Bone biomechanics Flashcards
What are the mech properties of bone?
main supporting structure in bod
High comp strength, low tensile strength
preotects internal organs
provides framework for locomotion
viscoelastic material - exhibits both viscous and elastic characteristics under deformation
Anisotropic elasticity - determined by variations in lamellar orientation
mineral density varies anisotropically
What are the mech properties of cortical bone?
form outershell of bone and has dense structure
stiff hollow shaft
Stiffner in directions of osteons
Specimens loaded perp to osteon fail in a more brittle manner with little elastic deformation before healing
Orthotropic elasticity - strength varies with direction (some directions assumed transversely isotropic)
Elasticity same in 2 directions (radial and transverse)
Viscoelastic (strain rate, frequency, temperature)
Expulsion of water during loading
Exhibits: stress relaxation, creep, rate dependent stiffness, hysteresis
What are the mech properties of cancellous bone?
dependent on density and trabecular architecture
E = c(ro)^n
23% reduction in desnity = 65% reduction in strength
Anisotropic: 165 MPa in supero-inferior direction to 43 MPa in lateral direction
tension = 15,6 MPa, compression = 20 Mpa
increased porosity of trabecular bone to cortical bone
What are the forces in bone?
Cancellous bone located where stresses are limited in direction
cancellous bone found where stresses are weake/mulitdirectional
Tension on lateral side of shaft
compression on medial side of shaft
How is bone damaged?
microdamage accumulation due to fatigue
Fatigue life of any material is a function of crack accumulation and growth
cracks accumulate fast - caapacity for bone repair exceeded and stress fractures result
accumulate at normal rates but repair mechanism lack, fragility fracture result
How are cracks detected?
Bulk staining bone sections using basic fuchsin with light micrsocopy
Fluorescent chlating detections
what are different types of bone fractures?
High impact, low impact, repetitive
Describe a compressive fatigue test carried out on bone specimens
Δσ= 80MPa Frequency=3 Hz
Sequence of dye application
➢ Alizarin Red Before testing
➢ Xylenol Orange First 10,000 cycle
➢ Calcein Green 10,000 to 50,000 cycles
➢ Calcein Blue 50,000 cycles to failure
What are the different types of bone fracture?
simple,butterfly, comminuted, greenstick
what are the bone fracture types II?
Open: puncture of skin - infection risk
Closed: no puncture - better prognosis
What are the three phases of secondary fracture repair?
inflammation, callus formation, remodelling
Describe the inflammation phase of fracture repair?
0-5 days post fracture -infiltration of immune cells (neutrophil and macophage)
Bleeding into fracture site, forming a
hematoma.
Blood cells + MSC + osteoprogenitors + fibroblasts invade fracture site
Growth of small blood cells
Describe the soft callus formation stage of fracture repair
➢ Begins when pain and swelling subsides
➢ Ends when bone fragment are united by
fibrous/cartilagenous tissue
➢ No longer mobile
➢ New blood vessels
➢ Not calcified
Describe the hard callus formation phase of fracture repair
➢ Conversion of cartilage to woven bone
➢ Restores strength and stiffness
➢ Endochondral ossification in internal callus
➢ Membranous ossification in periosteal callus
Describe the bone remodelling phase of fracture repair
woven bone replaced by lamellar bone
What are the local and systemic factors affecting fracture repair?
Local:
➢ Type of bone
➢ Amount of bone loss
➢ Degree of reduction/Immobilisation
➢ Degree of local trauma
➢ Vascular supply
➢ Infection
➢ Local disease
Systemic:
➢ Age
➢ Nutrition
➢ Functional activity
➢ Nerve function
➢ Hormones
what are the repair/union problems of fracture repair?
Delayed union - not within 3 months
Non- union - not within 9 months
what are the types of medical devices used in fracture repair?
Cast – avoid motion and damage to vessels/nerves
Compression plating - rigid (closes gap), dynamic (small movement enhances callus formation)
Dynamic hip screw - extracapsular fracture
External fixation
intramedullary nailing
grafts: autograft, allograft, xenograft, synthetic graft
What are the mechanical effects on fracture healing?
- mechanical deformation and motion can affect the course of fracture healing
-Interfragmentary strain theory - changes in fracture gap tissue related to strain magnitudes in the fracture gap - these magnitude determine diferrentiation of fracture gap tissue
Interfragmentary strain - relative displacement of the fracture gap ends divided by the initial fracture gap width.
Describe the different effects of the intergragmentary strains
above 100% = non-union.
between 10 and 100% = sustain initial fibrous tissue formation
between 2 and 10% = cartilage formation and an endochondral ossification formation.
Under 2% = direct bone formation and primary fracture healing.
What is fracture callus?
Increase in radius by callus greatly increases
moment of inertia and stiffness ( Moment of inertia proportional to r4 )
what are the functions of fixation?
➢ Obtain and maintain alignment
➢ Reduction
➢ Transmission of compressive forces
➢ Minimum motion across fracture site
➢ Achieve stability
➢ Avoid tensile/ shear/torsion forces across fracture site
➢ Prevent motion in most crucial plane
What do external fixators do?
used for trauma and deformity correction or limb lengthening
principal - bypass load to fracture site - provide stability
realign bones using pins, wires, clamps and bars/rings
types: Pin and ring systems
What fixator provides the max stability?
four pin construct - one pin as close to fracture site as possible and other as far from fracture as possible (more pins = distributed force and increased stiffness of system - more pins can increase damage for anatomic structures and provide portals for infection)
What are the types of external fixators?
unilateral, bilateral, triangular, semi- or circular
describe the role pins play in external fixators
incr core diam = bending strenth incr at rate prop to radius to fourth power. 6mm pin 5 times stiffer than a 4mm pin
maximise stiffness
dont exceed 1/3 of bony diameter (minimise risk of pin hole fractures -weak point of pin thread- shank junction)
Thermal necrosis of bone during pin insertion causes loosening and infection
Mech properties of bone changed at tempts greater than 50 degrees
How to increase stability of an external fixator?
incr pin diam
incr number of pins
incr spread of pins
multiplanar fixation
reduce bone-frame distance
predrilling and cooling (reduce thermal necrosi)
radially preload pins
90 degree tensioned wires
what are the advantages of external fixation?
easy application
good stability
excellant pain relief
adjustable
access to open wounds
What are the types of internal fixation?
comminuted - nail with interlocking screw
transverse/oblique - plates/screws
What are the principles of internal fixation
- Rigid, anatomic fixation and allows an early return to function
- For cases that cannot be reduced and immobilized by external means
- Good blood supply to undisturbed tissues
what are intramedullary (IM) rods?
align and stabilize fractures. Inserted into the bone marrow canal in the centre of the long bones of the extremities. IM rods share the load with the bone, rather that entirely support the bone.
What is a dynamic compression plate (DCP)?
compress the fracture
Offset screws exert force on specially designed holes in plate
Force between screw and plate moves bone until screw sits properly
Compressive forces are transmitted across the fracture
