23. Alveolar Bone Flashcards
What forms the tooth sockets?
- alveolar process of mandible
- maxilla
Explain bone composition
- mineralised, living connective tissue
- organic matrix permeated by hydroxyapatite deposited between type I collagen fibrils
- non collagenous proteins like bone sialoprotein, osteocalcin, osteonectin, osteopontin that bind to calcium of HA to control mineralisation
- proteoglycans, cytokines, growth factors, serum proteins
- varies on site and developmental stages
Functions of bone
- support
- protection
- locomotion
- mineral resevoir
Bone physiology is controlled by …
Such as …
- hormones
- resorption ones like PTH, glucocorticoids
- formation ones like calcitonin, vitamin D, oestrogen, leptin
Bone secretes … , … and …
- growth factors (BMP, TBG-beta, PDGF, IGF)
- cytokines (interleukins, TNF, RANKL)
- neuroendocrine
Bone responds to … forces
Explain
- mechanical
- like tooth movement
- resists compressive forces best and tensile forces least (fractures)
Explain woven bone
- bone formed during development
- characterised by randomly orientated collagen fibrils
- becomes replaced by lamellar bone
How does woven bone have a role in healing?
- in bone fracture repair
- it forms rapidly as part of wound healing response but eventually replaced by lamellar bone
Parts of adult bone
- compact bone (dense outer area)
- trabecular bone (cancellous, spongy) - cavity filled with bone marrow interrupted by a network of bone plates (trabeculae)
Both bone types (compact/trabecular) consists of different …
bone layers/lamellae
3 types of bone lamellae
- circumferential
- concentric/Haversian
- interstitial
Where is circumferential lamellae?
- encloses entire outer and inner perimeter of bone
Where is concentric lamellae?
forms basic unit of bone/osteon and makes up bulk of compact bone
Where is interstitial lamellae?
interspersed between adjacent osteons (remnants of remodelled osteons)
Define ‘osteon’
cylinder of bone - generally orientated parallel to long axis of bone
Osteons are also called …
Haversian systems
Structure of osteons/Haversian systems
- central canal (Haversian canal) including blood capillary lined by a layer of osteoblasts (bone-forming cells)
- adjacent Haversian canals are interconnected by Volkmann’s canals
Explain periosteum
- external surface
- connective tissue membrane consists of 2 layers
- outer fibrous layer - dense collagen fibres
- inner cellular layer - oesteoblasts and their precursors, highly vascularised
Explain endoosteum
- internal surface
- not well demarcated
- loose connective tissue including osteoblasts
- separates bone surface from marrow
- less active in bone formation than periosteum
Osteoblasts shape when active/inactive?
- when active, cuboidal
- when inactive, flat - bone lining cells
Osteoblasts are derived from …
mesenchymal stem cells
Role of osteoblasts
- synthesize organic bone matrix (osteoid - mainly collagen type I)
- produce alkaline phosphatase which cleaves inorganic phosphate to initiate and promote mineralisation
- produce growth factors (IGF1, TBF-beta, PDGF) that increase bone repair, considered for dental therapy, periodontal disease, dental implants
What are osteocytes?
- osteoblasts that become trapped in bone matrix (un/mineralised)
- become smaller in size and produce a space in matrix/bone (lacunae)
What do osteocytes do?
- form a network of cellular processes that connect adjacent osteocytes
- forms canaliculi in future bone
- function as sensors of changes in bone environment
- signalling centres to maintain bone integrity (e.g induce bone remodelling)
Osteoclasts are … cells derived from …
- large, multinucleated
- haematopoietic stem cells
What do osteoclasts do?
- produce Howship’s lacunae/resorption bays during bone resorption
- produce acid phosphatase and lysosomal enzymes
What’s the resorption sequence in osteoclasts?
- attachment to osteoclast to bone
- creation of acidic microenvironment for demineralisation
- degradation of exposed matrix by enzymes
- endocytosis of degradation products
Development of the alveolar process
- mandible takes the shape of a trough underneath the inferior alveolar bone
- the alveolar process begins to grow towards the tooth germ
- alveolar process almost surrounds incisor tooth germ. Inferior alveolar nerve (incisive branch) now enclosed in bony canal
- to accomodate the growing tooth germ (stellate reticulum), alveolar bone must be resorbed on the inner wall of alveolus (osteoclast) and new bone must be deposited on the outer wall (osteoblasts)
Structure of the alveolar bone
- outer compact layer - cortical plate
- central trabecular layer - spongiosa
- inner compact layer - alveolar plate/cribiform plate
- alveolar crest
What feature does the alveolar plate of the alveolar bone have that the others don’t?
