Structure Of Periodontal ligament 2
C. Alveolodental ligament
□ Alveolar crest group:
The fiber bundles radiate from the crest of the alveolar process and attach themselves to the cervical part of the cementum. These fibers limits vertical and intrusive movements.
• The horizontal group:
The fiber bundles run horizontally between the cementum and the rim of the alveolar bone in the neck and mid root region. They check horizontal and dipping forces.
• Oblique group:
These are the most numerous fiber bundles. They run obliquely inwards and apically from the alveolar bone to the cementum. They constitute the main support of the tooth against masticatory forces. They check vertical and intrusive forces.
• Apical group :
The fiber bundles of this group are irregularly arranged and radiate from the apical region of the root to the surrounding bone forming a cushion that check vertical forces.
• Interradicular group:
The fiber bundles extend from the crest of the interradicular septum to the furcation of the multi-rooted teeth. They check vertical and lateral forces.
The arrangement of the fiber bundles in these different groups is well adapted to counteract the forces applied to the tooth from different directions.
Controversy exists concerning the extent of individual principal fibers across the width of the periodontal ligament. One view holds that there are distinct tooth related (dental) and bone-related (alveolar) fibers, and that these intercalate near the middle of the ligament at an intermediate plexus.
However, recent evidence suggests that the fibers cross the entire width of the periodontal space but branch on route and join neighbouring fibers to form a complex three-dimensional network.
The intermediate plexus can only be distinguished in continually growing and erupting teeth such as rodent incisors (rats, rabbits)
2- Oxvtalan fibers
Throughout the PDL are fine fibers (of 150 A0 diameter) that appear to be immature elastic fibers or a variant of them. They are found to run obliquely between blood vessels and cementum and mainly perpendicular to the occlusal plane. They are also affiliated with neural elements. They are numerous and dense in the cervical region especially in teeth which carry abnormally high loads (carrying bridges).
Although their function is unknown, yet they may be part of the supportive system for the blood vessels and the principal fibers. It is thought that they regulate vascular flow in relation to tooth function.
3- Eluanin fibers:
They represent another variant of elastic fibers consisting of bundles of microfibrils within small amount of elastin. It may form a meshwork extending from cementum to bone sheathing the collagen fiber bundles.
4- Elastic fibers:
There is no elastic fibers in the human PDL except around the wall of blood vessels. However, the PDL of some animals may contain such fibers.
Between the bundles of the oblique fibers there are round or oval intervals where the periodontal tissue is much less dense. In these areas it forms a reticular framework of loose connective tissue through which the blood vessels, nerves and lymphatics run. These are termed interstitial tissue.
The arterial plexus of the PDL are derived from these sources.
1. Branches from the apical vessels that supply the dental pulp.
2. Branches from the intra-alveolar vessels which penetrate the alveolar bone and enter the ligament.
3.Branches from the gingival vessels.
These branches ramify and form a rich network. In the cervical part and at the root apex the capillaries form a denser network. The capillary vessels may take a coiled course resembling glomeruli.
During mastication, numerous vessels are emptied for a short time from the areas towards which the root moves. The blood is displaced into the surrounding alveolar bone through the inner connecting vessels, thus dissipating any pressure on the cellular elements. The blood can return when the force is removed.
There are numerous arterio-venous anastomosis. The veins tend to run axially and to drain to the apex.
A network of lymphatics, following the path of the blood vessels provides lymph drainage for the PDL. It starts in the gum and runs towards the apex where they join those emerging from the pulp.
The nerves usually are associated with blood vessels. Nerves run from the apical region towards the gingival margin and joined by nerves entering laterally through the foramina of the socket wall. These lateral nerves divide into two branches, one extending apically and the other gingivally.
The apical region of the PDL contains more nerve endings (except in the upper incisors where there are much endings both apically and cervically).
Non-myelinated nerve endings belonging to the autonomic nervous system run on the blood vessels and supply the smooth muscle in their walls and affect regional blood flow.
Myelinated sensory nerves run in small bundles along side the blood vessels. The nerves are either of:
a) Large diameter which lose their myelin sheath and end in a variety of endings e.g. Khob-like, spindle -like and Meissner-like which appear to be responsible for touch and pressure and especially found at root apex (Mechano-receptors).
b) Small diameter which lose their myelin sheath and end as free nerve endings which are concerned with detection of pain. These are located at regular intervals along the length of the root and extend to the cementoblast layer.
1- supportive: it maintains the relation of the tooth to the
surrounding hard and soft tissues.
2- Nutritive: through its blood vessels , the PDL provides nutritive
substances to the cells of the ligament, cementum and the more superficial bone cells. The blood vessels are also concerned with removal of catabolites.
3- Sensory: It provides a most efficient proprioceptive mechanism
through its’ nerve supply, allowing the organism to detect the
application of the most delicate forces to the teeth. This is important in protecting the substance of the tooth and its supporting structure.
4- Protective: it protects the tissues at the site of pressure. The
ligament transforms the pressure exerted on the teeth into
tension or traction on cementum and bone. This function is
performed by three mechanisms:
a) The arrangement of the capillaries (coiled).
b) The arrangement of the principal fibers.
c) The mechano-receptors (properioceptive stimuli).
5- Homeostatic: It is evident that the cells of the periodontal
ligament have the capacity to resorb and synthesize the
extracellular substance of the connective tissue of the ligament,
alveolar bone, and cementum.
Remodeling of the alveolar bone occurs at a higher rate than other bone tissue in the jaws. The PDL collagen shows fast turnover, and it appears that its connective tissue cells are also turned over. There is continual slow death of cells which are replaced by new cells that are provided by the progenitor cells in the PDL.
Cementum deposition, however seems to be a slow continuous process and resorption is not a regular occurrence.
It is evident that these processes are not activated haphazardly, however, the mechanisms controlling the processes of synthesis and resorption are unknown.
If these hemeostatic mechanisms are upset, derangement of the periodentium occurs, which may lead to its progressive destruction and loss of attachment between bone and the tooth occurs.
These are calcified masses seen in older individuals in the PDL. It is possible that degenerated epithelial cells form a nidus for their calcification.
Cementicles may be:
-Remain free in the PDL.
-Join the cementum and form excementosis (attached). -Become embedded into the cementum after cementum increases in thickness (hyper cementosis) by advancing age.