Cementum, which develops from the dental sac, forms on the root after the disintegration of Hertwig’s epithelial root sheath (see Figure 6-20). This disintegration allows the undifferentiated cells of the dental sac to come into contact with the newly formed surface of root dentin, inducing these cells to become cementoblasts. The cementoblasts then disperse to cover the root dentin area and undergo cementogenesis, laying down cementoid. Unlike ameloblasts and odontoblasts, which leave no cellular bodies in their secreted products, during the later steps within the stage of apposition, many of the cementoblasts become entrapped by the cementum they produce, becoming cementocytes (Figure 14-7). Thus again, cementum is more similar to alveolar bone, with its osteoblasts becoming entrapped osteocytes.

When the cementoid reaches the full thickness needed, the cementoid surrounding the cementocytes becomes mineralized, or matured, and is then considered cementum. Because of the apposition of cementum over the dentin, the dentinocemental junction (DCJ) is formed. This interface is not as defined, either clinically or histologically, as that of the dentinoenamel junction, given that cementum and dentin are of common embryological background, unlike that of enamel and dentin.

Cementum is composed of a mineralized fibrous matrix and cells (see Figures 14-2 and 14-11). The fibrous matrix consists of both Sharpey’s fibers and intrinsic nonperiodontal fibers. Sharpey’s fibers are a part of the collagen fibers from the periodontal ligament that are each partially inserted into the outer part of the cementum at 90 degrees, or at a right angle, to the cemental surface (as well as the alveolar bone on their other end) as they are inserted on the other end, the alveolar bone. These fibers are organized to function as a ligament between the tooth and alveolar bone. The intrinsic nonperiodontal ligament fibers of the cementum are collagen fibers made by the cementoblasts and laid down in a nonorganized pattern, yet all these fibers still run parallel to the DCJ.

The cells of cementum are the entrapped cementoblasts, the cementocytes (see Figure 14-7). Each cementocyte lies in its lacuna (plural, lacunae), similar to the pattern noted in bone. These lacunae also have canaliculi or canals. Unlike those in bone, however, these canals in cementum do not contain nerves, nor do they radiate outward. Instead, the canals are oriented toward the periodontal ligament and contain cementocytic processes that exist to diffuse nutrients from the ligament because it is vascularized.

After the apposition of cementum in layers, the cementoblasts that do not become entrapped in cementum line up along the cemental surface along the length of the outer covering of the periodontal ligament. These cementoblasts can form subsequent layers of cementum if the tooth is injured (discussed later).

Three possible types of transitional interfaces may be present at the CEJ. The traditional view was that certain interfaces dominated in certain oral cavities. Studies with the scanning electron microscope indicate that the CEJ may exhibit all of these interfaces in an individual’s oral cavity, and there is even considerable variation when one tooth is traced circumferentially (Figure 14-8; see Figure 14-3). In some cases, the cementum may overlap the enamel at the CEJ (but thankfully fewer times than previously thought) at less than 15%. Novice clinicians may have difficulty discerning the CEJ from calculus deposits around the cervix with this situation. However, compared with the usually spotty placement and roughness of calculus, cementum exhibits a more uniform placement and roughness when using an explorer.

Another possible interface that can occur the CEJ is that the cementum and enamel may meet end to end, presenting no problems for either the clinician or patient, and it is the most common finding at about 52% of cases. Finally, another possible interface at the CEJ is that a gap may exist between the cementum and enamel, exposing dentin in about 33% of cases (see Figure 14-8). Thus, patients may experience dentinal hypersensitivity (see Figure 13-9).

Similar to bone tissue such as alveolar bone, cementum can undergo removal within the tissue as a result of trauma (Figure 14-9). This removal involves resorption of cementum by the odontoclast, resulting in reversal lines. When viewed in a stained section of cementum, these reversal lines appear as scalloped lines, just as in bone. However, cementum is less readily resorbed than bone, an important consideration during orthodontic tooth movement (discussed later).

At the same time, there can be repair of traumatic resorption area by involving the apposition of cementum by cementoblasts in the adjacent periodontal ligament. Apposition of this recently formed protective cementum is noted by layers of growth, or arrest lines, which, when viewed in a stained section, look like smooth growth rings in a section of a tree similar to what occurs in bone tissue such as alveolar bone. Both reversal and arrest lines are prominent in cementum subjected to occlusal trauma or to orthodontic tooth movement, as well as during the shedding of primary teeth and eruption of the permanent tooth. However, unlike bone, cementum does not continually undergo remodeling or repair as part of its makeup, but only when severely traumatized.

Two basic types of cementum are formed by cementoblasts: acellular and cellular (Figure 14-10 and Table 14-1). Acellular cementum consists of the first layers of cementum deposited at the DCJ, and thus is also considered primary cementum. It is formed at a slower rate than other types and contains no embedded cementocytes. At least one layer of acellular cementum covers the entire outer surface of each root with many more layers covering the cervical one third near

Clinical Considerations about Cemental Formation

Cementicles are mineralized bodies of cementum found either attached to the cemental root surface or lying free in the periodontal ligament (see Figure 14-11). They form from the apposition of cementum around cellular debris in the periodontal ligament (PDL), possibly as a result of microtrauma to Sharpey’s fibers. They become attached or fused from the continued apposition of cementum, and thus may interfere with periodontal treatment, as well as being noted on radiographs.

Cemental spurs can be found at or near the CEJ. These are symmetrical spheres of cementum attached to the cemental root surface, similar to enamel pearls. Cemental spurs result from irregular deposition of cementum on the root. They can present some clinical problems in differentiation from calculus and may be noted on radiographs; yet, because they are hard dental tissue, they are not easily removed, and thus may also interfere with periodontal treatment.

Hypercementosis is the excessive production of cellular cementum, which mainly occurs at the apex or apices of the tooth (Figure 14-12). It may be noted on radiographs as a radiopaque (or lighter) mass at each root apex. This condition can result from occlusal trauma caused by occlusal forces and during certain pathological conditions (at a generalized level as noted with Paget’s disease), such as chronic periapical inflammation. It may also be a compensatory mechanism in response to attrition to increase occlusal tooth height. However, such deposits form bulbous enlargements on the roots and may interfere with extractions, especially if adjacent teeth become fused (concrescence). It may also result in pulpal necrosis by blocking blood supply via the apical foramen (see Chapter 13).