EVALUATION OF MECHANICAL PROPERTIES OF THREE COMMONLY USED SUTURE MATERIALS FOR CLINICAL ORAL APPLICATIONS: AN IN VITRO STUDY

Background/Aim: Appropriate selection of suture material is a crucial step in oral, maxillofacial and periodontal surgery for uneventful healing. We have scarcity of comprehensive studies comparing mechanical properties of commonly used suture material in oral surgeries. The present in vitro study sought to evaluate the effect of saliva on the strength, elongation and stiffness of the commonly used suture material over a period of two weeks. Methods: Three suture materials Silk (SL), polyglactin 910 (PG) and polypropylene (PP) were used in 4-0 gauge. A total of 120 suture samples (40 from each material) were used for the experiment. Artificial saliva was mixed with human serum in 1:1 concentration and maintained at pH of 7.4 to 8.1 to simulate oral environment. All samples were tested at pre-immersion (baseline), 3 rd , 7 th and 14 th day post-immersion periods. Universal testing machine was used to test the selected mechanical properties. The collected data were subjected to statistical analysis. Results: The distribution of mean baseline strength and percentage elongation was significantly higher in PP group (P-value<0.001), whereas stiffness score was highest with SL group (P-value<0.001). Inter-group comparison revealed that PP group had maximum tensile strength compared to PG and SL groups at all time points. When percentage elongation was compared, PP and PG groups showed highest values at 7 th and 14 th day respectively. PP group exhibited highest stiffness values as compared to SL and PG groups at 7 th and 14 th day post-immersion (P-value<0.001). Intra-group comparison showed that all suture materials had significant difference in mechanical properties when pre-immersion values were compared to 14 th day post-immersion values (P-value<0.001). Conclusion: PP sutures are strongest and have highest tensile strength and elongation property. PP seems to sustain its tensile strength better than SL and PG at the end of 14 th day. Controlled clinical experiments are necessary to verify this finding in an in-vivo setting.


Introduction
Important concerns of periodontal, oral and maxillofacial surgeons refer to the selection of proper suture material. The suture material should be biocompatible and easy to use, it should form proper knot, have the property of elongation, biodegradable in some circumstances and resist breakage during its use 1 . The mechanical properties of the suture materials play an important role in regulating their behavior.
Placing sutures in the oral cavity is challenging due to varied functions of mastication, speech, swallowing and high tissue vascularization along with continuous pooling of saliva 2 . Suitable sutures must possess specific physical and mechanical properties, among which the tensile strength is one of the most important properties. The function of the suture while in use is controlled by its elasticity, stiffness and tensile strength 3 .
The flap edges should remain in close apposition after suturing of the surgical site to help primary healing, failure of which can have negative effect on the desired results of the surgery. Tensile strength is an important feature that is required to be maintained because the suture material tends to lose between 70 and 80% of its original strength. Therefore, the required original tensile strength must always be there to avoid breakage of the suture material 4,5 . Moreover, a compromise in the strength of the suture material can result in incomplete adaptation of the flap and consequent healing by secondary intention 6 . Most of the published studies related to mechanical properties discussed mainly a breaking force.
There are very few reports that actually compare other useful aspects, like failure elongation, failure stress/strain and stiffness across suture materials. However, exhaustive studies that are cited on suture materials are comparatively less pertinent to materials used for oral and periodontal surgical procedures 7,8 .
A distinct suture materials show discrete behaviors in the oral cavity 9 . Various experimental researches indicated that the suture's tensile strength could be affected by saliva, various solutions or consumed fluids. It was found that there is a reduction in strength of Vicryl after it is immersed in saliva, bovine milk, and soy milk for 35 days 10 .
Another study described remarkable reduction in the strength of two different suture types (Vicryl and Monocryl) after they were submerged in artificial saliva, chlorhexidine and essential oil mouth rinse 11 .
Of the various commercially available suture materials, silk and polyglactin are most often used in oral and periodontal surgery. Silk is the most frequently used natural suture material, due to its better handling properties 12 .
Consequently, aim of this study is to assess and compare the tensile strength, percentage elongation and stiffness of silk, polyglactin 910 and polypropylene suture material in an environment simulating the oral cavity (immersed in artificial saliva) and a pre-immersed dry condition for an interval of fourteen days. The results mentioned in the present study are meant to provide a baseline data for oral surgeons and periodontist by assembling the mechanical and physical properties of these sutures under controlled conditions. So this data will help in the selection of suitable suture material depending upon the required area of surgical procedure.

