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Biomechanics of Fractures

February 19, 2016

Contributors: Daniel James Kaplan, BA; Kenneth A Egol, MD, FAAOS; Victor H Frankel, MD; Victor H Frankel, MD

BACKGROUND: Bone fracture biomechanics are central to the field of orthopaedic surgery. This video presents the digitized, original, reel to reel footage of these groundbreaking 1960s experiments demonstrating the viscoelastic properties and fracture mechanics of loaded bone. As described in his seminal paper, "A Standard Test for Laboratory Animal Bone" (1971), and seen here, Dr. Frankel developed a technique using novel instrumentation that resulted in an easily reproducible method of controlling bone loading rates (i.e. adjusting force with time). The innovation, and associated experiments, radically advanced our understanding of the mechanisms of acute fractures and bone's response to energy. In recognition of this work, and for his cumulative contribution to the orthopaedic discipline, Dr. Frankel was knighted by the king of Sweden.

METHODS: The video begins with a review of the types of energy including potential, kinetic, and strain. Using the standard torsion testing machine that he helped design, the author explains how the mechanical and functional properties of bone—strength (load capacity, energy storage capacity), rigidity (resistance to deformation), and torsional load capacity—are affected by alterations in structure. Examples used include an intact dog femur, a femur with a hole in the cortex (similar effect as a screw), and a femur with an open section defect (can be secondary to bone graft harvest or a tumor). Fractures are shown at 5,000-8,000 frames per second. The slow motion depiction allows the viewer to appreciate the potential soft tissue damage associated with bone fragmentation, and how this varies with energy input. The video concludes with a demonstration of the effect of torsional motion on arteries.

RESULTS: Graphs produced by the torsion tester display torque vs. angular deformation plots for each experiment. These illustrate the relationship between bone structure, fracture, and energy. Viewers will gain an understanding of the dangers of non-intact bone, and how defects diminish bone stress capacity. The x-ray contrast segment provides an example of associated non-osseous damage secondary to a fracture. The viewer will witness deformation of soft tissues, and arterial bending, elongation, segmentation in slow motion, furthering cognizance of these oft ignored, but critical injuries.

CONCLUSION: Biomechanics of bone is paramount to orthopaedic surgery, but its complexity can intimidate surgeons of all training levels. This previously unreleased piece of orthopaedic history provides viewers with perspective on the early days of biomechanical study, as well as an easy-to-understand tutorial on acute fracture mechanics and the role of energy in injury.

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