biomechanics of fractures and fixation · basic biomechanics • load to failure – continuous...
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Biomechanics of Fracturesand Fixation
Theodore Toan Le, MD
Original Author: Gary E. Benedetti, MD; March 2004New Author: Theodore Toan Le, MD; Revised October 09
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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)
ForceArea L
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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
PlasticElasticFailure
Yield
Stiffness
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StiffDuctile ToughStrongStiff
BrittleStrong
DuctileWeakBrittle
Weak
Strain
Stress
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FlexibleDuctile ToughStrong
FlexibleBrittleStrong
FlexibleDuctileWeak
FlexibleBrittleWeak
Strain
Stress
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Basic Biomechanics
• Load to Failure– Continuous application
of force until the material breaks (failure point at the ultimate load).
– Common mode of failure of bone and reported in the implant literature.
• Fatigue Failure– Cyclical sub-threshold
loading may result in failure due to fatigue.
– Common mode of failure of orthopaedic implants and fracture fixation constructs.
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Basic Biomechanics
• Anisotropic– Mechanical properties
dependent upon direction of loading
• Viscoelastic– Stress-Strain character
dependent upon rate of applied strain (time dependent).
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Bone Biomechanics
• Bone is anisotropic - its modulus is dependent upon the direction of loading.
• 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 density changes greatly changes 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
Trauma, Lippincott, 1994.
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Fracture Mechanics
• Combined bending & axial load– Oblique fracture– Butterfly fragment
Figure from: Tencer. Biomechanics in Orthopaedic
Trauma, Lippincott, 1994.
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Moments of Inertia
• Resistance to bending, twisting, compression or tension of an object is a 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 r4
– Increase in radius by callus greatly increases moment of inertia and stiffness
1.6 x stronger
0.5 x weakerFigure from: Browner et al, Skeletal Trauma
2nd Ed, Saunders, 1998. Figure from: Tencer et al: Biomechanics inOrthopaedic 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 NailsMoment 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 axial loads• 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 the zone of fxs that can be fixed at the expense of construct stability
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Biomechanics of Internal Fixation
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Biomechanics of Internal Fixation
• Screw Anatomy– Inner diameter– Outer diameter– Pitch
Figure from: Tencer et al, Biomechanics in
OrthopaedicTrauma, Lippincott, 1994.
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Biomechanics of Screw Fixation
• To increase strength of the screw & resist fatigue failure:– Increase the inner root
diameter
• To increase pull out strength of screw in bone:– Increase outer diameter– Decrease inner diameter– Increase thread density– Increase thickness of
cortex– Use cortex with more
density.
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Biomechanics of Screw Fixation
• Cannulated Screws– Increased inner
diameter required – Relatively smaller
thread width results in lower pull out strength
– Screw strength minimally affected(α r4
outer core - r4inner 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) of the plate to the 3rd
power.I= bh3/12Base (b)
Height (h)
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Biomechanics of Plate Fixation
• Functions of the plate– Compression– Neutralization– Buttress
• “The bone protects the plate”
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Biomechanics of Plate Fixation
• Unstable constructs– Severe comminution– Bone loss– Poor quality bone– Poor screw technique
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Biomechanics of Plate Fixation
• Fracture Gap /Comminution– Allows bending of plate with
applied loads– Fatigue failure
Applied Load
Bone Plate
Gap
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Biomechanics of Plate Fixation
• Fatigue Failure– Even stable constructs
may fail from fatigue if the fracture does not heal due to biological reasons.
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Biomechanics of Plate Fixation
• 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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Biomechanics of Plate Fixation
• The screws closest to the fracture see the most forces.
• The construct rigidity decreases as the distance between the innermost screws increases.
Screw Axial Force
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Biomechanics of Plate Fixation
• Number of screws (cortices) recommended on each side of the 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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Biomechanics of External Fixation
• Pin Size– {Radius}4
– Most significant factor in frame stability
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Biomechanics of External Fixation
• Number of Pins– Two per segment– Third pin
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Biomechanics of External Fixation
A
B
CThird pin (C) out of plane of two other pins (A & B) stabilizes that segment.
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Biomechanics of External Fixation
• 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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Biomechanics of External Fixation
• Bone-Frame Distance– Rods– Rings– Dynamization
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Biomechanics of External Fixation• 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 with
spanning ext fix – avoid zone of injury (far/near – far/far)
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Biomechanics of Locked Plating
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Patient Load
Conventional Plate Fixation
Patient Load
< =Friction Force
Patient Load
Courtesy of Synthes- Robi Frigg
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Locked Plate and Screw Fixation
Patient Load
=< Compressive Strength of the
Bone
Courtesy of Synthes- Robi Frigg
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Stress in the Bone
Patient Load
+Preload
Courtesy of Synthes- Robi Frigg
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Standard versus Locked LoadingCourtesy of Synthes- Robi Frigg
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Pullout of regular screws
by bending load
Courtesy of Synthes- Robi Frigg
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Higher resistant LHS against bending load
Larger resistant area
Courtesy of Synthes- Robi Frigg
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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
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Preservation of Blood SupplyPlate Design
DCP LCDCP
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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
Courtesy of Synthes- Robi Frigg
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Angular Stability of Screws
LockedNonlocked
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Biomechanical principlessimilar to those of external fixators
Stress distribution
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Surgical TechniqueCompression 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
Courtesy of Synthes- Robi Frigg
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Surgical TechniqueReduction
If the same technique is attempted with a locked plate and locking screws, an anatomical reduction will not be achieved. Locked Plating
Courtesy of Synthes- Robi Frigg
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Surgical TechniqueReduction
Instead, the fracture is first reduced and then the plate is applied.
Locked Plating
Courtesy of Synthes- Robi Frigg
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Surgical TechniquePrecontoured Plates
1. Contour of plate is important to maintain anatomic reduction.
Conventional Plating Locked Plating
1. Reduce fracture prior to applying locking screws.
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Biomechanical AdvantageUnlocked 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 integrity5. Improved purchase on osteoporotic bone
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Surgical TechniqueReduction with Combination Plate
Lag screws can be used to help reduce fragments and construct stability improved w/ locking screws Locked Plating
Courtesy of Synthes- Robi Frigg
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Surgical TechniqueReduction with Combination Hole Plate
Lag screw must be placed 1st if locking screw in same fragment is to be used.
Locked Plating
Courtesy of Synthes- Robi Frigg
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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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Length of Construct
• Longer spread with less screws– “Every other” rule (3 screws / 5 holes)
• < 50% of screw holes filled• Avoid too rigid construct
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Further Reading• Tencer, A.F. & Johnson, K.D., “Biomechanics in Orthopaedic Trauma,”
Lippincott.• “Orthopaedic Basic Science,” AAOS.• Browner, B.D., et al, “Skeletal Trauma,” Saunders.• Radin, E.L., et al, “Practical Biomechanics for the Orthopaedic Surgeon,”
Churchill-Livingstone.• Tornkvist H et al, “The Strength of Plate Fixation in Relation to the Number
and Spacing of Bone Screws,” JOT 10(3), 204-208• Egol K.A. et al, “Biomechanics of Locked Plates and Screws,” JOT 18(8),
488-493• Haidukewych GJ & Ricci W, “Locked Plating in Orthopaedic Trauma: A
Clinical Update,” JAAOS 16(6),347-355
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