Suturing, which is the final procedure of a surgery, is used to reattach the removed tissue, to control bleeding, and allow for primary healing [1]. In addition to the physical requirements for an ideal suture, such as maintaining proper form, ensuring the ease of sterilization and manipulation, and maintaining ligation and tension, the biological requirements of proper absorption time and minimum tissue response have also been suggested [2]. In intraoral surgeries, a variety of suture types have been applied [3]. Sutures can be divided into absorbable and non-absorbable ones and are classified as monofilament or multifilament types. Although microbial inflammation response due to the multifilament interstices has been demonstrated, silk has been widely used in intraoral surgeries because it has the advantages of easy manipulation, the capability of maintaining knot security, and less irritation than nylon [4,5]. Polyglycolic acid, a polymer of lactide and glycolide, is used to manufacture an absorbable suture. After being extracted as a melted polymer, a thin fiber is manufactured into a braided form to improve its ease of manipulation. Polyglycolic acid is absorbed in 60 to 90 days after insertion. It is hydrolyzed without any phagocytosis, which results in a weaker immune response than that of absorbable organic sutures [6]. One of the well-known non-absorbable monofilaments, nylon, is made from polyamide polymer and has been reported to show a high resistance to infection, high tension, and a low acute inflammation reaction [7].

Due to the oral conditions of moisture and susceptibility to infection by saliva, ingested food, microorganisms, etc., a suture placed in an oral cavity is affected differently than a suture in extraoral conditions [8]. In addition to the chronic inflammation response, a suture placed in an infectious condition is likely to show constant acute inflammation responses. Furthermore, as a result of the inflammation response due to the suture itself, a suture can interfere with optimal wound healing and can result in the formation of excessive scar tissue, thereby reducing the tensile strength of the wound site. This eventually negatively affects the immune response to infection. Therefore, the immune response to a suture is an important factor when choosing the proper suture type [9,10].

The suture type is one of the critical factors influencing infection occurrence post-surgery. It has been reported that braided sutures show a more severe inflammation reaction than do those without any braid, due to the potential for penetration and transfer by microorganisms [11,12]. On the other hand, it has been suggested that wound infection is not always associated with suture structure, but may also pertain to the number of fibers [13]. In addition, Gabrielli et al. [14] insisted that patient factors such as age and gender have an influence on the immune response and infection occurrence rate when they observed no significant differences in immune response among sutures.

Studies on suture-induced immune response according to oral mucosa tissue type have been conducted. Castelli et al. [15] observed that the inflammation due to a suture located on the buccal mucosa was more severe than that of a suture located on the gingival mucosa. However, it was reported that there was no significant difference in immune response between the gingiva and alveolar mucosa in an adult dog [8].

The purpose of this study was to compare and evaluate the inflammatory responses induced by the three suture types widely used in clinical dentistry in the keratinized gingiva and buccal mucosa of beagle dogs.

Adaptation and maintenance of the wound site by sutures are critical events for the success of a surgical procedure. However, the existence of foreign material within the wound can increase the susceptibility to tissue infection [17]. Infection caused by the incision of a wound site, spread of infection during suturing and suture removal, and the delay of healing after surgery can all lead to a serious situation for a patient with a chronic disease, such as diabetes mellitus or cardiovascular disease. It has been reported that the risk of infection is increased by the presence of a suture within a wound [18].

Therefore, the importance of yielding the minimum immune response, which is an ideal result of suturing, should be considered with great attention when selecting suture type. Accordingly, this study was conducted to compare and evaluate the function of three suture types, all of which are widely used in intraoral surgeries, through the comparison of their tissue affinities. One of the most important functions of a suture is to maintain the stability of the wound edges until they can sustain themselves. The removal of oral surgery sutures is routinely performed one week post-surgery. However, in the case of regeneration therapy, maintenance of the suture for at least two weeks is frequently allowed in clinical practice for the enhancement of tissue maturity [19].

Therefore, in this study, a long term evaluation was used to examine the early- and late-stage immune responses. The experiment was divided into three time periods for the observation of acute inflammation, the common period of suture removal, and the maximum tensile force of the incision: three days post suturing, seven days post suturing, and 14 days post suturing. As for the sutures in the experiment, silk, polyglycolic acid, and nylon sutures were selected based on their wide utilization in clinical practice.

According to the study by Cardaropoli et al. [20] healing of the soft tissue in the extraction site included highly organized fibrous connective tissues at 30 days post-extraction, while the isolation of the newly formed hard tissue was mainly composed of woven bone and marginal mucosa at 60 days post-extraction. In this study, the suturing was conducted two months after the extraction to rule out the influence of the immune response caused by the soft tissue healing process and the woven bone formation.