Methods
The present in-vitro experimental study design was approved by the King Khalid were evaluated: silk, which is observed by many surgeons as a benchmark due to it easy handling 12 , polyglactin 910 (Vicryl), which is a multifilament absorbable synthetic suture comprised of a copolymer of glycolide and L-lactide, and polypropylene monofilament, non-absorbable material made of an isotactic crystalline stereoisomer of polypropylene (Table1). Suture materials were divided into the control (pre-immersed) and the test group (immersed in artificial saliva - Table 2). All the test suture materials were exposed to thermo-cycling (alternate temperature change from 5 °C to 55 °C), so as to simulate the challenges in the oral cavity.

India
A total of 120 suture samples were collected from commercially available unexpired stocks. Forty samples were obtained from each suture material type. All the suture samples were measured at a uniform length of 18cm. Ten specimens from each group were tested for tensile strength before immersing into artificial saliva and referred as a control group.
Remaining suture specimens were kept in artificial saliva until exposed to an experimental procedure ( Figure 1). A detailed description of the study protocol has been described in figure 2 (flow chart).

Figure 1. Different suture specimens immersed in artificial saliva
Artificial saliva was formulated by mixing the compounds shown in table 2 in one liter of distilled water 13 . To prevent any chemical changes, the prepared mixture was kept secured in an amber color bottle until used for the experiment. During the experiment, the prepared artificial saliva was mixed with Human serum in 1:1 concentration, to simulate oral environment. This biologic mixture was kept at a pH of 7.4 to 8.1 in an incubator at The setup of the experiment and the testing machine are shown in figure 3. Length of each suture specimen for testing = 18cm Total number of specimens (n=120) Baseline Pre-immersion testing of suture specimen (n=10) from each group All the suture specimens were exposed to thermocycling by altering the temperature between 5 °C to 55 °C Testing of suture specimen after immersing into Artificial saliva (n=90) ) Distribution of suture material in groups; SL (n=30), PP (n=30), PG (n=30) Tests were repeated at interval of 3 rd , 7 th and 14 th days. While testing at specified intervals, (n=10) suture specimens were selected randomly.
Collected data were subjected to statistical analysis *SL= Silk, # PG= Polyglactin 910, $ PP= Polypropylene  Tensile Strength was defined as maximum load that can be applied to a suture material before the suture breaks; it was measured in Newtons (N). Elongation was defined as cumulative displacement exhibited by a suture material before it breaks when a gradual load is delivered and it was measured in millimeters (mm). Stiffness was defined as a measurement of the capacity of a suture material to elongate by application of gradual increasing load before it breaks and it was measured in Newtons per millimeter (N/mm).
The stiffer materials would exhibit lesser elongation.
The data on continuous variables is presented as mean and standard deviation (SD) across the study groups. Statistical test Analysis of Variance (ANOVA) was used for the inter-group and intra-group comparison. In the entire study, the p-values less than 0.05 were considered to be statistically significant. All the hypotheses were formulated using two-tailed alternatives against each null hypothesis (hypothesis of no difference). The entire data is statistically analyzed using Statistical Package for Social Sciences (SPSS version 21.0, IBM Corporation, USA) for MS Windows.

