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Research Papers

Characterization of Forces on the Sternal Midline Following Median Sternotomy in a Porcine Model

[+] Author and Article Information
Shruti Pai, George D. Pins

Department of Biomedical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609

Raymond M. Dunn, Russell Babbitt, Heather M. Strom, Janice F. Lalikos

Department of Surgery, Division of Plastic Surgery, University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, MA 01655

Kristen L. Billiar

Department of Biomedical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609; Department of Surgery, Division of Plastic Surgery, University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, MA 01655; Department of Mechanical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609kbilliar@wpi.edu

J Biomech Eng 130(5), 051004 (Jul 11, 2008) (7 pages) doi:10.1115/1.2948401 History: Received July 11, 2007; Revised April 09, 2008; Published July 11, 2008

The development of more effective fixation devices for reapproximating and immobilizing the sternum after open-heart surgery is limited by current methods for evaluating these devices. In particular, precise emulation of in vivo sternal loading has not been achieved in controlled model systems. The present study is an initial effort to determine the in vivo loading parameters needed to improve current in vitro and in silico (computational) models. Towards this goal, the direction, magnitude, and distribution of loading along a midline sternotomy were characterized in a porcine model. Two instrumented plating systems were used to measure the forces across the bisected sternum in four anaesthetized Yorkshire pigs during spontaneous breathing, ventilated breathing, and coughing for four treatments: live, cadaveric, embalmed, and refrigerated. Changes in forces incurred by death and embalming were also investigated to evaluate the potential applicability of cadavers as models for testing sternal fixation devices. The magnitudes of the respiratory forces in three orthogonal directions ranged from 0.4Nto43.8N, many fold smaller than previously estimated. Dynamic forces were highest in the lateral direction during coughing and low in all directions during normal breathing. No significant differences in force were found between the four treatments, most likely due to the unexpectedly low magnitude of forces in all groups. These results provide the first measurements of in vivo sternal forces and indicate that small cyclic fatigue loads rather than large quasistatic loads should be applied in future model systems to best evaluate the mechanical performance of fixation devices.

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Copyright © 2008 by American Society of Mechanical Engineers
Topics: Force
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Figures

Grahic Jump Location
Figure 1

Experimental setup showing (a) supine pig with skin and muscle retracted to expose sternum: A valsalva force was applied at the abdomen near the diaphragm and a phrenic cough was stimulated by applying a voltage to the phrenic nerve. An outline of the directional plating system is shown on the sternum. (b) Close-up of directional plating system used to measure forces in three orthogonal directions as indicated by the arrows; 222N(50lb) force transducers are shown from left to right in the (1) lateral direction at the manubrium, (2) cranial-caudal and (3) dorsal-ventral directions at the midsternum, and (4) lateral direction at the xiphoid. (c) Distributional plating system with 5lb(22.2N) force transducers placed laterally at the (1) manubrium and (2) xiphoid. Note that ball joints were attached to the force transducers from both plating systems to allow free movement in the directions not being measured.

Grahic Jump Location
Figure 2

Schematics showing 25s of a sample respiratory wave form. Static forces are equivalent to the signal base values and represent the force required to bring both sternal halves together without effects of breathing (seen in Schematics A and B). Dynamic forces are equivalent to the amplitude of the peak-to-peak signal and represent the additional force required to hold the sternal halves together due to respiration/coughing (Schematic C).

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