ALVEOLAR BONE

Contents

Introduction

Bone Histology

Cells and Intercellular Matrix

Bone Development

Remodelling

Age Changes

Clinical Considerations

Conclusion

References

Introduction

Bone- used to designate both an organ and a tissue

Specialized mineralized connective tissue

Functions:

mineralised supporting tissue

act as a reservoir for ions

(especially calcium).

provide a framework for bone marrow

gives attachment to muscles

its "plasticity', allows it to remodel according to the functional demand placed upon it.

CLASSIFICATION (Ten Cate )

DEVELOPMENTALLY, Endochondral bone Intramembranous bone

HISTOLOGICALLY, according to its density, mature bone can be divided into; Compact (cortical) bone Cancellous (spongy) bone

MICROSCOPICALLY:

Lamellar bone

Fibrous bone

LAMELLAR BONE:

Most of the bones, whether compact or cancellous, are composed of thin

plates of bony tissues called lamellae.

These are arranged in piles in a cancellous bone, but in concentric

cylinders (Haversian system or secondary osteon) in a compact bone.

FIBROUS BONE (WOVEN BONE):

It is found in young fetal bone. Collagen fibers - more variable diameter Irregular orientation giving it matted appearance

DEVELOPMENT OF ALVEOLAR PROCESS

Alveolar process is dependent on the presence of teeth for its

development and maintenance.

At the late bell stage, bony septa and bony bridge start to form,

and separate the individual tooth germs from another, keeping

individual tooth germs in clearly outlined bony compartment.

(BERKOVITZ)

Initially, this bone forms a

thin egg shell of support,

termed as the ‘tooth crypt’,

around each tooth germ.

FIG. 9-5 A developing root shown by a divergent apex around the dental papilla (arrow), which is enclosed by an opaque bony crypt.

Relationship between

a deciduous tooth & its

accompanying

succedaneous tooth

detailing the formation

of the alveolar bone

- Scoh, Symonds 1974

12/85

AT BIRTH AT 7MONTHS

AT 2½ YRS 7 YRS

Bone

67 %

Inorganic

Hydroxyapatite

33% (organic)

28% 5 %

Collagen type Ӏ Non coll.proteins

(ca10{po4}6{oh}2)

Osteocalcin, Osteonectin, Bone morphogenic proteins, Phosphoproteins and Proteoglycans

Ground substance- Glycosaminoglycans, proteoglycans and water

Osteopontin, Bone Sialoprotein- cell adhesion proteins (Mackie et al, 2003)

Osteocalcin (bone GLA protein)

Found in bone matrix

Expressed only by fully differentiated cells

Specifically localized to developing bone

Produced by osteoblasts and odontoblasts

Role in bone formation

Osteopontin

Glycosylated phosphoprotein

Role in bone formation and resorption

Synthetized by osteoblasts, osteoclasts, osteocytes, smooth muscles and epithelial cells

Role in cell adhesion

Significant amounts at mineralizing front

Bone sialoprotein

Structural protein of bone

Restricted to mineralized tissues

Secreted by osteoblasts

Osteonectin

Glycoprotein bound to HA

Calcium binding glycoprotein

Synthesized by fibroblasts and role in wound healing

Inorganic material- calcium, phosphate ,hydroxyl, carbonate, citrate

Trace amounts of sodium, magnesium and fluorine (Glimcher 1990)

Hydroxyapetite crystals of ultramicroscopic size

Enzymes like alkaline phosphatase, ATP and pyrophosphatase

Parallel to collagen fibres and contribute to lamellar appearance of bone

Alveolar bone

Portion of maxilla and mandible that forms and supports the tooth sockets (alveoli)

Forms when tooth erupts to provide osseous attachment to PDL

Disappears gradually after tooth loss

‘Tooth dependent bony structure’ (Schroeder et al, 1991)

Transverse section

Longitudinal section

Morphology determined by size, shape, function and location of teeth

Formed during fetal growth by intramembraneous ossification

Cancellous Bone

Compact Bone

Shelf like bone

Holds the tooth firmly in position during mastication

Aids in movement

Adapts to occlusal loads

Helps to move the teeth for better occlusion.

