Tensile strength testing in feline skin with different sizes of nylon suture material: a biomechanical in vitro study

Abstract

Objectives

This in vitro study aimed to evaluate the failure loads and failure types of sutured feline skin samples using different sizes of nylon (polyamide 6/6.6) suture material.

Methods

A total of 88 skin samples from the lateral thorax and abdomen of 11 cadaveric cats were harvested using an hourglass-shaped template. In total, 80 samples from 10 cats were bisected and sutured with three simple interrupted sutures using randomly assigned nylon suture sizes (2-0, 3-0, 4-0 and 5-0 USP). Control samples remained intact. Uniaxial tensile tests were performed using a universal testing machine. Failure load and failure type were recorded and analysed. Failure mechanisms were classified as either suture material failure or tissue failure, with tissue failure defined as linear tears perpendicular to the suture line at the penetration site.

Results

Suture size significantly influenced failure load. No significant reduction in force was observed between 2-0 and 3-0 (P = 0.089), but all other pairwise comparisons showed significant differences (P <0.05). Load to failure decreased by 25.7% between 3-0 and 4-0, and by 40.0% between 4-0 and 5-0. Suture failure rates increased as suture size decreased, with failure occurring in 4/20 samples for 3-0, 17/20 samples for 4-0 and all samples for 5-0. A strong correlation was found between suture failure and suture size (r = 1.000 and P <0.001, Spearman’s rank correlation), indicating a higher risk of breakage with smaller suture diameters. All 2-0 sutures failed because of tissue rupture.

Conclusions and relevance

Nylon 2-0 and 3-0 provided sufficient mechanical stability. Nylon 4-0 exhibited a high risk of suture breakage but may be suitable for thin-skinned areas with tension-relieving measures. Nylon 5-0 showed an unacceptably high failure rate and is not recommended for feline skin closure. Evidence-based suture selection guidelines could improve feline surgical outcomes.

Introduction

Wound closure is a critical step in surgical procedures, most commonly achieved using sutures. Sutures approximate tissues, maintain apposition and provide support until the tissue regains sufficient tensile strength to sustain itself independently.^1,2 However, needle penetration through soft tissue induces additional surgical trauma, and the presence of foreign material may increase susceptibility to infection.^3,4

The tensile strength of suture material is defined as the maximum load it can withstand before breaking. ⁵ Suture size selection depends on the mechanical properties of the tissue being repaired, with the smallest-diameter suture capable of maintaining tissue apposition recommended to minimise trauma. ² Excessively large sutures should be avoided, as they can increase tissue damage, disrupt tissue architecture and introduce unnecessary foreign material. ⁶

Despite the critical role of suture size, clear guidelines for selecting appropriate suture diameters in companion animal surgery are lacking. There are no standardised recommendations based on species, weight or anatomical location. Instead, selection is typically guided by clinical training, experience, knowledge of tissue healing and individual surgeon preference.^2,7 As a result of uncertainty regarding the tension requirements at the suture site, many surgeons opt for larger-gauge sutures with higher tensile strength to reduce the risk of suture failure and wound dehiscence.^7,8 However, this empirical approach is not evidence-based and may contribute to the routine use of inappropriate suture sizes. The absence of standardised guidelines is particularly challenging for young veterinary professionals early in their careers, as it makes suture size selection more difficult. Establishing objective, species-specific recommendations could improve surgical outcomes and support informed decision-making.

In small animal surgery, synthetic monofilament sutures with cutting or reverse-cutting needles are generally recommended for skin closure. ⁶ Nylon, a synthetic, monofilament, non-absorbable polyamide suture, is widely used because of its high tensile strength and low reactivity.^2,9,10 According to the product brochure of a major suture manufacturer, USP 4-0 is recommended for skin closure in cats, while USP 4-0 or 3-0 is recommended for dogs. ¹¹ However, to the authors’ knowledge, no existing data describe the tensile strength of nylon sutures in feline skin.

This study investigated the tensile strength of four nylon suture sizes (2-0, 3-0, 4-0 and 5-0 USP, Dafilon; B Braun) in feline skin samples. Understanding how suture size influences tensile strength in feline tissues can provide evidence-based guidance for optimising suture selection and minimising complications.

The objective of this study was to generate biomechanical data on feline skin wound strength by assessing the failure load and failure types associated with different suture sizes. We hypothesised that, from a biomechanical perspective, feline skin closure could be achieved with finer nylon suture materials (USP 4-0 or 5-0) than those commonly used in clinical practice.

