Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

fracture healing are available. 18,30,36,38,148,176 Specific guidance as to how to achieve the desired stiffness of external fixators, intramedullary nails, and plate constructs is provided in the following section of this chapter. FIXATION STRATEGIES FOR PRIMARY BONE HEALING Targeting direct bone healing with compression or locked plat- ing is a valuable fixation strategy for intra-articular fractures that rely on anatomic reduction to restore the continuity of joint sur- faces. However, unlike the forgiving nature of callus healing, the choice of primary bone healing in diaphyseal fractures requires an exacting compression plating technique. 168 A perfect, com- pressive reduction of fracture fragments is required to allow a healing environment that limits motion to less than 0.15 mm. Even for simple fractures, obtaining such a fracture environ- ment is rarely possible. 224 With the exception of intra-articular fractures, the lack of clinical evidence of its superiority and the unforgiving nature of the stringent procedure to achieve inter- fragmentary compression detract from its merit. 152,210 In summary, choosing the desired fracture-healing mode dictates the type and configuration of the required fixation con- struct. Stiff implants are reserved for cases in which direct bone healing is desired, while flexible implants favor callus healing. Most fractures, comminuted or simple, benefit from natural bone healing and require a fixation construct that enables early and controlled interfragmentary motion to stimulate callus for- mation. Targeting direct bone healing should remain reserved for anatomic reduction of intra-articular fractures, owing to its slow, 168 weak, 71 and invisible healing process. 210 Furthermore, attempting to mix the direct and callus healing strategies will not properly support either mode of healing. For example, the use of an interfragmentary lag screw in the setting of a relatively stable construct such as an external fixator or a plate construct with far cortical locking (FCL) screws mixes two different frac- ture-healing modes and should be avoided. 137 CREATING A DURABLE FIXATION CONSTRUCT Regardless of the mode of healing chosen, the osteosynthesis con- struct must be strong enough to withstand loads until the fracture is consolidated. There are two interrelated strategies to improve the strength of a fixation constructs: minimizing stress concen- trations and maximizing the working length. These fundamental concepts are explained in the following section by examples that can be translated to a wide range of fixation constructs. Minimizing Stress Concentrations When force is focused on a small area, it generates high stress that can lead to material failure or fracture. For example, a sharp drill focuses a cutting force on a small cutting edge to generate a stress concentration that can readily penetrate or fracture bone. A dull drill will apply less concentrated stress and will require a far greater force to penetrate bone. For the same reason, stress concentrations in fracture fixation constructs must be mini- mized to prevent early fixation failure. Stress concentrations in fracture fixation constructs can be reduced by three strategies: load distribution, reduction of stiffness gradients, and preven- tion of preloading during fixation.

may shift transversely relative to each other due to torsion around the external monolateral bar. 8,187 With bridge plating constructs, increasing the bridge span can induce up to three times more shear motion than axial motion, leading to shear-dominant interfrag- mentary motion 111 that can inhibit callus formation. 82 Targeting natural bone healing with callus allows robust fracture healing under a wide range of fracture fixation con- structs. Specific mechanical principles must be respected, but these allow a significant envelope of possible environ- ments for healing. Intramedullary nails and external fixa- tors have an adequately low axial stiffness, 202 and their low shear stiffness can be improved by modulating the implant configuration to achieve the desired torsional rigidity. 216 In contrast, plating constructs have an inherently high stiffness, which is difficult to effectively modulate to support natu- ral bone healing. 88 While a multitude of technical tricks and guidelines have been recommended to decrease the stiff- ness of contemporary plating constructs, such strategies can also reduce construct strength and may induce asym- metric or shear-dominant interfragmentary motion. 111,122,203 Novel plating strategies that incorporate engineered solu- tions capable of dynamizing the fracture to stimulate natural Figure 1-20.  A fixation construct should support the three stages of natural bone healing, all of which are directed by the mechanical envi- ronment at the fracture: First, the load-bearing construct must sup- port interfragmentary motion to simulate callus formation. Second, it must gradually share load with the bridging callus to stimulate callus maturation. Finally, the remodeling stage relies on a largely unshielded load transfer through the diaphysis to restore normal bone strength. An overly stiff fixation construct can not only delay all three healing stages by suppressing callus formation, maturation, and remodeling, but may also induce porosis under the plate by depriving the bridged bone seg- ment of sufficient mechanical loading.

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