Lecture 13: Bone Injury and Healing

Differentiate micro-damage and micro-fracture.

Microdamage = de-bonding

Microfracture = breaking of mineralized structures

Which occurs more often, micro-damage or micro-fracture?

Micro-damage

Micro-damage is more common in which type of bone?

Cortical bone

Micro-fracture is more common in which type of bone?

Trabecular bone

Minor damage of bone may occur at near ___% strain

Minor damage of bone may occur at near 0.8% strain

T/F: Bone can sustain high numbers of cracks and de-bonding before failing

True!

T/F: As loading increases, micro-damage increases

True

T/F: As cycles increase, more load is needed to reach failure

False!

Less load is needed to reach failure with more cycles

Discuss loading history and how it can cause deformation and fractures.

Isolated overloads that do not cause fracture of bone can cause subtle, cumulative, and permanent deformation resulting in clinical fractures years later

How does fracture occur differently with tensile and compressive loading?

Tensile: single crack initiation occurs at a threshold level followed by multiple cracks (crack propagation) and then a coalescence of cracks which happens at fracture

Compressive: single crack initiation occurs at a much higher load level followed by a sudden fracture of the structure

What are the aging effects on bone?

Progressive net loss of bone mass and a change in bone material properties (resulting in bone tissue of reduced strength and modulus)

Young bone is more ________ than older bone so more ______ before breakage is allowed in young bone.

Young bone is more ductile than older bone so more strain before breakage is allowed in young bone

Why is younger bone more ductile than older bone?

Water content

When is maximum bone present?

~3rd decade (slightly younger in females)

How is geometric remodeling affected by aging?

Cortex thins but the diaphysis becomes a cylinder of a larger diameter (as a mechanical compensation)

(Cortical areas remain almost constant in men, but they are decreased in women)

What contributes to degenerative joint disease?

Linked to failure of articular cartilage → shifts the load to the trabecular / subchondral bone → leads to more bone being laid down

How does compressive loading lead to degenerative joint disease?

Increased compressive loading → microfracture → bone remodeling → stiffening of subchondral bone → increased loading on already degenerated cartilage

T/F: Patients with osteoporosis are not as likely to get DJD.

Why or why not?

True!

Because osteoporotic bone is relatively soft and would act as a shock absorber

A joint with larger surface area relative to its load is (less/more) likely to develop DJD

A joint with larger surface area relative to its load is LESS LIKELY to develop DJD

OA in the knee is correlated with…

Body mass (especially in the medial compartment)

What are the pathomechanics of osteoporosis?

Osteoclasts continue to work but the osteoblasts become sluggish (result in greater bone resorption)

What is the best treatment of osteoporosis?

Exercise

Other options — drug cocktail of Ca2+, Vit D, and biphosphates

What medications can be used to treat pain for bone pathology?

• Steroids — negatively affects bone density (may be okay for short term usage; ~ 7 days)

• NSAIDS — not enough research to indicate negative impact on bone during fracture healing

What positive effects does regular exercise have on bone?

Positive effects on bone density, size, and most importantly shape

What is an important predictor of bone strength?

The location of increased bone mineral content/density (bone geometry)

Using the FITT Principle, how would you “strengthen bone?”

Frequency — at least 3x / week

Intensity — high impact exercises that produces large rates of deformation of bone matrix

Type — dynamic exercise (produces hydrostatic pressure changes on osteocytes which stimulates bone growth)

Time — short sessions (~ 20 -25 minutes) with longer rest breaks

For bone strengthening, is it better to increase the intensity, number, or time of the exercise session?

Better off increasing the intensity or number of sessions (rather than increasing the time of the exercise)

T/F: Vigorous exercise when you are a child and young adult can reduce your fracture risk later in life

True

After a reduction of bone mass (osteoporosis), what kind of effect can exercise have on bone mineral density?

• Exercise can only have a moderate effect on BMD

• Fracture risks may still be reduced (secondary to remodeling the geometry of the bone)

• Balance training and postural stability will help with avoiding falls

Failure of bone due to tension loads is related to…

Debonding at the cement lines and pulling out of the osteon

What is the resulting fracture of a tension load?

Transverse fracture

Where do pure tension fractures tend to occur?

Tends to occur in areas with large proportions of cancellous bone (e.g., calcaneus and base of 5th metatarsal)

Failure due to compressive loads is mainly seen as…

Oblique cracking of the osteons (~30 degree angle)

What is mostly responsible for fractures that occur due to compressive loads?

Shear forces that result from the compressive load

Where do compressive load fractures typically occur?

Mostly seen at the metaphysis where the cancellous bone is weaker (e.g., spondylolysis — vertebral fracture)

[However, few fractures occur in strictly compression mechanics]

Discuss the fracture mechanics of bone with bending loads

• Bending loads cause compression and tension but these forces are not equal (since bone is not symmetrical)

• Transverse fracture will start on the tension side (since bone is weaker in tension)

• As the fracture propagates, the neutral axis shifts towards the compression side

  • Causes an oblique fracture on this side

• When the cracks meet, a “butterfly” segment occurs

What is an example of a 3-pt bending fracture?

Ski boot fracture

In immature bone, bending loads cause what type of fracture?

