Lecture 44 Biology of Fracture Healing Flashcards
What are the four phases of skeletal development?
- Migration (of preskeletal cells to sites of future skeltogenesis)
- Epithelial- mesenchymal interaction
- condensation (of mesenchymal cells)
- Differentiation (into odontoblasts and chondrocytes)
What are the two types of bone formation?
- Endochondral bone formation
- Intramembranous Bone Formation
What is endochondral bone formation?
- Indirect (mesenchyme forms cartilage template first which is later replaced by bone)
- occurs in most bones in the skeleton esp. bones that bear weight and have joints
- also occurs during fracture repair
What is intramembranous bone formation?
- direct transformation of mesenchymal cells to osteoblasts (no cartilage intermediate)
- restricted to cranial vault, some facial bones, parts of the mandible and clavicle
- contributes to fracture repair
When do secondary ossification centers appear?
- around the time of birth
What produces VEGF and what does it do?
- hypertrophic chondrocytes
- attracts blood vessels that invade the cartilage model
When does growth plate fusion occur?
- around age 14-20 in humans
How do the flat bones of the skull form?
- intramembranous bone formation
What is the process of intramembranous bone formation?
- mesenchymal cells condense to produce osteoblasts which deposit osteoid (unmineralized) bone matrix
- osteoid matrix calcifies/osteoblasts become arrangled along calcified region of the matrix
- some osteoblasts trapped in bone matrix- become osteocytes
- NO CARTILAGE MODEL PRECEDING THE BONE
What is the immature bone that is produced first?
- woven bone aka primary bone (immature)
Woven bone is produced when osteoblasts need to form bone rapidly in situations like:
- embryonic development
- fracture healing
- disease states (paget’s disease)
What is immature woven bone remodeled and replaced with?
- Lamellar bone (aka secondary bone) (mature)
What are the features of woven bone?
- disorganized structure
- collagen fibrils in random orientation (lower birefringence w/polarized light)
- increased cell density
- reduced mineral content
What are the features of lamellar bone?
- highly organized
- bone lamellae concentrically arranged around central canal containing blood vessels and nerves
- collagen fibrils in parallel orientation (more birefringence w/polarized light)
- mechanically stronger
What can lamellar bone be further classified into?
- Compact
(cortical/haversian) - cancellous (spongy/trabecular)
What type of bone is the bone marrow located in?
- cancellous bone
What is skeletal healing essential for?
- resolution of orthopedic trauma that has caused fractures
- healing of corrective surgeries where bony injuries are created intentionally to correct bone deformities
- bone regeneration in oral surgical procedures/tooth extractions
Fracture healing requires coordinated activity of several cell types:
- inflammatory cells
- chondroprogenitors/chondrocytes
- osteoprogenitors
- osteoclasts
- vascular cells
What is the timeline of the inflammatory phase?
- peaks at 48 hrs and is diminished by 1 week
What is the timeline of the reparative phase?
- activated within a few days and persists for up to 2-3 months
What is the timeline of the remodeling phase?
- can continue for several years
John hunter (1935) described the 4 stages of fracture repair as what?
- Formation of vascular hematoma
- Formation of (fibrocartilage) callus
- Tissue metaplasia- callus replaced by mineralized bone
- Bone remodeling and turnover
What are the cytokines that the hematoma releases in the hematoma formation/inflammation phase?
- Tumor necrosis factor a (TNF-a)
- Interleukins (IL-1, -6, -11 and -18)
What do the cytokines do?
- lead to recruitment/infiltration of inflammatory cells
What do the inflammatory cells do?
- release more inflammatory cytokines and recruit mesenchymal stem cells (MSC)/osteogenic precursors to fracture site
What happens during formation of fibrocartilagenous callus?
- MSC/connective tissue stem cells/blood vessels invade hematoma
- hematoma degenerates/phagocytes clear debris
- fibrous connective tissue matrix laid down by fibroblasts (granulation tissue)
- some MSC differentiate towards chondrogenic/osteogenic lineages
- at borken ends of bones where blood supply was disrupted hypoxia/tissue necrosis occurs
- in hypoxic regions MSC differentiate into chondrocytes
- intramembranous bone may form in subperiosteal sites where vascular supply is intact= hard (external) callus
What initiates endochondral bone formation?
- MSC differentiating into chondrocytes
What are the cell sources of osteogenic precursors?
- periosteum
- muscle
- bone marrow
What are the cell types of osteogenic precursors?
- mesenchymal stem cell (MSC)
- pericyte
- muscle satellite cell
How does the bony callus form?
- intramembranous bone (formed where vascular supply is intact) contributes to bony callus
- ** cartilage under goes endochondral ossification
What is endochrondral ossification?
- hypertrophy –>calcification of cartilage –> removal by osteoclasts –> replacement with bone
When is the fracture considered healed?
- when bone stability is restored by bone tissue completely bridging the original fracture
“clinical union”
What is the timeline for formation of bony callus?
- several weeks up to 2-3 months
how long does formation of fibrocartilagenous callus take?
- 1 week
How quickly does the hematoma form at the fracture site?
- 0-2 days
T/F the remodeling phase of bone repair can last years
- T
What is the process of remodeling?
- osteoclasts resorb woven bone in fracture callous then osteoblasts lay down new lamellar bone (haversian) which is mechanically stronger
T/F same sequence of fracture healing events occurs for healing of alveolar bone in tooth socket after tooth extraction
- T
Early phase of fracture healing include?
