Principal Fibers of the Periodontal Ligament

  1. Principal fibers of the periodontal ligament

The periodontal fibers of the periodontal ligament are primarily composed of bundles of type I collagen fibrils.  They have been classified into several groups on the basis of their anatomic location.  The following constitute the principal fiber groups of the periodontal ligament:
  1. Alveolar crest fibers
  2. Horizontal fibers
  3. Oblique fibers
  4. Periapical fibers
  5. Interradicular fibers
In addition to the collagen fibers, the periodontal ligament also contains oxytalan fibers that are related to the microfibrillar component of elastic fibers (see Fig. 31).  They generally run parallel to the root surface, although they can occasionally insert into cementum.
Fig. 64 : This diagram shows the location of some of the principal fibers of the periodontal ligament. AC: alveolar crest fibers; H: horizontal fibers; OBL: oblique fibers; PA: periapical fibers; IR: Interradicular fibers.
The periodontal ligament is composed of collagen fibers that originate as fibers of different diameters at the mineralized surfaces where the fibers are continuous with their mineralized portion (Sharpey's fibers). These fibers are of a wider diameter on the bone side than the cementum side.  Whether they originate from bone or cementum,   they unravel into smaller fibers, which join up with those of adjacent fibers to produce a meshwork of interconnected fibers oriented between bone and cementum (Fig. 65).   Thus, the periodontal fibers do not stretch cable-like from cementum to bone, but form a meshwork of interconnected fibers.
Fig. 65:   Histological cross-section through a periodontal ligament on the distal surface of a single-rooted tooth. The periodontal ligament fibers (F) originate from relatively thin fibers (*) inserted into the cementum. After they unravel and become intermeshed with adjacent fibers, they form thicker fiber bundles that insert into bundle bone (BB), so named because it contains numerous Sharpey's fibers (SF).  Because of mesial drift, the tooth is slowly displaced mesially (to the left).  This requires continuous remodeling of the ligament and deposition of new bone on the distal alveolar surface in order to maintain the width of the ligament constant.  Neurovascular channels (NV) that house blood vessels, lymphatics and nerves in a loose connective tissue sheath course throughout the periodontal ligament.  The ligament also contains cell rests of Malassez (M).
Fig. 66: Higher magnification of the junction of the periodontal ligament and bone shown in Fig. 65. The Sharpey's fibers are the mineralized continuation of the thick fiber bundles (marked with a *) that originate in the periodontal ligament and help anchor the tooth to the bone. In this section the mineralized bone, that includes the Sharpey's fibers, appears light blue as compared to the purple color of the non-mineralized portions of the fibers.
Fig. 67: Histological cross-section through a periodontal ligament on the mesial surface of a single-rooted tooth.  
Mesial drift of the tooth results in cyclical episodes of alveolar bone (AB) resorption, followed by resting periods and short cycles of bone deposition.   A distinct reversal line (RL) indicates the location where the last cycle of bone resorption was followed by bone deposition.  The latter resulted from the production by osteoblasts (OB) of a thin seam of new bone in which very short Sharpey's fibers (SF) are imbedded.  Note the thin fibers of the periodontal ligament (PDL) inserting into the cementum (C) layer. 
Fig. 68:   Higher magnification of the junction of the periodontal ligament and bone shown in Fig. 67. Note the short Sharpey's fibers (SP) imbedded in the new osteoid seam (OS) secreted by osteoblasts (OB) in the periodontal ligament.  OC, osteocytes; RL, reversal line.