Fracture Mechanisms (Long Bones)Fracture Mechanisms (Long Bones)

Direct and IndirectDirect and Indirect DirectDirect

– Magnitude and area distribution of the forceMagnitude and area distribution of the force

– Rate at which force acts on the boneRate at which force acts on the bone

IndirectIndirect

Transverse/Spiral Transverse/Spiral Transverse fracture/Bending load

Spiral fracture/Torsion load

max load of failure 3 1 Energy 1 1Stiffness 8º of angulation 23 º of twist

VERY STIFF LOW STIFFNESSHealing slow fastArea smaller larger

MicrocracksMicrocracks

Factors Influencing Fracture HealingFactors Influencing Fracture Healing

FractureFracture

3 phases3 phases– InflammationInflammation

– ReparationReparation

– RemodelingRemodeling

Endosteal and Periosteal callus formation which is Endosteal and Periosteal callus formation which is replaced by haversian (secondary osteon) remodelingreplaced by haversian (secondary osteon) remodeling– Suppressed by rigid fixationSuppressed by rigid fixation

– Excessive motion is also harmfulExcessive motion is also harmful

Fracture Fixation DevicesFracture Fixation Devices

In vitro studies to determine structural In vitro studies to determine structural rigidityrigidity– CadaverCadaver– Synthetic boneSynthetic bone

Apply axial, bending, torsion to Apply axial, bending, torsion to determine load-formation or torque determine load-formation or torque deformation curvesdeformation curves– Axial stiffnessAxial stiffness– Bending stiffnessBending stiffness– Torsional stiffnessTorsional stiffness

External FixationExternal Fixation

Low axial stiffnessLow axial stiffness Success is dependent on type of fractureSuccess is dependent on type of fracture Mode of reductionMode of reduction

– Whether or not you have bones in contactWhether or not you have bones in contact– Maximum of about 1mm between them for Maximum of about 1mm between them for

successful remodelingsuccessful remodeling

External FixationExternal Fixation– Increased pin diameterIncreased pin diameter– Increased pin numberIncreased pin number– Decreased side-bar separationDecreased side-bar separation– Decreased pin separationDecreased pin separation– Increased pin group separationIncreased pin group separation

Think about as an engineering how you would Think about as an engineering how you would change the external fixation to make it more stiff, but change the external fixation to make it more stiff, but not too stiff and ugly to wear or open too much to not too stiff and ugly to wear or open too much to infection, (pin wear, stress concentrations)infection, (pin wear, stress concentrations)

– Pin geometry and thread designPin geometry and thread design– Bone thread preparationBone thread preparation– Pin insertion techniquePin insertion technique– Pin-bone stressPin-bone stress

Rigid Compression PlatingRigid Compression Plating

Definition:Definition:– Bringing entire fracture surfaces into contactBringing entire fracture surfaces into contact– Compressing the fracture surfacesCompressing the fracture surfaces

Developed to prevent micromotion (too much Developed to prevent micromotion (too much micromotion causes fracture end resorption)micromotion causes fracture end resorption)

ImplementationImplementation– Pre-load between 2 joined bonesPre-load between 2 joined bones– Friction between bone and plateFriction between bone and plate

Disadvantage?Disadvantage?

Rigid Compression PlatingRigid Compression Plating

Disadvantage:Disadvantage:– ““stress shielding”-no loading of bone so when stress shielding”-no loading of bone so when

remove the plate, bone refractures- also called remove the plate, bone refractures- also called postunion osteopenia.postunion osteopenia.

How can we overcome this disadvantage?How can we overcome this disadvantage?– Optimize plate removalOptimize plate removal– Biologically degradable material internallyBiologically degradable material internally– Reduce rigidityReduce rigidity

Intramedullary NailsIntramedullary Nails

Define intramedullaryDefine intramedullary Ream canal and place rod in canal-Ream canal and place rod in canal- Most chosen method by surgeonsMost chosen method by surgeons

Experimental StudiesExperimental Studies

Canine tibial shaft osteotomyCanine tibial shaft osteotomy Plate vs. IntramedullaryPlate vs. Intramedullary

– Rod-fixed osteotomies healed by periosteal callusRod-fixed osteotomies healed by periosteal callus– Plate-fixed healed by endosteal callusPlate-fixed healed by endosteal callus– No differences in final bone porosityNo differences in final bone porosity– Differences in torsional stiffness disappeared after 4 monthsDifferences in torsional stiffness disappeared after 4 months– Time to normal stiffness and strength was sameTime to normal stiffness and strength was same

Experimental ComparisonsExperimental Comparisons

Plate versus Ext. FixationPlate versus Ext. Fixation– Plate more rigid and bone had increased stiffness and max. Plate more rigid and bone had increased stiffness and max.

torquetorque– Increased porosity with ext. fixation (increased resorption)Increased porosity with ext. fixation (increased resorption)– Same biological union after 4 monthsSame biological union after 4 months

Experimental ComparisonExperimental Comparison

External FixationExternal Fixation– 4 versus 6 pins increased periosteal callus, but also increased 4 versus 6 pins increased periosteal callus, but also increased

porosityporosity– Static compression increased rigidity of fixation, but did not Static compression increased rigidity of fixation, but did not

change bone union at 4 monthschange bone union at 4 months– Bilateral stiffer and results in bone with less porosity and Bilateral stiffer and results in bone with less porosity and

stiffer unionstiffer union

What you have gainedWhat you have gained

Be able to list advantages and disadvantages of Be able to list advantages and disadvantages of various fixation methods, and how to overcome various fixation methods, and how to overcome the disadvantages with engineeringthe disadvantages with engineering

Be aware and understand why 1 type of fixation Be aware and understand why 1 type of fixation doesn’t work for all types of fracturesdoesn’t work for all types of fractures