Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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CHAPTER 1 • Biomechanics of Fractures and Fracture Fixation

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Figure 1-31.  Effect of relative stability on heal- ing strength 9 weeks after stabilization of an ovine tibial osteotomy with or without a 3-mm gap: rigid fixation in case of locked plating or com- pression plating yielded around 20% strength recovery. 39 Relative stabilization of a 3-mm gap with FCL or active plating restored between 36% and 80% of native strength. 36,38 Active plating allowed approximately twice the amount of axial motion than FCL. In case of a well-reduced oste- otomy, active plating restored over 60% of native strength by week 9. 39

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performance of implants in clinical studies is confounded by multiple patient- and fracture-specific factors, including varia- tion in fracture patterns, comorbidities, patient activity levels, and patient compliance. This considerable number of uncon- trolled variables necessarily limits the sensitivity by which differ- ences between interventions can be detected. Moreover, clinical studies are time- and cost-intensive and subject to a consider- able regulatory burden. Most important, there remains a lack of clinically practical tools for direct, quantitative assessment of fracture healing. 10,106 Because reliable assessment of fracture healing is a prerequisite for the evaluation of new implants and interventions aimed at improving fracture care, this deficiency has recently been termed “the bottleneck of evidence-based fracture care.” 30 Why does the clinical assessment of fracture healing remain so difficult despite advanced diagnostic technologies? First, fracture healing is a gradual process, whereby the detection of specific endpoints such as union, nonunion, or delayed union remains controversial. In fact, there exists no uniformly accepted definition of union or delayed union. 63 Secondly, the state of fracture healing in vivo can only be measured by indirect, noninvasive means, such as radiographic imaging, load transduction, wave transmission, or patient functional outcomes. 10 Consequently, outcome parameters of these indi- rect measurements can only estimate the actual stiffness and strength at the fracture site.

In contrast to clinical studies, biomechanical testing can mea- sure the actual strength of fixation constructs and the strength of fracture healing by loading bone specimens obtained from animal studies to failure. This destructive testing not only pro- vides a direct, quantitative measure of strength, but also demon- strates the mode and cause of failure. As such, biomechanical research provides a time- and cost-effective complementary strategy to analyze implant performance before a clinical study is attempted. A further advantage of biomechanical studies is their ability to quantify functional aspects of orthopedic implants under defined, reproducible bench-test conditions. By reducing the number of uncontrolled variables compared to clinical studies, biomechanical studies have a higher sensitivity to detect performance differences between interventions. Con- versely, evaluating the efficacy of an intervention under simpli- fied test conditions can lead to unrealistic oversimplifications that detract from the clinical merit of the study, whereby bio- mechanical results may not correlate with clinical results. 212 In fact, a single deficiency in the study design can limit or negate the clinical relevance of a biomechanical study. The challenge in both designing and interpreting biomechanical studies relies on selection of simplified, well-defined test parame- ters while avoiding inappropriate oversimplification of the clinical problem. This challenge is best addressed by designing a study pro- tocol with close collaboration between orthopedic surgeons and biomechanical engineers. After formulating a study hypothesis that

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