biomechanics

BIOMECHANICS OF FIXATION

IN ORTHOPEDICS

By

Dr.Mohammed Elbasheir Elhussein

Introduction

Biomechanics :Science of forces, internal or external,

on the living body.Biomaterials :• Is any matter, surface, or construct

that interacts with biological systems.

• Encompasses all synthetic and natural materials used during orthopedic procedures.

Introduction

.Biomaterials :• Is any matter, surface, or construct

that interacts with biological systems.

• Encompasses all synthetic and natural materials used during orthopedic procedures.

Kinesiology

• Study of human movements and motions.• Kinematics.• Kinetics.• Anatomy.• Physiology.• Motor control.

Dynamics

The study of forces and torques and their effect on motion, as opposed to kinematics, which studies the motion of objects without reference to its causes.

Kinematics

• Describes the study of motion (displacement, velocity, and acceleration) of the living bodies without consideration of the causes of motion.

Kinetics

Concerned with the relationship between the motion of bodies and its causes, namely forces and torques

Basic definitions

load :A force that actson a body.

A. Compression,B. tension,C. shear,D. torsion

Compression, tension, shear, and torsion

Basic definitions

Stress :• definition – intensity of an internal force or – Internal resistance of a body to a load

• calculation – force / area–Unit of measure: pascal (Pa) = N/m2

Types of stresses:

1-Tensional stress:• Stress applied in two opposite

directions.2- Compressive or tensile:

• Perpendicular to the surfaces on which they act.

3- Shear stress:• Parallel to the surfaces on

which they act.

Strain

Definition :relative

measureof thedeformationof an object.calculation change in

length / original length

Roman –era bridge in Switzerland

Mechanical property definitions

Deformation can be classified into :1. Elastic deformation .2. Plastic deformation.


Elastic deformation • reversible changes in shape to a material due to a load.• material returns to original shape when load is removed..


Plastic deformation • Irreversible changes in shape to a material due to a load.• Material DOES NOT return to original shape when load is

removed.


Toughness Definition;amount of energy pervolume a material canabsorb before failure(fracture).calculation

– area under the stress/strain curve

• units ;• joules per meter

cubed, J/m3


Creep ;increased loaddeformation withtime underconstant load.


load relaxation :• Decrease in applied stress under conditions of constant

strain.


Finite element analysis :1. Complex geometric forms and

material properties are modeled.2. A structure is modeled as a finite

number of simple geometric forms.3. Typically triangular or trapezoidal

elements.4. A computer matches forces and

moments between neighboring elements.

5. Finite element analysis is often used to estimate internal stresses and strains.

6. Example: stress/strain at bone-implant interface

Material Strength: Stress vs Strain Curve

• Stress strain curve is derived from axially loading an object and plotting the stress verses strain curve.


• Elastic zone :o The zone where a material will return to its

original shape for a given amount of stress.


• Plastic zone :o The zone where a material will not return to its

original shape for a given amount of stress.


• Breaking point .o The object fails and breaks.


• Yield point :o the transition point

between elastic and plastic deformation


• Yield strength :o the amount of stress necessary to produce a specific amount of

permanent deformation .


Ultimate(Tense)strength :Is the maximumstress that amaterial canwithstand whilebeing stretchedOr pulled beforefailing or

breaking


• Hooke's law :– when a material is loaded in the elastic zone, the stress is proportional

to the strain .– Basically, stress is proportional to strain up to a limit.


• Young's modulus of elasticity: • Measure of the stiffness (ability to

resist deformation) of a material in the elastic zone

• Calculated by measuring the slope of the stress/strain curve in the elastic zone.

• E = stress/strain E is the slope in the elastic range of

the stress-strain curve• A material with a higher E can

withstand greater force• The critical factor in load-sharing

capacity.

