Eur J Cardiothorac Surg 2008;33:769-773. doi:10.1016/j.ejcts.2008.01.026
Copyright © 2008, European Association for Cardio-thoracic Surgery. Published by Elsevier. All rights reserved.
Modified closure technique for reducing sternal dehiscence; a clinical and in vitro assessment
Lindsay C.H. John*
Department of Cardiothoracic Surgery, Kings College Hospital, Denmark Hill, London SE5 9RS, UK
Received 27 November 2007;
received in revised form 10 January 2008;
accepted 14 January 2008.
* Corresponding author. Tel.: +44 20 3299 4365; fax: +44 20 3299 3433. (Email: lindsay.john{at}kch.nhs.uk).
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Abstract
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Objective: Although the incidence of sternal dehiscence is low its mortality can be high. An alternative technique is described (modified closure) which aims to redistribute the dehiscence force into the longer longitudinal axis rather than the shorter transverse axis, thereby maximising the closure strength. Four ethibond sutures, which interlock anteriorly, are used in addition to eight transverse sternal wires. The aim of the study was to assess the modified closure using both an in vitro and a clinical study. Methods: (a) In vitro study: A weight and traction pulley system applied a force of 0.1 kN to pairs of silicone rubber hemisterna approximated to each other using alternative closure techniques. The dehiscence tendency (DT) was measured as the amount of separation under tension. Using 10 pairs of hemisterna for each closure technique the measured DT for the modified closure (MC) was compare with those for each of five alternative closures (two figure-of-eight and four transverse sutures (2C), 6 (6T), 8 (8T), 10 (10T) and 12 transverse sutures (12T)). (b) Clinical study: The incidence of sternal dehiscence for the first 4 years of a consultants’ practice (using 8T) was compared with the second 4 years (using MC). Results: (a) Measured DT (mean ± SEM), (MC: 149 ± 14; 6T: 256 ± 13; 8T: 223 ± 9; 10T: 213 ± 13; 12T: 203 ± 8; 2C: 294 ± 15). DT was significantly smaller for MC (p
< 0.003). (b) The incidence of dehiscence was significantly smaller in the second 4 years (MC) than in the first (8T): 0.2% (1/529) versus 1.6% (13/788); p
= 0.01 Conclusions: In vitro and clinical studies suggest that the modified closure technique can reduce the incidence of sternal dehiscence.
Key Words: Sternum • Wound closure • Wound dehiscence
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1. Introduction
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Although the incidence of sternal dehiscence is relatively low, 0.5–2.5% [1], its mortality can be high, 10–40% [2]. It has been estimated that it causes more than 1500 deaths per year in the USA [3]. A large number of alternative techniques for sternal closure have been described including the use of mersilene tapes [4], stainless steel bands [5], thermo reactive nitallium clips [6] and more recently plates with screws [7]. This wide range illustrates both the concern with sternal dehiscence as well as the failure to demonstrate one clearly superior technique. A structure fails when the force applied is greater than its strength. The maximum force to which a sternal closure is subjected is probably from a cough. A strong cough may generate a pressure of 40 kN m–2
[8]. By approximating the thorax to a cylindrical pressure vessel it has been estimated that such a pressure would develop a force of 1.5 kN across a sternotomy [9]. This is equivalent to 0.25 kN/wire for a six transverse wire closure which would develop a stress between the wire and sternum of 500,000 kN m–2
[10]. This is greater than the expected failure stress of human bone, 205,000 kN m–2
[11]. There are many patient related factors that decrease bone strength or otherwise increase the risk of dehiscence such as diabetes, chronic steroid use, morbid obesity, renal failure, chronic obstructive airways disease, concurrent infection or ageing which decreases bone strength by approximately 3–7% for each decade increase in age [12]. Therefore, it is extremely unlikely that any single method of sternal closure can eradicate sternal dehiscence.
An optimal sternal closure is one that maximises closure strength. This is the ratio between sternal resistance and the dehiscence force. The modified closure technique that has been developed aims to redistribute the dehiscence force into the longer longitudinal axis rather than the shorter transverse axis. In this closure the dehiscence force is opposed by a greater sternal volume (resistance) that maximises the closure strength. A mathematical analysis has demonstrated the potential advantages of this technique [13]. The aim of this study was to examine whether these advantages could be demonstrated in a clinical and in vitro assessment.
