The Major Types of Cementum

Cementum, is a mineralized tissue covering the entire root surface of the teeth, which fit into alveolar sockets of alveolar bone, and functions as a tooth-supporting device in concert with the periodontal principal fibers and alveolar bone. Cementum is often referred to as a bone-like tissue. It composes of around 50% organic and another 50% inorganic component. However, cementum is markedly different from bone as it is avascular, does not undergo dynamic remodeling, and increases in thickness throughout life. Cementum, which has the surface lining by cementoblasts, has been classified into cellular and acellular cementum by inclusion or non-inclusion of cementocytes. Generally, acellular cementum is thin and covers the cervical part of the root, whereas cellular cementum is thicker and covers the apical region of the root.

Cementoblasts are highly differentiated mesenchymal cells of the periodontal ligament (PDL) that build up cementum (Bosshardt, 2005). Disintegration of the Hertwig’s epithelial root sheath (HERS) into epithelial fragments during tooth root development allows ectomesenchymal cells from the dental follicle proper to come into contact with the dentin surface, where they differentiate into cementoblasts (Kim et al, 2009). Cementoblasts are responsible for cementum matrix synthesis, deposition and mineralization. Cementocytes are cementoblasts those trapped in their matrix during matrix deposition. Moreover, cementoblasts can also communicate with cementoclasts to regulate the resorption of cementum by the cementoclastogenesis. Therefore, cementoblasts are importance for the homeostasis of cementum (Oka et al, 2007).

Fibrillar collagen is the majority of the organic component, cementum contains two types of fibers, extrinsic (Sharpey's) fibers which are embedded ends of the principal fibers and intrinsic fibers which are fibers of cementum proper. It is believed that the extrinsic fibers are secreted by periodontal ligament fibroblasts and partly cementoblasts and that the intrinsic fibers are secreted by only cementoblasts.

There are three major types of cementum:

1. Acellular extrinsic fiber cementum (AEFC) contains densely packed extrinsic fibers and no cementocytes.
2. Cellular intrinsic fiber cementum (CIFC) contains intrinsic fibers and cementocytes.
3. Cellular mixed stratified cementum (CMSC) contains intrinsic fibers, cementocytes and densely packed extrinsic fibers.

Since main function of cementum is tooth anchorage together with the periodontal ligament and alveolar bone. AEFC is the most suitable cementum for tooth support. Function of CIFC is for adaptation such as reshaping the root surface during tooth movement and compensating for crown wear. CMSC thickness is parallel to the masticatory stress loaded on the tooth to provide loading absorption during mastication (Yamamoto et al, 2016). Other functions are to reparative sense, in the healing of root fracture and protective tooth from resorption and to confine tooth motion by way of the periodontal ligament (Jang et al, 2014).

The most common pathology effecting on cementum is orthodontics induced root resorption, which causes losing of root cementum (Ramanathan and Hofman, 2006). Once the resorption process is terminated, cementoblasts begin to refill the resorption lacunae with CIFC However, about 3% of patients fail to repair these resorptions with cementum. As a result, apparent increased mobility and length reduction in tooth root (Owman-Moll et al, 1995).