10 Bone Mechanics (Jastifer)
10.1 Bone Fracture Mechanics
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10.1.1 Functions of Bones
- Support
- Protection
- Movement
- Mineral Storage
- Hematopoietic (bone marrow makes the cells of blood)
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10.1.2
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10.1.3
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10.1.4 Microscopic observation
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- Composite material
- Basic unit: Haversian system (osteon)
- Weight %
- 2/3 inorganic material -Hydroxyapatite, \(3Ca^3(PO4)^2 Ca(OH)^2\)
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10.1.5 Haversian System
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Picture courtesy Gwen Childs, PhD.
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10.1.6 Bone Remodeling
- Coupled process of bone resporption and formation
- In the first year of life, almost 100% of the skeleton is replaced.
- In adults, about 10% per year
- An imbalance in the regulation of bone remodeling results in many metabolic bone diseases, such as osteoporosis
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10.1.7 Bone Remodeling
- Why do we need this process?
- Reshapes bones during growth
- Mineral balance (Calcium)
- Repair micro-damage
- Response to loads (Wolff)
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10.1.8
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10.1.9
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10.1.10
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10.1.11 Bone Multicellular Unit
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10.1.12 Microcracks
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10.1.13 Bone repair
- 2 methods of bone repair
- Direct bone healing
- Direct apposition (touching) of bone surfaces
- Indirect bone healing
- Does not require apposition of bone surfaces
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10.1.14 Direct bone healing
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- Surfaces are close enough together that body treats it as a remodeling process
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Credit: JBJS (br) evolution of internal fixation of long bone fractures
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10.1.15 Indirect Bone Healing
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- Gap in fracture surfaces
- Resorption of surfaces (more to come) -
JBJS (br) Evolution of internal fixation of long bone fractures
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10.1.16 Indirect repair summary
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10.1.17 Bone Repair- response to catastrophic failure
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10.1.18 Bone repair: Callus
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Figure from Brighton, et al, JBJS-A, 1991.
- Within days, callus begins to form
- Composed of woven bone
- Function: Gradually enlarges fracture ends and thereby fracture cross sectional area
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10.2 Collagen orientation in woven bone (Callus)
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With permission http://en.wikipedia.org/wiki/File:Woven_bone_matrix.jpg
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10.2.1 Also Example 8.1
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Figure from: Browner et al, Skeletal Trauma 2nd Ed, Saunders, 1998.
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10.2.2 Fracture Mechanics
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- Time of Healing
- Callus increases with time
- Stiffness increases with time
- Near normal stiffness at 27 days
- Does not correspond to radiographs
Figure from: Browner et al, Skeletal Trauma, 2nd Ed, Saunders, 1998.
10.2.3
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10.2.4
Credit: JBJS (br) Evolution of internal fixation of long bone fractures
- Indirect Bone Healing
- Gap in fracture surfaces
- Resorption of surfaces
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10.2.5 Why does resorption take place?
- Strain theory: “tissue cannot be produced under strain conditions which exceed the elongation at rupture of the given tissue element, such as a cell”
- Either absolute stability (direct bone healing) or elastic flexible fixation of a gap is required (indirect healing) -
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10.2.6 Resorption at fracture sites
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10.2.7 Interfragmentary strain
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10.2.8
It is easier to get a comminuted fracture to heal than a simple fracture
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www.orthosupersite.com
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10.3 Bone Biomechanics
Bone is anisotropic
Ultimate Stress at Failure Cortical Bone Compression < 212 N/m2 Tension < 146 N/m2 Shear < 82 N/m2
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10.4 Bone Biomechanics
10.5 Bone is viscoelastic:
Becomes stiffer in compression the faster it is loaded.
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10.6 Bone Mechanics
Bone Density - Subtle density changes greatly changes strength and elastic modulus - Density changes - Normal aging - Disease - Use - Disuse Figure from: Browner et al: Skeletal Trauma 2nd Ed. Saunders, 1998.
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Fracture Mechanics
Figure from: Browner et al: Skeletal Trauma 2nd Ed, Saunders, 1998.
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Intramedullary Nails - Device placed in intramedullary canal of long bones - Femur - Tibia - Humerus - Radius - Ulna - Metacarpals
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10.6.1 IM Nails
- Moment of Inertia
- Bending rigidity is
- Product E*I
- Axial rigidity
- Product E*A
- Bending rigidity is
10.6.2 Torsional rigidity
- Product G*J where G is elastic shear modulus and J is torsional constant (for cylinder is equal to I)
Figure from: Browner et al, Skeletal Trauma, 2nd Ed, Saunders, 1998.
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10.6.3 Cross-Section Design
- 92% drop in torsional rigidity
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10.6.4 Slotting-Torsion
- Slot in nail decreases torsional rigidity by 92%
(pp268-269)
Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.
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IM Nail Diameter
Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.
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Biomechanics of Internal Fixation
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10.7 Biomechanics of Plate Fixation
Plates: - Bending stiffness proportional to the thickness (h) of the plate to the 3rd power.
Base (b)
I= bh3/12
Heigh t (h)
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Biomechanics of Plate Fixation Applied Load
Applied Load
Bone-Screw-Plate Relationship - Bone via compression - Plate via boneplate friction - Screw via resistance to bending and pull out.
Fracture Gap - Plate alone resists loads
Plate
Gap
Bone
10.7.1
Construct stiffness is similar however, Locking plates do not allow motion at near cortex
10.7.2 Biomechanics of External Fixation
10.7.3 Biomechanics of External Fixation
Pin Size
{Radius}4 Most significant factor in frame stability
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10.8 Biomechanics of External Fixation
10.8.1 SUMMARY OF EXTERNAL FIXATOR STABILITY:
Can make a fixator more stable by:
- Increasing the pin diameter.
- Increasing the number of pins.
- Increasing the spread of the pins.
- Multiplanar fixation.
- Reducing the bone-frame distance.
- Predrilling and cooling during insertion (reducing thermal necrosis).
- Stacked frames.
**but a very rigid frame is not always good.
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10.9 Biomechanics of Screws
10.10 Screw Anatomy
Inner diameter Outer diameter Pitch
Figure from: Tencer et al, Biomechanics in OrthopaedicTrauma, Lippincott, 1994.
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10.11 Biomechanics of Screw Fixation
To increase strength of the screw, & resist fatigue: -
Increase the inner diameter
To increase pull out strength of screw in bone: -
Increase outer diameter Decrease inner diameter Increase thread density Increase thickness of cortex (of bone) Use cortex with more density (young patient).
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10.11.1 Biomechanics of Screw Fixation
- Cannulated Screws Increased inner root diameter required to compensate for cannulation.
- Relatively smaller thread width.
- Screw strength minimally affected
- (α r4outer core - r4inner core )
Figure from: Tencer et al, Biomechanics in OrthopaedicTrauma, Lippincott, 1994.
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10.11.2
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