Forces acting on bones and implants can cause fracture or implant failure. Bones experience compression, tension, bending, and torsional forces. Implants must resist these forces to provide stability for bone healing. Intramedullary nails share loading with bone to heal fractures. Nail properties like diameter, length, and material influence its bending stiffness and resistance to forces. Proper surgical technique including starting point, working length, and interlocking screws is needed for successful bone healing.

2. ● Basic Concepts/Definitions
● Biomechanics of Intact/Healing Bone
● Biomechanics of Fracture
● Bone Healing
● Biomechanics of Implants: Avoiding
problems

3. Forces Acting on Long Bones
● Force is a vector
(magnitude with direction)
● Moment: Force acting on
a bone causing rotation
● Moment Arm: lever that
force acts on (some
distance away from center
of rotation)

4. Basic Forces Causing Fracture
● Compression
● Tension
● Transverse Loading
● Torsion

5. Forces On Healing Fx
● Tension
● Compression
● Shear
● (Bending)

6. Forces Acting on Construct
● Stress = Force/(Area
force is acting on)
= (normalized force)
● Strain = (Change in
Ht)/(Original Ht)
● Elastic Modulus =
Stress/Strain
= measure of stiffness

7. Deformation
● Elastic = if load is
removed material returns to
original shape
● Plastic = residual
deformation after load
removed
● Yield Point = load when
plastic deformation takes
place
● Work = Force x (distance
of bending)
● Toughness = Amt of work
req to Fx material

8. Implant Shape
● Moment of Inertia:
further away material
is spread in an object,
greater the stiffness
● Stiffness and strength
are proportional to
radius4

9. Fatigue
● Cyclic Loads below failure level  progressive
cracks  failure
● Stress Concentrator = radical change in shape
● Galvanic Corosion = Flow of electrons from (-) to
(+) in 2 dissimilar materials in conductive fluid
● Fretting = rubbing of 2 surfaces together
(removing oxidative layer)
● Crevice Corrosion = impurities in material

10. Crevice Stress
Fretting Galvanic

11. Viscoelasticity
● Increased resistance with
increased loading rate
● Creep = under constant load
soft tissue will continue to
gradually deform
● If compressive force is applied
slowly, syringe offers little
resistance
● Increased rate of force,
increased resistance to rate of
motion of syringe

12. Viscoelasticity
● Stress Relaxation: Applied
force with constant
displacement  Decrease in
internal force as resistance
decreases
– resistance decreases as fluid is
forced from syringe
● Trabecular Bone: Trabecular
structure acts as elastic
component, Interstitial fluid
thru porous matrix is viscous
component
– Under higher loading rate there
is resistance to flow thus
increased internal pressure thus
increased stiffness of bone

13. Biomechanics Intact/Healing Bone
● Hierarchical structure
– Collagen embedded with
apatite
– Decreased modulus with
decreased apatite:collagen
ratio
● Fibrils organized to resist
force
– Fibers organized into
lamellae
– Concentric Lemellae make an
Osteon

14. Strength/Stiffness
● Strength proportional to
density2
● Modulus proportional to
density(2 to 3)
● Age: increased modulus,
bending strength from child
to adult, then decrease
● Holes/defects weaken bone
(round better than square)
● Strength proportional to
diameter4

15. Biomechanics of Bone Fx
● Weakest in Tension,
Strongest in
Compression
● Pure Bending 
Transverse Fx
● Torsion  Spiral
Fx
● Shear  Oblique
Fx
● Butterfly due to Bend
+ Shear

16. ● Smaller cross section of
bone fails 1st (distal 1/3 of
tibia)
● Osteoperosis
– decreased density of
trebecular bone
– decreased endosteal
thickness of cortical bone

17. Bone Healing
● Direct
– Primary bone healing
– Cutting cones
– Seen with absolute stability
● Indirect
– Secondary bone healing
– Callus formation; resorption at fx site;
– Seen with relative stability

18. Indirect Stages:
● Inflammation
– 1-7 days
● Soft callus
– 3 weeks
● Hard callus
– 3 – 4 months
● Remodeling
– months => years

19. ● Preload > external load => absolute
stability!
● External load > preload => relative stability!
● External load >> preload => frank
instability!!