Young's Modulus of Metals and Biologics

• Relative values of Young's modulus of elasticity; 1. Ceramic (Al2O3)2. Alloy (Co-Cr-Mo)3. Stainless steel4. Titanium 5. Cortical bon6. Matrix polymers7. PMMA8. Polyethylene9. Cancellous bone10. Tendon / ligament11. Cartilage

Material Descriptions• Brittle material : • Is the material that

exhibits linear stress strain relationship up until the point of failure.

• undergoes elastic deformation only, and little to no plastic deformation.

• characterized by the fact that rupture occurs without any noticeable prior change in the rate of elongation.

• examples :a. PMMA .b. Ceramics.c. Glass.d. Stone.

Material Descriptions• Ductile Material :

• undergoes large amount of plastic deformation before failure• example :–Metal.

Material Descriptions• Viscoelastic material :• Bones and ligaments.• Stress-strain behavior is time-rate

dependent.• Properties depend on load

magnitude and rate at which the load is applied.

• These materials exhibit both fluid (viscosity) and solid(elasticity) properties.

• These materials exhibit hysteresis.• Loading and unloading curves

differ.• Energy is dissipated during loading.

Material Descriptions• Isotropic materials :

possess the same mechanical properties in all directions of applied load. • Example: • Golf ball.

Material Descriptions• Anisotropic materials :

• possess different mechanical properties depending on the direction of the applied load• examples

a. Ligaments.b. Bone.

• For example, bone is stronger with axial load than with radial load.

Metal Characteristics• Fatigue failure :• Failure at a point below the

ultimate tensile strength secondary to repetitive loading.

• Depends on magnitude of stress and number of cycles.

.

Metal Characteristics• Endurance limit :

Defined as the maximal stress under which an object is immune to fatigue failure regardless of the number of cycles.

s

Metal Characteristics Creep :– phenomenon of

progressive deformation of metal in response to a constant force over an extended period of time .

Metal Characteristics• Corrosion :

– refers to the chemical dissolving of metal.

– Types include :1-Galvanic corrosion :

dissimilar metals leads to electrochemical destruction

mixing metals 316L stainless steel and cobalt chromium (Co-Cr) has highest risk of galvanic corrosion

can be reduced by using similar metal

electrochemical

Metal Characteristics2-Crevice corrosion :– occurs in fatigue cracks due

to differences in oxygen tension.– stainless steel most prone

to crevice corrosion .

.

Metal Characteristics3-Fretting corrosion: – occurs at contact sites between two

materials that are subject to micromotion– common at the head-neck junction

in hip arthroplasty

head-neck

Specific Metals

Specific Metals• Titanium

– uses • fracture plates• screws• intramedullary nails• some femoral stems

– advantages • very biocompatable• forms adherent oxide coating through self passivation

– corrosion resistant • low modulus of elasticity makes it more similar to biologic materials as cortical

bone– disadvantages

• poor resistance to wear (notch sensitivity) (do not use as a femoral head prosthesis)

• generates more metal debris than cobalt chrome

Specific Metals• Titanium – uses

• fracture plates• screws• intramedullary nails• some femoral stems

– advantages • very biocompatable• forms adherent oxide coating through self passivation

– corrosion resistant • low modulus of elasticity makes it more similar to biologic materials as cortical

bone– disadvantages

• poor resistance to wear (do not use as a femoral head prosthesis)• generates more metal debris than cobalt chrome

Specific Metals• Stainless Steel:– components • primarily iron-carbon alloy with lesser elements of

a. Chromium.b. Molybdenum.c. Manganese.

– advantages • very stiff• fracture resistant

– disadvantages • susceptible to corrosion• stress shielding of bone due to superior stiffness.