Buckle fracture (where you see the tension side start to crack, but it never propagates all the way to the compressive side)

T/F: All loads will produce a shear load

Why or why not?

True

Bone is asymmetrical (therefore difficult to have pure compression, tension, or bending)

Shear loads tend to affect what type of bone?

Affects trabecular bone (more than cortical bone)

How does a torsion fracture occur?

Begins with failure in shear with the creation of a crack parallel to the neutral axis; then followed by failure in tension

(results in twisting about a neutral axis)

What type of fracture occurs from torsion?

Spiral fracture

What are the different types of fractures?

What do high rates of loading imply regarding bone fractures?

Higher rates of loading imply that more energy is stored quickly within the bone prior to failure

When failure occurs this energy is released instantaneously which can lead to comminuted fractures

T/F: Comminuted fractures are possible with higher loads

True

How do fatigue fracture (aka stress fractures) occur?

• Repeated cycles of sub-failure load

• Can occur from a high load repeated a few times or a low load repeated many times

T/F: Fatigue fractures (stress fractures) can be complete fractures

False

Through the cortex only — not complete fractures

How does frequency affect the likelihood of stress fractures? Why?

As frequency of the load increases the likelihood of stress fracture increases

If loading occurs too frequently then there is no time for remodeling to occur

How can muscle fatigue potentially lead to stress fractures?

If a muscle is fatigued, it can affect their ability to transmit loads; this results in more load being transmitted to the bone (and the bone cannot sustain it)

What are the phases of healing for a bone fracture?

1. Inflammation

2. Initial Union

3. Remodeling of Callus

What occurs during the inflammation phase of fracture healing?

• Hematoma around fx site within 3 days

• Mesenchymal cells produce fibrous tissue (to surround broken ends)

• Injured tissue and platelets release vasoactive mediators

• Growth factors and other cytokines are released

  • Influences cell migration, proliferation, differentiation, and matrix synthesis

• Macrophages, PMNs, and mast cells begin process of removing debris

When does the inflammatory phase of fracture healing occur?

Occurs from the time of injury to 24-72 hours

What occurs during the initial union (reparative response) phase of fracture healing?

• Vasoactive substances cause neovascularization and local vasodilation

• Undifferentiated mesenchymal cells migrate to fx site (have ability to form cartilage, bone, fibrous tissue, etc.)

• Hematoma is organized; fibroblasts and chondroblasts appear between the bone ends

  • Cartilage is formed (type III collagen)

How does the stability of the fracture impact the following stages of healing?

• Stable Fracture — fibrous tissue is vascularized

  • Osteoblasts form bone along the periosteal surface of the fibrous layer and bridge the fracture site

• Unstable Fracture — fibrous tissue is non-vascularized

  • New bone is formed along the periosteal surface, but it does not bridge the fracture line

When does the initial union phase occur?

From 2 days to 2 weeks

During the initial union phase the amount of callus formed is ________ proportional to the amount of immobilization of the fracture

During the initial union phase, the amount of callus formed is inversely proportional to the amount of immobilization of the fracture

T/F: In fractures that are fixed with rigid compression plates, there can be primary bone healing with little or no visible callus formation

True

When does the remodeling phase of fracture healing occur?

Middle of repair phase and up to 7 years

During the remodeling phase of fracture healing, the remodeling of woven bone is dependent on what?

Dependent on the mechanical force applied

When is fracture healing complete?

When there is repopulation of the medullary canal

How does remodeling occur in cortical bone (from fracture)?

Occurs by invasion of osteoclasts (to “cut down the bone”) which is then followed by osteoblasts to lay down new lamellar bone (osteon)

How does remodeling occur in trabecular bone (from fracture)?

Occurs on the surface of the trabeculae which causes it to become thicker

T/F: More than one process/phase of healing can occur at any given time

True

Usually just one process dominates and therefore it is labeled as that phase

What are some systemic factors that could influence fracture healing?

• Age

• Hormones

• Functional activity

• Nerve function

• Nutrition

• Drugs (cortisone = decreased callus production)

• Pathologies (e.g., diabetes)

What are some local factors that could influence fracture healing?

• Degree of trauma (e.g., injury to surrounding soft tissue)

• Degree of bone loss

• Vascular injury

• Type of bone fractured

• Degree of immobilization

• Infection

• Local pathologic condition

What is the interfragmentary strain hypothesis?

States that the type of tissue formed in a healing gap depends on the strain that it experiences

• Strain is between 10-100% → granulation tissue can be expected to form

• Strain between 2-10% → fibrocartilage will form

• Strain <2% → bone will form

• 0% Strain → result in no growth

Using the inferfragmentary strain hypothesis, if the gap had movement of 0.5 mm (16% strain) what would occur to the bone mineral density?

Increased bone mineral density

Using the inferfragmentary strain hypothesis, if the gap had movement of 2.0 mm (66% strain) what would occur to the bone mineral density?

Decreased bone mineral density

T/F: Considering the interfragmentary strain hypothesis, the mecahnism of loading is not important regarding bone healing (e.g., tensile or compressive), but what is important is the strain produced

True (change in length is important)

What are the goals for fracture treatments?

• Successful reduction of fracture

• Provide good stability, rapid healing, and minimization of any subsequent deformity

• Prevent malunion