Periodontal ligament

Composition

a. Consists mostly of collagenous (alveolodental) fibers.
Note: the portions of the fibers embedded in cementum and the alveolar bone proper are known as Sharpey’s fibers.

b. Oxytalan fibers (a type of elastic fiber) are also present. Although their function is unknown, they may play a role in the regulation of vascular flow.

c. Contains mostly type I collagen, although smaller amounts of type III and XII collagen are also present.

d. Has a rich vascular and nerve supply.

Both sensory and autonomic nerves are present.

(1) The sensory nerves in the PDL differ from pulpal nerves in that PDL nerve endings can detect both proprioception (via mechanoreceptors) and pain (via nociceptors).

(2) The autonomic nerve fibers are associated with the regulation of periodontal vascular flow.

(3) Nerve fibers may be myelinated (sensory) or unmyelinated (sensory or autonomic).

Cells

a. Cells present in the PDL include fibroblasts; epithelial cells; cementoblasts and cementoclasts; osteoblasts and osteoclasts; and immune cells such as macrophages, mast cells, or eosinophils.

b. These cells play a role in forming or destroying cementum, alveolar bone, or PDL.

c. Epithelial cells often appear in clusters, known as rests of Malassez.

Types of alveolodental fibers

a. Alveolar crest fibers—radiate downward from cementum, just below the cementoenamel junction (CEJ), to the crest of alveolar bone.

b. Horizontal fibers—radiate perpendicular to the tooth surface from cementum to alveolar bone, just below the alveolar crest.

c. Oblique fibers

(1) Radiate downward from the alveolar bone to cementum.

(2) The most numerous type of PDL fiber.

(3) Resist occlusal forces that occur along the long axis of the tooth.

d. Apical fibers

(1) Radiate from the cementum at the apex of the tooth into the alveolar bone.

(2) Resist forces that pull the tooth in an occlusal direction (i.e., forces that try to pull the tooth from its socket).

e. Interradicular fibers

(1) Only found in the furcal area of multi-rooted teeth.

(2) Resist forces that pull the tooth in an occlusal direction.

Gingival fibers

a. The fibers of the gingival ligament are not strictly part of the PDL, but they play a role in the maintainence of the periodontium.

b. Gingival fibers are packed in groups and are found in the lamina propria of gingiva

c. Gingival fiber groups:

(1) Transseptal (interdental) fibers

(a) Extend from the cementum of one tooth (just apical to the junctional epithelium), over the alveolar crest, to the corresponding area of the cementum of the adjacent tooth.

(b) Collectively, these fibers form the interdental ligament , which functions to resist rotational forces and retain adjacent teeth in interproximal contact.

(c) These fibers have been implicated as a major cause of postretention relapse of teeth that have undergone orthodontic treatment.

(2) Circular (circumferential) fibers

(a) Extend around tooth near the CEJ.

(b) Function in binding free gingiva to the tooth and resisting rotational forces.

(3) Alveologingival fibers—extend from the alveolar crest to lamina propria of free and attached gingiva.

(4) Dentogingival fibers—extend from cervical cementum to the lamina propria of free and attached gingiva.

(5) Dentoperiosteal fibers—extend from cervical cementum, over the alveolar crest, to the periosteum of the alveolar bone.

The pre-dentition period.

-This is from birth to six months.

-At this stage, there are no teeth. Clinically, the infant is edentulous

-Both jaws undergo rapid growth; the growth is in three planes of space: downward, forward, and laterally (to the side). Forward growth for the mandible is greater.

-The maxillary and mandibular alveolar processes are not well developed at birth.

-occasionally, there is a neonatal tooth present at birth. It is a supernumerary and is often lost soon after birth.

-At birth, bulges in the developing alveoli precede eruption of the deciduous teeth. At birth, the molar pads can touch.

ARTICULAR SURFACES COVERED BY FIBROUS TISSUE
TMJ is an exception form other synovial joints. Two other joints, the acromio- and sternoclavicular joints are similar to the TMJ. Mandible & clavicle derive from intramembranous ossificiation.

