Periodontal Soft Tissues

The periodontium consists of 4 intimately collaborating structural components: periodontal ligament, cementum, alveolar bone, and gingiva. Although the cellular composition, foundational architecture, and biochemical mechanisms are extraordinarily diverse between the components, traumatic and pathologic insults to any of these 4 individual units have deleterious effects on the entire periodontal complex. The periodontal ligament is a vascular fibrous connective tissue that bridges the tooth root to the alveolar bone and measures 0.15 to 0.21 mm on average. The principal purpose of the periodontal ligament is to resist occlusal force impact, provide protection to nerve and vascular structures from mechanical insult, and communicate occlusal forces to bone. Cementum is the calcified outer covering of the anatomic root that is attached to Sharpey’s fibers of the periodontal ligament to transmit occlusal forces. The alveolar process is the portion of the maxilla and mandible that forms the tooth sockets. Although it is not the goal of this article to delve into an extended discussion of the individual anatomic and biochemical components of the periodontium, the gingival component is discussed in closer detail to build a foundation for the remainder of the article.

The gingiva can be classified into marginal, attached, and interdental areas and serves to protect against both mechanical and microbial insults. Evidence exists that gingival epithelium participates actively in states of infection. Marginal gingiva is the border that surrounds the dentition in a collar-like fashion. Typically it measures less than 1 mm in thickness and in its apicocoronal dimensions. On the tooth side of the marginal gingiva, the gingival sulcus is found. In healthy adult human gingiva, this sulcus is shallow and is typically measured to be an average of 1.8 mm with variations from 0 to 6 mm.

The attached gingiva is coronally continuous with the marginal gingiva. It receives its name from its tight attachment with the underlying periosteum of the alveolar bone and terminates apically at the mucogingival junction. In the esthetic zone, the attached gingiva retains its greatest dimensions, between 3.5 to 4.5 mm, while being much narrower in the posterior segments. The loss of width of attached gingiva is primarily attributed to changes in the position of the gingival margin as the mucogingival junction remains stationary in the adult anatomy.

Interdental gingiva is found in the interproximal space beneath tooth contacts and embodies the gingival embrasure. The presence of an adequate papilla depends on the presence of interproximal contact between the adjacent teeth and the distance of this contact from the alveolar crest.

Histologically, gingiva is composed of a highly cellular stratified squamous epithelium overlying a central compartment of connective tissue. Within the connective tissue, collagen fibers and ground substance are found. Keratinocytes constitute the primary cellular makeup of the gingival epithelium and alongside these cells, Merkel cells, Langerhans cells, and melanocytes are observed within the epithelium. Variations of this histologic makeup occur at different sites of the gingiva. Oral epithelium is keratinized and covers the outer surface of the marginal gingiva and the surface of the attached gingiva. On the other hand, sulcular epithelium that lines the gingival sulcus is nonkeratinized and extends from the coronal dimension of the junctional epithelium to the crest of the gingival margin. Sulcular epithelium can keratinize if exposed to the oral cavity, and the outer oral epithelium can “dekeratinize” once in contact with the dentition.

Biologic Width

Moving apically from the base of the gingival sulcus, the junctional epithelium is encountered, followed by connective tissue. The “biologic width” (also known as supracrestal tissue attachment according to the 2017 Classification of Periodontal and Peri-Implant Diseases and Conditions) encompasses these 2 tissue layers. The biologic width holds responsibility in establishing a seal around the tooth. Junctional epithelium forms a tight connection to afibrillar cementum and root cementum via an hemidesmosome attachment within the internal basal lamina. This tight attachment forms a physiologic barrier against penetration of oral pathogens to the subgingival tooth surface while providing an avenue for immunologic host defense components to gingival sulcus. ^, This barrier is often penetrated during periodontal probing, especially in the setting of gingival inflammation. ^, The cells of the junctional epithelium are characterized by a rapid rate of cellular turnover and a distinct ability to spread across both tooth and implant surfaces after mechanical postsurgical insults. ^,

The gingival connective tissue is principally composed of organized groups of collagenous fibers that insert into the cementum, bone, and soft gingival tissues. In close association with these fibers is an extracellular matrix of ground substance interspersed with fibroblasts, vessels, and nerves. Epithelial and connective tissue attachments around teeth were analyzed on cadaver specimen in a classic study, in which Gargiulo and colleagues found that the average width of the junctional epithelium was 1.07 mm and the connective tissue component measured on average of 0.97 mm with average biologic width of 2.04 mm. As with any average, there are physiologic individual variations to these measurements, with greater variation in the junctional epithelial component than the connective tissue. Infringement upon the biologic width has been theorized to cause gingival inflammation, alveolar bone loss, and eventual periodontitis. ^, Given these dimensions, it is recommended that restorations must allow for at least 3.0 mm to exist between the alveolar crest and the margin of the restoration. ^{, ,}

