Cementum is the calcified material that covers the root of a tooth. The cementum is anchored to the alveolar bone by the periodontal ligament and is part of the collection of specialized tissues that surround and support the teeth, called the periodontium. Cementum is a specialized mineral substance that is similar in composition to bone. Whereas the small canals in bone contain nerves, however, the canals in cementum transmit nutrients between the cementum and the periodontal ligament. Because of the specific structure and composition of the cementum, which consists of entrapped cementoblasts called cementocytes, if the tooth is injured or damaged, the cementum can generate new layers and promote its own healing. Interestingly, recent studies indicate that cementum contains five times the amount of mitochondrial DNA as dentin, which may have profound implications for the identification of human remains, as dentin is adversely affected by age and disease but cementum is not.
Because the cementum covers the root of the teeth, it is not visible in a healthy mouth. If, however, the gums have receded due to periodontal disease, the cementum may be exposed. The place where the cementum joins the enamel, which covers the anatomical crown of the tooth, or the part of the tooth that is visible above the gumline, is referred to as the cervical line. Cementum is made up of about 45-50% of inorganic material and 50-55% organic material. The organic material is primarily collagen and specific types of protein called proteoglycans. Cementum is avascular, meaning it lacks blood vessels, and it derives nutrients from the surrounding periodontal ligament, which is vascular. Cementum is light yellow, slightly lighter in color than dentin, which is the material that surrounds the entire pulp of the teeth and is surrounded by cementum below the gums and enamel above them. Cementum has the highest fluoride content of any mineralized tissue and is permeable to an array of materials. As the outer layer of cementum ages, new layers of cementum are deposited, which means that cementum is formed continuously throughout the life of the tooth; additionally, cementum is not resorbed like bone is. The cells that secrete cementum and allow it to continuously develop and repair itself are called cementoblasts.
Because of its relative thinness and low mineral content, if the cementum is exposed, it can become abraded very easily. If this abrasion leads to exposure of the dentin, this can lead to tenderness and sensitivity. The cementum may become exposed, particularly, in cases of periodontal disease, which has multiple other negative effects. In older adults, who are more likely to experience gingival recession and periodontal disease, cavities may appear in the cementum, forming a shallow lesion and eventually invading the dentin and leading to chronic infection of the pulp. Because these lesions are often painless until they are relatively developed, this is a significant contributing factor to tooth loss in aging populations. Cementum can also form cemental spurs, which are spheres of irregularly deposited cementum that can develop on the cementum. Because these are made of hard tissue, they are challenging to remove and may cause complications with periodontal treatment.