Interactive Cardiovascular and Thoracic Surgery 2:3-5(2003)
© 2003 European Association of Cardio-Thoracic Surgery
Institutional review - Cardiac general |
Strength of wired sternotomy closures: effect of number of wire twists
Susan Glenniea,
Duncan E.T. Shepherda,* and
Rajwinder S. Jutleyb
a Department of Bio-Medical Physics and Bio-Engineering, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, UK
b Department of Cardiac Surgery, Nottingham City Hospital, Hucknall Road, Nottingham, NG5 1PB, UK
* Corresponding author. Tel.: +44-1224-553489; fax: +44-1224-685645
d.shepherd{at}biomed.abdn.ac.uk
Received August 7, 2002;
received in revised form October 4, 2002;
accepted October 7, 2002
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Abstract
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The objective of this study was to investigate the effect of the number of wire twists on the strength of wired sternotomy closures. A custom-built test rig, fitted to a materials testing machine, was used to apply an increasing tension to wire closures, until they failed. The number of twists in the wire closure was varied between one and ten. Stainless steel wires of diameter 0.7 mm (No. 5) and 0.9 mm (No. 7) were tested. Initially, there is an increase in the maximum strength of the wire closure with increasing number of wire twists. After three wire twists greater strength is not achieved with increasing the number of wire twists. The highest mean force taken by the 0.7 mm diameter wire was 613 N ( 63 kg), at nine wire twists, whereas the highest force taken by the 0.9 mm diameter wire was 887 N (90 kg), at eight wire twists. However, by three wire twists, 80% and 88% of the maximum force has been achieved for the 0.7 and 0.9 mm diameter wire, respectively. Twisting wires many times in a sternotomy closure does not result in increased strength. Three or four twists would appear sufficient to sustain the forces across a sternotomy.
Key Words: Median sternotomy; Strength; Twists; Wire
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1. Introduction
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The median sternotomy is the most commonly used incision for surgical access to the heart, lungs and great vessels [1]. After surgery the incision is usually closed by placing stainless steel wires through or around the sternum [2,3]. The wire ends are then twisted together to close the incision. Sternal dehiscence can occur by the wire untwisting or breaking, or from the wire cutting through the bone [4]. It has been suggested that the wires can be used in conjunction with grommets [5] or cannulated screws [6] to prevent dehiscence occurring by the wire cutting through the bone. Biomechanical testing has been previously used to investigate the strength of sternotomy closures [1,2,7]. However, no studies have investigated the effect of wire twists on the strength of the wire closure. The aim of this study was to investigate how the strength of a twisted wire varies with the number of times the wire has been twisted.
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2. Materials and methods
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A custom-built test rig was used in this study (Fig. 1) and consisted of two ultra high molecular weight polyethylene blocks (Nylonic Engineering, Cumbernauld, UK). Each block had a hole, of diameter 1.4 mm, through which a wire could be passed. Metal brackets were used to fix one block to the base of an Instron materials testing machine (Instron Ltd., High Wycombe, UK), and one block to the cross-head, via the load cell, of the testing machine. A 180 mm length of wire was threaded through the holes in the blocks and the ends of the wire were twisted manually. The cross-head of the testing machine was set to rise at a rate of 25 mm/min, thus applying an increasing tension to the wire closure. This loading rate has previously been used for the mechanical testing of sternotomy closures [6,8]. During the testing, force and displacement were recorded, using Instron MAX software (Instron Ltd., High Wycombe, UK). Testing continued until the wire closure failed.
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Fig. 1 Test rig. The test rig consisted of two polyethylene blocks, attached to a materials testing machine via metal brackets. The wire was threaded through holes in the polyethylene blocks.
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No. 5 and No. 7 stainless steel wires (Ethicon Ltd., Edinburgh, UK) of diameter 0.7 and 0.9 mm, respectively, were used in this study. Testing was undertaken to determine the effect of the number of wire twists on the strength of the wire closure. This involved varying the number of twists in a wire from one to ten twists. The wires were then pulled apart using the test rig and materials testing machine. Each wire configuration was tested three times, with a new wire used for each test.
Graphs of force against displacement were plotted and the maximum force taken by a wire closure to fail was determined. A typical graph of force against displacement is shown in Fig. 2, for a wire of diameter 0.9 mm that has been twisted five times.
