Mechanisms of Bone Healing PDF

CVM 737A Introduction to Surgery Mechanisms of Bone Healing Spring 2025 Robert S. Gilley, DVM, PhD, DACVS Professor and Chief Small Animal Surgery  Learning Objectives 1. Describe the requirements for primary (contact & gap) versus secondary (ind...

CVM 737A Introduction to Surgery Mechanisms of Bone Healing Spring 2025 Robert S. Gilley, DVM, PhD, DACVS Professor and Chief Small Animal Surgery  Learning Objectives 1. Describe the requirements for primary (contact & gap) versus secondary (indirect) bone healing to occur 2. Describe the phases of primary (direct) gap bone healing 3. Describe the phases of secondary (indirect) bone healing 4. Describe the difference between intramembranous and endochondral ossification  Learning Objectives 5. List and describe types of fracture healing failures (delayed union, nonunion, malunion) 6. List and describe the different types of bone grafts 7. List and describe the common locations for the collection of cancellous bone grafts Bone Composition Cells Extracellular matrix Osteoprogenitor cells (ECM) Directly from MSCs Osteoblasts – Organic & Water: 35% Produce bone Type I collagen: 90% Osteoclasts Osteocalcin, osteonectin, Remove bone proteoglycans, Osteocytes glycosaminoglycans, lipids Resident support – Inorganic: 65% Bone lining Mostly hydroxyapatite Control & differentiation Anatomy of Bone Fracture Healing Direct (primary) Osteonal reconstruction Requires rigid internal fixation Less than 2 % strain Minimal or no fracture gap Indirect (secondary) Intermediate callus formation Direct bone formation (intramembranous) Endochondral ossification = transformation cartilage into bone within Strain in Fracture Healing Strain: fracture gap length Change in length / original length Described as a percentage Strains withstood by tissue Granulation tissue 100% strain Cartilage 10% strain Bone (osteoblasts) 2% strain  Direct Healing Contact healing Gaps less than 300 microns Osteons (cutting cones) Cross fracture plane - one fragment to the other 50-80 microns/day Welding metal similar to Spot · Gap healing Gaps less than 1 mm Blood vessels and connective tissue form Osteoblasts deposit perpendicular lamellar bone in gap Cutting cones traverse fracture plane Lamellar bone becomes longitudinally oriented Direct (Primary) Healing Contact healing Transverse fracture cutting Cones V L T cuthnga ~  Intra-cortical Remodeling Formation of new osteons “Cutting cones” Osteoclasts down bond (Osteoblasts) = produce new bone Direct (Primary) Healing Gap Healing pancarpal arthrodesis frigid fixation compression , of fracture Gap Direct (Primary) Healing Requirements 1. Rigid fixation Inter-fragmentary strain 2. Adequate reduction together Stabilizing = put (2)pieces 2 them Only possible with simple fractures 3. Sufficient blood supply Indirect (Secondary) Healing J usuallycombinationa Most common type of fracture healing Even in repaired fractures Enhanced by motion More motion = more callus Inhibited by rigid stabilization Requires callus formation Pinned Fracture Callus Formation Ideal callus formation Pin Removed · Whoever performed SX Stabilized forces but bending not rotational (but luckily dog healed fine) Indirect (Secondary) Healing 4 General Phases of Fracture Healing & 1. Hematoma formation (inflammation) phase 2. Soft callus formation (proliferative) phase 3. Hard callus formation (maturing or modeling) Jereparative phase 4. Remodeling phase Some references combine 2 and 3 =>Reparative phase  ⑪ Indirect (Secondary) Bone Healing Specific: Four Stages (Dr. Tom Einhorn) 1. Hematoma formation/inflammation Release of inflammatory cells and mediators Bone Morphogenetic protein (BMP) Other *TGF- proteins 2. Intramembranous bone formation = direct bone formation (from periosteum) 3. Chondrogenesis 4. Endochondral ossification Very similar to metaphyseal growth plate *Transforming Growth Factor Indirect Fracture Healing Soft Callus - Intramembranas Indirect Fracture Healing Hard Callus T N Forms inwards from both sides to meet in middle Summary of 4 Phases Fracture Healing Summary Fracture healing is a continuum Indirect bone healing faster process than direct bone healing Direct fracture healing Rigid stabilization bone plate Indirect fracture healing Less rigid fixation with callus formation Pins/ wires, interlocking nail, external fixator, LCPs Fracture Complications Can happen even if repair principles adhered to Less likely if ‘rules’ are followed To avoid- understand Bone healing Bone blood supply Growth factors Surgical principles Fracture Nonunion ① All evidence of osteogenic activity at fracture site has ceased Fracture union NOT possible w/o surgical intervention Weber-Cech classification Viable (Vascular) Nonviable (Avascular) Viable- Hypertrophic Nonunion Abundant callus but NOT bridging the fracture site Called ‘the elephant foot’ Causes: Inadequate stabilization Premature