Materials and methods

Tissue collection, sample preparation and testing

In total, 88 skin samples from 11 cats were tested biomechanically. Four skin samples were collected from the thoracic and abdominal walls on each side of the body, resulting in eight skin samples per cat. One cat was designated as a control, leaving 80 samples from 10 cats for the experimental tests.

All cats were euthanased for reasons unrelated to this study. Before euthanasia, owner consent for inclusion in the research project was obtained. The breed, age, body weight (BW) and sex of each cat were documented.

Tissue samples were harvested within 1 h of euthanasia. The cadavers were positioned in left lateral recumbency, and the hair of the thoracic and abdominal regions was clipped. From cranial to caudal, skin samples were excised using a #10 blade and Metzenbaum scissors. The cadaver was then turned over, and the same procedure was repeated on the opposite side. Skin samples with visible structural abnormalities were excluded from the study.

The samples were standardised using a reusable hourglass-shaped template.^12,13 This template included bilateral holding sections and a central measuring portion. The template measured 100 mm in length and had a maximum width of 40 mm at the holding sections (Figure 1). Skin samples were cut to identical sizes to ensure comparability and consistency.

Figure 1

Schematic view of the hourglass-shaped template placed over a bisected skin sample sutured with three simple interrupted sutures. All dimensions are in mm. R10 = reduction of 10 mm

Each sample was wrapped in glycerol-soaked gauze (Glycerol 85%; Thermo Scientific) and placed in cylindrical synthetic containers. Containers were labelled with the identification number and collection date and stored at –80°C. ¹⁴

Biomechanical tests were conducted at the Faculty of Mechanical Engineering, Poznan University of Technology, Poland. Before testing, the samples were thawed to room temperature. Each specimen was bisected and sutured with three simple interrupted sutures by a single board-certified surgeon (PS). Sutures were tied using a surgeon’s knot followed by three additional throws, and loose ends were trimmed to 1 cm.

Four nylon suture sizes (2-0, 3-0, 4-0 and 5-0 USP) of Dafilon (B Braun), a blue-dyed monofilament suture made from polyamide 6/6.6, were tested. For each cat, all four suture sizes were tested on all four thoracic skin samples and all four abdominal skin samples. To ensure randomisation, both the suture size and the skin samples were assigned randomly, with suture size selected by blindly drawing the material from a prepared box. All sutures were placed using 3/8 circle cutting needles. Needle length varied by suture size: USP 2-0 and 3-0 were placed with a 24 mm needle (DS24), while USP 4-0 and 5-0 were placed with a 19 mm needle (DS19). Suture bites were placed 3–5 mm apart with a suture length:wound length ratio of 4:1. The control group samples did not undergo midline incision and suturing before the biomechanical testing.

Uniaxial tensile tests were performed immediately after suturing using the universal testing machine Inspekt 20k (Hegewald & Peschke) equipped with a 1 kN load cell (type S2M; Hottinger Brüel & Kjær). To ensure proper grip and prevent slippage, sandpaper pads were placed between the samples and the machine jaws (Figure 2). All samples were pre-tensioned to an initial force of 1 N. Testing was conducted at a travel rate of 100 mm/min, with data recorded at 100 Hz. The automatic stop condition was defined as a drop of 80% from the peak tensile force. This threshold was chosen based on empirical pre-tests, in which such a force reduction consistently corresponded to the point of tissue or suture rupture. It allowed the reliable identification of mechanical failure without overstressing already failed samples. Time (s), tensile force (N) and displacement (mm) were recorded during the testing. The tests were conducted using LabMaster testing software, and samples were secured using HP154 mechanical grips (Hegewald & Peschke). The tests were recorded, and failure modes were visually analysed. Failure mechanisms were classified into two types: suture material failure and tissue failure. Tissue failure was defined as linear tears in the skin perpendicular to the suture line at the site of suture penetration (Figure 3).

Figure 2

Overview of Hegewald & Peschke MPT GmbH Universal testing machine Inspekt 20k. 1 = upper and lower holders; 2 = tested sample; 3 = control panel; 4 = ball-screw feed drive; 5 = casing; 6 = load cell. Courtesy of Jaenich et al ¹³

Figure 3

(a) Series of images showing progressive tissue elongation until tissue failure in a representative skin sample sutured with USP 2-0. (b) Series of images showing suture failure in a representative skin sample sutured with USP 4-0

Statistical analysis

The minimum required sample size (n ⩾80) was determined through power analysis using an ANOVA test, assuming a medium effect size of f = 0.25, a significance level of α = 0.05 and a statistical power of β = 0.8.