20. Relative Stability
● Motion between fracture fragments that is
compatible with fracture healing.
● Motion is below the critical strain level of tissue
repair.
● Promotes indirect bone healing!
● Examples:
– IM nails
– Bridge plate
– External Fixator

21. Absolute Stability
● Compression of two anatomically reduced
fracture fragments.
● No displacement of the fracture under
functional load.
● Promotes direct bone healing!
● Examples:
– Lag screw
– Plate => compression, buttress, neutralization
– Tension band

22. Biomechanics of Implants:
Avoiding Problems
● Cerclage Wire: strength proportional to
diameter
– Solid wire sensitive to scratch/notch (cable
better)
– Optimal # of turns 4-8 when twisted

23. Screw fixation
● Rotary forces  compression between
objects (inclined plane on spiral pulls object
toward head)
● Four part construction: head, shaft, thread,
tip
● Thread defined by root diameter, thread
diameter, pitch

24. Screws
● Larger core diameter has
higher resistance to fatigue
& shear failure
– 4th power of the diameter
● Pullout strength (maximum
force screw can support along
its axis)
– outer diameter, length of
engagement, shear
strength/density of bone

25. Screws
● Tapping
– increase compressive forces,
decrease interface friction
● Cyclic Loading: If plate is
not tight enough to bone
– Force transferred to long axis
of screw
– Stress corrosion of plate
rubbing

26. Plates & Bending failure
● Leaving gap opposite plate
makes it a fulcrum
● increased stress at holes
● avoid holes over fracture sites
● greater the span between
screws
– less stiff
– more bending

27. Prebending

28. Femoral Nail
● Contact Forces expand
femur (hoop stresses)
may cause it to split if
too large
● Starting hole: too
medial, too anterior
● Initial curve of IM rod,
rod stiffness

29. Femoral Nail
● Distal Fx:
– Longer moment arm of external force
thus greater bending moment in rod
– Greatest area of stress in rod (Fx site)
is near screw holes (stress riser)
– Locking screw supported only by
cortices
● Stiffness & strength to bend decrease with
length3
● Possible to nick border of rod hole
w/ screw  accentuate fatigue

30. Ex-Fix
● Self-tapping pins  local heat 
thermal necrosis & microcracking
(thus corrosion/fatigue)
● Pin Micromotion  bone
resorption at interface
– Undersize hole 0.1 mm  decr
micromotion
– Undersize >0.3 mm  incr
microcracking

31. Ex-Fix
● Deformation of Pin or Side Bar
– Stiffness & Strength proportional to diameter4
– Stiffness & Strength inversely proportional to lenght3
● To increase strength:
– Decrease sidebar to bone distance
– Increase pin diameter
– Put pins closer to fracture
– Increase # of sidebars
– Bury pin thread completely within cortex
● Add Sidebar at 90 degree plane also resist torsion

32. Test Questions

43. Intramedullary Nails
● “Internal Splint”, Load Sharing
● Nail itself should resist bending and torsion
● The bone should resist axial loading
● Strength => wall thickness, diameter, and
material
● stiffness => 4th power of the diameter
● Type of fracture –transverse, oblique, or
complex determines some stability

44. Intramedullary Nails
● Working length is area that spans fracture
between points of fixation.
● In bending, stiffness inversely proportional
to square of length
● Torsional rigidity is inversely proportional
to length

45. Intramedullary Nails
● Gripping strength is resistance to slipping at
bone-implant interface. Increased by
increasing cortical contact.
● Nail can twist or slip with torsional loading
which allows angulation

46. Intramedullary Nails
interlocking
● Dynamic fixation controls bending and
rotation, but allows axial loading
● Static locking controls bending, rotation,
and axial loading

47. Intramedullary Nails
interlocking

49. ● femoral splitting
● starting point
● length of proximal
fragment
● initial curvature
● stiffness

50. ● distal fracture=> longer
moment arm
● fracture site to distal
locking screw distance
< 4cm
● screw breakage in
distal fracture