Bone Healing and Fracture Repair Quiz

Introduction to Surgery - Mechanisms of Bone Healing

• Course: CVM 737A, Introduction to Surgery
• Topic: Mechanisms of Bone Healing
• Instructor: Robert S. Gilley, DVM, PhD, DACVS, Professor and Chief of Small Animal Surgery
• Institution: LMU College of Veterinary Medicine, Lincoln Memorial University

Learning Objectives

• Describe primary (contact & gap) versus secondary (indirect) bone healing requirements.
• Detail the phases of primary (direct) gap bone healing.
• Detail the phases of secondary (indirect) bone healing.
• Explain the difference between intramembranous and endochondral ossification.
• List and describe types of fracture healing failures (delayed union, nonunion, malunion).
• Characterize different types of bone grafts.
• Describe the common locations for cancellous bone graft collection.

Bone Composition

• Cells:
  • Osteoprogenitor cells (directly from MSCs)
  • Osteoblasts (produce bone)
  • Osteoclasts (remove bone)
  • Osteocytes (resident support)
  • Bone lining (control & differentiation)
• Extracellular Matrix (ECM):
  • Organic & Water: 35% (Type I collagen: 90%, osteocalcin, osteonectin, proteoglycans, glycosaminoglycans, lipids)
  • Inorganic: 65% (Mostly hydroxyapatite)

Anatomy of Bone

• The diagram illustrates the parts of a long bone, including the:
  • Epiphysis
  • Metaphysis
  • Diaphysis
  • Articular cartilage
  • Spongy bone
  • Epiphyseal plate
  • Medullary cavity
  • Endosteum
  • Periosteum

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 into bone within cartilage)

Strain in Fracture Healing

• Strain is defined as fracture gap length, change in length / original length, described as a percentage.
• Tissues withstand various strains:
  • Granulation tissue (100% strain)
  • Cartilage (10% strain)
  • Bone (osteoblasts) (2% strain)

Direct Healing

• Contact healing: Gaps less than 300 microns, osteons (cutting cones), fracture plane across one fragment. (50-80 microns/day)
• Gap healing: Gaps less than 1 mm, blood vessels & connective tissue formation, osteoblasts deposit perpendicular lamellar bone, cutting cones traverse the fracture plane, lamellar bone becomes longitudinally oriented.

Intra-cortical Remodeling

• Formation of new osteons ("cutting cones")

Direct (Primary) Healing - Gap Healing

• Detailed graphic depictions of this method

Direct (Primary) Healing - Requirements

• Rigid fixation to reduce inter-fragmentary strain.
• Adequate reduction of fracture fragments to proper anatomical alignment.
• Sufficient blood supply to facilitate healing.

Indirect (Secondary) Healing

• Most common fracture healing type (even in repaired fractures).
• Enhanced by motion; more motion = more callus.
• Inhibited by rigid stabilization.
• Requires callus formation.

Pinned Fracture Callus Formation

• Diagrams illustrating ideal callus formation in pinned fractures.

Pin Removed

• X-ray images showing post-pin removal.

Indirect (Secondary) Healing - General Phases

• Hematoma formation (inflammation)
• Soft callus formation (proliferative)
• Hard callus formation (maturing/modeling)
• Remodeling

Indirect (Secondary) Healing - Specific Stages (Dr. Tom Einhorn)

• Hematoma formation/inflammation (release of inflammatory cells/mediators, bone morphogenetic protein (BMP), other TGF-β proteins)
• Intramembranous bone formation (direct bone formation from periosteum)
• Chondrogenesis
• Endochondral ossification (very similar to metaphyseal growth plate; Transforming Growth Factor β)

Indirect Fracture Healing - Soft Callus

• Diagram demonstrating the stages involved in soft callus formation.

Indirect Fracture Healing - Hard Callus

• Diagram illustrating the stages of hard callus formation.

Summary of 4 Phases

• Chronological diagrams/illustrations of the four phases of fracture healing, labeled with A, B, C, and D. 

Fracture Healing Summary

• Fracture healing is a continuum.
• Indirect bone healing is faster than direct bone healing.
• Direct fracture healing involves rigid stabilization (bone plate).
• Indirect fracture healing involves less rigid fixation, callus formation (pins/wires, interlocking nails, external fixators, LCPs).

Fracture Complications

• Can occur even if repair principles are followed.
• Less likely if rules are followed.
• Factors to consider:
  • Bone healing
  • Bone blood supply
  • Growth factors
  • Surgical principles

Fracture Nonunion

• All evidence of osteogenic activity at fracture site has ceased.
• Fracture union not possible without surgical intervention.
• Various types/classifications (e.g., Weber-Cech classification: viable [vascular], nonviable [avascular]).

Viable- Hypertrophic Nonunion

• Abundant callus, but callus does not bridge fracture site.
• Causes:
  • Inadequate stabilization
  • Premature weight-bearing
  • Excessive patient activity.