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2. Material and methods
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2.1 In vitro study
This was based upon a weight and traction pulley system (Fig. 1
). ‘Hemisterna’ were manufactured using silicone rubber (Alec Tiranti Ltd., Reading, UK), with dimensions of a ‘standard’ hemisternum [13]. Sternal closure was simulated by approximating two such ‘hemisterna’ with two separator bars between them, using the relevant closure technique. As weights are applied to the test system a traction force then acts upon the separator bars via the pulleys. As the traction force is applied the separator bars become distracted (Fig. 2
). The greatest distraction is at either end of the silicone rubber hemisterna and the least in the middle. The degree of separation of the hemisterna depends upon four factors: (i) the total traction force; (ii) the deformability of the silicone rubber; (iii) the deformability of the metal separator bars; (iv) the distribution of the distraction force in the transverse axis. The first three factors are fixed, the fourth will depend upon the type of closure. Of the different forces that act upon a sternal closure it is those acting in the transverse axis that are most important in causing dehiscence. Therefore, a closure that redistributes a traction force to minimise its transverse component is less likely to dehisce. Thus, in order to compare different closures the areas of separation of the silicone rubber hemisterna when under traction were measured. This area of separation was called the dehiscence tendency (DT). It was estimated by measuring the greatest separation of the hemisterna at each end using electronic digital callipers together with the length for which there was a significant separation. The area of separation could then be calculated by simple geometry.
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Fig. 2. Silicone rubber hemisterna and separator bars with an applied distraction force of 0.1 kN. (The greatest separation is at both ends of the hemisterna and the measured area of separation is the dehiscence tendency (DT).)
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Six types of sternal closure were assessed by this technique: transverse sutures alone (6,8,10,12), two figure-of-eight sutures combined with four transverse sutures and the modified closure technique. In the modified closure the sutures are placed longitudinally in each hemisternum, passing in and out twice. A loop from each suture interlocks anteriorly with a similar loop from the corresponding suture in the opposite hemi-sternum (Fig. 3
). Four such ethibond sutures (Ethicon, Johnson and Johnson Intl., Belgium) are shown in Figs. 4 and 5
, which illustrates both the anterior longitudinal component as well as the lack of a ‘complete’ transverse or oblique component. In clinical practice the modified closure technique consists of four such ethibond sutures together with eight transverse wires (Fig. 6
).
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Fig. 4. The four interlocking sutures of the modified closure technique in a model sternum (anterior view).
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Fig. 5. The four interlocking sutures of the modified closure technique in a model sternum (posterior view).
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Fig. 6. The modified closure technique in clinical practice showing the interlocking sutures tied before approximating the remaining transverse sternal wires.
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Using a distraction force of 0.1 kN the dehiscence tendency was measured using 10 different silicone rubber sterna for each of the six different types of sternal closure.
2.2 Clinical study
For the first 4 years of the authors’ consultant practice he closed all the sterna of his patients having cardiac surgery with eight transverse wires. He then changed to only using the modified closure technique. After further 4 years a comparison was made between these two patient cohorts. The patient characteristics together with the incidence of sternal dehiscence were compared between the first group (A) which had all been closed by one surgeon using eight transverse wires with the second group (B) which had all been closed by the same surgeon using the modified closure technique. All cases of dehiscence underwent secondary closure with a modified Robicsek closure and pectoral flaps.
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3. Statistical methods
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A statistical comparison was made between relevant parameters using Student's t-test or 
2-analysis as appropriate. A p-value of less than 0.05 was considered statistically significant.
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4. Results
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4.1 In vitro study
The measured dehiscence tendencies (mean ± standard error of the mean) for each of the six closure techniques compared are shown in Table 1
. The modified closure technique gave a statistically significant smaller dehiscence tendency than each of the five alternative techniques (p
< 0.003). This was not purely a consequence of the modified closure using 12 sutures (eight transverse wires and four interlocking ethibonds) as one of the five alternative closures was the use of 12 transverse sutures.
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Table 1 The dehiscence tendency (DT) (mean ± standard error of the mean) measured for each of six different sternal closure techniques (n
= 10) with a distraction force of 0.1 kN
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4.2 Clinical study
A comparison of the population characteristics and incidence of sternal dehiscence between groups A and B is shown in Table 2
. There was a statistically significant higher incidence of both chronic obstructive airways disease (COAD) and insulin dependent diabetes (IDDM) in group B (modified closure technique) compared to group A (eight transverse wire sternal closure). Despite this there was a significantly smaller incidence of sternal dehiscence in group B (1/529 [0.2%]) than in group A (13/788 [1.6%]) (p
= 0.01).
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Table 2 Comparison of the population characteristics and incidence of sternal dehiscence between group A (closed using eight transverse wires) and group B (modified closure)
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5. Discussion
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The most common method to close a median sternotomy is to twist together six or more transverse steel wires. A very large number of alternative techniques have also been described. These have included differences in the sternal incision [14], differences in the number and orientation of the stainless steel or ethibond sutures, differences in the circumferential closure materials used either in addition to or instead of stainless steel wire [5] or completely different techniques such as the use of poly L-lactide (PLLA) sternal pins [15] and thermo reactive nitallium clips [6].
