The periodontal ligament

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TITLE: PDL, Alveolar bone, Cementum.

DATE: 19/9/2013

DONE BY: Eman N. Smadi.


The periodontal ligament is the connective tissue that surrounds the root and connects the tooth to the bone. The width of the periodontal ligament is 0.2 to 0.4 mm, the average is 0.25 mm and the narrowest point is around the middle third of the root, the width decreases with age because of continuous cementum deposition.

The PDL components are:

  1. Fibers:

There are collagen fibers type I and elastic fibers. The part of the PDL fibers that present in the alveolar bone and cementum are called Sharpey’s fibers (the PDL doesn’t contain Sharpey’s fibers).

  1. Cells:

Fibroblasts, epithelial cell rests of malassez, osteoblasts, cementoblasts, neural cells, vascular cells and undifferentiated mesenchymal cells.

  1. Ground substance.

The fibers in the PDL are arranged in groups, the major groups are:

  1. Alveolar crest fibers:

Prevent the extrusion of the tooth and resist the lateral tooth movements, if they injured by surgery, they don’t affect tooth mobility.

  1. Horizontal fibers.

  2. Oblique fibers (the most numerous group):

They stand all the masticatory vertical forces and convert them into tension on the alveolar bone.

  1. Apical fibers.

  2. Interradicular fibers.

The PDL developmentally derived from the dental follicle that surrounds the tooth bud, the PDL fibers develop with tooth eruption (eg. When the apex closed, the apical fibers form).

How the fibers of the PDL form? The fibrils exit from the surface of the bone and the surface of the cementum then the fibers that arising from the side of bone start to increase in number and thickness and radiate toward the connective tissue in the PDL, while from the side of cementum they still short and thin then the fibers from the cementum begin to increase in size, number and thickness and they will radiate toward the PDL space to join the fibers arising from the bone and fuse with them.

We said that the PDL contains connective tissue cells:

Fibroblast… they synthesize and degrade collagen fibers and the collagen fibers in the PDL have a high turnover rate because they are under continues functional load.

Epithelial cell rests of malassez… arises after the disintegration of hertwig’s epithelial root sheath (HERS), they might be implicated in cementum proliferation (because originally they involved in the formation of cementum) and they form three-dimensional network around the root.

Now moving to the Ground substance:

We have proteoglycan and glycoprotein.

Functions of the PDL:

  1. Physical function:

Soft tissue casing for protection of nerves and vessels in the PDL, transmission of occlusal forces to the bone, attachment of the teeth to the bone, maintenance of dimensions between teeth and gingiva and shock absorption.

  1. Formative and remodeling function:

In response to physiological stimuli and presence of normal collagen turnover and mesenchymal stem cells. The tissue of PDL resembles the fetal mesenchyme (fetal connective tissue) because it is under continuous stimuli so the turnover rate of it is very high.

  1. Nutrional function:

Blood vessels.

  1. Sensory function:

Nerve endings.

Cementum is a hard, avascular connective tissue covering the root, there are no nerves in it, it doesn’t undergo physiologic resorption nor remodeling, but there is continuous deposition throughout life and that is why the width of PDL decreases with age.

We can classify the cementum either according to the presence of cells or not (Cellular and A cellular), or according to the origin of fibers (Extrinsic and Intrinsic).

Extrinsic a cellular fiber cementum… in the coronal 2/3 of the root.

Cellular mixed stratified cementum… mixed because it has extrinsic and intrinsic fibers, it is found apically in the furcation area, and it is formed after the termination of eruption and in response to the mechanical forces.

Cellular intrinsic fiber cementum… found in the apical part of the root.

A cellular a fibrillar cement… near CEJ.

Thickness of cementum increases from cervical region (25 to 50 microns) to the apical region (up to 250 microns).

Formation of cementum:

From the disentigration of hertwig’s epithelial root sheath. There are two theories for the formation of cementum either:

  1. HERS sends signals to the ectomesenchymal stem cells to produce the cementoblasts.


  1. The ectomesenchymal stem cells undergo differentiation into cementoblast.

Cementoenamel junction (CEJ):

It is the interface between cementum and enamel at the cervical region of the root.

It is 3 types:

  1. Overlapping … 60% of the sections.

  2. Edge to edge (butt joint)… 30% of the sections.

  3. Gap joint… 10% of the sections.

Note … (of the sections) means that in the same tooth, if we take 100 sections we get 60% overlapping, 30% butt joint and 10% gap joint.

Alveolar bone is the socket that surrounds the tooth.

It is composed of two parts:

  1. Alveolar bone proper (bundle bone).

  2. Supporting bone.

In the maxilla, thickness of bone buccally lesser than lingually that is why in surgery when we extract maxillary tooth we move it buccally (to the side of the lowest resistance). The opposite is in the mandible.

Dehiscence and fenestration:

Dehiscence… A dehiscence is loss of alveolar bone on the facial (rarely lingual) aspect of a tooth that leaves a characteristic oval, root-exposed defect from the cementoenamel junction apically. The defect may be one or two millimeters long or extend the full length of the root. The three features of dehiscence include gingival recession, alveolar bone loss and root exposure.

Fenestration… is a "window" of bone loss on the facial or lingual aspect of a tooth that places the exposed root surface directly in contact with gingiva or alveolar mucosa. It can be distinguished from the dehiscence in that the fenestration is bordered by alveolar bone along its coronal aspect.

Periosteum and endosteum:

Periosteum… nearest to the outside, the periosteum contains osteoblasts, osteoclasts, osteocytes, bone lining cells (resting osteoblasts).

Endosteum… nearest to the bone marrow space.

Bone formation:

At the beginning we have a clot and within 2-3 weeks, the formation of osteoid (immature bone) begins then the immature bone starts to connect with the old bone then it undergoes maturation and within 2-4 months we can’t differentiate between old bone and the newly formed bone.

Bone remodeling:

Resting phase >>> resorption phase >>> transition between osteoclast and osteoblast >>> bone deposition >>> resting phase.

When the osteoblast finishing its function, the macrophage comes to clean all the debris and return the PH to the neutral then the bone formation begins.

Blood supply:

Branches from the external carotid artery:

Sublingual artery, mental artery, buccal artery, facial artery, greater palatine artery, infraorbital artery and posterior superior alveolar artery.

In the gingiva, the subepithelial plexus (highly fenestrated) fonund in the connective tissue papillae (in the sulcular epithelium), but not in the junctional epithelium because it doesn’t contain connective tissue papillae.

Blood supply of the PDL is from superior or inferior alveolar artery, dental artery, intraseptal arteries… that is mean the blood supply of the PDL comes from the gingiva itself or from the arteries that go to the pulp or from branches penetrate the bone to reach PDL.

Lymphatic system:

Submental lymph nodes… drainage from lingual and labial gingiva of the mandible in addition to the mandibular incisors.

Deep cervical lymph nodes… palatal gingiva of the of the maxilla.

Submandibular lymph nodes… buccal gingiva of the maxilla, buccal and lingual gingiva of mandibular molars and the periodontium.

Jugulodigastric lymph nodes… all the third molars.

Nerve supply:

Infraorbital nerve >>> buccal side of the maxilla from the premolar to the premolar.

Posterior superior alveolar nerve >>> buccal side of the maxilla for the molars.

Nasopalatine nerve >>> palatal side of the maxilla for anterior teeth.

Greater palatine nerve >>> palatal side of the maxilla for posterior teeth.

Buccal nerve >>> buccal side posteriorly in the mandible.

Mental nerve >>> buccal side anteriorly in the mandible.

Sublingual nerve >>> lingual side of the mandible.

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