- perforations for nerves and blood vessels
- connecting to PDL
How far is alveolar crest from CEJ?
around 1.5-2 mm
Why does alveolar bone have the structure it does?
- a compromise between maximum stability and minimum weight
- e.g birds have hollow bones adapted for flying
Structural variation in the cortical plate
- surface layer of lamellar bone supported by osteons/Haversian systems
- thinner in maxilla than mandible
- thickest on buccal aspect of mandibular premolars/molars
Structural variations in alveolar bone
- trabecular/cancellous bone
- bone marrow spaces rich in adipose tissue - yellow bone marrow
- absent in anterior teeth - cortical and alveolar plate are fused
Structural variations for alveolar plate in alveolar bone
- lamellar bone and bundle bone
- they contain Sharpey’s fibres
What is bundle bone?
- innermost layer of alveolar plate - directly lines socket
- named due to collagen fibre bundles of PDL embedded - Sharpey’s fibres
- provides attachment for PDL fibres
- bundle bone apposed to lamellar bone
Define ‘interdental and interradicular septum’
- alveolar bone between two teeth
- alveolar bone between two roots
4 stages of bone remodelling
- resorption
- reversal
- formation
- resting
What happens in resorption?
- recruitment, migration and activation of osteoclasts
- bone resorption
What happens in reversal?
- cessation of resorption
- disappearance of osteoclasts (apoptosis or migration)
What happens in formation?
- recruitment, migration and differentiation of osteoblasts
- bone formation
What happens in resting?
- cessation of bone formation
- surface covered by flat bone-lining cells
What structural lines appear in bone?
- resting lines (parallel), these are pauses in bone deposition
- reversal lines (scalloped) - these are the change from bone resorption to deposition
Explain tooth drift and orthodontic treatment
- resorption on the right side of the alveolar plate creates space that tooth can move into
- to compensate for bone loss, new bone is formed onto cortical plate on opposite side
- since tooth moves right, space in socket on left must be filled by bone deposition onto alveolar plate
- excess bone must be resorbed from cortical plate on opposite side
- alveolar bone remodelling proceeds through same stages but occurs at same site, so no bone displacement
Explain mesial drift
- unworn teeth have very few interproximal contact points
- attrition causes hard tissue loss on occlusal and interproximal surfaces
- increase in interproximal distance is compensated by mesial /forward drift of teeth resulting in broader interproximal contact points
- for example during 6-18 (abrasive diet) the 6s drift 4mm mesially - creates space for 8s to erupt and softer diet in modern pops, higher incidence of impacted 3rd molars
Explain infraocclusion
- dental trauma or infection can cause tooth ankylosis - fusion of tooth root to alveolar bone
- most common in primary molars
- prevents exfoliation - leads to impaction of successor tooth
- further growth of alveolar bone results in ‘submergence’ of ankylosed tooth - clinically called infraocclusion
- extraction to prevent malocclusion or periodontal problems
Explain lamina dura
- on dental radiographs, alveolar plate is called lamina dura
- increased radiopacity due to thick cortical bone
- interrupted lamina dura in apical region indicates periapical abscess
Explain alveolar bone resorption
- after tooth extraction or chronic periodontitis
- alveolar process can resorb
- placement of dental implants is difficult
- implanting soon after tooth loss decreases rate of alveolar ridge resorption
- bone loss impacts on construction of removable prostheses
Explain maxillary sinus perforation
- close proximity of premolar/molar roots and alveolar bone to maxillary sinus floor
- during tooth extraction, bone can fracture (oro-antral connection/fistula - infection)
Explain alveolar osteitis
- after tooth extraction, socket fills with blood and forms clot
- important step in wound healing
- detachment of blood clot - dry socket
- leads to painful bone inflammation - bad odour
- most common post-extraction problem (mainly affects mandibular teeth like 1-2%)
