Bone biomechanics

Question 1

What are the mech properties of bone?

Answer 1

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

Question 2

What are the mech properties of cortical bone?

Answer 2

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

Question 3

What are the mech properties of cancellous bone?

Answer 3

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

Question 4

What are the forces in bone?

Answer 4

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

Question 5

How is bone damaged?

Answer 5

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

Question 6

How are cracks detected?

Answer 6

Bulk staining bone sections using basic fuchsin with light micrsocopy
Fluorescent chlating detections

Question 7

what are different types of bone fractures?

Answer 7

High impact, low impact, repetitive

Question 7

Describe a compressive fatigue test carried out on bone specimens

Answer 7

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

Question 8

What are the different types of bone fracture?

Answer 8

simple,butterfly, comminuted, greenstick

Question 9

what are the bone fracture types II?

Answer 9

Open: puncture of skin - infection risk
Closed: no puncture - better prognosis

Question 10

What are the three phases of secondary fracture repair?

Answer 10

inflammation, callus formation, remodelling

Question 11

Describe the inflammation phase of fracture repair?

Answer 11

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

Question 12

Describe the soft callus formation stage of fracture repair

Answer 12

➢ Begins when pain and swelling subsides
➢ Ends when bone fragment are united by
fibrous/cartilagenous tissue
➢ No longer mobile
➢ New blood vessels
➢ Not calcified

Question 13

Describe the hard callus formation phase of fracture repair

Answer 13

➢ Conversion of cartilage to woven bone
➢ Restores strength and stiffness
➢ Endochondral ossification in internal callus
➢ Membranous ossification in periosteal callus

Question 14

Describe the bone remodelling phase of fracture repair

Answer 14

woven bone replaced by lamellar bone

Question 15

What are the local and systemic factors affecting fracture repair?

Answer 15

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

Question 16

what are the repair/union problems of fracture repair?

Answer 16

Delayed union - not within 3 months
Non- union - not within 9 months

Question 17

what are the types of medical devices used in fracture repair?

Answer 17

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

Question 18

What are the mechanical effects on fracture healing?

Answer 18

• 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.

Question 19

Describe the different effects of the intergragmentary strains

Answer 19

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.

Question 20

What is fracture callus?

Answer 20

Increase in radius by callus greatly increases
moment of inertia and stiffness ( Moment of inertia proportional to r4 )

Question 21

what are the functions of fixation?

Answer 21

➢ 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

Question 22

What do external fixators do?

Answer 22

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

Question 23

What fixator provides the max stability?

Answer 23

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)

Question 24

What are the types of external fixators?

Answer 24

unilateral, bilateral, triangular, semi- or circular

Question 25

describe the role pins play in external fixators

Answer 25

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

Question 26

How to increase stability of an external fixator?

Answer 26

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

Question 27

what are the advantages of external fixation?

Answer 27

easy application
good stability
excellant pain relief
adjustable
access to open wounds

Question 27

What are the types of internal fixation?

Answer 27

comminuted - nail with interlocking screw
transverse/oblique - plates/screws

Question 28

What are the principles of internal fixation

Answer 28

• 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

Question 29

what are intramedullary (IM) rods?

Answer 29

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.

Question 30

What is a dynamic compression plate (DCP)?

Answer 30

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