Functions of alveolar bone

Supplies vessels to the PDL.

Houses & protects developing permanent teeth while supporting primary teeth.

Organizes successive eruptions of primary & secondary teeth.

Bone Histology

Three parts

1) External plate of cortical bone

2) Inner socket wall

3) Cancellous trabeculae (between two compact layers)- function of support

Cortical bone

1) Circumferential lamellae (encloses entire adult bone and forms the outer perimeter

2) Concentric lamellae (make up bulk of compact bone and forms the basic metabolic unit of bone, the osteon)

3) Interstitial lamellae (inter-spread between adjacent concentric lamellae and fill the spaces between them..actually fragments of pre-existing concentric lamellae and can be of many shapes)

Osteon –cylinder of bone parallel to long axis of bone (structural and metabolic units)

Haversian canal –in centre of osteon, lined by single layer of bone cells

Each canal has a capillary

Haversian canals interconnected by Volkmann canals

System for dense bones like cortical plates and alveolar bone proper, where surface vessels are unable to supply blood

Socket wall

Dense , lamellated bone – alveolar bone proper (contains sharpeys fibers and circumferential lamellae)

Cribriform plate (anatomic term)

Lamina dura (radiographic term)

Bundle bone (histologic term)

Bundle bone

Bone adjacent to PDL that contain sharpeys fibers

Contains higher calcium than other areas

Many features in common with cementum layer on root surface

Collagen fibers larger in diameter, less numerous , less mature

Localized within alveolar bone proper

Sharpeys fibers completely calcified or partially calcified with uncalcified core

Not unique to jaw -occurs wherever ligaments and muscles are attached

Thickness of 100-200 microns

High turnover rate

FIBER ARRANGEMENT IN ABP

DOUBLE FIBRILLAR ORIENTATION:

Extrinsic fibers- Sharpey’s fibers

run perpendicular to bone surface

produced by PDL fibroblast

At their insertion in bone, they become mineralized, with their periphery

being hypermineralized than cores.

Intrinsic fibers

Laid down by osteoblasts between Sharpey’s fibers

Irregularly arranged & less dense.

Cancellous bone

Presence of trabeculae enclosing irregular marrow spaces lined with a layer of thin, flattened endosteal cells

Variation in trabeculae pattern depending upon occlusal forces and genetically

Matrix consists of irregularly arranged lamellae separated by incremental and resorption lines

Found in inter-radicular and inter-dental spaces

Maxilla>mandible

Trabeculae alligned in path of tensile and compressive stresses to provide maximal resistance to occlusal forces with minimum bone substance (Glickman et al 1970)

in thickness and number with force

Spongy bone (anatomic term)

Trabecular bone (radiographic term)

Cancellous bone (histologic term)

CANCELLOUS BONE:

Type 1: The interdental and interradicular trabeculae are regular

and horizontal in a ladder like arrangement.

Type 2: Shows irregularly arranged numerous delicate

interdental and interradicular trabeculae

CORTICAL BONE SPONGY BONE

About 85% of bone About 15% of bone

Lesser turnover than spongy Higher turnover

Remodel about 3% of its mass each year

remodel about 25% of its mass each year

Mechanical/protective role More metabolic function

Interdental septum

Consists of cancellous bone bordered by alveolar bone proper of approximating teeth and facial and lingual cortical plates

Narrow septa- only cribriform plate

Irregular window

Study by Heins et al 1986

Area Cribriform plate+cancellous bone

Only cribriform plate

Irregular window

Maxillary molars

66.6% 20.8% 12.5%

Mandibular premolar and molar

85% 15% 0%

Mesiodistal angulation of IDS is parallel to line drawn between CEJ of approximating teeth (Ritchey et al, 1953)

Shape and size of IDS depends on

1) Size and convexity of crowns of approximating teeth

2) Position of teeth

3) Degree of eruption

Crest of IDS located 1-2 mm apical to CEJ of adjacent teeth

Diagram of relation between CE junction of adjacent teeth shape of crest of alveolar septa

Bone marrow

• Embryo and newborn,

• Ribs, sternum, vertebrae, skull, humerus

• Hemopoiesis

Red hematopoieti

c marrow

• Adult

• Red marrow foci found sometimes in maxillary tuberosity, symphysis and angle of ramus

• Storage of energyYellow fatty

marrow

Cells

CELL TYPES IN BONE

Determined osteogenic precursor cells

Inducible osteogenic precursor cells

Muscles.

