Microsoft PowerPoint - 15 Periodontal_Ligament().ppt [] Jerry Lin DDS, DMSc Diplomate, American Board of Periodontology



1.Illustrated Dental Embryology, Histology, and Anatomy: Mary Bath-Balogh, Margaret J. Fehrenbach, 2nd Edition, Elsevier Saunders,2006
2 E t ti l f O l Hi t l d E b l A li i l2.Esstential of Oral Histology and Embryology--A clinical approach: James K Avery, Denial J Chiego,Jr 3rd Edition, Elsevier Mosby, 2006
3.Ten Cate's Oral Histology: Development, Structure, and Function: Nanci Anatonio, 6th ed. Mosby, 2003
Summary
Periodontium
The attachment apparatus of the tooth and consists of cementum, PDL, bone lining the
alveolus, and part of the gingiva
Periodontal Ligament • The PDL is composed of a
complex vascular and highly cellular connective tissue that surrounds the tooth root and connects it to the inner wall of the alveolar bone
• The average width of the PDL is about 0.2 mm, ranging from 0.15 to 0.38 mm. The width decrease with age
• The thinnest portion is around the middle third of the root
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Connective tissue cells Epithelial rest cells Immune system cells Cells associated with neurovascular elements Undifferentiated mesenchymal cells
Connective Tissue Cells • Fibroblasts
– Most common cells in PDL – Appear as ovoid or elongated cells along
the principal fibers – Pseudopodia-like process – Producing collagen which has high
turnover rate • Cementoblasts
– Most often seen in resting phase – Cementum does not remodel,
cementoclasts (odontoclasts) are not normally found in the ligament
• Osteoblasts • Osteoclasts • Odontoclasts
Epithelial rest cells • The epithelial rests of
Malassez form a latticework in the PDL and appear as either isolated clusters of cells or interlacing standsg
• Considered remnants of Hertwig’s root sheath
• Contain keratinocyte growth factors
• Participate in the formation of periapical cysts and lateral root cysts
Defense Cells • Neutrophils • Lymphocytes • MacrophagesMacrophages • Mast cells • Eosinophils
Undifferentiated mesenchymal cells (Progenitor cells)
• Although it has been demonstrated that they are a source of new cells for the PDL, it is not known whether a single progenitor cell gives rise to daughter cells that differentiate into fibroblasts, osteoblasts, and cementoblasts, or whether there are separate progenitors for each cell line
Periodontal Fibers • The most important
elements of PDL are the principal fibers, collagenous and g arranged in bundles
• The terminal portions of the principal fibers that are inserted into cementum and bone are termed Sharpey’s fibers
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Collagen • The term collagen derives from the greek work
for “glue”, it yields gelatin on boiling • The main constituent of skin, bone, cartilage, , , g ,
tendon and is also present in specialized structure such as basement membranes, vitreous, cornea, aorta and other tissue
• Triple-chain helix
Collagen • A protein composed of different amino acids, the most
important of which are glycine, proline, hydroxylysine and hydroxyproline
• At least 19 recognized collagen species encoded by t l t 25 t di d 12at least 25 separate genes, dispersed among 12
chromosome • The principal fibers are composed mainly of collagen
type I, whereas reticular fibers are composed of collagen type III. Collagen type IV is found in the basal lamina
Collagen Biosynthesis • Occurs inside the fibroblasts to
form tropocollagen molecules • Tropocollagen molecules
aggregate into microfibril thataggregate into microfibril that are packed together to form fibrils
• Collagen is synthesized by fibroblasts, chondroblasts, osteoblasts, odontoblasts, and other cells
Collagen Biosynthesis • Intracellular steps
resid esresidues – Helix formation & secretion
• Extracellular steps – Processing – Fibril formation – Cross linking & fibril stabilization
Principal Fibers • Transseptal group • Alveolar crest group
– Prevent the extrusion of the tooth
• Horizontal group Obli• Oblique group – Bear the brunt of vertical
masticatory stress & transform them into tension on the alveolar bone
• Apical group • Interradicular group
– In the furcation area
Oxytalan & Eluanin • Although the PDL does not contain mature elastin,
two immature forms are found: oxytalan and eluanin • Oxytalan fibers run parallel to the root surface in a
ti l di ti d b d t tt h t th tvertical direction and bend to attach to the cementum • Regulate vascular flow • Oxytalan fibers have been shown to develop de novo
in the regenerated PDL
Other fibers • Small collagen fibers associated with the larger
principal collagen finers have been described • Run in all directions • Indifferent fiber plexus
Extracellular Matrix
Ground substance
& cells • Main components:
