biomecanica fracturi

8/18/2019 Biomecanica fracturi

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Biomechanics of Fractures

and Fixation


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Most movements arelikely a combination of

both linear and

angular motion


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SETUP ANALYSIS

CALIBRATION

(

(50,490)

(10, 570)


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GROUND REACTION FORCES


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1. Extended swingaround bar

2.

Extended swingaround central axis3. Pike4. Tuck

Assuming:

!md2

Where:

M = massd = distance from

axis of rotation


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U.:8(&% 2:(;

Water particles attract other water particles and

will increase with “roughness of skin”


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V:#30% 2:(;

Low pressure pocket forms

and “ holds back ” the

cyclist. As velocity doubles this

resistive force quadruples!!!!

Important factors:• Shape

•

smoothness

•

orientation (crouch can lower

resistance ~30%


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e%C.&"); 2:(;•

Frame designson bikes are often

“ tear-shaped ” to

reduce drag

•

Drafting within 1 m

can reduce drag

accounting for 6% of

energy cost (e.g., ducks flying)


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@"f

\#$7#)%)/ #8 (": :%*"*/()&% /'(/ "* C":%&/%C

(/ :";'/ ();0%* /# /'% C:(; 8#:&%Lift

Air Flow

Drag

Resultant


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@"f c $$#) %O($70%High velocity/Low Pressure

Low velocity/High Pressure

According to Bernoulli's Law , faster air has lower

air pressure, and thus the high pressure beneath the

wing pushes up to cause lift.


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• Fluid dynamics

– drag

–

lift


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Basic Biomechanics

•

Material Properties

–

Elastic-Plastic

–

Yield point

–

Brittle-Ductile

–

Toughness

•

Independent of

Shape!

•

Structural Properties

–

Bending Stiffness

–

Torsional Stiffness

–

Axial Stiffness

•

Depends on Shape

and Material!


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Basic BiomechanicsForce, Displacement & Stiffness

Force

Displacement

Slope = Stiffness =

Force/Displacement


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Basic Biomechanics

Stress = Force/Area Strain Change Height (!L) /Original Height(L0)

Force

AreaL


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Basic BiomechanicsStress-Strain & Elastic Modulus

Stress =

Force/Area

Strain =Change in Length/Original Length (!L/ L0)

Slope =

Elastic Modulus =

Stress/Strain


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Basic BiomechanicsCommon Materials in Orthopaedics

•

Elastic Modulus

(GPa)

•

Stainless Steel 200

•

Titanium 100

•

Cortical Bone 7-21

•

Bone Cement 2.5-3.5

•

Cancellous Bone

0.7-4.9

•

UHMW-PE 1.4-4.2

Stress

Strain


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Basic Biomechanics

•

Elastic Deformation

•

Plastic Deformation

•

Energy

Energy

Absorbed

Force

Displacement

PlasticElastic


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Basic Biomechanics

• Stiffness-Flexibility

• Yield Point

•

Failure Point

•

Brittle-Ductile

• Toughness-Weakness

Force

Displacement

PlasticElastic

Failure

Yield

Stiffness


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Stiff

Ductile

Tough

StrongStiff

BrittleStrong

Ductile

WeakBrittle

Weak

Strain

Stress


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Flexible

Ductile

Tough

Strong

Flexible

Brittle

Strong

Flexible

DuctileWeak

Flexible

Brittle

Weak

Strain

Stress


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Basic Biomechanics

•

Load to Failure

–

Continuous

application of force

until the materialbreaks (failure point

at the ultimate load).

– Common mode offailure of bone and

reported in theimplant literature.

•

Fatigue Failure

–

Cyclical sub-

threshold loading

may result in failuredue to fatigue.

–

Common mode of

failure of orthopaedicimplants and fracture

fixation constructs.


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Basic Biomechanics

•

Anisotropic

–

Mechanical

properties dependent

upon direction ofloading

•

Viscoelastic

–

Stress-Strain

character dependent

upon rate of appliedstrain (time

dependent).


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Bone Biomechanics

•

Bone is anisotropic - its modulus isdependent upon the direction ofloading.

•

Bone is weakest in shear, then tension,then compression.

•

Ultimate Stress at Failure Cortical BoneCompression < 212 N/m2

Tension < 146 N/m2

Shear < 82 N/m2


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Bone Biomechanics

•

Bone is viscoelastic: its force-

deformation characteristics are

dependent upon the rate of loading.

•

Trabecular bone becomes stiffer in

compression the faster it is loaded.


