Composition and Structure of Cementum: Strategies for Bonding 8 D

Composition and Structure of Cementum: Strategies for Bonding 8 D. Tziafas Introduction;; The cervical margin of class-II and class-V cavities is usually located below the cemento-enamel junction. This area poses a clinical problem in adhesive resin restorations, since the margins of the cavities are limited by cementum (Fig. 8.1), a calcified tissue exhibiting several histo-morphological and functional variations from the dental structures that have been extensively evaluated as bonding sub- strates (enamel and dentin). Cementum is a specialized connective tissue covering the outermost layer of calcified matrix on root surface, with a primary role to connect the periodontal ligament to the root surface.Cementum has not been largely encountered as a substrate from the bonding point of view, despite the fact that cervical microleakage has been early recognized to contribute to high incidence of secondary caries and Fig. 8.1. Light micrograph of a ground section of an adult human canine. Enamel (E), dentin (D) and cementum (C ) at cervical tooth situation 178 D.Tziafas fail of restorations [1, 2]. Very few studies have evaluated cementum-bonding restorations and the techniques used in present clinical practice in terms of their effectiveness and predictability [2–6]. A systematic approach to bonding at the cervical margins of class-II and class- V restorations requires understanding of anatomy and histo-physiology of the root dental structures. This chapter reviews the main structural, compositional, and functional aspects of cementum, and its primary evaluation as a bonding substrate. Developmental and Structural Aspects Cementum Varieties and Their Locations Three varieties of cementum cover the root of the human tooth: acellular afibrillar cementum; acellular extrinsic fiber cementum, and cellular mixed-stratified cementum. Their location and structural characteristics are associated with their different functional identity [7–10]. Acellular afibrillar cementum. Acellular afibrillar cementum is located mainly around the cemento-enamel junction covering further small enamel areas. Its distribution pattern varies from tooth to tooth and along the cemento-enamel junction of the same tooth [9]. By light and electron microscopy acellular afibrillar cementum represents a non-homogenous matrix composed of multiple calcified layers without collagen fibers [11]. Acellular extrinsic fiber cementum. The acellular extrinsic fiber cementum con- stitutes the majority of the cementum. It represents a thin layer of mineralized tissue located along the coronal two-thirds of the root surface (Fig. 8.2a, b). In ante- rior teeth acellular extrinsic fiber cementum is further extended to the apical root third. The formation of this type of cementum (Fig. 8.2c) is a continuous process characterized by an extremely slow rate of new matrix deposition [12]; thus, it reaches a thickness of approximately 15 mm during the prefunctional stage of root development, and a total thickness of approximately 50 mm during the functional stage throughout the life of the tooth [13, 14]. The periodic deposition of this type of cementum is seen in light microscopy as multiple layers of matrix interspersed by growth lines. Cellular mixed stratified cementum. Cellular mixed stratified cementum is composed of multiple interposed layers of acellular extrinsic fiber cementum and cellular intrinsic fiber cementum (Fig. 8.2d, e), which is mainly deposited on the apical root surface area and in the furcation areas. In fact, pure cellular intrinsic fiber cementum is only deposited as a part of the reparative process following root re- sorption [15]. The appositional growth of cellular cementum is much faster that acellular cementum,reaching a mean thickness of more than 200 mm [16].In adult teeth, impacted teeth, and in teeth without antagonists, extremely thick layers of cellular cementum could be found around the apical third of their roots [9, 17]. Composition and Structure of Cementum: Strategies for Bonding 179 c a d b e Fig. 8.2 a–e. Light micrographs of the main types of cementum.a Demineralized section of adult dog molar.Acellular extrinsic fiber cementum. b Ground section of adult human premolar.Acel- lular extrinsic fiber cementum. Note the dentino-cemental junction. c Demineralized section of developing dog molar. Cuboidal-shaped cementoblasts forming acellular extrinsic fiber cementum. d Apical situation from the section seen in a. Cellular mixed stratified cementum. e Apical situation from the section seen in d. Cellular mixed stratified cementum with numerous cemen- tocyte lacunae The primary role of acellular extrinsic fiber cementum is to attach periodontal ligament fibers to the root surface, supporting tooth mobility