Viable- Mildly Hypertrophic Nonunion

• Mild callus, does not bridge fracture site.
• Called "horse's foot."
• Causes:
  • Inadequate stabilization
  • Implant failure (plate breaks/screw pulls out).

Nonviable- Oligotrophic

• No callus. Just fibrous tissue & blood vessels joining fracture edges..
• Resorption of bone and shortening of fragments.
• Causes:
  • Displacement of fracture fragments or inadequate apposition.

Nonviable - Dystrophic Nonunion

• Intermediate fragments heal to only one main fragment.
• Causes:
  • Poor blood supply on non-healing side.
  • Instability on avascular side
  • More common in older animals

Nonviable - Necrotic Nonunion

• Fragments lack blood supply; cannot heal to main fragment.
• Classical "sequestrum."
• Avascularity of fragment leads to implant loosening.
• Cause:
  • Poor blood supply
  • Infection (but does not always have to be infection related)

Nonviable - Defect Nonunion

• Large bone defect.
• Ends have blood supply, but cannot bridge bone.
• Causes:
  • Massive loss of bone at fracture site.

Nonviable - Atrophic Nonunion

• End result of other three nonviable forms.
• Uncommon.
• Most difficult cases to treat.

Nonunion Causes

• Infection
• Ischemia (poor blood supply)
• Distraction of bone ends
• Excessive bone end compression
• Interposition of soft tissue at fracture site
• Improper surgical implant fixation
• Systemic factors (e.g., diabetes, Cushing's)

Clinical Signs of Nonunion

• Pain at fracture site
• Lameness (usually non-weight-bearing)
• Atrophy of limb/Muscle wasting
• Movement at fracture site

Radiographic Signs of Nonunion

• Fracture gap
• Lack of activity at fracture ends
• Obliteration of marrow cavity
• Osteopenia (thinning) of surrounding bone.
• Callus present, but does not bridge the gap.

Treatment of Nonunions

• Rigid stabilization of fracture.
• Enhance blood supply (bone grafting).
• Address underlying cause of nonunion (infection).
• Contaminated wounds in open fractures are more likely to develop complications.
• Ensure fracture gaps are not metabolically compromised.

Malunions

• Fracture heals in non-anatomic position.
• Causes:
  • Untreated fracture
  • Improperly treated fracture
  • Premature excessive weight-bearing.

Clinical Results - Malunion

• Angular limb deformities
• Limb shortening
• Gait abnormalities
• Degenerative joint disease

Delayed Union

• Fracture does not heal in expected time
• Factors to consider:
  • Patient characteristics
  • Fracture environment

Final outcome after grafting

• X-ray images showing changes in fracture.

Blood Supply to Normal Bone

• Nutrient artery to bone marrow (80-85% supply)
• Periosteal vessels
• Epiphyseal & Metaphyseal vessels.
• Differences exist based on growth vs. maturity phase.

Blood Supply After Injury

• Extraosseous blood supply (supplies early periosteal callus)
• Medullary supply eventually takes over.

Principles of Bone Grafts

• Very important tool for fracture complications.
• Types of grafts:
  • Autogenous (within same individual.
  • Allograft (different individuals, same species).
  • Xenograft (different individuals/species).

Bone Graft Characteristics

• Osteogenesis (osteoblasts surviving transfer)
• Osteoconduction (graft acting as a scaffold for new bone)
• Osteoinduction (inducing cells to form new bone)
• Bone Morphogenetic Protein (BMP)
• Osteopromotion

Types of Bone Grafts- Cancellous

• From trabecular bone.
• Works due to "O's" of grafting.

Bone Graft Physiology- Cancellous

• Bone graft separated from blood supply results in few osteogenic cells surviving.
• Mesenchymal stem cells form new bone cell lines.
• Deposition of new bone from osteoprogenitor cells.
• Resorption of necrotic bone

Indications for Bone Grafts

• Orthopedic fractures/arthrodesis
• Infected fractures
• Delayed/nonunions
• Bone loss (cysts/fractures)
• Limb-sparing for tumors

Harvesting a Cancellous Bone Graft

• Aseptic technique is critical
• Preparation of donor site in advance.
• Common donor sites: Ilial wing, proximal tibia, humerus, distal femur.
• Separate surgical instruments.
• Minimize exposure to air.
• Graft storage in blood-soaked sponges.
• Debridement of graft area is crucial..
• Careful not to create additional fracture.

Sites for Cancellous Bone Grafts

• Diagrams/illustrations showing donor sites for grafts.

Course Information

• Small Animal Surgery, 5th Edition
• Chapter 32; pages 991-995 & 1025-1032
• Theresa Welch Fossum, DVM, MS, PhD, et al.
• Copyright © 2018 by Mosby, Inc., an affiliate of Elsevier.