Periodontal Ligament (PDL)

• The periodontal ligament (PDL) is a connective tissue that surrounds the root of the tooth, connecting it to the bone
• The PDL is continuous with the gingival connective tissue and communicates with bone marrow spaces through vascular channels
• At the root apex, the PDL merges with the dental pulp

Functions of the Periodontal Ligament

• Sensory
• Physical
• Nutritive
• Formative & (remodelling)resorptive

PDL Width

• The average width of PDL is 0.2mm
• PDL width range spans from 0.15mm to 0.38mm
• Near the crest of the alveolus, the width of PDL is 0.35mm when heavily loaded, 0.14mm when normally loaded, and 0.10mm when functionless
• In the middle of the alveolus, the width of PDL measures 0.28mm when heavily loaded, 0.10mm when normally loaded, and 0.06mm when functionless
• Approaching the fundus of the alveolus, the PDL width is 0.30mm in heavy loading, 0.12mm under normal loading, and 0.06mm when functionless

PDL on Radiographs

• It appears as periodontal space on radiographs
• It is a radiolucent area between the lamina dura of alveolar bone proper and radiopaque cementum
• It is narrower in permanent teeth compared to deciduous teeth

Development of PDL

• Originates from the dental follicle shortly after root development begins
• As crown approaches the oral mucosa, fibroblasts in the follicle become active
• Fibres initially lack organization, becoming oblique to the tooth
• First fibres are apical to the cementoenamel junction (CEJ), giving rise to gingivodental fibres
• With eruption, more fibres develop, attaching to new cementum and bone
• During eruption, cemental Sharpey’s fibres appear first, followed by Sharpey’s fibres in bone
• Sharpey's fibres in bone are fewer
• Alveolar fibres extend into the middle zone, joining the lengthening cemental fibres, gaining thickness and strength during occlusal function

Structural Elements of PDL

• Cells
• Fibres
• Ground substance
• Extracellular substance
• Epithelial rests of malassez
• Defence
• Progenitor
• Resorptive
• Synthetic

Blood Supply

• The principal blood supply comes from the superior and inferior alveolar arteries
• Apical vessels
• Penetrating vessels from the alveolar bone
• Anastomosing vessels from the gingiva
• Vessels anastomose freely within the ligament, occupying interstitial spaces

Nerve supply

• Sensory nerves
• Autonomic nerves
• Branches of the second & third division of the 5th cranial nerve (trigeminal nerve)

Fibres of Periodontal Ligament

• All fibres are made from collagen
• PDL is wider near the apex and cervical margin of the tooth
• Most fibres are principal fibres, organized into groups, resembling spliced ropes
• Fibres distribute forces of mastication & speech
• Principal fibres are embedded into cementum & bone, known as Sharpey's fibres

Principal Fibre Groups

• Main principal fibres consists of 5 groups
• Alveolar crest group
• Horizontal group
• Oblique group
• Apical group
• Interradicular group
• The overall function of the five groups is to resist rotational forces

Alveolar Crest Group Fibres

• Located just beneath the junctional epithelium
• Originates from cementum, below the CEJ
• Runs downwards and outwards, inserting into the alveolar crest
• Prevents tooth extrusion from the socket and resists lateral, tilting, and intrusive tooth displacement

Horizontal Group Fibres

• Located limited to the coronal one-fourth of PDL
• Originates from cementum, apical to the alveolar crest group
• Runs at right angles to the long axis of the tooth, inserting into the bone apical to the alveolar crest
• Resists horizontal and tipping forces

Oblique Group Fibres

• Located in the two thirds of length of the PDL
• Originates from cementum
• Runs obliquely in the coronal direction and inserts into the alveolar bone
• Largest group of fibres
• It resists vertical masticatory forces, preventing tooth intrusion

Apical Group Fibres

• Located at the root tip
• Originates from cementum, around the apex of the tooth
• Fans out in an irregular fashion and inserts into the apex of the socket
• Not found in incompletely formed roots
• They resist vertical, luxative & twisting forces, protecting delicate nerves and vessels at the apex

Inter-radicular Group Fibres

• Located roots of multi rooted teeth
• Originates from cementum
• Insertion located to interradicular septum
• Resists vertical & lateral movement
• Assist with Tipping, torquing & luxation
• Periodontal disease results in total loss of fibres

Interdental Ligament (Transseptal Fibres)

• Important fibres which run through gingival mucosa to connect neighboring teeth
• Inserts interdentally into cervical cementum of neighboring teeth over alveolar crest
• Fibres travel from cementum to cementum without any bony attachment
• Resists rotational forces & keeps teeth in alignment

Gingival Fibre Group

• A name given to separated but adjacent fibre groups found within the lamina propria of marginal gingiva

Ground Substance

• Collagen fibres
• Water 70%
• Tissue fluid pressure high, 10mm Hg above atmospheric pressure
• Functions include the exchange of metabolites, ion & water binding, fibre orientation, and tooth support & binding mechanisms

Age Changes

• Number of cells decrease
• Increase in fibrous tissue
• Decrease in vascularity and mitotic activity
• Narrowing of PDL width
• Scalloping is seen on calcified tissues with PDL attached to the peaks of scallops