Histologic

1. Fibrous layer: collagen type I, avascular (self-contained and replicating)
2. Proliferating zone that formes condylar cartilage
3. Condylar cartilage is fibrocartilage that does not play role in articulation nor has formal function
4. Capsule: dense collagenous tissue (includes the articular eminence)
5. Synovial membrane: lines capsule (does not cover disk except posterior region); contains folds (increase in pathologic conditions) and villi
   Two layers: a cellular intima (synovial cells in fiber-free matrix) and a vascular subintima
   Synovial cells: A (macrophage-like) syntesize hyaluronate
   B (fibroblast-like) add protein in the fluid
   Synovial fluid: plasma with mucin and proteins, cells
   Liquid environment: lubrication, ?nutrition
6. Disk: separates the cavity into two comprartments, type I collagen
   anterior and posterior portions
   anetiorly it divides into two lamellae one towards the capsule, the other towards the condyle
   vascular in the preiphery, avascular in the center
7. Ligaments: nonelastic collagenous structures. One ligament worth mentioning is the lateral or temporomandibular ligament. Also there are the spheno- and stylomandibular with debatable functional role.

Innervations
 

Cementum

Composition

a. Inorganic (50%)—calcium hydroxyapatite crystals.

b. Organic (50%)—water, proteins, and type I collagen.

c. Note: Compared to the other dental tissues, the composition of cementum is most similar to bone; however, unlike bone, cementum is avascular (i.e., no Haversian systems or other vessels are present).

Main function of cementum is to attach PDL fibers to the root surface.

Cementum is generally thickest at the root apex and in interradicular areas of multirooted 

Types of cementum

a. Acellular (primary) cementum

(1) A thin layer of cementum that surrounds the root, adjacent to the dentin.

(2) May be covered by a layer of cellular cementum, which most often occurs in the middle and apical root.

(3) It does not contain any cells.

b. Cellular (secondary) cementum

(1) A thicker, less-mineralized layer of cementum that is most prevalent along the apical root and in interradicular (furcal) areas of multirooted teeth.

(2) Contains cementocytes.

(3) Lacunae and canaliculi:

(a) Cementocytes (cementoblasts that become trapped in the extracellular matrix during cementogenesis) are observed in their entrapped spaces, known as lacunae.

(b) The processes of cementocytes extend through narrow channels called canaliculi.

(4) Microscopically, the best way to differentiate between acellular and cellular cementum is the presence of lacunae in cellular cementum.

MAXILLARY SECOND MOLAR

The second molars are often called 12-year molars because they erupt when a child is about 12 years

Facial: The crown is shorter occluso-cervically and narrower mesiodistally whe compared to the first molar. The distobuccal cusp is visibly smaller than the mesiobuccal cusp. The two buccal roots are more nearly parallel. The roots are more parallel; the apex of the mesial root is on line with the with the buccal developmental groove. Mesial and distal roots tend to be about the same length.

Lingual: The distolingual cusp is smaller than the mesiolingual cusp. The Carabelli trait is absent.

Proximal: The crown is shorter than the first molar and the palatal root has less diverence. The roots tend to remain within the crown profile.

Occlusal: The distolingual cusp is smaller on the second than on the first molar. When it is much reduced in size, the crown outline is described as 'heart-shaped.' The Carabelli trait is usually absent. The order of cusp size, largest to smallest, is the same as the first but is more exaggerated: mesiolingual, mesiobuccal, distobuccal, and distolingual.

Contact Points; Height of Curvature: Both mesial and distal contacts tend to be centered buccolingually below the marginal ridges. Since themolars become shorter, moving from first to this molar, the contacts tend to appear more toward the center of the proximal surfaces.

Roots: There are three roots, two buccal and one lingual. The roots are less divergent than the first with their apices usually falling within the crown profile. The buccal roots tend to incline to the distal.

Note: The distolingual cusp is the most variable feature of this tooth. When it is large, the occlusal is somewhat rhomboidal; when reduced in size the crown is described as triangual or 'heart-shaped.' At times, the root may be fused.