Results
Baseline (pre-immersion) comparison of mean tensile strength, percentage elongation and stiffness are presented in table 3. The distribution of mean baseline strength and percentage elongation was significantly higher in the PP group, followed by PG group and the least with SL group (P-value<0.001 for all). However, stiffness score was highest with the SL group as compared to PP and PG groups, respectively (P-value<0.001 for all).       Table 6 shows distribution and comparison of mean stiffness among three suture groups at the 3 rd , 7 th and 14 th day post immersion in the saliva. The highest stiffness was recorded by the SL group, followed by the PP and PG groups at baseline and the 3 rd day post-immersion (P-value<0.001), whereas the PP group exhibited higher stiffness as compared to the SL and PG groups at the 7 th and 14 th day post-immersion (P-value<0.001). Table 7   Tensile strength 0.001 *** 0.001 *** 0.001 *** % elongation 0.001 *** 0.001 *** 0.001 *** Stiffness 0.001 *** 0.001 *** 0.001 *** *** p-value<0.001= Highly significant

Discussion
The key step of surgery is meticulous wound closure. The main purpose of wound closure is eradication of dead space, apposition of wound margins to generate a closed secure atmosphere and preservation of tensile strength at the wound margins till the tissue tensile strength is enough to bear external load 15 . Previously, materials like animal hair, natural fibers, silk, nylon and gut mucosa were used to seal the surgical sites 16 . A surgeon always desires for better handling characteristics and tensile strength of a suture while choosing appropriate suture material. The tensile strength of a suture material is an essential property that helps suture material to bear the tissue traction at the flap margin 17 .
Suture materials manifesting low tensile strength are more liable to break during the healing phase because of pull created by edema and tissue tension.
Suture materials are mainly categorized as absorbable and non-absorbable, natural and synthetic, braided polyfilament and monofilament fibers 18 . Distinct suture materials bearing the same diameter size may differ significantly in their tensile strengths. Most of the reported studies on mechanical properties of sutures are done on skin and subcutaneous tissues 18,19,20 . In these exploratory studies, sutures were exposed to few environmental conditions that can influence physical and mechanical properties of the sutures. Studies associated with oral cavity present a number of difficulties, like presence of saliva, reflux gastric juice, pressure from the surrounding soft tissues and occlusal forces that can markedly change the properties of suture materials 21,22 .
In the present study the suture gauge designation was fixed at 4-0 in order to help in Sutures were immersed in artificial saliva to be used as a control group as previous studies indicated its possible harmful effect on suture material's mechanical properties 24,25 . To the best of authors' knowledge, it's a first original study that assesses the mechanical properties of different suture materials used intraorally by simulating a natural environment.
All the experiments were done by a single investigator to circumvent any inter-  Earlier studies on the PG sutures exhibited good handling properties, high initial tensile strength, and less tissue reactions during healing 30,31 . A strong correlation between suture degradation and tensile strength has been described in various studies under controlled in vitro and in vivo settings. PG degradation in-vivo is mostly due to proteolytic enzymes. The PG sutures preserved more than two-thirds of their initial tensile strength at the 14 th day post-immersion period 32 . The results of the present study were similar to this study as more than two thirds of the initial tensile strength was conserved at the 14 th day post immersion. Some studies state that, when PG is immersed in saliva, it shows a fast tensile strength loss, especially after 7 days 10 . This is in contradiction to the findings of the present experimental study.
SL is the most frequently used suture material in the surgical procedures even though it exhibits inferior mechanical properties. Even though, SL is said to be a non-resorbable suture but acknowledged to be subject to proteolytic degradation over a longer period 33 .
Studies indicate that SL is one of the most vulnerable sutures to differences in pH conditions 24 . In the present study, it was found that mechanical properties of the SL sutures As the present study design is in-vitro, it has certain constraints as mentioned below.
The outcome of the current experiment may not be completely similar to the oral clinical situations. There are various possible confounding factors such as diet, habits, occlusal forces and medications in the oral cavity that may affect the oral environment and cause variation in the mechanical properties of sutures. More information can be collected by performing molecular interpretation of the selected suture materials upon their reaction with saliva. However, this was beyond the scope of the current experimental study.

Conclusion
The present study affirms that the suture material tends to lose a significant amount of tensile strength when exposed to oral environment. The PP sutures showed highest mechanical properties when compared to the PG, and SL suture. Under the limitation of the present study, authors conclude that the PP is best suture material for wound closure after oral and periodontal surgeries, followed by the PG and SL, respectively.