In Vitro Tensile Strength Study on Suturing Technique and Material

Wound dehiscences are a common complication after surgeries, especially in procedures that require a tension-free primary closure, such as oroantral fistula treatment,¹  bone block grafts,²  or guided bone regeneration (GBR)³  techniques. In implantology, dehiscence may increase the risk of infection by exposing the wound to the oral cavity. Such exposure is of special concern after GBR procedures with nonresorbable membranes because it often jeopardizes the result of the bone graft and even precludes placing an implant.⁴ 

Dehiscence of the surgical wound is frequently related to flap and suture tension. An in vitro study found a 10% dehiscence rate when the flaps were sutured at low tensions (from 0.01 to 0.1 N), while those sutured at more than 0.1 N showed a significantly higher incidence of wound exposure (40%).⁵  Several authors have proposed different techniques to reduce flap tension, including periosteal releasing incisions, to allow primary closure of a wound.^6–10 

Equally, suture type and caliber can be decisive factors for preventing dehiscences of surgical wounds. One in vitro study¹¹  reported finding the highest tensile strength (TS) with 5-0 sutures, which showed a good balance between tissue and suture resistance, with a mean value of 14.6 N for each.

When comparing different suture materials, Kim et al.¹²  found a mean TS and knot security values of 14.3 N and 11.9 N for nylon, 10.8 N and 10.1 N for polyester, 11.7 N and 9.7 N for polypropylene, and 10.0 N and 10.4 N for silk. Nevertheless, the literature on some materials—such as expanded polytetrafluoroethylene (e-PTFE) or Supramid (Resorba Medical GmbH, Nüremberg, Germany)—is scarce. Supramid is a type of polyamide that presents a different internal structure to monofilament nylon in suture gauges over 5-0. This structure, called a pseudomonofilament, has an internal multifilament structure of polyamide 6.6 coated in a layer of polyamide 6, which gives it a monofilament-like surface.

Some authors recommend performing a deep horizontal mattress suture to release tension, followed by simple sutures in the superficial layer to avoid wound dehiscences after vertical GBR procedures.^6,7  However, no studies appear to have compared the TS of such techniques. Therefore, the aim of this study was to determine the TS of simple sutures, mattress sutures, and their combination, comparing gauge 4-0 and 5-0 silk, monofilament polyamide, Supramid polyamide, and e-PTFE. Secondarily, the study also tried to identify which event took place after 5 mm traction: breakage, untying, or nothing.

An in vitro study was performed to assess the TS of 3 suture techniques—simple suture, horizontal mattress suture, and a combination of both—using 4 different materials in 2 different gauges.

The device used to test the sutures (Microtensile Tester, Bisco Dental Products, Schaumburg, Ill) pulls 2 test jaws in opposite directions to measure the maximum TS. The original test jaws were metallic, so they were duplicated to perform the test with different techniques. The duplicate was made with self-curing acrylic resin. Three perforations were made in each jaw. The perforations were placed 4 mm apart, 3 mm from the outer edge.

A sample calculation was performed with G*Power 3.1 software (G*Power Team, Heinrich-Heine University, Düsseldorf, Germany), taking the results reported by Kim et al.¹²  as a reference (14.3 N with a standard deviation [SD] of 1.6 N for nylon and 10.0 N with a SD of 1.2 N for silk). An alpha error was set at 0.01 and power at 0.99. Eight sutures were needed in each group. This number was increased to 10 to compensate for possible errors during the procedure.

Two researchers (A.G.B. and O.C.F.) independently performed 160 sutures under the same environmental conditions using braided silk (Resorba), monofilament polyamide (Resorba), Supramid polyamide (Resorba), and e-PTFE (Gore-Tex, Gore Medical, Flagstaff, Ariz). For the 4-0 sutures, 10 simple sutures (Figure 1a), 10 horizontal mattress sutures (Figure 1b), and 10 combinations of the two sutures (Figure 1c) were performed with each material. In addition, 10 simple sutures were performed with each gauge 5-0 material. All the sutures were tied with a triple knot (clockwise–counterclockwise–clockwise) and were cut leaving 5-mm tails.

The 2 jaws were pulled in opposite directions using the Microtensile Tester device (Bisco; Figure 2). Traction was performed at a speed of 2.46 mm/min to reach a maximum opening of 5 mm. The event that took place was registered (breakage, untying, or nothing). The maximum load (in N) was registered when the suture untied or broke.

The statistical analysis was performed with the IBM SPSS Statistics 22.0 software (IBM Corp, Armonk, NY). A descriptive bivariate analysis was conducted. Pearson's chi-squared and Fisher exact tests were applied to compare categorical variables. Normality of the scale variable (tensile strength) was explored using the Kolmogorov–Smirnov test. Where distribution was compatible with normality (simple sutures), t-tests for independent samples were used. Where normality was rejected, nonparametric tests were employed (Kruskal–Wallis test). The statistical significance was set at P ≤ .05.

The distribution of events and the TS values of the sutures performed were similar for both investigators (P ≤ .05).

The main events registered after 5 mm of traction of the different materials are shown in Table 1. The e-PTFE showed significantly fewer broken or untied sutures than did the rest of the materials tested, and monofilament polyamide showed fewer broken or untied sutures than silk or Supramid.