Gingival Biotype

Gingival biotype (more recently known as gingival phenotype according to the 2017 Classification of Periodontal and Peri-Implant Diseases and Conditions) is defined as the thickness of gingiva in the faciopalatal or faciolingual dimension. Accurate diagnosis of the gingival biotype is essential to both esthetic dentistry and implant planning. In 1989, Seibert and Lindhe introduced the categories of “thick-flat” and “thin-scalloped” gingival biotypes. Periodontal probe visibility has been held as the clinical gold standard to identify thick from thin gingival biotype. As such, difficulty arises in the subjective interpretation of what defines visibility, and studies have since attempted to quantify gingival thickness in lieu of subjective analysis.

Thin-scalloped biotype is distinguished by small contact dimension and is located near the incisal edge of the tooth, scarce attached gingiva, a highly scalloped soft tissue and bone architecture, and thin underlying osseous form. ^, This is clinically significant because thin biotype is subjected toward a tendency to gingival recession, apical migration of attachment, loss of underlying alveolar volume, dehiscence, and fenestration after surgical or prosthetic manipulation or after sustaining irritation. Removal of supracrestal implant abutments often results in immediate collapse of peri-implant soft tissue. In this biotype, every effort must be made to use minimally traumatic surgical techniques. Some investigators advocate a conservative approach to preserve circulation and soft-tissue volume at implant sites. Flapless surgery techniques such as a tissue punch or U-shaped peninsula are recommended for these patients. Despite the use of minimally invasive techniques, patients with this biotype should still be warned of increased risk of soft-tissue loss, and consideration for soft-tissue grafting procedures should be given. However, studies have shown that gingival biotype does not have significant influence in surgical root coverage results associated with connective tissue grafting, nor does it have significant effects on hard-tissue augmentation results.

Thick-flat biotype is characterized by broader contact areas located closer to the gingiva accompanied by distinct cervical convexities, dense fibrotic tissue, and often a thick underlying osseous form ( Box 1 ). In contrast to the thin-scalloped biotype, patients with this characteristic gingival biotype are more resistant to recession but more susceptible to development of scarring after implant therapy.

Peri-Implant Soft Tissues

Biologic width must also be considered for implant-supported restorations. Multiple studies have revealed that the peri-implant soft tissues closely mimic those of natural dentition. In comparison with the biologic width around implants and teeth, the major equivalence between natural teeth and implants is the junctional epithelial attachment, whereas the chief disparity is the absence of collagenous supracrestal connective tissue fiber insertions into the implant and lack of periodontal ligament fiber attachments within alveolar bone. After surgical interruption of the gingiva, the cut edge of remaining attached gingiva rapidly differentiates and reattaches to the surface of a clean, uncontaminated implant to form the junctional epithelial attachment.

This brings us to the discussion of importance of attached gingiva around an implant site. Stable peri-implant mucosa is necessary for the preservation of bone around an implant. However, there is much debate in the literature regarding the necessity of peri-implant keratinized attached gingiva versus nonkeratinized mucosa. Some clinicians warn of an increased risk for peri-implant mucosal recession and attachment loss when surrounded by inadequate keratinized attached mucosa, whereas others report that there is no difference in peri-implant bone levels when placed in alveolar mucosa. ^, Most studies do agree, however, that aside from the obvious final esthetic advantage, keratinized attached gingiva also provides a stable background for the prosthetic phase and facilitates oral hygiene care. Inadequate keratinized tissue can impede proper oral hygiene and result in increased plaque accumulation. Subsequently, gingival inflammation results and may potentially lead to peri-implantitis. Thus, the peri-implant soft-tissue seal provided by attached peri-implant soft tissues can simplify oral hygiene care and facilitate a “prosthetic-friendly” environment that is essential for long-term implant success.