Friedenstein (1973) divided osteoprogenitor cells into:

Osteoblast

Produce organic matrix of bone

Differentiated from pluripotent follicle cells

No decrease with age

Uninuclear cells

Secrets collagen as well as non collagenous proteins

Present on outer bone surface

Have high levels of alkaline phosphatase (this feature distinguishes it from fibroblasts)

Alkaline phosphatase believed to cleave organically bound phosphate and help in bone growth

Active-plump, cuboidal

Inactive-flattened

Secrete type Ӏ and V collagen, variety of cytokines and several members of BMP such as BMP-2, BMP-7, TGF-ß, IGF-1, IGF-2

BMP family helps in bone formation and repair

Under physiologic condition which support resorption- release of IL-6 and hydrolytic enzymes

Osteocyte

Enclosed within spaces called lacunae within calcified matrix

Entrapped Osteoblasts

Reduction in size and loss of matrix synthesizing ability after being entrapped

Excess space-lacunae

Extend processes into canaliculi that radiate from lacunae

Anastomosing system

Bring O2 and nutrients to osteocytes through blood and remove metabolic waste products

More rapid the bone formation-more osteoblasts get entrapped – more osteocytes (eg- bone formed during repair)

Osteolytic osteolysis- osteocytes capable of resorption

Three functional states of osteocytes

Quiescent osteocytes:

paucity of rER, diminished golgi apparatus

An osmiophilic lamina representing mature calcified matrix is seen in close apposition to cell

membrane.

Formative osteocyte:

abundant rER & golgi apparatus

evidence of osteoid in pericellular space within the lacuna.

Resorptive osteocyte:

Numerous ER & well developed golgi apparatus.

The pericellular space is devoid of collagen fibrils & may contain a flocculent material suggestive

of breakdown product.

‘Osteocytic osteolysis’.

Osteoclast

Originate from hematopoietic tissue

Fusion of mononuclear cells (blood derived monocytes) to form a multinucleated cell

Very large, 5-50 nuclei

Active on less than 1% of bone surface

Mobile and capable of migrating

Lie in Howships lacunae

Acidophilic cytoplasm

Active osteoclasts- ruffled border facing bone (hydrolytic enzymes are secreted)

Increases surface area

Clear zone devoid of organelles but rich in actin filament, vinculin, talin (site of adhesion of osteoclast to bone)

Sealing zone

Ruffled border-enzymes like tartarate resistant acid phosphatase, carbonic anhydrase, proton pump ATP’s

Cathepsin containing cytoplasmic vesicles near ruffled border

OSTEOCLASTIC FUNCTION IN BONE RESORPTION

1. Attachment of the osteoclast to mineralized bone surface

2. Creation of sealed acidic environment through action of proton pump which demineralizes bone & exposes the organic matrix

3. Degradation of the exposed organic matrix to its constituent amino acids by the action of released enzymes like acid phosphatase & cathepsin

4. Sequestering of the mineral ions & amino acids within the osteoclasts.

Tencate 1994- Described sequence of events of resorptive process:

Bone Lining Cells

- When bone is no longer forming…..surface

osteoblasts become inactive ….. Lining cells.

- Thin flat nucleus, few cytoplasmic organelles

- Retain gap junctions with osteocytes….functions

to control mineral homeostasis & endure bone

vitality.

