Equine Bone Healing

Question 1

What is good about bone healing?

Answer 1

Bone has ability to heal completely after fracture and return to original structure and mechanical properties (if given the right mechanical environment to do so)

Question 2

What is the immediate injury response <48h to bone injury?

Answer 2

Structural damage

• Haematoma
• Inflammatory mediators
  
  • Kinins, complement, histamine, serotonin, prostaglandins, leukotrienes
    
    • Promote vasodilation
    • Chemotaxis of white blood cells
    • Platelet aggregation and release
    • Mesenchymal cell proliferation

Question 3

Name some inflammatory mediators that appear during the immedaite injury response in bone

Answer 3

Kinins, complement, histamine, serotonin, prostaglandins, leukotrienes

Question 4

Inflammatory mediators are released in bone during the immediate injury response. What do they do?

Answer 4

• Promote vasodilation
• Chemotaxis of white blood cells
• Platelet aggregation and release
• Mesenchymal cell proliferation

Question 5

What are the phases of fracture healing?

Answer 5

1. Inflammation
2. Repair
3. Remodelling

The phases are not distinct –> lots of overlap

Question 6

What happens during the inflammatory phase of bone healing?

When does it occur?

Answer 6

• Occurs in first 2-3 weeks
• Growth factors
• Cytokines
• Phagocytosis
• Fragment end resorption
• Inflammatory phase results in clearance of debris/necrotic/devitalised bone. Sets the stage for healing to occur. Fracture gap widens.
• One of the first things that happens is fragment reabsorption
• Fracture becomes more obvious during healing because of the resorption
• On a radiograph a fracture will look its worst at 2-4 weeks post injury

Question 7

At what point does the fracture become more obvious?

Answer 7

On a radiograph the fracture will look its worst at 2-4 weeks post injury

This is during the inflammatory phase

Question 8

During the inflammatory phase, does the fracture get smaller or larger?

Answer 8

Fracture gap widens

Inflammatory phase results in clearance of debris/necrotic/devitalised bone

Fragment reabsorption

Question 9

For how long does the reparative phase occur?

Answer 9

2-12 months duration

Question 10

What happens during the reparative phase of bone healing?

Answer 10

• Angiogenesis
  
  • Adjacent tissues (muscles)
  • Medulla
  • Angiogenesis –early supply from surrounding soft tissue, latterly from re- establishment of medullary blood supply.
• Periosteal and endosteal callus formation
  
  • Interfragmentary stabilisation
  • Suppressed by
    
    • Rigid immobilisation –will be more likely to trigger 1y bone healing
    • Excessive mobilisation
  • Callus –cartilage and bone, stabilise fragments –> callus is really important for stabilisation
  • Cartilage component becomes mineralised, resulting in cancellous bony union (subsequently remodelled into lamellar bone)
• Bone union
  
  • Cancellous bone, formed by a combination of intramembranous and endochondral ossification

Question 11

What is periosteal and endosteal callus formation suppressed by?

Answer 11

Suppresed by:

• Rigid immobilisation
• Excessive immobilisation

Question 12

What is this image showing with regards to bone healing?

Answer 12

Intramembranous ossification during the reparative phase

Question 13

What happens during intramembranous ossification during the reparative phase of bone healing?

Answer 13

• Peri/endosteal surfaces
  
  • low strain/motion
• Fibrous scaffold
• Buttress for ossification
• Endochondral ossification can form between the bone ends
• Intramembranous ossification goes straight from fibrous scaffold to mineralisation

Question 14

What is this arrow pointing to with regards to bone healing during the reparative phase?

Answer 14

Soft callus - endochondral ossification

Question 15

How does the haematoma formed during immediate injury response change during the reparative phase of bone healing?

Answer 15

• Differentiation of haematoma
  
  • Fibroplasia
    
    • Chondrogenesis
      
      • Mineralisation

Question 16

Briefly outline the steps in the indirect fracture healing process

Answer 16

Question 17

Briefly, what happens during the remodelling phase of bone healing?

Answer 17

Bone resorption

Bone formation

Cortical callus - Haversian remodelling

Question 18

During the remodelling phase of bone healing, how can there be some correction of malalignment possible in juveniles?

