Development and Function of Tooth Supporting Structures

Tooth supporting structures such as the periodontal ligament, cementum, alveolar process are critical to the health and vitality of the dentition. Development of the attachment apparatus, external forces and the periodontium, and vascularization of the supporting structures are discussed in this article.

The periodontal ligament is the connective tissue that surrounds the root and connects it to bone. The principal fibers, composed of collagen, are responsible for connecting the tooth to bone. These principal fibers can be arranged into six groups: transseptal, alveolar, horizontal, oblique, apical, and interradicular. All fibers have a physiologic as well as supportive responsibilities to the dentition. Four cell types have been identified in the periodontal ligament: connective, epithelial rest cells, immune system cells, and neurovascular elements. The ligament functions to provide a protective sheath for vessels and nerves, transmit occlusal forces to bone, attach teeth to bone, maintain gingival tissues and resistance the impact of occlusal forces.

Cementum is the calcified avascular mesenchymal tissue that forms the outer covering of the anatomic root. It exists as acellular and cellular. Acellular cementum is formed first and covers approximately the cervical third or half of the root. Cellular cementum is formed after the tooth reaches the occlusal plane and is more irregular. Cementum is very permeable in the young and diminishes with age. Normal thickness ranges from 16 to 30 ì at the coronal half and grows to 150 to 200 ì at the apex. Pathologic conditions associated with cementum include resorption (caused by either local or systemic causes) or ankylosis (fusion of cementum and alveolar bone).

The alveolar process is the bone that forms and supports the tooth sockets. It forms during tooth eruption and provides the osseous attachment to the periodontal ligament. It consists of an external plate of cortical bone, the inner socket of thick, compact bone, and cancellous trabeculae. During fetal growth, alveolar bone is formed by intramembranous ossification and consistently remodels by activity of osteoblasts and osteoclasts. Outer bone surfaces are covered by periosteum and inner surfaces are lined with endosteum. The contour of bone follows the topography of tooth roots. Defects in the bone can be seen as isolated areas in which root is denuded of bone (fenestrations) or defects through the marginal bone (dehiscence).

Development of the attachment apparatus is initiated after crown formation. Hertwig's root sheath ruptures and allows mesenchymal cells to contact dentin thus forming cementum. While the crown approaches the oral mucosa fibroblasts become active and begin to produce collagen fibrils which eventually become the periodontal ligament. The alveolar bone is developed right before mineralization by osteoblasts which produce matrix vesicles. These vesicles contain enzymes which help the nucleation of hydroxyapatite crystals. The crystals are aligned with their long axes parallel to collagen fibers and appear to be deposited within the fibers in mature lamellar bone.

The periodontium main responsibility is to support teeth during function and depends on the stimulation it receives from function for the preservation of its structure. Due to external forces the alveolar bone undergoes constant remodeling. The periodontal ligament can also accommodate increased function by increasing width. When forces are reduced the periodontal ligament will atrophy.

Vascularization of supporting structures are derived from the inferior and superior alveolar arteries. The periodontal ligament receives blood from the apical vessels, penetrating vessels from the bone and anastomosing vessels from the gingiva. Venous drainage accompanies the arterial supply while lymphatics supplement the venous drainage system.