Periosteum and endosteum Both are layers of differentiated osteogenic connective tissue

Periosteum covers outer surface of bone and endosteum lines the internal bone cavities

Bundles of collagen fibres from outer layer penetrate bone and bind periosteum to bone

Endosteum composed of a single layer of osteoblasts with some connective tissue

Periosteum

• Rich in blood vessels, nerves• Contains collagen fibres and

fibroblasts• Fibrous periosteum

Outer layer(fibrou

s)

• Composed of osteoblasts and osteoprogenitor cells

• Cellular periosteumInner layer (osteogenic

)

Functions of periosteum

Medium through which muscles, tendons and ligaments are attached to bone

Nutritive function to the bone

Osteoprogenitor cells – Important role during development and repair after fracture

Fibrous layer- acts as limiting membrane (exostoses in cases of periosteal tear)

Bone Development

1) Endochondral bone formation

2) Intramembranous bone formation

3) Sutural bone formation

Endochondral bone formation Cartilage replaced by bone

Shape of cartilage resembles miniature version of bone to be formed

At end of long bones, vertebrae, ribs, head of mandible and base of skull

Condensation of mesenchymal cells

Perichondrium at the periphery

Rapid growth of cartilage

Cartilage replaced by bone gradually by osteoblasts at periphery

Intramembranous bone formation

Occurs directly within mesenchyme

Bone develops directly within the soft connective tissue

Vascularity increases and osteoblasts differentiate and lay down bone

Occurs at multiple sites (primary ossification center)

Ossification centers grow radially

Cranial vault, maxilla, body of mandible and mid shafts of long bones

Proceeds at extremely rapid rate

Woven bone formed first in form of radiating spikules which ultimately fuse to form plates

Transition of woven bone to lamellar bone

Mesenchymal condensation followed by increase in vascularity

Some mesenchymal cells lay down collagen fibre bundles forming a membrane

Some differentiate into osteoblasts and lay down osteoid Which then gets calcified

Mineralization always lags behind the production of bone matrix

Sutural bone growth

Bone forms along suture margins

Found in skull

Fibrous joints between bones

Allow only limited movement

Helps skull and face to accommodate growing organs like eyes and brain

Vascular supply

Derived from inferior and superior alveolar arteries of maxilla and mandible

Lymphatic drainage

Submandibular lymph nodes

Nerve supply

Branches from anterior, middle and posterior superior alveolar nerves for maxilla and branches from inferior alveolar nerve for mandible

Osseous topography

Bone contour follows root prominence

Intervening vertical depressions that taper towards margin

Height of facial/lingual plates affected by 1) Allignment of teeth 2) Angulation of root to bone 3) Occlusal force

Osseus topography:

Normally: prominence of the roots with the intervening vertical depressions that taper toward the margin.

On the labial version: the margins of the labial bone is thinned to a knife edge & presents an accentuated arc in the direction of the apex.

On the lingual version: the margins of the labial bone is blunt & rounded & horizontal rather than arcuate.

Buttressing bone- adaptive mechanism against occlusal force (thickened cervical portion of alveolar plate)

Fenestration and Dehiscence

Fenestration- Isolated areas in which root is denuded of bone and root surface covered only by periosteum and overlying gingiva

Dehiscence- Denuded area extends through marginal bone

Facial > lingual

Anteriors > posteriors

Frequently bilateral

20% of all teeth affected

Caused due to malposition, root prominence, labial protrusion and a thin cortical plate

Can complicate procedure and outcome of periodontal surgery

Remodelling

Remodeling of alveolar bone Least stable of periodontal tissues Structure in a constant state of flux

• Functional requirements• Age related changes in bone

cells

Local influence

s• Hormones (PTH, vit D,

calcitonin)Systemic influence

s

Remodeling is the major pathway of bone changes in shape, resistance to forces, repair of wounds, and calcium and phosphate homeostasis in the body.

REMODELING

Regulation of bone remodelling is a complex process involving hormones and local factors acting in a autocrine and paracrine manner on the generation and activity of differentiated bone cells – Sodek et al 2000

Bone-99% of body calcium ions

Major source of calcium release when blood Ca

Monitored by parathyroid gland

Bone coupling

Decrease in blood Ca

Detected by receptors on chief cells of parathyroid gland

Release of PTH

Stimulate osteoblasts to release IL-1 and IL-6

Stimulates monocytes to migrate to area

Monocytes coalesces to form multinucleated osteoclasts in presence of LIF-

Leukemia inhibiting factor released by osteoblasts

Bone resorption

Release of Ca ions from hydroxyapetite crystals

Normal blood calcium levels

PTH secretion stopped by feedback mechanism

Organic matrix resorbed with hydroxyapetite

Collagen breakdown

Release of organic substrate which are covalently bound to collagen

Stimulates differentiation of osteoblasts

Bone deposition

‘COUPLING’ refers to interdependency of osteoclasts and osteoblasts in remodelling

