Periodontal Ligament - Functions

III. Periodontal ligament


III. Periodontal ligament

The periodontal ligament is the fibrous connective tissue structure, with neural and vascular components, that joins the cementum covering the root to the alveolar bone (Fig. 57).

Fig. 57: Histologic section of a maxillary central incisor. The cementum layer (C) is connected to the adjacent alveolar bone (AB) by a thin periodontal ligament (PDL).

Despite its fibrous nature, the periodontal ligament is a highly cellular structure that is able to perform a number of important functions that are essential for the long-term health of the masticatory apparatus. 

A. Functions of the periodontal ligament

The periodontal ligament serves primarily a supportive function by attaching the tooth to the surrounding alveolar bone proper.  This function is mediated primarily by the principal fibers of the periodontal ligament that form a strong fibrous union between the root cementum and the bone.  The periodontal ligament also serves as a shock-absorber by mechanisms that provide resistance to light as well as heavy forces.  Light forces are cushioned by intravascular fluid that is forced out of the blood vessels.  Moderate forces are also absorbed by extravascular tissue fluid that is forced out of the periodontal ligament space into the adjacent marrow spaces.  The heavier forces are taken up by the principal fibers.  
The periodontal ligament also serves a major remodeling function by providing cells that are able to form as well as resorb all the tissues that make up the attachment apparatus, i.e. bone, cementum and the periodontal ligament   Undifferentiated ectomesenchymal cells, located around blood vessels, can differentiate into the specialized cells that form bone (osteoblasts), cementum (cementoblasts), and connective tissue fibers (fibroblasts). Bone- and tooth-resorbing cells (osteoclasts and odontoclasts) are generally multinucleated cells derived from blood-borne macrophages.
The periodontal ligament also serves a sensory function.   The myelinated dental nerves that perforate the fundus of the alveoli rapidly lose their myelinated sheath as they branch to supply both the pulp and periodontal ligament.  The periodontal ligament is richly supplied with nerve endings that are primarily receptors for pain and pressure.
Finally, the periodontal ligament provides a nutritive function that maintains the vitality of its various cells. The ligament is well-vascularized, with the major blood supply originating from the dental arteries that enter the ligament through the fundus of the alveoli.  Major anastomoses exist between blood vessels in the adjacent marrow spaces and the gingiva.