Strain in fractures is calculated by dividing the interfragmentary movement by the size of the fracture gap. Strain must be very low (2%) for primary bone healing to occur, and should be between 10-30% for secondary bone healing to occur. Stiffness refers the ability of the construct to resist movement under applied loads. Answer 3 describes a situation where primary bone healing is the goal. For this to occur, there must be no significant gapping at the fracture site, there must be low strain between fracture fragments, and the construct must be stiff. None of the other answers would accomplish this. Functional bracing and intramedullary fixation both accomplish healing through formation of fracture callus, or secondary healing. A comminuted fracture treated with locked bridge plating relies on less stiffness to allow for secondary healing between fragments. However, since there are many fracture fragments, the strain is distributed among them and therefore remains low. A uniplane external fixator is very unlikely to accomplish low strain and high stiffness in this setting.
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Mazzoca AD, DeAngelis JP, Caputo AE, et al. Principles of internal fixation. In: Browner BD, Jupiter JB, Levine AM, Trafton PG, Krettek C, eds. Skeletal Trauma: Basic Science, Management, and Reconstruction, 4th ed. Philadelphia, PA: WB Saunders; 2009:83-142.