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3. Results
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The mean maximum force against number of twists in the wire is shown in Fig. 3, for the 0.7 and 0.9 mm diameter wires. For both wires it can be seen that initially there is an increase in the maximum force with the number of wire twists. After three twists the graphs level off and greater strength is not achieved with increasing the number of twists. The highest mean force taken by the 0.7 mm diameter wire was 613 N (63 kg), at nine wire twists, whereas the highest force taken by the 0.9 mm diameter wire was 887 N (90 kg), at eight wire twists. The strength of the 0.9 mm diameter wire was significantly greater than the 0.7 mm diameter wire ( ), as assessed using a Mann–Whitney test, since the data were non-parametric.
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Fig. 3 Mean force against number of wire twists for the 0.7 and 0.9 mm diameter wires. Error bars represent ±1 standard deviation.
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4. Discussion
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The aim of this study was to determine the optimum number of wire twists for closure of the median sternotomy. Moreover, it also has a practical application in other surgical specialities such as orthopaedics where wire twist closure may be employed. The aim was achieved by investigating how the strength of a twisted wire varies with the number of times the wire has been twisted. The results show that initially there is an increase in maximum strength with increasing number of wire twists. After three wire twists, the twist strengths level off and not much greater strength is achieved with increasing the number of twists. Indeed, by three wire twists 80% and 88% of the maximum force has been achieved for the 0.7 and 0.9 mm diameter wires, respectively. The 0.9 mm diameter wire shows greater strength for all number of twists compared to the 0.7 mm wire, with the exception of the first twist where both wires produce approximately equal results.
It has been estimated that the total force across a sternotomy, during a large cough, is 1500 N (150 kg) [2]. Given that surgeons generally use six wires to close a median sternotomy, each wire would be required to sustain 250 N. It would, therefore, take a minimum of three twists of the 0.7 mm wire to withstand a large cough. In comparison, it would only take two twists of the 0.9 mm wire. Surgeons generally use five to seven twists of the wire in a sternotomy closure [3] and, based on the results of this study, this would appear sufficient to gain maximum strength. Twisting a wire too many times could cause damage to the wire that leads to early failure. In conclusion, twisting wires many times in a sternotomy closure does not result in increased strength. Three or four twists would appear sufficient to sustain the forces across a sternotomy.
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Appendix A
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ICVTS on-line discussion
Author: Dr. Carlos A. Mestres, Hospital Clinico, University of Barcelona, Department of Cardiovascular Surgery, Villarroel 170, Barcelona 08036, Spain
Date: 31-Oct-2002 23:25
Message: I found this brief report on biomechanics of sternal closure truly interesting. It is actually true that on a regular basis we the surgeons tend to neglect facts that we consider of less importance. We open a number of incisions and the problem is that we have to close them ensuring that our patients do go back to normal as soon as possible. Sternal closure is an important part of our daily surgical practice and it is part of the surgical success. This report tries to recall our attention to this last part of any major cardiac operation done through the front. I personally confess to probably tie the wires too much for the sake of making myself comfortable. The truth is that each and every of the readers or potential readers of this contribution have broken many wires during our surgical careers. I read this with great interest.
Author: Andrew Muir, Registrar in Cardiothoracic Surgery, Royal Victoria Hospital, Department of Cardiac Surgery, Royal Victoria Hospital, Grosvenor Road, Belfast BT12 6BA, Northern Ireland, UK
Date: 03-Nov-2002 14:32
Message: It is true that on many occasions all surgeons will break one or more of the sternal wires. However, the reason for breaking wires is usually not due to the surgeon wanting to achieve a set number of twists. The ultimate aim of adequate wiring of the sternum is to appose the sternal halves accurately, and with enough tension to prevent movement upon respiration, coughing or arm movement. In attempting to achieve this tension between the sternal halves, each wire is twisted until all slack wire is removed from behind the sternum, and the sternum is able to resist moderate distraction. It is this attempt to eliminate any excess wire that results in wires being overtightened, deforming and ultimately fracturing. The study shows that from 3 twists, 80–90% of total strength is achieved. There is no information as to the mechanism of failure of each tested wire. I suspect that the tested wires failed due to unravelling of the twists, instead of wire fracture. This is the opposite of what we find in clinical practise, where sternal dehiscence, without infection is usually due to wire fracture. The value of this paper is to show that excess wire can be removed from the wound, thereby reducing the amount of foreign material, and limiting infective complications such as sinus formation.
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Acknowledgements
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The Leverhulme Trust are gratefully acknowledged for providing funding for a Special Research Fellowship for D.E.T.S.
doi:10.1016/S1569-9293(02)00068-3
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References
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Abstract
1. Introduction