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Bone Mechanics

•

Bone Density

–

Subtle densitychanges greatlychanges strength

and elastic modulus•

Density changes

– Normal aging

–

Disease

–

Use –

Disuse

Cortical Bone

Trabecular Bone

Figure from: Browner et al: Skeletal Trauma

2nd Ed. Saunders, 1998.


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Basic Biomechanics

•

Bending

•

Axial Loading

–

Tension –

Compression

•

Torsion

Bending Compression Torsion


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Fracture Mechanics

Figure from: Browner et al: Skeletal Trauma 2nd Ed, Saunders, 1998.


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Fracture Mechanics

•

Bending load:

–

Compression

strength greater than

tensile strength –

Fails in tension

Figure from: Tencer. Biomechanics in Orthopaedic

Trauma, Lippincott, 1994.


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Fracture Mechanics

•

Torsion

–

The diagonal in the direction of the applied force is in

tension – cracks perpendicular to this tension diagonal

–

Spiral fracture 45º to the long axis

Figures from: Tencer. Biomechanics in Orthopaedic



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Fracture Mechanics

•

Combined bending

& axial load

–

Oblique fracture

–

Butterfly fragment

Figure from: Tencer. Biomechanics in Orthopaedic



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Moments of Inertia

•

Resistance to

bending, twisting,

compression or

tension of an object isa function of its shape

•

Relationship of

applied force to

distribution of mass(shape) with respect

to an axis. Figure from: Browner et al, Skeletal Trauma 2nd Ed,Saunders, 1998.


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Fracture Mechanics

•

Fracture Callus

–

Moment of inertia

proportional to r 4

–

Increase in radius bycallus greatly

increases moment of

inertia and stiffness

1.6 x stronger

0.5 x weakerFigure from: Browner et al, Skeletal Trauma2nd Ed, Saunders, 1998. Figure from: Tencer et al: Biomechanics in

Orthopaedic Trauma, Lippincott, 1994.


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Fracture Mechanics

•

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.


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IM Nails

Moment of Inertia•

Stiffness

proportional to the

4th power.

Figure from: Browner et al, Skeletal Trauma, 2nd Ed,

Saunders, 1998.


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IM Nail Diameter

Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.


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Slotting

Figure from: Tencer et al, Biomechanics

in Orthopaedic Trauma, Lippincott, 1994.

Figure from Rockwood and Green’s, 4th Ed

• Allows more flexibility

•

In bending

• Decreases torsional strength


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Slotting-Torsion

Figure from: Tencer et al, Biomechanics

in Orthopaedic Trauma, Lippincott, 1994.


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Interlocking Screws

• Controls torsion and axialloads

• Advantages – Axial and rotational stability

– Angular stability

•

Disadvantages – Time and radiation

exposure

– Stress riser in nail

•

Location of screws – Screws closer to the end

of the nail expand thezone of fxs that can befixed at the expense ofconstruct stability


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Biomechanics of Internal Fixation


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Biomechanics of Screw Fixation

•

To increase strength

of the screw & resist

fatigue failure:

–

Increase the innerroot diameter

•

To increase pull out

strength of screw in

bone:

–

Increase outerdiameter

–

Decrease inner

diameter

– Increase thread density

– Increase thickness ofcortex

–

Use cortex with more


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Biomechanics of Screw Fixation

•

Cannulated Screws

–

Increased inner

diameter required

–

Relatively smallerthread width results in

lower pull out strength

–

Screw strength

minimally affected

(! r 4outer core - r 4inner core )

Figure from: Tencer et al, Biomechanics in

OrthopaedicTrauma, Lippincott, 1994.


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Biomechanics of Plate Fixation

•

Plates:

–

Bending stiffness

proportional to the

thickness (h) ofthe plate to the 3rd

power.I= bh3/12

Base (b)

Height

(h)


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•

Functions of the plate

–

Compression

–

Neutralization

–

Buttress

• “The bone protects the

plate”


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•

Unstable constructs

–

Severe comminution

–

Bone loss

–

Poor quality bone –

Poor screw

technique


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•

Fracture Gap /Comminution

–

Allows bending of plate with

applied loads

–

Fatigue failure

Applied Load

BonePlate

Gap


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•

Fatigue Failure

–

Even stable

constructs may fail

from fatigue if thefracture does not

heal due to biological

reasons.


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•

Bone-Screw-Plate

Relationship

–

Bone via

compression –

Plate via bone-plate

friction

–

Screw via resistance

to bending and pull

out.

Applied Load


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•

The screws closest

to the fracture see

the most forces.