Data were evaluated for normality using the Shapiro–Wilk test. Descriptive statistics were performed to determine the mean ± SD for load to failure. Mixed model linear analysis was performed to evaluate the effects of suture material, sex, age, BW and body location (BL) of the skin harvest on load to failure (dependent variable). Suture material, sex and BL were considered to be categorical variables in the statistical model. The effect of suture material on the frequency of suture failure was determined by calculating Spearman’s rank correlation. A significant relationship between dependent and independent variables was determined using ANOVA. P <0.05 was considered statistically significant. The Kruskal–Wallis test was performed for the final statistical model to compare mean load to failure between different suture groups, followed by Dunn’s post-hoc analysis.

Results

A total of 11 cats were included in the study. The mean BW was 3.98 ± 0.79 kg (range 2.60–5.40), and the mean age was 8.00 ± 5.82 years (range 1.0–17.0). There were no significant differences in load to failure between male and female cats (P = 0.984), regardless of neutering status (P = 0.155). BL was also not a significant factor (P = 0.512).

After removing these non-significant factors from the statistical model, age (P = 0.047), BW (P = 0.032) and suture material (P <0.001) remained as the only significant factors influencing load to failure. Although age and suture material together had a statistically significant effect (P = 0.016), the combination of BW and suture material did not (P = 0.110). The relationship between load to failure, age and BW is presented in Figure 4.

Figure 4

Effect of (a) age and (b) body mass on tensile strength presented for different suture size groups

Suture material size was the most significant factor affecting load to failure. The measured values were in the range of 23.73–100.82 N, with a mean of 60.17 ± 19.33 N. Standard deviations are provided in Table 1 and data are summarised as box and whisker plots in Figure 5.

Table 1

Load to failure (Fmax) values for different suture material sizes

Suture material	Fmax (N)
2-0	75.73 ± 17.80 a
3-0	72.53 ± 15.16 a
4-0	53.90 ± 5.13 b
5-0	38.51 ± 5.46 c

Data are mean ± SD. Superscript letters denote significant differences between groups according to Dunn’s post-hoc test

Figure 5

Box and whisker plots illustrating the effect of suture material size on tensile strength (Fmax). Each box represents the middle 50% of observed values, with the bottom and top indicating the first quartile (25th percentile) and third quartile (75th percentile), respectively. The median (50th percentile) is shown as a line, and ‘×’ represents the mean. Whiskers indicate the range, excluding outliers. Different letters a, b and c denote significant differences between groups (Dunn‘s post-hoc test)

Comparing suture sizes 2-0 and 3-0, no statistically significant reduction in force was observed (P = 0.089). However, all other pairwise comparisons showed significant differences (P <0.05). The load to failure decreased significantly by 25.7% between 3-0 and 4-0, and by an additional 40.0% between 4-0 and 5-0.

The mean load to failure for the control group (non-sutured skin) was 287.18 ± 55.65 N.

Among 80 tested samples, suture failure occurred 41 times, while tissue failure was observed 39 times. All samples sutured with 2-0 exhibited tissue failure. The frequency of suture failure increased with decreasing suture size, occurring in four samples with 3-0, 17 samples with 4-0 and 20 samples with 5-0 (Figure 6).

Figure 6

Types of failure observed for different suture material sizes. S = suture failure, T = tissue failure

A strong correlation was found between suture material failure and suture size (P = 1.000 and P <0.001, Spearman’s rank correlation), indicating that the likelihood of failure due to suture breakage increases as suture diameter decreases.

Discussion

This study evaluated the tensile strength of four nylon suture sizes (2-0, 3-0, 4-0 and 5-0 USP, Dafilon; B Braun) in feline skin to provide biomechanical data for evidence-based suture selection. Our hypothesis – that feline skin closure could be achieved with finer nylon sutures (USP 4-0 or 5-0) than those empirically used in clinical practice – was not supported.

Smaller sutures exhibited significantly lower load to failure, with 5-0 showing a 40% reduction compared with 4-0, and a 25.7% reduction between 4-0 and 3-0. In contrast, 2-0 and 3-0 demonstrated similar biomechanical properties. Failure analysis revealed that suture breakage became more frequent as suture size decreased, highlighting the mechanical limitations of finer sutures. The strong correlation between suture diameter and breakage (P = 1.000, P <0.001) reinforces this finding. Although finer sutures may be suitable for areas with thinner skin, such as the distal extremities, they appear inadequate for routine feline skin closure.

