Cementum

Anatomy and Composition

Cementum is a specialized calcified connective tissue that covers the anatomic root of a tooth. It is a vital component of the periodontium, the supporting structures of the tooth. Unlike bone, cementum is avascular, meaning it does not contain blood vessels, and it lacks innervation, so it does not contain nerves. Its composition is approximately 45-50% inorganic material (primarily hydroxyapatite), 50-55% organic material (collagen fibers and non-collagenous proteins), and water. The collagen fibers, predominantly Type I, are arranged in a complex network, providing structural integrity. Cementum is produced by cells called cementoblasts, which originate from the dental follicle.

At the most coronal aspect of the root, cementum meets the enamel at the cementoenamel junction (CEJ), also known as the cervical line. The relationship between cementum and enamel at the CEJ can vary among individuals and even within different teeth of the same individual. The most common relationship is cementum overlapping enamel (about 60-65% of cases), followed by an edge-to-edge relationship (about 30%), and in a smaller percentage of cases (about 5-10%), a gap exists where enamel and cementum do not meet, exposing underlying dentin.

Types and Distribution

Cementum is generally classified into two main types based on the presence or absence of cementocytes (cells within the cementum matrix) and the origin of its collagen fibers:

• Acellular (Primary) Cementum: This type is typically found covering the cervical (neck) and middle thirds of the root. It forms before the tooth erupts into the oral cavity and prior to occlusal function. Acellular cementum is thin, ranging from 30 to 230 micrometers, and contains no cells within its matrix. It is characterized by Sharpey's fibers, which are the terminal ends of the principal fibers of the periodontal ligament (PDL) embedded firmly into its surface. These fibers run perpendicular to the root surface and are crucial for anchoring the tooth to the alveolar bone.
• Cellular (Secondary) Cementum: This type is usually located in the apical (tip) third of the root and in interradicular areas of multi-rooted teeth. It forms after the tooth has erupted and is in functional occlusion. Cellular cementum is thicker and contains cementocytes, which are cementoblasts trapped within the lacunae (small spaces) of the calcified matrix. Canaliculi, tiny channels, extend from the lacunae towards the PDL, providing nutrients to the cementocytes. Cellular cementum contains both extrinsic (Sharpey's) and intrinsic collagen fibers (produced by cementoblasts), often arranged more irregularly than in acellular cementum. It is less calcified than acellular cementum and is capable of continuous deposition throughout life.

A third, less common type, Afibrillar Cementum, may be found on or near the CEJ and contains no collagen fibers, only calcified matrix.

Function and Clinical Significance

The primary and most critical function of cementum is to provide a medium for the attachment of the periodontal ligament fibers. These fibers, known as Sharpey's fibers, embed into the cementum on one side and into the alveolar bone on the other, effectively suspending the tooth in its socket and distributing occlusal forces. This attachment mechanism is fundamental for the stability and physiological movement of the tooth.

Other important functions and clinical considerations include:

• Root Protection: Cementum covers and protects the underlying dentin, particularly from chemical and mechanical insult.
• Compensatory Mechanism: Continuous deposition of cellular cementum throughout life compensates for tooth wear (attrition) on the occlusal surfaces, maintaining the vertical dimension of occlusion. This process is slow and generally uniform.
• Repair and Regeneration: Cementum has a limited capacity for repair. In cases of root surface damage or resorption, new cementum can be deposited, although this process is more complex than bone repair.
• Periodontal Disease: In periodontal disease, cementum can become exposed to the oral environment due to gingival recession and attachment loss. The exposed cementum may undergo changes, becoming rougher and more permeable, and can absorb toxins and bacterial byproducts, making it a reservoir for pathogens and hindering reattachment.
• Hypersensitivity: If a gap exists at the CEJ, or if gingival recession exposes the cementum, the underlying dentin tubules can be exposed, leading to dentinal hypersensitivity when stimulated by temperature, touch, or osmotic changes.
• Hypercementosis: This condition involves the excessive deposition of cellular cementum, typically in the apical region. It can be generalized or localized and may be associated with inflammation, trauma, or developmental anomalies. While often asymptomatic, severe hypercementosis can complicate tooth extraction or endodontic treatment.

Assessing Cementum Health

Clinicians assess the health and integrity of cementum primarily indirectly through evaluating the surrounding periodontal tissues. A healthy periodontium implies healthy cementum, providing firm attachment for the PDL. Clinical signs of cementum involvement in disease include:

• Gingival Recession: Exposure of the root surface, including cementum, due to apical migration of the gingival margin.
• Clinical Attachment Loss: A key indicator of periodontal disease, measured from the CEJ to the base of the periodontal pocket, reflecting destruction of the cementum, PDL, and alveolar bone.
• Root Caries: Caries that develops on the root surface, primarily affecting exposed cementum and underlying dentin. This is more prevalent in older populations with gingival recession.
• Resorption: Internal or external resorption of the root can involve the cementum layer. External root resorption can be inflammatory, replacement (ankylosis), or surface resorption, often initiated by damage to the cementum and PDL.

Radiographic examination can reveal changes in cementum thickness (e.g., hypercementosis) and bone levels relative to the CEJ. Tactile sensation with a periodontal probe or explorer can detect calculus deposits on exposed cementum or surface irregularities indicating prior disease activity or anatomical variations at the CEJ. Understanding cementum's role is fundamental for diagnosis, treatment planning, and prognostic evaluation in periodontology and restorative dentistry.