Cementum is the calcified or mineralized tissue layer covering the root of the tooth which sits inside the gum socket. The tooth is held in place in the jaw by four periodontal tissues including:
- Alveolar bone or the jaw bone
- The periodontal ligament
- Gingivae or gums
- The cementum
In some mammals such as herbivores, cementum may extend over the crown of the tooth. In humans, however, cementum only surrounds the root of the tooth.
Structure of cementum
The thickness of the cementum varies at different levels of the tooth root. It is thickest at the apex of the root and between two roots when a tooth has more than one root. It is thinnest at the upper part of the root where it is 10-15Âµm thick. At the apex of the root, it is around 50-200Âµm thick and can even reach up to 600Âµm in thickness. The cementum is a continuous structure that spans the length of the periodontal ligament on the outside of the root and the dentine on the inside.
Cementum is very similar to bone but has no blood or nerve supply. In addition, cementum moves with movement of the tooth.
Development of the Cementum
The cementum is derived from the investing layer of the dental follicle. Over the surface of the cementum, there lies a thin layer about 3-5 Âµm in thickness that is not calcified called the precementum.
Cementum is pale yellow in colour and has a dull surface. It is softer and usually more permeable than dentine, although this permeability declines with age.
Classification of cementum
Cementum is classified into two types:
- Cellular cementum - Present at the apex of the root, this cementum contains cells called cementocytes.
- Acellular cementum (not containing cells) - This cemetum does not contain cells and covers the upper half or third of the root towards the crown
Functions of the cementum
The main function of cementum is to provide attachment to the collagen fibres present in the periodontal ligament. This helps maintain the integrity of the root and its position in the gum and bone. Cementum is also deeply involved in the repair and regeneration of teeth.
As a person grows older, gums may recede and the expose the roots of the tooth. This can lead to the cementum being abraded away and dentine being exposed.