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Eur J Cardiothorac Surg 2001;20:433-434
© 2001 Elsevier Science NL
Letter to the Editor |
a Department of Cardiothoracic Surgery, Leeds General Infirmary, Leeds LS1 3EX, UK
b Department of Anaesthesia and Intensive Care, University of Wales College of Medicine, Cardiff CF14 4XN, UK
c Department of Biomechanics, Northern General Hospital, Sheffield S5 7AU, UK
d Department of Cardiothoracic Surgery, Northern General Hospital, Sheffield S5 7AU, UK
Received 13 May 2001; accepted 14 May 2001.
Corresponding author. Tel./fax: +44-113-245-6170
e-mail: aaron{at}casha.fsbusiness.co.uk
We thank Dr Losanoff and associates for their comments. Analysis of bone quality would not include only bone densitometry, but also compressive strength and compressive modulus (elasticity). However, bone is a complex material because of its anisotropic (differing strength and elasticity depending on direction of loading) and viscoelastic (differing strength and elasticity depending on speed of load or applied strain rate) properties and also because of its complex geometric variation in thickness in both cortex and medulla [1]. Also, the bone mineral density is not the same throughout the skeleton. This discordance can be caused by several factors, including differences in bone accretion and loss at various sites, variation in the variations in the accuracy of measuring bone mineral density by different techniques, and differences in the normal ranges for young adults between devices [2]. There is significant variation in the prevalence of osteoporosis with measurements at different peripheral and central sites, suggesting potential for misdiagnosis if the WHO criteria are applied strictly [3,4]. Therefore, analysis of bone quality does not simply mean bone densitometry.
A word on the ‘complex statistics’ that we applied [5]. The design of the study was such to avoid the influence of sternal bone quality on the results. We used paired bone samples from adjacent parts of the same sternum and therefore of presumably identical bone quality. Each type of closure was compared to standard steel wire closure as a control using these paired bone samples. This permitted the use of a statistical test that used paired results (due to the skewed distribution of the data, logarithmic transformation of the raw data was required to perform the paired t-test). Therefore, any variability in the sheep's bone densitometry status affected both the test specimen and the control, and there would not be any error resulting from this. The design study is such as to eliminate differing material properties of the bone samples.
As regards Puc's report of reduced tissue damage with polyester tapes [6], we note that in this report both stainless steel wires and Mersilene ribbon were used. Sternal cortical bone behaves rather like femoral trabecular bone since it fails by yielding; and the rate of yield (or rate of wire cutting through bone) is proportional to the force and inversely proportional to the area of contact:
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Synthetic material closures have been used mostly in a paediatric setting, e.g. polydiaxone (PDS Ethicon) reported by Kreitmann in 1992 [9], Schwab in 1994 [10] and Keceligil in 2000 [11]. Our mathematical model, T=rlP, predicts that in such a population the forces generated on coughing would be low due to the small thorax size, and that these forces diminish as a square of the linear dimensions.
References
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