Periodontology: Cementum

The Normal Periodontium Cementum & Alveolar Bone Cementum The cementum is the calcified layer that covers the dentine and forms the outer layer of the root. It is into this calcified tissue that part of the periodontal ligament (Sharpey's fibers) is embedded to attach the tooth to the bone. Sources The main sources of collagen fibers in cementum are derived from two sources: 一 Inner epithelial root sheath (Intrinsic source):

After enamel is formed, the outer and inner enamel epithelial layers start extending to form the root of the tooth. This produces intermediate cementum. You can see the enamel being formed in the enamel bud in the pictures. At the bottom, the inner enamel epithelium starts to extend to give the root sheath where cementum starts to be laid. The cementoblasts in the cementum matrix form collagen fibers. After the formation of cementum, there is continuous turnover and remodeling of it.

一 Periodontal ligament fibroblasts (Extrinsic

source): Sharpey's fibers extend into the cementum and constitute a great proportion of the bulk of cementum. 一 Mixed:

Intrinsic cementum fibers come in between Sharpey's fibers. (They interdigitate.)

Composition Cementum's thickness varies along the root. Apically, it's 150-200 μm thick; almost ten times as thick as cervical cementum (10-15 μm). It is firmly attached to root dentin on one side, and to the periodontal ligament on the other to anchor the tooth securely within the alveolus. Cementum is very dynamic. It is highly responsive as the tooth is in continuous function; due to mastication, brushing or even using toothpicks. There is micromovement of teeth within their sockets to make them able to do their job. We need teeth to be strong, but we also need them to be able to continuously remodel and adapt. Cementum is avascular, but it's connected to the periodontal ligament where the nerve endings are. So the sense of the tooth in its socket comes basically from the innervation of the tissues around the cementum. So although cementum is avascular, this doesn't mean that it's not alive: it interacts with the periodontal ligament and it is dynamic. The major component of cementum is collagen. Type I collagen is predominant in cementum (90 %). Type III collagen comprises 5% of cementum. Types 一 Acellular (primary) If cementum is formed slowly during the normal development of the tooth, while it is within the bone (the crypt), it takes its time to mature properly, so the cementoblasts are no longer needed and so they disappear. Acellular dentin is formed first; before the tooth reaches occlusion. It is present in the cervical 1/3-1/2 of the root. The difference in color in cementum here (the longitudinal lines) is due to its increment nature of formation. A matrix is produced, there is a rest, calcification takes place, and another layer is formed with a minimal difference in mineral content.

一 Cellular (secondary)

When the tooth reaches occlusion, the reactionary mechanism takes place. If cementum is being formed under stimulation (by external stimulants, like trauma), an irregular type of cementum is laid down in reaction to the stimuli, and some of the cells are trapped within the layers of cementum, and maintained within the cementum after its mineralization. It is reparative cementum; and that is why it is found mainly at the root's apex: the apex is the part of the tooth mostly responsible for adaptation and reacting to external stimuli. It is also found at the interradicular areas as well; because between the roots, the bottom part of cementum covers bone in the same manner that the apical area of the root does. So the tooth rests on three (or four) points: the apices, and the area in between. The cells in this type of cementum, cementocytes, are not functioning (they are not cementoblasts). Cementocytes are mature, non-functioning, retired, and exhausted cells of cementum. These cementocytes are present in lacunae and interact with each other by processes that pass through canaliculi. Cementum's Relationship with enamel

Cementum's relationship with enamel is very important because it has clinical implications. Enamel is formed before cementum. So in no case will enamel cover cementum. After enamel is formed, the inner and outer enamel epithelia unite to form the root

sheath, and then cementum's formation begins. So cementum can overlap enamel, but it cannot be under enamel. Because under enamel there is dentine, so that space is already taken. So there are three relationships: 一 Cementum covers enamel (C in the picture above). 60-65% of cases. 一 Edge to edge contact (B in the picture above). 30% of cases. 一 No contact (A in the picture above). 5-10% of cases. A gap between the two structures forms when part of Hertwig's root sheath disintegrates and does not produce cementum at that site. It is very uncommon, but very significant; because at the site of the gap, dentine will be exposed, causing dentine hypersensitivity. One of the functions of cementum is to cover dentine, and when dentinal tubules are exposed, stimulation of the fluid within the dentinal tubules leads to micromovement reaching the pulp, causing pain.

Function Cementum provides a seal for the dentinal tubules. And, of course, it provides anchorage (attachment) for the periodontal ligament.

Physical properties Normal cementum is pale yellow in color; and this is how we differentiate it from enamel. But if it's exposed for a long time, and some abrasion takes place, it might become difficult to differentiate between them in color. It's very important to locate the cemento-enamel junction; because it's a starting point to differentiate between gingivitis and periodontitis. In gingivitis, the gums are inflamed, but there is no loss of structure attachment to the tooth. Normally, gingival tissue covers cementum and the alveolar bone, and the attachment of this tissue is at the cemento-enamel junction. This attachment might move apically, and this is called attachment loss. So if we can locate the cemento-enamel junction and we can insert an instrument below the cementoenamel junction, we know that the gingiva is not attached to the tooth at that point, and we have attachment loss, which indicates periodontitis.

Cementum's permeability varies with age and type of cementum. With age cementum and dentine (and even bone) undergo mineralization (sclerosis) sometimes, which leads to a decrease in their permeability. Cementicles One source of cementum is the periodontal ligament fibroblasts, and sometimes these start forming cementum within the periodontal ligament as ovoid or round nodules called cementicles. Cementicles can either be free in PDL, or embedded in cementum. Cementicles occur with aging and at sites of trauma.

Chemical properties Cementum is composed mainly of inorganic matter. But still, there is water in its composition. There is Calcium, Phosphorus, and Fluoride in cementum. Fluoride being present in higher levels in acellular cementum. INORGANIC ORGANIC WATER BY WEIGHT

65%

23%

12%

BY VOLUME

45%

33%

22%

Resorption and repair of cementum Cementum undergoes a continuous process of resorption and formation in order to react well in a physiologic way to external stimuli. Teeth are in continuous function. This needs bone, PDL, and cementum to be able to remodel to adapt to this function. Cementum is less susceptible to resorption and remodeling than bone. Localized areas of resorption may be found in cementum depending one the site of the stimulus. Tooth function may exceed its normal level causing occlusal trauma. One of the reactions of the periodontium and tooth structure to occlusal trauma is cementum resorption. While cementoblasts form cementum, resorption of cementum is done by odontoclasts. (There are no cementoclasts.) 一一一