Marginal Tissue Recession

When marginal soft tissue is displaced in an apical direction away from the cementoenamel junction with resultant exposure of the tooth root surface, it is referred to as marginal tissue recession. The term gingival recession is also often used interchangeably, but a study by Maynard and Wilson ^, in 1979 suggested that marginal tissue may originally have been alveolar mucosa rather than gingiva. The exposure of a tooth root may be associated with hypersensitivity to extremes in temperature, exposure of root surfaces to potentially carious microbiota, and pronounced esthetic defects. The primary causes of marginal tissue recession are inadequate oral hygiene that leads to plaque accumulation and subsequent inflammation, orthodontic tooth movement, and mechanical, thermal, and chemical insults. Patients who are especially susceptible to gingival recession include those with thin gingival biotype and those with reduced or absent keratinized tissue. ^,

Proper identification and classification of marginal tissue recession is essential to preoperative evaluation for periodontal plastic surgical procedures on tooth-borne defects and for implant planning to decide whether soft-tissue grafting is indicated in conjunction with implant therapy. Several classification systems exist to classify the amount of marginal tissue recession. ^, In 1968, Sullivan and Atkins introduced a system that organized defects into “shallow-narrow,” “shallow-wide,” “deep-narrow,” and “deep-wide” categories based on the width and length of the recession defect. These categories were used to predict the success of tissue-grafting procedures.

Subsequently, in 1985 Miller proposed an expanded classification system ( Box 2 ) that correlates the location of the apicalmost aspect of the marginal tissue defect with the location of the mucogingival junction while taking account of the degree of interdental hard-tissue and soft-tissue loss. He considered that many cases of recession were impossible to identify using the classification system developed by Sullivan and Atkins. Based on the Miller classification, prognosis of soft-tissue grafts used for root coverage could be systematically predicted. Class I and class II defects resulted in 100% root coverage. Partial root coverage could be expected in class III defects, and the amount of root coverage could be predicted by the surgeon by placing a periodontal probe horizontally at the midfacial tissue level of the 2 adjacent teeth to the tooth exhibiting recession. No root coverage is often anticipated in class IV defects.

Most recently, in 2017, the American Academy of Periodontology agreed to undertake the 2017 World Workshop classification system of periodontal and peri-implant diseases and conditions as the new classification system for periodontal conditions around teeth and edentulous ridges. This new system incorporates a multidimensional staging and grading system for the classification of periodontitis. The system also incorporates a novel classification for peri-implant diseases and conditions.

Peri-Implant Soft-Tissue Dehiscence

It has been well established in the literature that the dental implant success rate is more than 95% when measured in terms of osseointegration. However, esthetic and biological implant complications are relatively abundant. Apical migration of the peri-implant soft tissues is often referred to as soft-tissue dehiscence, mucosal recession, or mucosal dehiscence. There is no agreed classification of mucosal recession around implants, and most investigators advocate the use of exposure of the metallic implant or abutment surface as a reference point, or the adjacent levels of the mucosal margins surrounding natural dentition. Lack of adequate dimensions of keratinized tissue paired with inadequate oral hygiene routines has been shown to increase plaque accumulation around implants and increase the potential for peri-implant inflammation.

Recipient Site

There are several principles in oral soft-tissue grafting that must be strongly considered and adhered to for graft success. The primary concern is the sufficient nourishment of the newly placed graft. Soft-tissue grafts initially survive by plasmic imbibition and later on through neovascularization. To achieve this, firstly the recipient site must be vascularized. Second, recipient sites must facilitate rigid immobilization and intimate adaptation of the donor tissue. Excess movement of the graft precludes angiogenesis and plasmic imbibition, thus starving the site of adequate nutrition. In large areas of decreased vascularity such as a tooth root surface or implant abutment, treatment planning should include the use of a pedicled graft rather than free grafts. An intimate adaptation of the soft tissues will decrease the distance over which plasmic diffusion and capillary development will occur. In addition, hemostasis must be achieved at the recipient site. Blood products and active hemorrhage will prevent the ability of the graft to properly adapt to the recipient site. Loss of delivery of nutrition to the graft site often leads to the sloughing and inevitable loss of the graft. Consideration of these principles explains why periosteum is widely considered as an excellent recipient site for oral soft-tissue grafts. It is characterized by an abundant vasculature, is immobile, and can facilitate rigid and intimate adaptation of donor tissue.

Donor Site

With strong understanding of the essential features of a suitable recipient graft site, one would realize the necessity for proper planning and delicate harvesting of the graft from the donor site. The harvested graft should have a uniform thickness to facilitate intimate adaptation and immobilization to the recipient site which, in turn, would enable efficient angiogenesis and nutrient diffusion. Secondary contracture is often a concern when considering the thickness of a harvested graft. Thin grafts are highly subject to this phenomenon, whereas thicker grafts are better able to maintain their physical dimensions. Thicker grafts are generally preferred because they must have dimensions even after contracture.