Alveolar Bone and Periodontium

• The periodontium consists of cementum, alveolar bone, periodontal ligaments (PDL), and gingiva.
• The alveolar bone is the part of the maxilla and mandible that supports and protects teeth.
• Functions of the alveolar bone include protection, attachment, support, and shock absorption.
• Bone's ability to continuously remodel and adapt to changes is a key property.

Functions of Alveolar bone

• Protection of sockets for teeth.
• Attachment of principal fibers (Sharpey's fibers).
• Support of tooth roots on facial, palatal, and lingual sides.
• Shock absorption by disseminating force.

Remodeling

• Remodeling includes bone resorption and new bone deposition.
• Cementum is less readily resorbed compared to bone.

Classifications of Bone

• Bone type Flat: Skull, pelvis, scapula
• Bone type Long: Appendicular skeleton
• Compact bone is mature bone.
• Spongy/cancellous/trabecular bone is early embryonic bone.
• Intramembranous bones are direct transformation of mesenchyme.
• Endochondral bones come from a cartilage model.
• Embryonic/woven bone have irregular collagen network.
• Lamellar bone has collagen arranged in concentric layers.
• Circumferential lamellae is found on periosteal/endosteal surfaces.
• Osteonic lamellae are concentric forming osteons.
• Interstitial lamellae are residual fragments between osteons.

Bone Structure and Organization

• Woven bone: Immature with random collagen.
• Lamellar bone: Successive layers (lamellae).
• Interdental septa are bone between two teeth.
• Inter-radicular septa are between different roots

Bone Tissue Composition

• Inorganic component: Calcium hydroxyapatite forms 60% of bone weight.
• Mineral is carbonated hydroxyapatite in needle-like crystallites or thin plates.
• Organic component: Collagen (mainly type I) forms 90% of the extracellular matrix.
• Serum proteins, acidic glycoproteins, and small proteoglycans
• Non-collagenous proteins make up a heterogeneous group.
• Endogenous proteins are secreted by bone cells; exogenous come from blood.
• Proteoglycans chondroitin and heparin regulate collagen fibril diameters.
• Glycoproteins such as osteonectin, osteopontin, and fibronectin are present.
• Osteonectin binds calcium and is synthesized by osteoblasts/odontoblasts.
• Exogenous proteins (cytokines, growth factors) are locked in bone.
• Bone morphogenetic proteins (BMPs) are present and induce bone formation.

Bone Cell Types

• Osteoblasts: Secrete the organic extracellular matrix of new bone, Osteoid which rapidly undergoes mineralization to form bone
• Osteocytes: Trapped in lacunae
• Osteoclasts: Multinuclear cells involved in bone remodeling
• Bone lining cells: Flat cells line bone during inactivity

Osteoblasts

• Bone forming cells lining the alveolar bone.
• Prominent during active bone formation.
• Cuboidal with basophilic cytoplasm and protein synthesizing organelles.
• Inactive osteoblasts are flat.
• Desmosomes and tight junctions.

Osteocytes

• Trapped osteoblasts in lacunae.
• Play a role in bone remodeling throughout life.
• 42 billion cells in an adult human body.
• Live average half life of 25 years.
• Canaliculi enable communication.
• Send signals upon mechanical stress to remodel bone.
• Assist in mineral metabolism of calcium and phosphorus.
  • Produce proteins/hormones affecting mineral absorption, storage, and release.

Osteoclasts

• Involved in the resorption of bone.
• Originate from blood cells of the macrophage type.
• Leave resorption concavities called Howship's lacunae in bone.
• Vary in size from mononuclear to giant multinuclear.
• Have a brush border adjacent to bone, representing microvilli.
• Contain many vesicles with acid phosphatase.

RANK Signaling System

• Bone homeostasis goes from one side toward the other.
• RANK ligand is released from osteoblasts.
• The released RANK ligand attaches to RANK receptors on osteoclast progenitors.
• This activates osteoclast differentiation.

Parathyroid Hormone

• Stimulates osteoclast formation.
• Binding to its receptor on osteoblasts triggers the production of receptor activator of NFkappaB ligand (RANKL) and inhibiting the expression of osteoprotegerin (OPG)

Estrogen

• Inhibits resorption by direct action on osteoclasts.
• Increases OPG expression and inhibits RANKL signaling.
• Reduced estrogen during menopause leads to decreased bone density.

Histology of Bone

• Osteoid is (5 to 10) micrometers thick, unmineralized.
• Mineralization is a linear front style, where mineral content deposition happens.
• Compact bone is made of parallel bone columns along long bone axis.
• Columns are concentric bony layers (lamellae) and Haversian canals.
• Haversian systems (osteon) are Haversian canals with lamella.
• Volkmann's canals interact with the bundles and pierce columns.
• Osteocytes in lacunae interact with the central canal

Compact Bone

• Cylindrical and weight bearing.
• Able to withstand high levels of mechanical stress.
• The innermost aspect, the lamellae merge with trabeculae of - cancellous (spongy bone)

Spongy Bone

• Spongy bone gives bone its flexibility due to the presence of marrow spaces
• About 50 µm thick
• Aligned along lines of stress while giving bone mass
• Marrow space provide nutrients and supplement the bone
• Bone marrow contain hematopoietic stem cells and mesenchymal stem cells

Bone Resorption and Remodeling

• Howship's lacunae are longer; resorption lacunae
• Resting bone: projections marking the sites of extrinsic mineralized Sharpey's fibers
• bone deposition exceeds resorption during childhood but resorption exceeds deposition as you age.

Bone Remodeling

• Bone remodeling is constant to adapt to pressure
• Spongy bone remodels 25% of its mass every year.
• Compact bone remodels 3% of its mass per year.

Sharpey's Fibers

• Extrinsic fibers enter perpendicular to the surface, less numerous but thicker than those in cementum.
• Unmineralized fibers remain in centers, and mineralized projections project beyond bone surface.

Clinical Considerations

• Periodontitis can cause loss of alveolar bone and tooth loss.
• Orthodontic loading, healing extraction sockets, bone atrophy.
• Healing of extraction sockets include epithelium mobilization.
• Osteoporosis means that bone resorption exceeds bone deposition, increase bone mass density
• Osteopetrosis is affected from different hormones that decrease the bone density in abnormal pattern
• Titanium screws are used for dental implants and bond the bone by osseointegration.
• Implants are attached directly, unlike the cushion of periodontal ligament in natural teeth.
• Implants rely on spongy bone for support, and have a different gum attachment.
• Socket preservation maintains the bone and area for future treatments.
• Dental implants provide a natural looking solution to missing teeth to improve both appearance and function.