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Ann Thorac Surg 2004;77:2261
© 2004 The Society of Thoracic Surgeons
Department of Surgery, University of Columbia School of Medicine, M580 HSC, One Hospital Dr, Columbia, MO 65212, USA
e-mail: jonesjw{at}health.missouri.edu
To the Editor:
We read with interest the article by Dasika and colleagues [1] on lower sternal reinforcement after median sternotomy. The group's experimental model is based on static traction across the sternal closure to a maximum force of 400 newtons (N). The study does not appear to include assessment of repetitive cycling loads, a realistic replication of the forces associated with breathing, coughing, and bodily movement that cause wire to cut through the bone of patients [2].
Our ongoing sternotomy-closure study has used fresh cadaveric sternums attached to a biomechanical testing device (TAHDi Texture Analyzer; Texture Technologies Corporation, Scarsdale, NY) set for repetitive cyclic loads of 400 and 800 N and speeds of 0.04 and 0.5 mm/s. Preliminary assessment of failed sternotomy closure at forces lower than 400 N cycling for 12 hours showed that differences in sternal displacement after a variety of closure techniques, including single or figure-of-eight transsternal or peristernal wires, were insignificant. The sternal manubria were uniformly closed with three single wires and remained the stablest parts. Testing at 800 N for 40 minutes showed statistically significant differences in sternal displacement associated with individual closure methods, including wide gaps at both the manubrial and xiphoid ends. Dasika and associates used single wire loops to close the manubria; this might not be appropriate in live humans, who can produce substantial sternal displacement forces in the postoperative period.
In more than 60 adult cadaveric specimens tested to date by our group, separation typically occurred first in the xiphoid end of the closure, and this result validates findings in a human model the findings of Dasika and co-workers. Nevertheless, they used static traction forces; our preliminary experience with dynamic testing using variable loads equal to or higher than those the Dasika team employed suggest that their model might respond differently if tested under repetitive loads.
Dasika and colleagues noted an absence of information on the mechanical forces that can disrupt sternal union. A German median sternotomy study [3] done in the late 1980s used increasing traction and found that wires cut through the bone at forces between 390 and 490 N. These results are not consistent with our experience with repetitive cycling loads of 800 N, which do not typically cause wires to cut through bone or fracture the sternum.
The study of Dasika and associates illuminates largely unresolved issue in demonstrating that early sternal separation starts at or near the xiphoid end of the closure. Despite some questions about study design and its relationship to earlier reports, their work represents an important step that should be caused farther with comparable studies of a wider variety of closure techniques and a much larger sample.
References
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