Table 2 shows the mean TS values (tensile loads at breakage or untying) of the different sutures and materials tested. On comparing the TS values of the 4-0 and 5-0 gauge simple sutures, only e-PTFE showed similar results for the 2 gauges (P ≤ .05). For silk, Supramid, and monofilament polyamide, the 5-0 gauge was significantly weaker than the 4-0 gauge sutures (Table 3).

The maximum tensile force at breakage or untying of the sutures made with the 4-0 materials was significantly lower for e-PTFE than for monofilament polyamide and Supramid but similar to silk (Table 2, Figure 3). When comparing the 5-0 sutures, silk was significantly weaker than Supramid but of similar strength to monofilament polyamide and e-PTFE (Table 4, Figure 4).

Regarding the technique employed, the combination of a simple and a mattress suture achieved the highest tensile strength values when compared with both simple and mattress sutures alone (Table 2, Figure 5).

A possible limitation of this study is related to the fact that soft tissue resistance could not be assessed. Further, variables such as gingival tissue thickness are extremely difficult to control and may be a source of bias. Given these limitations, the results should be interpreted with caution; in vivo studies would be needed to confirm the results. The fact that the sutures were made by two operators does not seem to reduce the internal validity of the study since no significant inter-operator differences were found. However, it might increase the external validity.

According to the results of this in vitro study, the combination of a simple suture and a horizontal mattress suture resists significantly higher tension than either of the techniques performed separately. In addition, 4-0 sutures have a higher TS than do 5-0 sutures, except when e-PTFE is used. This seems to indicate that the combined technique may be of interest in high wound dehiscence risk cases.

When comparing different 4-0 materials, e-PTFE showed the lowest tensile force at suture breakage or untying of the knot. However, this material untying took place in only 40% of cases and breakage in only 2.5% after 5 mm of traction, which are low figures compared with the 100% silk and Supramid^, and the 87.5% monofilament polyamide. The elasticity of e-PTFE and its tendency to untie are factors that may explain the disproportional relationship between a higher resistance to traction without untying or breaking and the lower tensile strength values in comparison to the other studied materials. It is important to point out that the vast majority of e-PTFE sutures (21/23) that resisted the 5-mm traction showed some degree of shortening of the knots' tails. In view of these results, surgeons must take special care in tying sutures when using this material; otherwise, wound dehiscences may occur. Leaving longer tails and increasing the number of loops may prevent this complication.

Postoperative swelling is a common complication after GBR procedures and can increase tension on the suture. Therefore, knowing that 57.5% of e-PTFE sutures would resist 5 mm of traction or more, the elasticity of this material may be crucial for maintaining tension-free primary closure, not only at the time of the procedure but also during the initial postoperative period. Additionally, as no differences in TS values have been found between gauges 4-0 and 5-0, the latter might be preferable to minimize the trauma to the tissue and avoid its tearing.¹¹ 

That said, monofilament and Supramid withstood higher tensile forces compared to e-PTFE. Since monofilament polyamide resisted higher traction but resisted a similar tensile force to Supramid, it seems to be the more elastic of the two. Consequently, a higher risk of exposure may be expected. However, further in vivo investigation is needed to reach a final recommendation concerning the use of one type of polyamide over another.

In our opinion, e-PTFE could be an excellent choice for releasing the tension of the flap but should be combined with another material. Since the results of this study show that the combination of a simple and a mattress suture achieves the highest TS, a good option would be to perform a deep mattress suture with e-PTFE followed by superficial individual sutures with another material such as Supramid or monofilament polyamide. Further research is needed to prove this hypothesis.

Silk not only performed poorly from a mechanical perspective but also seems to accumulate more bacterial plaque than polyamide,¹³  e-PTFE,¹⁴  or Monocryl Plus.¹⁵  Therefore, this material seems to present no advantages apart from cost.

Moreover, to minimize the risk of bacterial colonization, the knot of the interrupted suture must be kept away from the incision line because this area is more prone to plaque accumulation. The findings of the present study are consistent with the results obtained by Kim et al.¹²  Many studies have addressed this issue, but most of them employed higher gauges that are not used in dentistry.^16–19  Regarding resorbable sutures, several authors^20,21  have shown a significant reduction in TS over time due to the action of the saliva.

The combined suture technique withstands a higher tensile force when compared to a simple suture or a horizontal mattress suture. The e-PTFE 4-0 resists greater traction without untying/breaking compared with all other studied materials. However, this event (untying/breaking) takes place at a lower level of force. The 4-0 sutures possess greater tensile strength than the 5-0 sutures, except in the case of e-PTFE.

Abbreviations

e-PTFE

expanded polytetrafluoroethylene

GBR

guided bone regeneration

MFP

monofilament polyamide

N

Newtons

TS

tensile strength

The authors wish to thank Mary-Georgina Hardinge for English language editing of the manuscript.

Dr Rui Figueiredo and Dr Eduard Valmaseda-Castellón collaborate with Inibsa Dental for sponsored lectures. This company is the distributor of Resorba suture materials in Spain. The remaining authors report no conflicts of interest related to this study.