The closure method described in this study, the modified closure technique, is an example of a variation in the number and orientation of the sternal sutures. It redistributes a transverse force into an oblique direction so that it is opposed by a larger resistance from the greater volume of sternum, thereby increasing the closure strength. Forces acting in the transverse axis are those that are most relevant in the aetiology of sternal dehiscence. It has been demonstrated in a cadaver model that the forces needed to cause dehiscence are 16% smaller when acting in a transverse direction than when acting in an antero-posterior direction, and 32% smaller than when in a rostro-caudal direction [16]. This is a consequence of both the smaller volume of sternum opposing the force in the transverse axis as well as the absence of bone-on-bone frictional forces [3]. In addition the largest force that acts upon a sternotomy closure that, due to coughing, acts predominantly in a transverse direction. Therefore the modified closure technique by redistributing a transversely acting force would be expected to reduce the incidence of sternal dehiscence [13]. There is a similar rationale in the use of figure-of-eight sutures [17]. However, with the latter suture there are transverse components as well as oblique components whereas in the modified closure technique there are only partial oblique and longitudinal components. It was also noted that the sternal closure incorporating two figure-of-eight sutures had the highest dehiscence tendency in the currently reported in vitro study. Similarly an in vitro study using polyurethane foam sterna concluded that figure-of-eight sutures should not be considered reliable enough to be used in primary sternal closures [18].
Other potentially useful modifications to the number and orientation of the sternal sutures are to increase the number of transverse sutures or to use longitudinal wire sutures for ‘lateral reinforcement’ as in the Robicsek closure [19]. As the number of wires are increased, so the dehiscence force acting upon each wire is decreased, however with the increase in the number of wires used there is also a decrease in the inter wire distance. This has a negative effect upon the sternal bone strength in the longitudinal axis. It is very unlikely that more than twelve transverse wires could be used in routine clinical practice.
The Robicsek closure has principally been used for secondary closure following dehiscence. However, variations of this technique have also been used in primary closure. These have included the use of parallel wires twisted together in the longitudinal axis and again in the transverse axis [20], the use of peristernal criss-cross wires [21], and the combination of lateral reinforcement together with the use of figure-of-eight sutures [22].
There have been a large number of reports of in vitro biomechanical assessments of sternal closures using sheep sterna [23], metal [9], polyurethane sternal models [24] and cadaveric sterna [25]. A common method is to apply an increasing or intermittent load and measure displacement using motion transducers placed along the sternum [16,18]. The measured stress-strain curve can be used to derive stiffness, elastic limit, yield and rupture points. An alternative technique was used in this study. Potential criticisms of it include: (i) the use of silicone rubber which has markedly different characteristics than sternal bone; (ii) the relatively small force applied (0.1 kN); (iii) the use of a constantly applied distraction force rather than an intermittent one. The reason for these differences from previously described methods is that the aim of this in vitro study was not to mimic the clinical situation and test to ‘failure’ but instead to investigate potential differences in the distraction force in the transverse axis. In this context it is necessary for the sternal substitute to measurably deform rather than to ‘fracture.’ For this purpose silicone rubber is more appropriate than material that more closely resembles the human sternum. In order to measure this deformation the force must be constantly applied rather than intermittently. A relatively small force is applied because of the characteristics of silicone rubber. If a greater force, approximating that in clinical practice, were applied then the sutures would ‘cut-through’ making comparative measurements impossible. A further potential criticism of the in vitro methodology is the use of flexible separator bars and the application of the disrupting forces at the ends rather than along the full length of these bars. This does not directly mimic the clinical situation. However, it does allow the easy measurement of deformation of the silicone rubber sternums and thereby facilitates the comparison between different methods of sternal closure of the distribution of the distraction force in the transverse axis. This was the aim of the in vitro study.
The clinical component of the reported study demonstrated a significantly smaller incidence of dehiscence using the modified closure technique compared to using eight transverse wires alone. A limitation of this study was that it was not prospectively randomised but instead included a retrospective comparison of one surgeon's results prior to changing over to the modified closure technique. It is possible that the results reflect the increasing experience of the surgeon. However, this is unlikely as he was an experienced cardiac surgeon prior to consultant appointment with over 10 years cardiac surgical practice.
In conclusion an alternative method for sternal closure has been described based upon a technique that optimally redistributes a transversely acting force. It is both simple to perform and has a minimal additional cost. Both an in vitro and a clinical study are suggestive that the modified closure technique can significantly reduce sternal dehiscence.
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Abstract
1. Introduction