weight-bearing Too much activity of patient Hypertrophic Nonunion Case Viable- Mildly Hypertrophic Nonunion Mild callus but NOT bridging the fracture site Called ‘the horses foot’ Causes: Inadequate stabilization usually due to implant failure Examples- plate breaks, screw pulls out Mildly Hypertrophic case Viable- Oligotrophic No callus- just fibrous tissue & blood vessels joining ends See rounding of fracture edges, resorption of bone & shortening of fragments Causes: Displacement of fracture fragments or inadequately apposed fragments Oligotrophic Nonunion Case & Do Not pin the radius !! Nonviable- Dystrophic Nonunion Intermediate fragments of fracture heal to one main fragment & not the other Causes: Poor blood supply on non-healing side Instability on avascular side More common in older animals w/ poorer blood supply Nonviable- Necrotic Nonunion Fragments have no blood supply & cannot heal to any of main fragments The classical ‘sequestrum’-mustbesurgically reme at Avascularity of fragment can lead to implant loosening Cause: Poor blood supply Infection at the fracture site But does not have to be an infection! ⑪ Nonviable- Defect Nonunion Large defect - even if ends have blood supply, they cannot bridge bone Causes: Massive loss of bone at fracture site Nonviable- Atrophic Nonunion End result of other 3 nonviable nonunions Uncommon The most difficult cases to treat Nonunion causes Infection Ischemia Distraction of bone ends Excessive compression of bone ends (c. " Surgical plates Interposition of soft tissue at fracture Improper implant fixation Systemic factors - Diabetes Cushings , , etc. Clinical signs of nonunion Pain at the fracture site Lameness: usually non-wt-bearing (NWB) Disuse atrophy of limb Movement felt at fracture site Radiographic signs of nonunion Fracture gap No activity at fracture ends Obliteration of marrow cavity Want > - medullary cavity to be open Osteopenia of surrounding bone [= of bone itself atrophy If callus even present: Does not bridge fracture gap! Treatment of Nonunions Rigid stabilization of fracture Enhancing blood supply (bone grafting) Treat underlying cause of nonunion Infection Contaminated wounds in open fractures are 5X more likely to develop complications Fracture gaps Make sure animal is metabolically healthy Example- nonunion treatment Monteggia fracture Example- nonunion treatment brokenscrewed bi remove It Malunions Fracture that heals in a non- anatomic position Causes Untreated fracture Improperly treated fracture Premature excessive wt-bearing on fracture Clinical results- Malunion Angular limb deformities Limb shortening Gait abnormalities Degenerative joint disease Delayed union Fracture not healed in expected time Considering GsW(metal patient & shrapnel/bullet fragments) fracture environment 3 months postoperatively  Final outcome after grafting 6 months Postop = Destabilized 6 months postoperatively Blood supply to normal bone Nutrient artery to bone marrow 80-85% of supply Periosteal vessels Epiphyseal & Metaphyseal vessels Differences exist depending on growth versus maturity phase Blood supply after injury Extraosseous Supplies early periosteal callus Medullary supply eventually takes back over the blood supply Principles of Bone Grafts Very important tool for fracture complications Graft Transplanting Autogenous- within same individual Allograft- different individuals, same species Xenograft- different individuals, different species  Bone graft characteristics:“O’s” of grafting Osteogenesis- osteoblasts that survive transfer Very few survive Osteoconduction- graft acts as scaffold in which new bone is laid down Osteoinduction- graft induces cells to promote new bone Bone Morphogenetic Protein (BMP) Osteopromotion- material that enhances regeneration of bone Platelet-Rich Plasma Types of bone grafts: Cancellous- from trabecular bone works by the O’s of grafting Bone graft physiology- cancellous Bone graft separated from its blood supply Few cells survive- osteogenic cells Mesenchymal stem cells are induced to form bone cell lines Inductive Proteins (BMPs) Deposition new bone from osteoprogenitor cells Resorption of necrotic bone Indications for bone grafts: Any orthopedic fracture or arthrodesis Infected fractures Delayed / nonunions Bone loss- cysts, fractures Limb- sparing for bone tumors Harvesting a cancellous bone graft Asepsis Prepare surgical donor site in advance Most Common: -2 yPrimary (proximal) Ilial wing, proximal tibia & humerus, distal femur Separate surgical instruments Minimize grafts exposure time to air Keep in blood-soaked sponges Debride graft area Be careful not to create a fracture! Sites for cancellous bone grafts Sites for cancellous bone grafts  Small Animal Surgery Fifth Edition CHAPTER 32 Pages 991-995 & 1025-1032 Theresa Welch Fossum, DVM, MS, PhD Dewey, Horn, Johnson, MacPhail, Radlinsky, Schulz, Willard Copyright © 2018 by Mosby, Inc. an affiliate of Elsevier