Answer 18

• Convex surface; positive charge – attract osteoclasts
• Concave surface; negative charge – attract osteoblasts (which are going to lay down bone)

Question 19

Explain what is happening in these radiographs

Answer 19

• Type II fracture extending through the growth plate
• Dense cortex is re-establishing down dorsal and plantar aspect
• Haversian remodelling is also taking place

Question 20

What are some regulatory molecules that are necessary in the regulation of fracture repair

Answer 20

Soz, too lazy to try and put this into bullet points so have the powerpoint slide instead

Question 21

Buddy the dog was wearing a 2 foot leash. He sees a squirrel and wants nothing more than to chase after it. Lucky for him Buddy was finally able to catch the squirrel. How did he accomplish this?

Answer 21

Because the leash is not tied on to anything else

Question 22

Name some regulatory molecules that are involved in bone healing, remodelling phase?

Answer 22

• Growth factors
  
  • TGF-βsuperfamily (including BMPs) –progression
  • PDGF
  • IGF-1/2
  • FGF
• Cytokines
  
  • IL1 (differentiation), IL3, CSF
• Others
  
  • osteopontin, osteocalcin, osteonectin
  • PGs

Question 23

What are the 2 basic patterns of bone repair?

Answer 23

Direct (primary) repair

Indirect (secondary) repair

Question 24

What occurs during diirect (primary) fracture healing?

Answer 24

• Exact alignment
• Rigid fixation (usually internal fixation)
• Sufficient blood supply

Question 25

What is contact healing and what is gap healing with regards to direct (primary) repair under stability?

Answer 25

Contact healing * Secondary osteonal (Haversian) remodelling of the fracture at interfragmentary contact points Gap healing * Small gaps filled with woven then lamellar bone * Secondary osteonal remodelling * Grow across In contrast, in primary bone healing we’re relying on residual strength of bone (if incomplete fracture) or strength of the surgical implants

Question 26

What are the arrows pointing at in this photo with regards to a bone structural unit - cutting cone

Answer 26

Multinuclested osteoclasts resorbing bone at the leading edge (white arrows) New bone is in red along the edges (being put down by osteoblasts)

Question 27

What are the AO principles 196o for direct fracture repair

Answer 27

O principles 1960 * Anatomic reduction * Stable internal fixation – interfragmentary compression * Preservation of blood supply * Aim is to achieve early active, pain-free mobilisation

Question 28

What are some methods of direct (indirect) bone repair?

Answer 28

* External fixation * Casts and braces * External fixators * Internal fixation * Screws * Plates * Intramedullary nails In horses external fixation has more associated complications and isn’t as useful for stabilisation

Question 29

When surigcally fixing an ulna in the horse, where should you avoid making the incision?

Answer 29

Avoid making incision over point of elbow --\> problems with healing

Question 30

What is a dynamic compression plate? How should it be placed?

Answer 30

* Standard bone plate * Stability of construct relies on friction between the plate and bone * Plate must be contoured to the bone * Relies on contact between plate and bone to achieve stability * Accurate anatomic reconstruction necessary * Fixed to the bone with cortical screws * Sloping holes in the plate allow compression to be applied to the fracture

Question 31

What is indirect (secondary) bone repair?

Answer 31

* Rapid stabilisation * Callus formation * Rapid restoration of mechanical integrity * Long period of remodelling * Repair pattern influenced by both biology and mechanics

Question 32

What are some problems in fracture repair?

Answer 32

* Accurate anatomical reduction: * Articular misalignment (step) causes osteoarthritis and cyclical implant loading * Accurate reduction often difficult: * Eburnation * Muscle contraction * Severe comminution (may simply be unable to put all the pieces back in proper anatomical alignment) * For repair of articular fractures it is absolutely vital that the bones are properly aligned * Can be difficult to get accurate reduction in long bone fractures * Contraction of surrounding muscles can impact on reduction

Question 33

What happens if you have no motion at the fracture gap?

Answer 33

No callus Atrophic non-union

Question 34

What happens if you have excessive motion at a fracture gap?