Our results demonstrated a strong correlation between suture size and failure type. Suture breakage occurred in 51.25% of samples, increasing with smaller suture sizes. Nylon 2-0 exhibited no failures, while failure rates were 20% for 3-0, 85% for 4-0 and 100% for 5-0. Tissue failure occurred in 48.75% of cases, with 98.75% occurring at the suture–tissue interface, indicating suture perforations as biomechanical weak points.

These findings align with previous research. Burkhardt et al ¹⁵ analysed the tearing characteristics of porcine soft tissues and found that larger-diameter sutures (3-0 USP) primarily caused tissue rupture, whereas smaller sutures (7-0 USP) failed predominantly because of suture breakage. Intermediate-sized sutures (5-0 USP) exhibited both failure modes. Similarly, González-Barnadas et al ¹⁶ demonstrated that 5-0 sutures had significantly lower tensile strength than 4-0 sutures across different suture materials. Their study, conducted by two independent investigators, found no inter-operator differences, reinforcing its reliability.

Larger sutures significantly increase knot volume and induce greater tissue reaction, whereas additional throws have minimal impact. Thick-gauge sutures contribute disproportionately to the foreign body burden and inflammatory response, potentially impairing wound healing. ¹⁷ Therefore, excessively large sutures should be avoided to minimise tissue trauma and the presence of foreign material. ⁶

Despite the critical role of suture size, standardised species-specific guidelines are lacking. A 2017 survey of UK veterinary surgeons found that practitioners without postgraduate qualifications relied more on practice policies and peer recommendations than on the intrinsic properties of suture materials. In addition, diploma holders were more likely to select smaller suture sizes than non-specialists. These findings highlight variability in suture selection based on qualification level and emphasise the need for evidence-based guidelines to support consistent suture selection across the profession. ⁷

A recent study investigated the biomechanical properties of three polypropylene suture sizes (2-0, 3-0 and 4-0 USP) in cadaveric canine midline skin incisions. Consistent with our findings, USP 4-0 exhibited significantly lower tensile strength compared with USP 2-0 and 3-0. ⁸ Similar to our results, this study identified two primary failure modes: suture breakage and tissue rupture. Larger sutures caused tissue tearing under high tension, whereas smaller sutures were prone to breakage. ⁸ Beyond biomechanical assessment, Li et al ⁸ also analysed inflammatory markers and histological changes associated with different suture sizes, reporting increased inflammation and tissue reactivity with larger sutures. Based on these findings and their previous work, which reported a median skin tension of only 0.67 N in the canine abdominal wall, they concluded that 4-0 polypropylene provides adequate mechanical stability while minimising tissue reactivity.^8,18 The mean tensile strength of polypropylene 4-0 in canine skin (92.5 N) ⁸ was substantially higher than the 53.9 N measured for nylon 4-0 in feline skin in our study.

Several factors likely explain this discrepancy. Our study examined feline skin and used nylon rather than polypropylene. In addition, species-specific differences in wound healing influence tensile strength. Bohling et al ¹⁹ reported that feline sutured cutaneous wounds were only half as strong as canine wounds 1 week postoperatively (4 N vs 8 N), indicating slower collagen production and maturation. Consequently, they recommended leaving skin sutures in place for longer in cats, particularly in high-motion areas.

In addition, the study by Li et al ⁸ lacks details on sample storage, stating only that samples were frozen for several months. Freezing can cause cellular damage due to ice crystal formation and osmotic stress. In skin graft preservation, tissue viability depends on proper handling, including the addition of cryoprotective agents such as glycerol, which, at an 85% concentration, minimises tissue degradation. Storage temperature is also critical: at 4°C, skin remains viable for 3–7 days, at –18°C to –20°C for up to 60 days, and in ultra-low-temperature freezers (–80°C or lower) for up to 1 year with approximately 60% viability. ¹⁴ Li et al ⁸ did not specify freezing rates, storage temperature or the use of cryoprotectants, limiting comparability with studies using controlled preservation methods.

An in vivo biomechanical study by Yang et al ¹² found that larger-diameter sutures (4-0 USP) provided greater tensile strength and faster recovery of skin defects in rats compared with smaller sutures (6-0 USP), which led to prolonged inflammation and delayed wound healing due to inadequate stability. Although tensile strength did not differ between groups at 2 weeks, by 4 and 6 weeks the 4-0 sutures exhibited significantly greater tensile strength. Histological analysis revealed more inflammatory cells in the 6-0 group in the first postoperative week, suggesting a prolonged inflammatory phase and delayed recovery.