– Glycoprotein • Fibronectin • Laminin
– Calcified epi rests – Calcified Sharpey’s fiber – Calcified, thrombosed vessels
Functions of PDL Physical
Formative Remodeling Nutritional sensory
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Physical Functions • Provision of a soft tissue “casing” to protect the
vessels and nerves from injury by mechanical forces • Transmission of occlusal forces to the bone • Attachment of the teeth to the bone• Attachment of the teeth to the bone • Maintenance of the gingival tissues in their proper
relationship to the teeth • Resistance to the impact of occlusal forces (shock
absorption)
Shock absorption • Tensional theory
– When a force is applied to the crown, the principal fibers first unfold and straighten and then transmit forces to the alveolar bone, cause an elastic deformation of the bony socket.
– Finally when the alveolar bone has reached its limit, the load is transmitted to the basal bone
Shock absorption • Viscoelastic system theory
– The displacement of the tooth is largely controlled by fluid movements, with fibers having only a second role
– When forces are transmitted to the tooth, the extracellular fluid passes from the PDL into the marrow spaces of bone throughpasses from the PDL into the marrow spaces of bone through foramina in the cribriform plate
– After depletion of tissue fluids, the fiber bundles absorb the slack and tighten, which lead to a blood vessel stenosis. Arterial back pressure causes ballooning of the vessel and passage of the blood ultrafitrates into the tissue, thereby replenishing the tissue fluid
Transmission of Occlusal Forces to the Bone • The arrangement of the principal fibers is
similar to a suspension bridge or a hammock.
• When an axial force is applied to a tooth, a tendency toward displacement of the root into the alveolar occurs
• The oblique fibers alter their wavy, untensed pattern; assume their full length; and sustain the major part of the axial force
• The apical portion of the root moves in a direction opposite to the coronal portion
• In areas of tension, the principal fiber bundles are taut rather than wavy. In areas of pressure, the fibers are compressed.
Formative & Remodeling Function • Cells of the PDL participate in the formation and
resorption of cementum and bone, which occur in physiologic tooth movement; in the accommodation of the periodontium to occlusal forces; and in the repair of injuries
• The PDL is constantly undergoing remodeling. Fibroblasts form the collagen fibers, and the residual mesenchymal cells develop into osteoblasts, cememtoblasts, and fibroblasts affect the rate of formation of collagen, cementum and bone
• The rate of collagen synthesis is twice as fast as that in the gingiva and four times as fast as that in the skin
Nutritional & Sensory Function • The PDL supplies nutrients to the cementum, bone, and
gingiva by way of the blood vessels and also provides lymphatic drainage I l ti t th li t d t d PDL i hi hl• In relation to other ligaments and tendons, PDL is highly vascularized
• This high blood vessel content may provide hydrodynamic damping to applied forces, as well as high perfusion rates to the PDL
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Nutritional & Sensory Function • The PDL is abundantly supplied with sensory nerve
fibers capable of transmitting tactile, pressure, and pain sensations by the trigeminal pathways
• The bundles divide into single myelinated fibers, hi h lti t l l th i li h th d d iwhich ultimately lose their myelin sheaths and end in
one of four types of neural termination: – Free ending – Ruffini-like mechanoreceptors, primarily in the apical area – Coiled meissner’s corpuscles, mainly in the midroot region – Spindlelike pressure and vibration endings, mainly in the
apex
Free ending
Ruffini-like mechanoreceptors
Spindlelike endings
Cementum
The calcified, avascular mesenchymal tissue that forms the outer covering of
the anatomic root
Cementum • Acellular (primary)
cementum • Cellular (secondary)
• Both consist of a calcified interfibrillar matrix and collagen fibrilscementum fibrils
Cementum • The two main sources of collagen fibers in cementum are
– Sharpey’s fiber (extrinsic): embedded portion of the principal fibers of the PDL and formed by fibroblasts Fibers that belong to the cementum matrix (intrinsic) and are– Fibers that belong to the cementum matrix (intrinsic) and are produced by the cememtoblasts
• Cementoblasts also form the noncollagenous components of the interfibrillar ground substance, such as proteoglycans, glycoproteins, and phosphoproteins
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Cementum • The major proportion of the organic matrix of a
cementum is composed of type I (90%) and type III (about 5%) collagens.
