Surgery Lecture 23

Orthopedic Surgery I: Bone Structure, Biomechanics, and Fracture Healing

What is cortical bone

Compact bone found in the shaft of long bones, made of lamellar bone

What is cancellous bone?

Trabecular bone found in the center and ends of long bones and flat bones, made of lamellar bone

Lamellar Bone

Major portion of cortical and cancellous bone, deposited layer upon layer (slow rate of formation; remodelling)

How much lamellar bone do osteoblasts form per day?

2 um

Woven Bone

Bone that occurs during rapid growth, rapid bone repair, and at insertions of tendons/ligaments; mechanically inferior to lamellar bone

Periosteum

Outer fibrous layer covering bones (except where covered by cartilage), containing an inner osteogenic layer

Haversian Canals

Channels within bone that contain blood vessels

Volkmann's Canals

Connect Haversian Canals

Bone Cells

Osteocytes, osteoblasts, and osteoclasts

Hydroxyapatite

Inorganic part of bone matrix (71% of wet weight), composed of calcium and phosphorus

Osteoid

Organic matrix of bone (21% of wet weight), mostly Type I collagen; made by osteoblasts

Membranous Bone Formation

• Ossification by direct mineral deposition into the organic matrix of mesenchymal connective tissue

• Used for skull, mandible, and appositional growth

Endochondral Bone Formation

• Cartilaginous template formed first, then transformed via calcification and replacement by bone

• Used for longitudinal growth of long bones.

What are the 3 growth cartilage types?

• Discoid growth plate → longitudinal growth

• Sperical growth plate → centrifugal growth

• Apophysis → traction epiphysis

What are the different zones of growth cartilage?

• Resting / Reserve zone

• Proliferative zone

• Hypertrophic zone

• Calcification zone

• Ossification zone (primary spongiosa of bone)

Wolff's Law

Bone adapts to the loads placed on it, increasing mass where needed and decreasing where not needed

What curves are used to demonstrate biomechanisms of bone?

• Load deformation curve

• Stress-strain curve

What are the loading modes of bone?

Bending, torsion, shear, tension, and compression

Failure configurations in bone

• Transverse

• Oblique

• Butterfly fragment

• Spiral

Causes of Bone Fractures

• Single ultimate failure load

• Repeated cyclic loading

Fracture Classifications

• Complete vs. incomplete

• Displaced vs. non-displaced

• Open vs. closed

What are the 3 open fracture classifications?

• Type 1: small skin laceration without bone exposure, no gross contamination

• Type 2: skin laceration with little tissue loss, inimally exposed bone, minimal gross contamination

• Type 3: extensive laceration, significant tissue loss, extensive gross contamination

What are different fracture configurations?

• Fissure/hairline/greenstick

• Transverse

• Oblique

• Spiral

• Comminuted/compound

• Avulsion

Salter-Harris Classifications (all)

Type 1: Complete separation of the physis

Type 2: Physeal Fx breaks out in metaphysis

Type 3: Intra-articular Fx, through epiphysis and physis

Type 4: Intra-articular Fx through epiphysis, physis, and metaphysis

Type 5: Crushing injury leading to closure of physis in limited area

Type 6: Periosteal bridge between metaphysis and epiphysis slows growth on affected side

What are the 2 types of bone healing?

1. Primary / direct Fx healing

2. Secondary / indirect Fx healing

Primary/Direct Fracture Healing

Rigid internal fixation, adequate reduction, and sufficient blood supply leading to fracture ends uniting via Haversian remodeling

Secondary/Indirect Fracture Healing

• Small movement in fracture gaps

• Healing via periosteal, intercortical, and endosteal callus formation

Describe sequence of bone healing

Hematoma → granulation tissue → fibrous tissue→ fibrocartilage → mineralized → woven bone → lamellar bone

Describe the elongation interfragmentary strain tolerance on granulation tissue, cartilage, and bone

• Granulation tissue - 100%

• Cartilage - 10-15%

• Bone - 2%

Describe the bending interfragmentary strain tolerance on granulation tissue, cartilage, and bone

• Granulation tissue - 40 degrees

• Cartilage - 5 degrees

• Bone - 0.5 degrees

Phases of Bone Healing

• Inflammatory

• Reparative

• Remodeling

Describe inflammatory phase of bone healing

• Vasodilation, leukocyte migration, chemotaxis

• Mesenchymal cell proliferation, angiogenesis

• 2-3 weeks post injury

Describe reparative phase of bone healing

• Overlaps with inflammatory phase

• 2-12 months duration

• Repair tissue highly vulnerable to interfragmentary motion

• Periosteal and endosteal callus formation by endochondral and membranous ossification

Describe remodeling phase of bone healing

• Overlaps with reparative phase

• Ostional remodeling

fracture gap(s) widen ___-___ weeks post injury

2-3

Size of callus reflects degree of _______ motion but may not predict degree of Fx ________

interfragmentary;

stability

Ultimate Goals of Bone Healing

• Reconstruction of original cortices

• Early return to total limb function

• Rapid bone union

• Prevention of fracture disease

What are examples of fracture diseases?

• Soft tissue contracture

• Loss of muscle

• Osteopenia

• Loss of joint function - prevent OA

• Contralateral limb disease - breakdown or laminitis

• Malunion / nonunion

Most Desirable Type of Bone Healing

Primary fracture healing by Haversian remodeling rather than callus formation, achieved by rigid fixation, adequate reduction, and preservation of blood supply

Advantages & disadvantages of Plating

Advantage

• Direct bone union if fracture compression

• Stress protection

• Rigid fixation

• Adequate reduction

Disadvantage

• Disrupt periosteum and Fx hematoma to apply

Biologic Osteosynthesis

• Closed reduction + external fixator

• Encouraging early callus growth

• Emphasizes preservation of blood supply over anatomic reconstruction

• Provide for early dynamization to stimulate callus growth

• Difficult in horses-large biomechanical stress often dictates fixation technique

Dynamization

Strategic reduction in rigidity of fixation to allow micromotion of fracture ends, ONLY in smaller animals

Main Functions of Bone Grafts

[OOO]

• Osteogenesis → by fresh osteoblasts

• Osteoinduction → growth factors, cytokines

• Osteoconduction → matrix scaffold

Types of Bone Grafts and Replacements

• Autogenous fresh cancellous bone grafts

• Cortical bone

• Bone replacements (hydroxyapatite, tricalcium phosphate, etc.)

• Growth factors (BMP-2)