Does skin preparation alter suture strength characteristics? Assessing the effect of chlorhexidine and isopropyl alcohol on common skin closure suture material

Abstract

Sutures are essential to approximate tissues and enable healing by first intention until a wound regains its original tensile strength. The mechanical properties of sutures are well documented, but the effects of exposing sutures to skin preparation solutions used in surgery are not. This study was performed to investigate whether 2% chlorhexidine and 70% isopropyl alcohol skin preparation, commonly used prior to incision and prior to closure, has any effect on the mechanical properties of several commonly used surgical suture types. Four suture types were soaked in either 2% chlorhexidine and 70% isopropyl alcohol or Hartmann's solution for 5 minutes. All sutures were left to dry for 11 days before being tested to failure using an Instron 3367 tensile testing machine. Testing revealed significant differences in failure load, ultimate tensile stress, and Young's modulus between suture types (P < .05). No significant differences in failure load (P = .98), ultimate tensile stress (P = .21), or Young's modulus (P = .22) were observed between the test group and the control group when comparing sutures of the same type. This study demonstrates that chlorhexidine/isopropyl skin preparation solutions do not significantly change the mechanical properties of suture materials exposed to them.

1. INTRODUCTION

Sutures are essential to approximate tissues and enable healing by first intention until a wound regains its original tensile strength. The selection of an appropriate suture for a particular application depends upon a number of factors including its tensile strength, handling properties, timescale of absorption, potential for tissue reaction, and surface area available for microbial colonisation. ¹ , ² To ensure adequate opposition of the target tissues until wound healing occurs, a suture of appropriate tensile strength must be selected. This tensile strength must be maintained until the tissue healing process has progressed sufficiently to ensure wound opposition without support from the suture material; otherwise, the wound could dehisce and lead to infection. While the mechanical properties of sutures are well documented in the body of literature, ³ , ⁴ it is not clear whether agents which come into contact with suture materials at the time of surgery may modify these properties. These properties include ultimate tensile strength (failure load), stress (force per unit area), and Young's modulus of elasticity (stiffness of a material). If a suture comes into contact with an agent which reduces its failure load, it might not withstand the force which is applied across it. Poor wound opposition, because of iatrogenic causes such as too much, or too little, tension in the suture across the wound, as well as poorly opposed edges, can lead to significant wound dehiscence and lead to scarring or surgical site infections. ⁵ The use of chlorhexidine to irrigate wounds is widespread across surgical specialties. Some surgeons also apply chlorhexidine skin preparation solutions to wound edges at the time of closure. It is presumed that this will decrease colonisation and reduce rates of surgical site infection. Despite these practices being common, they are not evidence‐based and the potential effect on the mechanical properties of the sutures has not been evaluated. Chlorhexidine solution has the potential to contribute to suture failure rates, wound dehiscence, and surgical site infections.

Research suggests soaking sutures in chlorhexidine mouthwashes, commonly used in dental procedures, can alter their mechanical properties leading to complications in wound healing. ⁶ , ⁷ , ⁸ , ⁹ Moharsan et al showed that soaking 4‐0 Vicryl sutures in 0.2% chlorhexidine did significantly reduce the ultimate tensile strength of the suture. ⁸ The aim of this study was to assess whether the exposure of suture materials to surgical chlorhexidine solutions changed their mechanical properties and therefore whether chlorhexidine washes affect the integrity of skin closure.

2. MATERIALS AND METHODS

The mechanical properties of four commonly used skin closure sutures were tested: 1 Coated Vicryl, 2‐0 Monocryl, 3‐0 Ethilon, and 3‐0 Vicryl Rapide (Table 1).

TABLE 1.

Summary of the four sutures used

Suture	USP (diameter/mm)	Manufacturer	Material
Absorbable
Vicryl Rapide	3‐0 (0.2)	Ethicon	Braided polyglactin 910
Coated Vicryl	1 (0.4)	Ethicon	Braided polyglactin 910
Moncryl	2‐0 (0.3)	Ethicon	Monofilament poliglecaprone 25
Non‐absorbable
Ethilon	3‐0 (0.2)	Ethicon	Poly (ethylene terephthalate)

Testing of the sutures' mechanical properties was carried out on an Instron 3367 tensile testing machine equipped with a 1 KN load cell (Instron Corporation, Norwood, Massachusetts) (Figure 1) using a custom‐made suture mount (Figure 2). Sutures were attached to the mount by wrapping each end 2 to 4 times (dependent upon suture length) around the spools of a custom thread testing mount with the sutures clamped distal to this to ensure load distribution through the body of the suture rather than at the clamp interface. A standardised de‐tensioned active length of 15 mm was used for each suture before the application of a preload of 1 N. Initiation of linear load tensile testing at a rate of 25 mm/minute was commenced until repair failure. The load (N)/clamp displacement (mm) was recorded every 0.1 seconds. Tests were performed in ambient air.