• Sharpey’s fibers are composed of mainly type I collagen where the type III collagen appears to coat the type I collagen of Sharpey’s fibers
Acellular Cementum • Acellular cememtum is the first cementum formed,
covers approximately the cervical third or half of the root • Formed before the tooth reaches the occlusal plane. Its
thickness ranges from 30 to 230 μmthickness ranges from 30 to 230 μm • Sharpey’s fibers make up most of the structure of
acellular cementum. Most fibers are inserted at approximately right angle into the root surface. Their size, number and distribution increase with function
Acellular Cementum • Sharpey’s fibers are
completely calcified, with the mineral crystals oriented parallel to theoriented parallel to the fibrils
• Acellular cementum also contains calcified intrinsic collagen fibrils
Cellular Cementum • Formed after the tooth
reaches the occlusal plane • Cellular cementum is less
l ifi d th th ll lcalcified than the acellular type
Cellular Cementum • More irregular and
contains cells (cementocytes) in individual spaces p (lacunae) that communicate with each other through a system of anastomosing canaliculi
Cementum • Both acellular cementum and cellular cementum are arranged in
lamellae separated by incremental lines parallel to the long axis of the root
• These lines represent “rest periods” in cementum formation and i li d th th dj t tare more mineralized than the adjacent cementum
• The inorganic content of cementum (hydroxyapatite) is 45% to 50%, which is less than that of bone (65%), enamel (97%), or dentin (70%)
• Acellular cementum and cellular cementum are very permeable. The permeability of cementum diminishes with age
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Schroeder’s Classification • Acellular afibrillar cementum (AAC)
– Contains neither cells nor extrinsic or intrinsic collagen fibers, except for mineralized ground substance. Coronal cementum. (1-15 μm)
• Acellular extrinsic fiber cementum (AEFC)Acellular extrinsic fiber cementum (AEFC) – Composed almost entirely of densely packed bundles of
Sharpey’s fibers. Cervical third of roots. (30-230 μm) • Cellular mixed stratified cementum (CMSC)
– Composed of extrinsic & intrinsic fibers and may contain cells. Co-product of fibroblasts and cementoblasts. Apical third of the roots, apices and furcation areas. (100-1000 μm)
Schroeder’s Classification • Cellular intrinsic fiber cementum (CIFC)
– Contains cells but no extrinsic collagen fibers. Formed by cementoblasts. It fills resorption lacunae
I t di t t• Intermediate cementum – Poorly defined zone near the cementodentinal
junction of certain teeth – Contain cellular remnants of Hertwig’s sheath
embeded in calcified ground substance
Cementoenamel Junction • Three types of
relationships: – In about 60-65%,
– In about 30%, edge-to- edge butt joint exists
– In 5-10%, the cementum and enamel fail to meet
Cementoenamel Junction • Studies with the SEM
indicate that the cementoenamel junction exhibits all of the aboveexhibits all of the above forms and shows considerable variation when trace circumferentially
Cementoenamel Junction