Specific Metals• Stainless Steel:– components • primarily iron-carbon alloy with lesser elements of

A. chromium B. molybdenumC. manganese



Specific Metals• Stainless Steel:– components

• primarily iron-carbon alloy with lesser elements of

– chromium – molybdenum– manganese



Specific Metals• Cobalt alloy – components • cobalt• chromium• molybdenum

– advantages • very strong • better resistance to corrosion than stainless steel

Specific Non-Metals• Ultra-high-molecular-weight polyethylene – advantages • tough• ductile• resilient• resistant to wear

– disadvantages • susceptible to abrasion –wear usually caused by third body inclusions

• thermoplastic (may be altered by extreme temperatures)• weaker than bone in tension

Polymethylmethacrylate (PMMA, bone cement)

Functions :• used for fixation and load distribution in

conjunction with orthopeadic implants.• functions by interlocking with bone.• may be used to fill tumor defects and minimize

local recurrence.


Functions :• used for fixation and load distribution in conjunction

with orthopeadic implants.• functions by interlocking with bone.• may be used to fill tumor defects and minimize local

recurrence.Properties:

• 2 component material – powder

» polymer» benzoyl peroxide (initiator)» barium sulfate (radio-opacifier)

– liquid » monomer» hydroquinone (stabilizer


advantages :• reaches ultimate strength at 24 hours• strongest in compression• Young's modulus between cortical and cancellous

bone disadvantages :• poor tensile and shear strength• insertion can lead to dangerous drop in blood

pressure• failure often caused by microfracture and

fragmentation

Silicones• polymers that are often used for replacement in non-

weight bearing joints• Disadvantages:

poor strength and wear capability responsible for frequent synovitis

Ceramics

– advantages :• best wear characteristics with PE• high compressive strength

– disadvantages :• typically brittle, low fracture toughness • high Young's modulus• low tensile strength• poor crack resistance characteristics

Ligaments & Tendons• Characteristics :

– viscoelastic with nonlinear elasticity – displays hysteresis .

• Advantages :– strong in tension (can withstand 5-10% as opposed to 1-4% in

bone)• Disadvantages :

– demonstrate creep and stress relaxation

Bone• Bone composition: – composed of collagen and hydroxyapatite1-collagen • low Young's modulus• good tensile strength• poor compressive strength

2-hydroxyapatite :• stiff and brittle• good compressive strength

Bone• Mechanical properties – advantages :• strongest in compression• a dynamic structure – remodels geometry to increase inner and outer

cortex to alter the moment of inertia and minimize bending stresses

– disadvantages :• weakest in shear

Bone• Failure (fracture) – tension • usually leads to transverse fracture

secondary to muscle pull .

.

.

Bone• Failure (fracture) :– compression :• due to axial loading• leading to a crush type fracture• bone is strongest in resisting compression

Bone• Failure (fracture) – bending • leads to butterfly fragment.

Bone• Failure (fracture) – torsion • leads to spiral fracture.• the longer the bone the greater the stresses on the outer cortex under torsion.

Orthopedics implants

Screws

• Definitions – pitch • distance between threads

Screws

• Definitions – lead • distance advanced with one revolution

Screws

• Definitions – screw working distance (length) :• defined as the length of bone

traversed by the screw

Screws

• Definitions – screw working distance

(length) :• defined as the length of

bone traversed by the screw

Warking distance

Screws

• Definitions – pullout strength • maximized by – large inner outer diameter difference.– fine pitch

• pedicle screw pullout most affected by degree of osteoporosis

• Types of screws 1. cortical screws .2. cancellous screws .3. locking screws.

Plates Properties• Overview & definitions :

– a load-bearing device that is most effective when placed on the tension side.

– Load bearingIn load-bearing fixation the plate assumes 100% of the functional loads.

plate working distancedefined as thelength from thefracture to theClosest screw on either side of the fracture.

• decreasing the working distance increases the stiffness of the fixation construct

• Biomechanics – absolute stability • constructs heal with primary

(Haversian) healing .• must eliminate micromotion with lag

screw fixation.• must be low strain at fracture site

with high fixation stiffness .

– Relative stability • constructs heal with enchondral healing.• strain rates must be <15%, or fibrous union will

predominate.–Material Properties • bending rigidity proportional to thickness.