FIGURE 1.

Instron 3367 tensile testing machine

FIGURE 2.

Custom‐made suture mounts

The study methodology was based upon previous studies investigating the mechanical properties of various suture materials. ³ , ⁴ Variability in testing procedure was minimised by using a single examiner who prepared and tested all of the samples on the same testing rig using a standardised protocol.

This in vitro study was designed and completed at the University of Exeter. Testing was carried out by one examiner upon 56 individual sutures. The sutures were divided into two equal groups: a control group (n = 28) and a test group (n = 28). Both the control group and the test group contained seven samples of each of the four suture types (1 Coated Vicryl (n = 7), 2‐0 Monocryl (n = 7), 3‐0 Ethilon (n = 7), and 3‐0 Vicryl Rapide (n = 7)). Samples in the control group were soaked in Hartmann's solution for 5 minutes. Samples in the test group were soaked in a solution of 2% chlorhexidine and 70% isopropyl alcohol for 5 minutes which was selected as a primary benchmark as it represents the most concentrated chlorhexidine solution commonly used as a skin preparation. The soak time of 5 minutes was chosen to allow the chlorhexidine solution time to fully penetrate the sutures in an attempt to ensure a uniform effect across each suture's cross‐sectional area and as a proxy of the highest time period the suture would be likely to be in contact with a concentrated solution. Both sample groups were left to dry at 21°C. Testing was carried out 11 days after soaking to represent the length of time that sutures might be expected to support a wound for while skin healing occurs. The combination of a lengthy soak, a long period of drying time and a concentrated solution of chlorhexidine and isopropyl alcohol was used to give a “worst case scenario” and to allow any time or concentration‐dependent weakening of the material to occur before testing.

For the purpose of data collection, extension (Δl) was defined as the maximal increase in total suture length prior to failure. Failure load (P) was defined as the maximal load that could be applied across the suture prior to failure. Stress (σ) was derived by dividing the failure load (P) of the suture tested by the mean cross‐sectional area (A ₀) of the suture type tested. Strain (ε) was calculated by dividing extension (Δl) by the original active length (l ₀) of the sample. Young's modulus (E) or stiffness of each suture type was expressed as a ratio of load to displacement (N/mm) and was derived from the linear elastic portion of the stress/strain graph through the addition of a least square linear trendline.

Seven sutures of each type (n = 7) were soaked in either Hartmann's solution or the Chlorhexidine preparation. Seven was chosen because it has been previously described in literature as adequate to reduce alpha and beta errors. ³

The results obtained during testing were analysed using a two‐way ANOVA for the dependent variables of failure load, ultimate tensile stress, and Young's modulus to determine the effect of the independent variables, suture type, and fluid exposure (chlorhexidine or Hartmann's solution). For the determination of pair‐wise statistical differences, post‐hoc analysis was carried out using Tukey's test and considered statistically significant if P < .05.

3. RESULTS

The results showed significant differences in failure load, ultimate tensile stress and Young's modulus between suture types (P < .05) as would be expected for sutures of differing composition and diameter.

However, no significant differences in failure load (P = .98), ultimate tensile stress (P = .21), or Young's modulus (P = .22) were observed between the test group (chlorhexidine/isopropyl) and the control group (Hartmann's) when comparing sutures of the same type. During the testing process, failure of every suture sample occurred at, or near, the centre of the active length, suggesting that mount design had no effect upon results.

As each suture type had a different USP size, we cannot directly compare sutures. However, in the Chlorhexidine solution, failure load was highest in Vicryl 126.8 (±4.2 N), with Monocryl 78.8 (±4.33 N), Ethilon 27.7 (±0.44 N) and lowest in the synthetic absorbable suture, Vicryl rapide, 18.6 (±4.4 N).

In chlorhexidine, average ultimate tensile stress was highest in Monocryl 1109.3 (±61.0 Nmm²) and lowest in Vicryl rapide 685.1 (±81.3 Nmm²), Ethilon recorded 882.7 (±14.3 Nmm²) and Vicryl 1008.7 (±33.6 Nmm²).

