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J Thorac Cardiovasc Surg 1999;118:1157-1158
© 1999 Mosby, Inc.

LETTERS TO THE EDITOR

Measurement of chest wall forces on coughing with the use of human cadavers

Department of Biomechanics^a, Department of Cardiothoracic Surgery^b, Northern General Hospital
Sheffield S5 7AU, United Kingdom

To the Editor:

We read with interest the recent article by McGregor and associates ¹ on biomechanical testing of median sternotomy closures performed on human cadavers. This work provides the first measurement in the literature of chest wall forces on coughing. One needs to know the magnitude of forces across a sternotomy to test methods of sternotomy closure properly. The norm in biomechanics is that a closure should be able to withstand twice the potential maximum stresses.

These first measurements of chest wall forces on coughing are provided only indirectly. The article says that a force of 220 ± 40 N produced a lateral displacement of 1.85 ± 0.14 mm (ie, 260 N displaces 1.99 mm). Also, an intrathoracic pressure of 63 ± 21 mm Hg displaced 2.14 ± 0.11 mm (ie, 42 mm Hg displaces 2.03 mm). Therefore, indirectly, a cough generating 42 mm Hg produces a lateral force across a sternotomy of 260 N (~26 kg).

This work validates our mathematical model, ² which describes the force placed across a sternotomy closure. With our model, where P is the distending pressure, r is the radius, l is the height of the chest, and T is the force across the sternotomy,

T = rlP = 0.17m x 0.25m x 5.6 kPa = 238 N ~24 kg

the pressure of 42 mm Hg (~5.6 kPa) results in a predicted disrupting force of 24 kg as compared with the measured value of 26 kg. However, a normal cough reaches 100 mm Hg, producing a force of 56 kg, whereas maximal coughing can generate a pressure of 300 mm Hg, ³ producing a force of 168 kg.

We do not share McGregor and associates’ enthusiasm for human cadavers. The problem in using whole human sternums for biomechanical testing is in the wide biologic variation (eg, osteoporosis, metastases, age) in the samples and the difficulty in quantifying this variation. Hence a large number of samples is required to include bones of different quality. Also, multiple re-use of sternums weakens the bone, introducing further error. Therefore we use sheep sternum as our biologic model of sternotomy closures.

References

1. McGregor WE, Trumble DR, Magovern JA. Mechanical analysis of midline sternotomy wound closure. J Thorac Cardiovasc Surg 1999;117:1144-50. [Abstract/Free Full Text]
   
2. Casha A, Yang L, Kay PH, Saleh M, Cooper GJ. A biomechanical study of median sternotomy closures. Eur J Cardiothorac Surg 1999;15:365-9. [Abstract/Free Full Text]
   
3. Murray JF, Nadel JA. Textbook of respiratory medicine. 2nd ed. Vol 1. Philadelphia: WB Saunders; 1994. pg. 531-2.
   

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This Article Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Graham J. Cooper Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Casha, A. R. Articles by Cooper, G. J. Search for Related Content PubMed PubMed Citation Articles by Casha, A. R. Articles by Cooper, G. J.