Locking plates

Indications for locking plate technology

1. indirect fracture reduction.2. diaphyseal/metaphyseal fractures

in osteoporotic bone.3. bridging severely comminuted

fractures.4. plating of fractures where

anatomical constraints prevent plating on the tension side of the bone (e.g. short segment fixation).

Locking plates

– locking plate screw biomechanics: • Locked plate/screw constructs compared to non-locked

plate/screw constructs result in less angulation in comminuted metaphyseal fractures. • unicortical locking screws have less torsion fixation strength

than non-locking bicortical constructs.

Percutaneous locking plates

• application has less soft-tissue stripping but higher chance of malunion

Hybrid locked plates

• utilize locking and nonlocking screws in order to assist with fracture reduction (nonlocking screws) as well as provide a fixed angle construct (locking screws).

– locking plate construct stability increases with: • bicortical

locking screws• increased

number of screws

• screw divergence from screw hole < 5 degrees

• longer plate

Bridging plates – provides relative

stability, relative length and alignment

– preserves the blood supply to the fracture fragments as the fracture site is undisturbed during the operative procedure

– allows some motion at fracture site; relative stability leads to callus formation

Intramedullary nails:

– a load-sharing device• Material Properties :– stiffness

• torsional rigidity – defined as amount of torque

needed to produce torsional (rotational) deformation.

– proportional to the radius to the 4th power

– depends on » shear modulus» polar moment of inertia

– increased by reaming– decreased by slotting of nail t

• bending rigidity – depends on

» material properties • Young modulus of elasticity of material

» structural properties • area moment of inertia• Length.

• External fixatorsFactors that increase stability of conventional external fixators 1. contact of ends of fracture 2. larger diameter pins (most important) 3. additional pins4. decreased bone to rod distance5. pins in different planes6. increasing size or stacking rods7. rods in different planes8. increased spacing between pins

Factors that increase stability of circular (Ilizarov) external fixators

1. larger diameter wires2. decreased ring diameter3. olive wires4. extra wires5. wires cross

perpendicular to each other

6. increased wire tension7. placement of two central

rings close to fracture8. increased number of

rings

Total Hip Implants

• Material Properties – rigidity depends on length and radius of

femoral stem • Biomechanics

– place femoral component in neutral or slight valgus to reduce moment arm and stress on cement.

– increasing femoral offset does the following • advantages

– moves abductor moment away from center of rotation

– increase abductor moment arm– reduces abductor force required for

normal gait• disadvantages

– increased strain on implant– increases strain on medial cement

mantle

Moment (M): Moment is the rotational effect of a force. Moment = force (F) multiplied by the perpendicular

distance (the moment arm or lever arm = d) from point

of rotation:

M = F × d

BIOMECHANICS OF FRACTURE HEALING

Stability and fracture healing

1. Stability determines strain.

Absolute stabilityRelative stability2. Strain determines the

type of healing.

Buttressplate

Compression plateInsertion order:B - Neutral screwE-Compression srewC - Lag screw

BridgeplateNeutraliz

ation plate(with lag screw)

ABCDE

Plating modalities for various fracture patterns.

Strain less than 2% results in primary bone healing (endosteal healing).

Strain 2% to 10% results in secondary bone healing (enchondral ossification).

Strain greater than 10% does not permit bone formation.Strain is defined as change in fracture gap divided by

the fracture gap (ΔL/L).

C. Absolute stability:1. No motion at fracture site under physiologic load2. Bone heals through direct healing (no callus).3. Strain is low or zero.2. Healing times are longer and more difficult to

confirm onby radiography.5. Implants must have longer fatigue life.6. Examples: lag screws, compression plating,

rigidlocked plating (in nonbridging mode)

Samuel Dixon uses the moment of inertia of the long rod to help maintain balance as he crosses the Niagara river (1890).

biomechanics

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