and function. Cellu- lar cementum has important functional role in maintaining the occlusal relation- ship of the tooth and repairing the resorbed root surface areas. Furthermore, acellular afibrillar cementum seems to have only adaptive function. Cementum Formation;; Cellular and molecular mechanisms, which regulate formation of periodontal tis- sues, including cementum, have not been fully understood. It is well known that dental follicle cells of ectomesenchymal origin are capable of forming cementum 180 D.Tziafas when properly stimulated [18–20].As to the nature of the cementum-forming cells (cementoblasts), there is a widely accepted opinion that they represent a popula- tion of periodontal cells [9, 21], although some authors suggest that epithelial cells may also contribute to the process of cementum formation [22–24]. The differentiation of dental follicle cells into cementoblasts is closely related to root dentin formation at a band extending 200–300 mm coronally to the developing root edge [9].Progenitor cells migrate towards the newly deposited,but not yet mineralized, dentin matrix, as the Hertwig’s root sheath breaks down. Numerous cytoplasmic processes are interposed between the dentinal collagen fibers, forming the dentino-cemental junction [14]. A layer of new matrix (precementum or cementoid) is deposited onto the external surface of the dentin by the surface root lining cells in a polar pattern. Recently, Nociti et al. [20] reported that the levels of inorganic pyrophosphate within the extracellular matrix of periodontal ligament is one of the crucial factors that regulate both initiation and maturation of cementum. Early mineralization of precementum into acellular extrinsic fiber cementum by deposition of hydroxyapatite crystals initially between and then within the collagen fibers, before the complete mineralization of the mantle root dentin, is usually found [25, 26]. At the apical third, highly active cementum-forming cells [12] deposit matrix in a non-polar bone-like pattern (cellular cementum). Cells entrapped into cellular cementum are called cementocytes. In mature teeth acellular cementum is not covered by precementum, a 3- to 5-mm-width layer which is usually seen along the cellular cementum [27, 28]. The formation of cementum is characterized by apposition throughout the functional stage of the tooth life. Since the collagen fibers of the periodontal ligament undergo remodeling, acellular cementum formation providing attachment to Sharpey’s fibers is a continuous process. New cementoblasts seem to be contin- uously recruited from the periodontal ligament pool to replace destroyed cementoblasts or cementoblasts that have reached the end of their life span [9]; however, it is not generally agreed that a specialized cell type exists in the periodontal ligament for the continuous cementum formation. Whether cementoblasts-forming acellular extrinsic fiber cementum are phenotypically different from the periodontal fibroblasts, or further cementoblasts-forming cellular cementum are specific cells or “osteoblast variants,”is still not known [30]. The root surface covered by the acellular extrinsic fiber cementum is an area at risk of carious demineralization or invasion by plaque bacteria. Regeneration of cervical dental structures is of clinical importance, and it seems challenging to devise new therapeutic methods enhancing cementum regeneration in affected root surfaces.More particularly,with regard to restorative strategies,novel bioma- terials aiming to induce controlled formation of acellular extrinsic fiber cementum might be designed. Structural Characteristics of Acellular Extrinsic Fiber Cementum;; Acellular extrinsic fiber cementum is not a homogenously calcified tissue (Fig. 8.3). Cementum is characterized by amorphous mineralized microstructures and a dense network of unmineralized Sharpey’s fibers, oriented perpendicularly to the root dentin.At the onset of cementum formation these fibers are extremely Composition and Structure of Cementum: Strategies for Bonding 181 b a c Fig. 8.3 a–c. Scanning electron micrographs of adult human acellular extrinsic fiber cementum at cervical tooth situation. a Root surface. The non-homogenously calcified structure of cementum. b Longitudinal section from the same root area. The uncalcified core of Sharpey’s fiber oriented perpendicularly to the root dentin (D). c Higher magnification of b.Atypical organization of apatite crystals short, whereas in mature teeth they are continuous with the long periodontal ligament fibers. These fiber types are clearly distinguishable from a small number of intrinsic collagen fibers, which are also found in acellular cementum. Hydroxyap- atite crystals, perpendicularly deposited to the root

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