The keratinized gingiva of the mandibular premolar regions and the buccal mucosa of the maxillary premolar regions were selected as the suture application sites for this study. Castelli et al. [15] reported that the mucosa is susceptible to the infiltration of inflammatory cells due to its composition of muscle fibers, minor saliva glands, and common connective tissues. Romanos et al. [21] immunohistologically identified type V collagen as containing the largest portion of lamina propria in sound keratinized gingiva. They also showed that type V collagen has great resistance to collagenase and acts as a mechanical barrier to bacterial penetration. It was also shown in this study that the sutures located on the buccal mucosa resulted in a higher infiltration of inflammatory cells into the internal tissue of the sutures than did those on the keratinized gingiva. However, the difference in the inflammation reaction between suture locations at 14 days post-suturing was insignificant.

The keratinized gingiva of the mandibular premolar regions and the buccal mucosa of the maxillary premolar regions were selected as the suture application sites for this study. Castelli et al. [15] reported that the mucosa is susceptible to the infiltration of inflammatory cells due to its composition of muscle fibers, minor saliva glands, and common connective tissues. Romanos et al. [21] immunohistologically identified type V collagen as containing the largest portion of lamina propria in sound keratinized gingiva. They also showed that type V collagen has great resistance to collagenase and acts as a mechanical barrier to bacterial penetration. It was also shown in this study that the sutures located on the buccal mucosa resulted in a higher infiltration of inflammatory cells into the internal tissue of the sutures than did those on the keratinized gingiva. However, the difference in the inflammation reaction between suture locations at 14 days post-suturing was insignificant.

In addition, it has been suggested that the physical form of a suture determines the degree of the inflammation reaction [15]. In many studies, the cause of the severe inflammation reaction observed in multifilament sutures, compared with monofilament sutures, has been referred to as 'the wicking effect' and results in the spread of the infection among the multifilament sutures and the presence of bacteria within the suture interstices [8,12,22]. However, Grigg et al. [4] found that silk did not produce more fluid movement by capillary action than coated Vicryl, and produced less than a polymer suture. According to a clinical study on the influence of suture absorption in two types of multifilament sutures and the resulting complication occurrence of wound healing and the failure of dental implants by Ivanoff and Windmark, neither type of suture showed any infection or implant failure [5]. Sortino et al. [23] counted the number of microorganisms with a light microscope in order to evaluate the inflammation reactions after removal of silk and polymer sutures from an intraoral environment eight days post-suturing. They reported that there was no significant difference between the treatment outcomes with regard to suture type, due to similar distributions of aerobic and anaerobic bacteria within the sutures. Also, it was reported that comparisons of infection, dehiscence, redness, scar hyperplasia, and patient satisfaction between nylon and polymer sutures from a skin biopsy at two weeks and six months after suturing did not show any significant differences [24]. In our study, blood clots from the trauma of suturing and infiltration of inflammatory cells around the suture and within the connective tissue due to acute inflammation reaction were observed around the suture and within the submucosa during the entire period of observation. However, there were no significant differences in the degree of inflammation among the three types of suture located on the keratinized gingiva. As for the silk and polyglycolic acid sutures, the existence of neutrophils within suture interstices and the migration of leukocytes were identified throughout the study.

In this study, intramuscular injection was administered with 2 mL of gentamicin for three days after suturing, and plaque control was performed with normal saline and 2% chlorhexidine. According to the study of Leknes et al. [25] on immune responses due to the use of silk and expanded polytetrafluoroethylene sutured onto the mandibular keratinized gingiva of a beagle, the administration of 2% chlorhexidine and systemic broad-spectrum antibiotics reduced the formation of biofilm and inflammation within the suture path. On the other hand, it was reported that the use of chlorhexidine had no effect on the species or existence of microorganisms, regardless of the kind of suture utilized [23]. However, because it used human participants, the latter study was considered to be incapable of strict control. Moreover, considering that the surgery sites were at the mandibular right angle area, the maintenance of oral hygiene was regarded as unmanageable. In this study, post suturing oral disinfection was conducted daily with 2% chlorhexidine, and unlike the study of Sortino et al. [23], the oral hygiene control of the suture sites in the mandibular premolar region (P1-P4) was possible, resulting in a definite outcome.

In conclusion, the nylon monofilament suture resulted in a favorable treatment outcome when placed in the alveolar mucosa, lesions arising from acute inflammation or an environment in which oral hygiene maintenance was unavailable. On the other hand, the structural difference in the monofilament and the multifilament sutures, with regard to immune response, did not affect the treatment outcome when placed in histologically dense keratinized gingiva with good oral hygiene.