Bone multicellular unit (BMU)

Reversal line

MEDIATORS OF BONE RESORPTION

STIMULI FACTORS REGULATING OSTEOCLAST FORMATION & FUNCTION

POTENTIAL THERAPEUTIC STRATEGIES TO TREAT BONE RESORPTION

Age changes

Similar to those occurring in remainder of skeletal system

Osteoporosis with ageing

Decreased vascularity

Reduction in metabolic rate and healing capacity(implants, extraction sockets, bone grafts)

Bone resorption may be increased or decreased

More irregular periodontal surface

Thinning of cortical plates

Rarification of bone

Reduction in no of trabeculae

Lacunar resorption more prominent

Susceptibility to fracture

Thickening of collagen fibers

Decrease in water content

Clinical and implant considerations

- Gingival margins …follows the contour of alveolar process.

Abnormalities such as ledges, exostosis & tori…reflect on

gingiva.

- Areas of fenestrations & dehiscence - partial thickness flap.

Clinical Considerations

- Process of bone remodeling - in orthodontic treatment.

- Knowledge of the various factors regulating bone formation

has resulted in their use for regeneration of bone.

Buccal-lingual/palatal ridge resorption during first 3 months after extraction about 30%... Reaching 50% at the end of 1 year (Schropp et al , 2003)

Resorption more pronounced at buccal than lingual/palatal aspect of ridge leading to shift of center of ridge towards lingual/palatal side

Socket preservation

Classification (Lekhom and Zarb- 1985)

4 bone qualities for the anterior regions of the jaw bone:

Quality1, Quality 2, Quality 3, Quality 4

Misch Bone Density Classification

D1-dense cortical

D2-porus cortical and coarse trabecular

D3-porus cortical and fine trabecular

D4-fine trabecular

Regional Acceleratory Phenomena

Local response to a noxious stimulus.

A process by which tissue forms faster than the normal regional

regeneration process.

- Frost et al, 1983

By enhancing the various healing stages, this phenomena makes the

healing process occur 2 – 10 times faster than normal physiologic

healing.

RAP begins within a few days of injury, typically peaks at 1 - 2

months, usually lasts 4 months in bone, & may take 6 - >24 months

to subside.

Duration & intensity of RAP α type & amount of stimulus & the

site where it was produced.

Noxious stimuli of sufficient magnitude: can evoke RAP.

Fractures

Mechanical abuses

Noninfectious inflammatory injuries: dental implant procedures

Bone grafting surgeries

Internal fixation procedures

Mucoperiosteal surgery

Injury to bone: Pathologic process

Arthrofibrosis

Neuropathic soft tissue problems

Rheumatoid phenomena

Secondary osteoporosis

Excessice heat

RAP is delayed / not initiated.

Formation of biologically delayed union / nonunion.

RAP does not result in a change in bone volume.

Restricted to bone remodelling.

More evident in cortical bone.

Usually accompanied by a systemic response: Systemic

Acceleratory Phenomena

Biochemical agents also appear to facilitate the RAP.

PG E1

Bisphosphonate

Inadequate RAP is associated with:

DM

Peripheral neuropathies

Regional sensory denervation

Severe radiation damage

Severe malnutrition

Conclusion

Thus a sound knowledge of bone anatomy, histology and physiology, will help the clinician in diagnosing and treatment planning, and lead to a favorable outcome of surgical procedures performed

References

Carranza’s Clinical Periodontology- 10th edition

Clinical Periodontology and Implant Dentistry- Jan Lindhe- 5th edition

Contemporary Implant Dentistry- Carl Misch- 3rd edition

Orban’s Oral Histology and Embryology- 11th edition

Structure of Periodontal Tissues in Health and Disease- Periodontology 2000, vol 40, 2006, 11-28

Thank you