These findings indicate that excessively small sutures may not only lack mechanical stability but also contribute to increased inflammation due to micro-movements at the wound edges, disrupting the healing process. This aligns with our results, reinforcing concerns that using very fine suture material, such as 5-0, could compromise tissue integrity. Although smaller sutures may initially reduce tissue trauma, they may fail to provide sufficient holding strength, leading to prolonged inflammation and weaker wound healing.

Another finding was that as cats aged, less force was required to break USP 3-0 sutures, indicating a moderate negative correlation between load to failure and age. This may be explained by increased skin stiffness with age, which places greater stress on sutures during mechanical loading. Correspondingly, decreased skin elasticity and brittle claws have been reported in older cats. ²⁰

This observation aligns with human studies showing that ageing reduces skin elasticity, increases stiffness and lowers tensile strength. The Young’s modulus, which defines a material’s resistance to elastic deformation, increases with age, indicating reduced flexibility. ²¹ In vivo studies measuring uniaxial skin tension found that skin elasticity in children (70 MPa) was higher than in elderly adults (60 MPa), while the ultimate skin deformation before bursting declined from 75% in newborns to 60% in older individuals.^{21 –23} These changes suggest that aged feline skin, like human skin, may exhibit reduced extensibility and increased fragility, placing greater strain on sutures and potentially leading to earlier fatigue. Clinically, this may necessitate adjustments in suture selection for older cats, such as using larger suture sizes or incorporating tension-relieving patterns to account for age-related biomechanical changes.

This study has several limitations. Although in vitro tests provide valuable biomechanical data, they do not fully replicate the physiological conditions of sutures in vivo. Factors such as tissue remodelling, biological degradation and dynamic mechanical forces may lead to suture failure at lower loads in vivo than are observed in vitro.^24,25

Moreover, gap formation and yield strength were not directly assessed. This study used load to failure and failure mode (suture vs tissue) as surrogate markers of closure integrity. However, these do not fully reflect the mechanical behaviour of sutures under physiological conditions. In tendon repair, gaps exceeding 3 mm have been shown to impair healing, and in cutaneous closures, even smaller separations may delay healing or increase the risk of infection. ²⁶ Future studies should incorporate direct measurements of gap formation and yield strength to provide a more comprehensive evaluation of suture performance.

Another limitation is the lack of standardisation among the examined cats, which varied in age and BW, leading to differences in skin thickness. Unlike the studies by Li et al ⁸ and Yang et al, ¹² we did not measure or normalise skin thickness in our analysis, which may have influenced biomechanical outcomes.

In addition, nylon suture properties vary between manufacturers. Callahan et al ²⁷ demonstrated significant differences in tensile strength across nylon suture brands, and Naleway et al ²⁸ reported considerable variation in suture diameter within the same USP category. As we did not measure the precise diameter of the sutures used in this study, variations in thickness may have influenced mechanical strength. Consequently, our results apply specifically to the tested suture material (Dafilon; B Braun) and should not be generalised to all nylon sutures without further validation.

Conclusions

Taken together, our findings suggest that although finer sutures (4-0 and 5-0 USP) may reduce tissue trauma, they exhibited lower tensile strength under in vitro conditions and may be insufficient to maintain mechanical wound closure in feline skin, where even small gaps could compromise healing. Although both nylon 2-0 and 3-0 provided sufficient mechanical stability, 3-0 may be preferred in clinical situations where minimising knot volume and foreign material burden is important. Nylon 4-0 showed a high probability of suture breakage but may still be considered in thin-skinned regions if supported by adequate tension-relieving measures. In contrast, nylon 5-0 exhibited an unacceptably high failure rate and, based on our findings, cannot be recommended for simple interrupted sutures in feline skin under in vitro conditions. Further evaluation under in vivo conditions is warranted.

Future studies should address the limitations of this study by standardising sample characteristics, measuring skin thickness, controlling for suture diameter and incorporating additional biomechanical parameters to improve comparability and reproducibility. A cyclic loading model simulating the dynamic conditions of a moving animal would be a valuable next step to bridge the gap between bench-top testing and clinical application. Establishing evidence-based guidelines for suture selection in feline surgery could further enhance surgical outcomes and support clinical decision-making.