healing outcome. 26,28 If used for fractures in the vicinity of the narrow isthmus of the medullary canal, reamed nails can yield a sufficiently high shear stiffness and good bone healing is expected. However, if used for fractures that are located distal or proximal where the intramedullary diameter is considerably greater than the nail diameter, shear stiffness can decrease consid- erably. 12 In this case, angle-stable interlocking screws can increase the stiffness of the bone–nail construct to some degree. 116 Unilateral external fixators normally display very low shear and axial stiffness. Simple changes in the application of the external fixator can correct this deficiency. The stabilization frame of the fixator can be moved close to the tibia, which is readily possible on the anteromedial surface where no muscles cover the tibia. This possibility does not exist in the femur, where large muscles prevent this strategy. Instead, the use of larger 6-mm diameter pins will optimize construct stiffness. In addition, placing one pin near the fracture and the second pin far from the fracture will increase the working length and will improve the stability of the fixator construct. The conven- tional ring-fixator has a very low stiffness in all loading direc- tions 77 with the exception of rotation around the longitudinal

axis. Higher stiffness can be achieved by adding more rings, by increasing the distance between rings, by maintaining the pretension of the wires, and by using half-pins in place of wires. Plate constructs provide only limited options to control stiff- ness, unlike intramedullary nails and external fixator constructs that allow surgeons to modify construct stiffness. The stiffness of plating constructs may be altered by the number of screws and screw pattern employed, although this will mainly affect bending and rotational stiffness. 203 Employing more flexible plates made of titanium or polyether-ether-ketone (PEEK) decreases bending stiffness, but has little effect on deficient axial motion adjacent to the plate. 54 Limitations to control stiffness with existing plates stimulated the development of new implants that are specifically designed to enable controlled axial motion. The quest for axial dynamization of plates dates back over three decades, 89,134,159.210 but only became practical with the advent of locking plates that no longer require rigid compression of the plate against the bone sur- face. Since then, two categories of implants have been introduced that enable controlled axial motion by either screw dynamiza- tion or plate dynamization (Fig. 1-29). Screw dynamization with dynamic Locking Screws (DLS) 75 or Far Cortical Locking (FCL) screws 34 employs a thinner screw shaft that elastically flexes relative to the near cortex adjacent to the plate, which in turn enables sym- metric axial dynamization at the fracture site. Plate dynamization with active locking plates employs locking hole elements that are elastically suspended within the plate by means of an elastomer envelope. 39 A biomechanical study illustrated that screw dynam- ization and plate dynamization deliver controlled axial motion without introducing shear-dominant motion (Fig. 1-30). 111 In vivo evaluation of sheep tibia osteotomies demonstrated that screw dynamization and plate dynamization yielded faster, circumfer- ential and significantly stronger healing compared to both rigid locking constructs 36,38,171 and compression plating constructs (Fig. 1-31). 39 Clinical studies of screw dynamization constructs have

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