•

The construct rigiditydecreases as the

distance between

the innermost

screws increases. Screw Axial Force


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•

Number of screws (cortices)recommended on each side ofthe fracture:

Forearm 3 (5-6)

Humerus 3-4 (6-8)

Tibia 4 (7-8)

Femur 4-5 (8)


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Biomechanics of Plating

•

Tornkvist H. et al: JOT 10(3) 1996, p

204-208

•

Strength of plate fixation ~ number of

screws & spacing (1 3 5 > 123)

•

Torsional strength ~ number of screws

but not spacing


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Biomechanics of External Fixation


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•

Pin Size

–

{Radius}4

–

Most significant

factor in framestability


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•

Number of Pins

–

Two per segment

–

Third pin


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A

B

CThird pin (C)

out of plane of

two other pins

(A & B)

stabilizes that

segment.


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• Pin Location

–

Avoid zone of injury or future ORIF

– Pins close to fracture as possible

–

Pins spread far apart in each fragment

•

Wires

– 90º


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•

SUMMARY OF EXTERNAL FIXATOR STABILITY:Increase stability by:

1] Increasing the pin diameter.

2] Increasing the number of pins.

3] Increasing the spread of the pins.

4] Multiplanar fixation.5] Reducing the bone-frame distance.

6] Predrilling and cooling (reduces thermal necrosis).

7] Radially preload pins.

8] 90° tensioned wires.

9] Stacked frames.

**but a very rigid frame is not always good.


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Ideal Construct

• Far/Near - Near/Far on either side of fx

• Third pin in middle to increase stability

•

Construct stability compromised withspanning ext fix – avoid zone of injury (far/

near – far/far)


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Biomechanics of Locked

Plating

Conventional Plate Fixation

Courtesy of Synthes- Robi Frigg


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Patient Load

Conventional Plate Fixation

Patient Load


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Locked Plate and Screw Fixation

Patient Load =<Compressive

Strength of theBone

St i th B



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Stress in the Bone

Patient Load

+ Preload

St d d L k d L di



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Standard versus Locked Loading

Pullout of regular screwsCourtesy of Synthes- Robi Frigg


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Pullout of regular screws

by bending load

Higher resistant LHSCourtesy of Synthes- Robi Frigg


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g

against bending load

Larger resistant area


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Biomechanical Advantages of

Locked Plate Fixation

•

Purchase of screws to bone not critical

(osteoporotic bone)

• Preservation of periosteal blood supply

•

Strength of fixation rely on the fixed angle

construct of screws to plate

• Acts as “internal” external fixator

Preservation of Blood Supply


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Preservation of Blood Supply

Plate Design

DCP LCDCP

Preservation of Blood SupplyCourtesy of Synthes- Robi Frigg


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Preservation of Blood SupplyLess bone pre-stress

Conventional Plating

•

Bone is pre-stressed

• Periosteum strangled

Locked Plating

•

Plate (not bone) is pre-stressed

• Periosteum preserved

Angular Stability of ScrewsCourtesy of Synthes- Robi Frigg


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Angular Stability of Screws

LockedNonlocked

Biomechanical principlesCourtesy of Synthes- Robi Frigg


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similar to those of external fixators

Stress distribution

Surgical TechniqueCourtesy of Synthes- Robi Frigg


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g qCompression Plating

•

The contoured plate maintains anatomical

reduction as compression between plate and bone

is generated. •

A well contoured plate can then be used to help reduce

the fracture. Traditional Plating



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g qReduction

If the same technique is attempted with a locked plate and locking screws, an anatomical reduction

will not be achieved. Locked Plating


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S i l T h i


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Surgical TechniquePrecontoured Plates

1. Contour of plate is importantto maintain anatomic

reduction.

Conventional Plating Locked Plating

1. Reduce fracture prior toapplying locking screws.

Unlocked vs Locked Screws


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Biomechanical Advantage

Unlocked vs Locked Screws

1. Force distribution2. Prevent primary reduction loss

Sequential Screw Pullout Larger area of resistance

3. Prevent secondary reduction loss4. “Ignores” opposite cortex integrity

5. Improved purchase on osteoporotic bone



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g qReduction with Combination Plate

Lag screws can be used to help reduce fragmentsand construct stability improved w/ locking screws

Locked Plating


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Hybrid Fixation

•

Combine benefits of both standard &

locked screws

•

Precontoured plate

•

Reduce bone to plate, compress & lag

through plate

•

Increase fixation with locked screws at

end of construct


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