- Formation of hematoma
- recruitment of MSC
- cell proliferation
- initiation of chondrogenesis/osteogenesis
- vascular ingrowth/angiogenesis
Signaling molecules important in the fracture healing?
- pro- inflammatory cytokines
- TGFB superfamily members
- angiogenic factors
Who secretes the pro-inflammatory cytokines?
- macrophages, mesenchymal cells, inflammatory cells
Mice null for TNFa receptor show?
- impaired fracture healing
What are the pro-inflammatory cytokines?
- Tumor necrosis factor (TNFa)
- Interluekins
What is the function of the pro-inflammatory cytokines?
- recruit other inflammatory cells/promote MSC recruitment
- induce apoptosis of hypertrophic chondrocytes
- recruit fibrogenic cells/promote formation of granulation tissue/ECM formation
- can promote osteoclast formation
What is the TGFB superfamily members job in fracture healing?
- promote ECM synthesis and assembly/initiation of callus formation
- promote osteogenic differentiation
- GDF-8 has a role in cell proliferation
What are the TGFB superfamily members involved in fracture healing?
- Transforming growth factor B (TGFB)
- Bone morphogenetic protein-2 BMP2 (also 5,6)
- Growth and differentiation factor (GDF-8)
What produces the TGFB superfamily members?
- produced by hematoma (platelets)/granulation tissue/differentiating MSC/periosteal callus
What are the angiogenic factors?
- VEGF- Vascular endothelial growth factor
- PDGF - platelet derived growth factor
- ANGPT- Angiopoietin
What do the angiogenic factors do?
- promote vascular ingrowth vessels in periostum (brings oxygen/osteogenic precursors [pericytes])
VEGF:
- promotes chemotaxis of osteoprogenitors
- up regulated in regions of hypoxia (under control of transcription factor HIF1a)
- HIF1a overexpressing mice show enhanced bone regeneration
Why is vascularization important? (why are angiogenic factors important)
- critical for fracture repair/bone formation
- brings in calcium and phosphate for mineralization
What dictates why type of healing will occur?
- fracture stability (mechanical environment)
If strain <2% _______ bone healing will occur
- intramembranous
If strain is >2% <10% _________
- endrochondral bone healing will occur
High strain > 15% promotes
- fibrous tissue
Bone repair could be enhanced by:
- improving vascularization
- attracting progenitor cells
- accelerating bone formation
- accelerating remodeling
How have BMPS been used to help grow bone?
- appear to be effective alternative to autologous bone graft for repair of fracture non union/open tibial fractures
- controversy about clinical use due to cost effectiveness and potential safety drawbacks
Why have platelet rich plasma been tried for osteogenesis?
- contains multiple growth factors
- evaluated in preclinical and clinical trials
- appears effective in promoting bone heling
Why is FGFs being used for osteogenesis?
- FGF signaling is important in skeletal healing
- FGF2 shown to enhance fracture healing in various in vivo experiments dating back to 1990s
- continued elevation of FGF2 may impair mineralization so timing of treatment needs to be optimized
What are the cell based therapies for bone growth/healing?
- autologous bone marrow - collected from iliac crest/injected into non-union site (increases # of progenitor cells)
- purified stem cell sources (MSC- mesenchymal stem cells, EPC- endothelial progenitor cells)
Other approaches to bone healing treatments?
- anti resorptives (bisphosphonates, denosumab)
- bone anabolic agents (sclerostinn abs, teriparitide)
- gene therapy (still experimental)
Sclerostin
- inhibitor of Wnt/b catenin signaling
- anitbodies to sclerostin being developed as anabolic treatment for osteoporosis
_____ showed sclerostin abs enhaced bone regeneration in rat model of periodontitis
- Taut
_______ a patient with a BMD> 2.5 standard deviations below average for a young healthy male or female
- osteoporosis definition
Is osteoporosis associated with menopause and or aging?
- Yes
Osteoporosis facts
- causes > 8.9 million fractures annually
- worldwide 1 in 3 women and 1 in 5 men over 50 will experience osteoporotic fractures
- Hip fractures with mortality rates of up to 20-24% in first year after fracture. greater risk of dying may persist for at least 5 years
What are the anti resorptive medecines? (prevent further bone degredation)
- amino bishosphonates
- hormone replacement therapy
- selective estrogen receptor modulators (SERMS) e.g raloxifene
- Denosumab
- Cathepsin K inhibitors
What are the anabolic agents?
- PTH 1-84, Teriparitide (PTH 1-34)
- Anti sclerostin antibodies
_____ preferentially bind to hydroxyapatite. Inhibit activity of osteoclasts by inhibiting mevalonate pathway important for prenylation of GTPases important for vesicular trafficking
-Amino bisphosphonates
__________ restores hormone levels following menopause
- Hormone replacement therapy
Serms
- work as a partial antagonists of estrogen receptor but mechanism of action not fullly understood
Denosumab
- antibody against Rankl (inhibits osteocalst formation)
- comparable efficacy as bishosphonates
Cathepsin K inhibitors
- currently in clinical trails (inhibit gone degrading enzyme Cathepsin K)
PTH 1-84, Teriparitide (PTH 1-34)
- intermitten administration of teriparatide stimulates bone formation, partly through inhibition of sclerostin