Young's modulus was demonstrated to be lowest in Vicryl, at 1.44 (±0.2 Nmm²), Vicryl rapide 2.8 (±0.2 Nmm²), Ethilon 15.6 (±1.1 Nmm²), and Monocryl 18.0 (±3.3 Nmm²).

Standardised stress/strain curves for each suture type are shown (Figure 3) with Hartmann's solution and Chlorhexidine solution group curves overlaid for each suture type.

FIGURE 3.

Standardised average stress‐strain curves by suture type

Box and whisker plots demonstrate the median, Q1 and Q3 (box) and minimum and maximum (whiskers) for ultimate stress (Figure 4), and Young's modulus (Figure 5) of the four suture types.

FIGURE 4.

Box plot diagram to show maximal stress by suture type. Suture type: 1. Vicryl rapide. 2. Monocryl. 3. Ethilon. 4. Coated Vicryl

FIGURE 5.

Box plot diagram showing Young's modulus by suture type. Suture type: 1.Vicryl rapide. 2. Monocryl. 3. Ethilon. 4. Coated Vicryl

4. DISCUSSION

Surgical sutures are used to approximate a number of soft tissues such as ligaments, fascia, and skin. Retention of a suture within the tissue is one problem but failure to seat the stitch or knot appropriately can lead to loop elongation and gap healing, which in turn will lead to failure of wound healing. Failure of the suture itself is also common, which demonstrates that it is essential for the surgeon to choose a suture with the correct mechanical properties. ⁵ , ¹⁰ Orthopaedic surgery often places repetitive loading forces on the repair site which can jeopardise wound healing itself. Surgical wound infections are particularly important to avoid in orthopaedic surgery due the risk of infected metalwork and the long‐term complications of this. Some surgeons are soaking sutures in chlorhexidine skin preparation with the aim to reduce wound infection rates but without the knowledge that the integrity of the suture is not affected.

Various types of suture are available for use including synthetic, biodegradable and braided. Sutures with different combinations of these properties all have a purpose in different operative procedures. Orthopaedic wounds may require a suture with higher failure load and a higher modulus of elasticity and are possibly more unlikely to choose a biodegradable suture because of their varying rate of hydrolysis and therefore integrity, over time.

One limitation of this study was the lack of testing with cyclical loading. Cyclical loading is the repetitive stress or strain exerted on the suture. In orthopaedic wounds, often people are asked to mobilise early to avoid stiffness and therefore in the example of walking, wounds come under cycling loading. ¹¹

Chlorhexidine's antimicrobial properties are well known and it is used in many hand washes and surgical skin preparations, in varying concentrations. It is a biguanide that damages bacteria at the level of the cell membrane, affecting their ability to produce ATP. ¹² Thomas, Gregory, et al have demonstrated that chlorhexidine in higher concentrations does alter wound healing by inhibiting fibroblast growth and production of matrix metalloproteinases. However, at concentrations as low as 0.05%, chlorhexidine solutions can upregulate fibroblast growth and be bactericidal against common skin flora such as Staphylococcus aureus. This shows that soaking sutures in low concentrations of chlorhexidine offers a low‐cost antiseptic benefit.

This study successfully demonstrated that in vitro and at 11 days, there is no significant difference in the failure load, Young's modulus, or ultimate tensile stress of four different, commonly used suture types when soaked in 2% chlorhexidine and 70% isopropyl alcohol. We demonstrated this in a number of sutures with varying properties, including being biodegradable, braided and synthetic. This suggests that while soaking sutures and preparing the skin edges with chlorhexidine is not evidence based, it is unlikely to compromise the mechanical properties of the sutures used for closure and may offer a microbiological advantage.

Further work is needed to extensively investigate any microbiological benefit of using chlorhexidine skin prep during wound closure. We will look to determine the effect of alternative skin preparations on a greater variety of suture types.

This study shows that soaking sutures in chlorhexidine solution does not affect their mechanical properties. It demonstrates there is no statistically significant difference in failure load, maximal stress or Young's modulus of four common skin sutures when soaked in 2% chlorhexidine solution. Clinically, chlorhexidine solution can be applied to surgical wounds without increasing rates of dehiscence and wound infection due to suture failure. It may also offer a microbiological advantage but further research is required to investigate this.

CONFLICT OF INTEREST

All of the authors declare that they have no conflict of interest.

Gaukroger AJ, Jones RJS, Evans JP, Dixon SM. Does skin preparation alter suture strength characteristics? Assessing the effect of chlorhexidine and isopropyl alcohol on common skin closure suture material. Int Wound J. 2020;17:1857–1862. 10.1111/iwj.13475