 So Peridontal Principle Fibers, okay so these fibres are collagenous in nature and it follows a wavy pattern. So you can see a wavy pattern when viewed in a longitudinal section okay this is a cross section this is the longitudinal section so you can see a wavy pattern. So they are thought to contribute the regulation of mineralization and tissue corrosion at sites of increased biomechanical strain. So when there is a strain so in orthodontic movement when there is strain and stress there will be remodelling and changes in the Peridontal ligaments or the fibres. So we have basically five groups principle fibres they are alveolar crest group horizontal group oblique group apical group and inter radical group it's a very important sub topic in Peridontal ligament it might be asked as a short note or a long essay. So let's see the five principle fibres alveolar crest horizontal group oblique group apical group and inter radical group. So I have drawn all these the green color is apical the black color is oblique the pink color is horizontal the violet is inter radicaler the orange is alveolar crest. So let's see one by one alveolar crest group is extends obliquely from cementum this is a cementum just beneath the junction epithelium to the alveolar crest and to the fibres layer of periosteum covering the alveolar bone. So this is the alveolar bone and it is a crest so it is starting from the cementum that is obliquely you can see this is not horizontal this is not perpendicular this is at an angle okay so this is oblique this is horizontal this is oblique so it extends from cementum just beneath the junction epithelium and to the alveolar crest so this is alveolar crest and to the fibres layer of periosteum covering the alveolar bone. So the horizontal fibres they extends from cementum to alveolar bone in horizontal direction at right angle to the long axis of tooth so this is a long axis of tooth and horizontal is at right angle which is starting from cementum to the alveolar bone and it occupy 10 to 15 percentage of coronal root surface. Now we have oblique group oblique group it is the largest group of periodontal ligament fibres and it is around 80 to 85 percentage of root surface it extends from cementum in a coronal direction obliquely to bone so it is extends from cementum going at oblique direction towards the bone okay so this is the alveolar crest group which is running oblique direction this is the oblique group it's the largest periodontal ligament fibres and it is running from cementum to alveolar bone it is around 80 to 85 percentage of total root surface. Now we have apical group so this is apical group the apical fibres radiate in a rather irregular fashion from cementum from cementum to the apical region of socket and they do not occur in incompletely formed roots so these may occur in incompletely formed roots but apical will not occur until and unless the root formation is complete so this going in irregular fashion from the cementum to the alveolar to the socket okay so this is apical group and the last one is interradicular group so this is the interradicular group this violet one they fan out from cementum to the tooth in the forkation areas of multi rotor tooth okay so this is like multi rotor tooth they fan out from cementum to the forkation areas. Now let's see what are the functions of these fibres the alveolar crest are involved in retaining the tooth in the socket which opposes lateral forces forces and it prevents extrusion and intrusion extrusion is moving away from the socket and intrusion is moving towards the socket so it prevents extrusion and intrusion of tooth protect deeper periodontal ligament structures the horizontal group is restraining the lateral tooth movement whereas the oblique group which bear the vertical master category stresses and transform them into tension on alveolar one and resist intrusive forces so intrusive forces is towards apex so it resist vertical master category forces and the next one is apical group so the basic function of apical group fibres are it prevents tooth tipping and it resist lexation it protect blade lymph and nose of place to the tooth since it is at the apical position and the interradicular group which helps in resisting tipping talking and lexation moments of the tooth so these are the principal fibres of periodontal ligament it's a very important topic so alveolar crest horizontal oblique apical and interradicular you need to draw this picture very neatly and explain the origin and its end and the basic functions now if we have various other fibres that is sharpies fibres we already seen these sharpies fibres they are collagen bundles of periodontal ligament embedded into cementum and alveolar bone the orientation is similar to that of adjacent periodontal ligament bundles and they are more numerous but smaller at their attachment into cementum than alveolar bone these fibres in cellular cementum will be fully mineralized but in cellular cementum it is partially mineralized and few sharpies fibres pass uninterruptedly through bone of alveolar process which is known as trans alveolar fibres so trans alveolar fibres are nothing but sharpies fibres which pass uninterruptedly through bone of alveolar process and the next one is intermediate plexus so intermediate plexus in the beginning it was believe that principal fibres follow a wavy course from cementum to bone and are joined in the mid region of the periodontal space giving rise to a zone of distinct appearance which is known as intermediate plexus so just starting from cementum to bone in a wavy course at the middle region a peculiar appearance periodontal space giving a zone of distinct appearance which is known as intermediate plexus in recent time it does the concept has little bit changed so the recent concept is that fibres cross the entire width of periodontal space but branches are branches and join neighboring fibres to form a complex three-dimensional network so forming a three-dimensional network is the reason for that peculiar appearance of that mid region okay next is the elastic fibres elastic fibres there are three types of elastic fibres which are histochemically and ultra structurally different they are mature elastic fibres euline fibres and oxytaline fibres euline fibres and oxytaline fibres have been described as immature elastic fibres so mature elastic fibres they consist of microfibrillar component which is surrounding an amorphous core of elastin protein whereas oxytaline fibres they are microfibres which run in apico coronal direction to bend and attach to cervical third of fruit and this the function of oxytaline fibres basically unknown but it has a role in supporting blood vessels of periodontal ligament and there is also tooth supporting function and euline fibres euline fibres are bundles of microfibres embedded in small amount of amorphous elastin their basic functions regulate vascular flow and role in tooth support and facilitate fibroblast attachment and migration the reticular fibres they are immature collagen fibres with staining properties and are related to basement membrane of blood vessels and epithelial cells which is adjacent to periodontal ligament whereas the secondary fibres they represent the newly formed collagenous element but still it is not incorporated into the principal fibres so located between these principal fibres they are basically non-directional and randomly oriented they appear to transverse the periodontal ligament space in coronal apical and are often associated with the vasculature and nervous elements but so that is all about principal fibres so we have seen the principal fibres and the other fibres so next we have the functions of periodontal ligament so principal fibres are very important question it might be asked as short assay or long assay and even a short knot either any group of principal fibres so I will come up with functions of periodontal ligament thank you