Fracture Healing & Bone Remodeling & Healing Complications Flashcards
Describe the three phases of fracture healing in detail.
The inflammatory phase (hours to days) involves hematoma formation, inflammation, and the recruitment of immune and mesenchymal stem cells. The reparative phase (weeks) includes soft callus formation via fibrocartilage deposition, followed by hard callus formation as woven bone replaces the cartilage. The remodeling phase (months to years) involves the gradual replacement of woven bone with lamellar bone to restore strength and structure.
What key events occur during the inflammatory phase of bone healing?
A hematoma forms at the fracture site due to ruptured blood vessels. Inflammatory cells like neutrophils and macrophages migrate to the area to remove necrotic debris. Mesenchymal stem cells and fibroblasts are recruited to initiate repair.
What is the role of mesenchymal stem cells in fracture healing?
They differentiate into chondrocytes, osteoblasts, and fibroblasts, which contribute to cartilage and bone formation during the reparative phase.
What differentiates primary and secondary bone healing?
Primary healing occurs when fracture fragments are rigidly stabilized, allowing direct remodeling without callus formation. Secondary healing occurs when movement is allowed, leading to a callus formation followed by remodeling.
How do osteoblasts and osteoclasts coordinate during bone remodeling?
Osteoclasts resorb bone in response to mechanical stress or microdamage, creating resorption pits. Osteoblasts then deposit new bone matrix, which mineralizes into mature bone.
What molecular signals regulate bone healing?
Key regulators include BMPs (bone morphogenetic proteins), TGF-β, VEGF, Wnt signaling, and RANKL-OPG interaction, which control osteoblast and osteoclast activity.
Why is cortical bone stronger than trabecular bone?
Cortical bone has a dense, highly organized osteon structure, providing resistance to compressive forces, whereas trabecular bone is porous and adapted for absorbing impact and reducing mechanical stress.
What is Wolff’s Law, and how does it relate to bone remodeling?
Wolff’s Law states that bone adapts to mechanical stress by remodeling its structure. Increased stress leads to bone deposition, while reduced stress results in bone resorption.
How does osteoporosis affect fracture healing?
Osteoporosis weakens bone due to decreased mineral density and impaired osteoblast activity, leading to increased fracture risk and delayed healing due to inadequate callus formation.
Explain the process of endochondral ossification in fracture healing.
Chondrocytes from the periosteum create a cartilage template at the fracture site. The cartilage is then gradually replaced by woven bone through endochondral ossification, forming a hard callus before remodeling into lamellar bone.
What are the risk factors for non-union fractures?
Severe soft tissue damage, poor blood supply, infection, inadequate immobilization, smoking, diabetes, and corticosteroid use.
What histological changes occur in osteonecrosis?
Bone lacunae become empty due to osteocyte death. Surrounding bone marrow shows fat necrosis and fibrosis. Later, resorption and collapse occur due to impaired repair mechanisms.
Why does osteonecrosis commonly affect the femoral head?
The femoral head has a limited collateral blood supply, relying mainly on the medial circumflex femoral artery. Any disruption, such as trauma or steroid use, can cause ischemia and bone necrosis.
Describe the differences between woven and lamellar bone.
Woven bone is immature, rapidly produced with a haphazard collagen arrangement, making it weak. Lamellar bone is mature, with parallel collagen fibers providing high tensile strength.
How does infection impair fracture healing?
Infections introduce inflammatory cytokines, disrupt blood flow, impair osteoblast function, and can lead to osteomyelitis, which may prevent callus formation or cause bone destruction.
What role does vascularization play in bone healing?
Blood vessels deliver oxygen, nutrients, and growth factors essential for osteoblast proliferation, callus formation, and remodeling.
How does diabetes mellitus affect bone healing?
Diabetes leads to microvascular damage, reducing blood flow to the fracture site, impairing osteoblast function, and increasing the risk of infection and delayed healing.
Explain how mechanical stress influences bone formation.
Mechanical stress stimulates osteoblast activity via mechanotransduction, increasing bone formation. Lack of stress leads to bone resorption, as seen in immobilized patients.
What are the long-term consequences of malunion?
Malunion can lead to angular deformities, altered joint biomechanics, chronic pain, reduced mobility, and an increased risk of secondary osteoarthritis.
What surgical interventions can promote fracture healing?
Internal fixation (plates, screws, rods), external fixation, bone grafting, and biologic agents like BMPs to stimulate osteogenesis.
How does glucocorticoid-induced osteoporosis impair fracture healing?
Glucocorticoids suppress osteoblast activity, increase osteoclast resorption, and reduce bone formation, leading to weaker callus formation and increased risk of non-union.
What are the stages of bone healing visible in histological analysis?
Histology shows hematoma formation with inflammatory cells, fibrocartilage callus formation, woven bone deposition, and final remodeling into lamellar bone.
Why are elderly patients at higher risk for fracture complications?
Aging leads to reduced osteoblast function, slower bone turnover, impaired angiogenesis, and increased comorbidities such as osteoporosis, all contributing to delayed healing.
How do bisphosphonates affect bone remodeling?
Bisphosphonates inhibit osteoclast-mediated bone resorption, increasing bone density but potentially impairing normal remodeling and fracture healing.
Explain the importance of angiogenesis in bone repair.
New blood vessel formation supplies essential nutrients, oxygen, and progenitor cells, facilitating tissue regeneration and successful fracture healing.
What is the primary cause of stress fractures?
Repeated mechanical load without adequate recovery leads to microfractures that accumulate, weakening the bone.
What is the clinical presentation of osteonecrosis in early vs. late stages?
Early osteonecrosis is often asymptomatic, but later stages present with pain during weight-bearing, progressing to pain at rest, joint stiffness, and eventual collapse.
What imaging modalities are used to diagnose osteonecrosis?
X-rays show late-stage bone collapse, but MRI is the gold standard for early detection, revealing bone marrow edema and necrotic areas.
What are the primary treatment options for osteonecrosis?
Early-stage treatments include bisphosphonates, core decompression, and vascularized bone grafts, while late-stage cases may require joint replacement.