Answer 34

Hypertophic non-union due to persistence of fibrous tissue

Question 35

What is circled here with regards to union during indirect (secondary) bone repair?

Answer 35

Large mass of callus is disproportionate to the size of the fracture and the fracture line has persisted Hypertrophic delayed union

Question 36

What is going on here?

Answer 36

Fibrous Malunion Rather than getting an ossified repair it just forms a fibrous bridge that persists Not as stable or strong so high risk of re-fracture

Question 37

What is the problem with excessive motion on implants for fracture repair?

Answer 37

Excessive motion - implant cycling Excessive motion is something we are always fighting against in the horse because of the mechanical loads We need the bone to heal within 2-3 months otherwise the implants will reach their fatigue point and break

Question 38

What are some problems in fracture repair in horses?

Answer 38

* Large animals (often \>500kg) * Enormous mechanical loads on recovery from general anaesthesia * Rapid weight bearing * Bed rest not an option A recipe for disaster!!

Question 39

What are some problems in fracture repair in horses with regards to soft tissues?

Answer 39

* Condition of the soft tissues * Integrity of soft tissue cover: * Protection against infection * Integrity of blood supply * Tendency to open fractures: * Poor soft tissue cover distal limbs * Explosive nature of fractures * Poor first aid stabilisation * Prox. Limb fractures --\> they have a lot of muscle coverage In the distal limb there isn’t much more than subcutaenosu tissue covering the bone * Can get problems with implants impinging on the skin and closing the skin can be an issue too

Question 40

What are some problems in fracture repair in horses with regards to load?

Answer 40

* Contralateral overload (‘fracture disease’) in the horse: * Laminitis * Angular deformity (in younger animals) * Elongation of the stay apparatus * Cartilage degeneration * Pain-free weight bearing is necessary objective of fracture repair in the horse * Requires accurate reduction and rigid fixation

Question 41

What are some problems with contralateral limb overload in horses with fracture repair?

Answer 41

* Contralateral overload (‘fracture disease’) in the horse: * Laminitis * Angular deformity (in younger animals) * Elongation of the stay apparatus * Cartilage degeneration

Question 42

What are some problems in fracture repair with prolonged immobilisation ('cast disease')?

Answer 42

* Prolonged immobilisation (‘cast disease’): * Osteoporosis (loss of bone density) --\> reversible but have to be careful because if they have developed severe osteoporosis they have increased risk of re-fracture * Joint laxity * Tendon laxity * Cartilage degeneration * Pressure or rub sores (be very careful not to apply bandages too tightly)

Question 43

What are some problems in fracture repair with regards to infection?

Answer 43

* Infection (osteomyelitis) * Open fractures * Destruction of blood supply in overlying soft tissues * Prolonged surgery time * Implants are ‘foreign bodies’ * Osteomyelitis could be considered as one of the most major causes of fracture repair failure

Question 44

What is and what are the pros and cons of biological fracture healing?

Answer 44

Minimally invasive surgery * Small incisions * Minimal disruption to fracture haematoma * Periosteum largely left intact * Quicker and better fracture healing BUT * Increased reliance on complex (expensive) intra-operative imaging (CR/DR/fluoroscopy) * Increased technical demands * Expensive implants * Less accurate anatomic reduction –increased reliance of implants to load share Can slide under the soft tissues and screw them in via small stab incisions --\> good for maintaining blood supply and avoiding damage to the periosteum

Question 45

How can you help enhancement of fracture repair?

Answer 45

* Cancellous Bone Grafting * Gold standard * Morbitity? * Availability * Osteoconduction -scaffold * Osteoinduction –stimulation of osteogenesis * Source of osteoblasts / osteoprogenitor cells * Alternatives * Active area of research due to desirability in human field * No risk of disease transmission * No morbidit * Ready availability * Synthetic osteoinductive agents * Bone morphogenic proteins * Deminaralised bone matrix * Parathyroid hormone gene * Synthetic osteoconductive agents * Beta tricalcium phosphate (lots of products) * Mesenchymal stem cell based techniques * Gene therapy (BMP, VEGF)