Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

TABLE 1-7. Mechanical Factors for Callus Healing

Factor

Goal

Avoid

< 0.2 mm

Interfragmentary motion 0.2–1 mm

Timing of motion

Early

Late

Direction of motion

Axial compressive

Shear

Residual gap

Small

Large

allows controlled fracture motion. 103,167,185,210,224 Functional brac- ing, external fixation, intramedullary nails, and bridge plating constructs all provide an environment of relative stability, and fractures will heal with callus formation. The guiding principles of fracture fixation constructs used to promote callus healing are based on the large volume of basic science studies evaluating the mechanical conditions under which callus forms. 36,61,103,125,210 The surgeon must evaluate each possible construct based on the four mechanical factors that appear to affect healing with callus, namely the amount of interfragmentary motion, 61 the timing of the introduction of motion, 103 the direction of motion, 8,187,216 and the residual fracture gap 56 (Table 1-7). The amount of interfragmentary motion allowed by an osteo- synthesis construct is determined by its stiffness. A low stiffness construct allows fracture motion, while a high stiffness construct limits motion. It is important to differentiate between the stiff- ness and strength of the construct, which are two independent factors. A construct such as an intramedullary nail has low stiff- ness and high strength of fixation, allowing healing with callus. Alternatively, a high stiffness and high strength construct may not allow adequate fracture site motion to reliably promote callus formation. 147 In general, compression plates and locking plates both produce a high stiffness environment, which is several times stiffer than intramedullary nails and external fixators. 88 A low stiffness and low strength construct permits interfragmentary motion, but because of its low strength it fails before the fracture has consolidated. When targeting natural bone healing with cal- lus a low stiffness, high strength implant is ideal. 210,224 The final stage of callus healing is remodeling. This is guided by the load transfer through the fracture site according to Wolff’s law. The paradox of internal fixation is that fixation is required to achieve union, while flexibility is necessary to restore nor- mal mechanical properties of bone after union. 3 As long as the stiffness of the fracture site remains less than that of the fixation construct, most loading will be shared by the fixation construct (Fig. 1-20). Because of the low stiffness of external fixators and intramedullary nail constructs, much of loading can be shared by the fracture site after successful callus bridging. However, stress-shielding due to overly rigid plating constructs can not only delay remodeling, but may also cause porosis by depriving the underlying cortex of sufficient mechanical stimulation. 53,210,224 Callus bridging may be delayed or prevented if fracture motion is dominated by shear due to transverse motion of the fracture sur- faces rather than axial compressive motion. 8,187,216 For example, torsional “twisting” of a thin intramedullary nail will permit rota- tion of the diaphysis, leading to shear-dominant motion between the fracture surfaces. 216 With external fixators, fracture surfaces

Figure 1-19.  Bilateral distal femur fractures, 6 months postoperative radiographs. Both fractures were treated with biologic techniques and locked plates. Residual gaps were left at the fracture site, and an overly stiff fixation construct was employed. Atrophic nonunions resulted.

FRACTURE FIXATION STRATEGIES

Osteosynthesis is a race between fracture healing and failure of the fracture fixation construct. If a fracture does not heal, the fixation construct will eventually fail by mechanical fatigue under pro- longed loading. The ideal osteosynthesis construct will promote biologic fracture healing while providing adequate mechanical durability as the fracture heals. 125,168,210 The surgeon has a vast array of constructs and techniques available for stabilization of fractures. It is necessary that a logical approach to the choice of implant and technique be applied to each fracture. This section will address general strategies for targeting a specific fracture-healing mode and for designing a durable fixation construct that will promote frac- ture healing. The next section will provide guidance concerning specific implants to implement these strategies. TARGETING A FRACTURE-HEALING MODE The first and most important decision in planning a fracture fixation strategy is determining which healing mode is desired. As previously discussed, the mechanical environment at the fracture site largely determines the mode of healing. 61,84,202 The mechanical environment is affected by multiple patient-related factors, including bone quality, fracture type, degree of comminu- tion, and fracture location. A principal factor in determining the mechanical environment at the fracture is the fixation construct chosen by the surgeon. Based on the technique and construct chosen, the fracture will heal either by natural bone healing with callus formation or by direct bone healing without callus forma- tion. This decision is critical, since the two healing modes have directly opposing mechanical requirements. 172 Specifically, a fix- ation strategy aimed at preventing fracture site motion to target direct bone healing will necessarily suppress natural bone healing by callus formation. Therefore, the most important question a surgeon should ask in choosing an osteosynthesis construct is: “What mode of fracture healing am I targeting?” FIXATION STRATEGIES FOR NATURAL BONE HEALING The decision to target natural bone healing by callus formation requires the use of a fixation construct with relative stability that

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