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Biomechanics of Fractures and Fixation

Basic Biomechanics. Material PropertiesElastic-PlasticYield pointBrittle-DuctileToughnessIndependent of Shape!. Structural PropertiesBending StiffnessTorsional StiffnessAxial StiffnessDepends on Shape and Material!. Basic Biomechanics Force, Displacement

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Biomechanics of Fractures and Fixation

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    1. Biomechanics of Fractures and Fixation Theodore Toan Le, MD Original Author: Gary E. Benedetti, MD; March 2004 New Author: Theodore Toan Le, MD; Revised September 2005

    2. 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! Material Properties: fundamental behaviors of a substance independent of its geometry. Structural Properties: the ability of an object to resist bending under torsion, axial load, or bending is a function of its shape and distribution of material around the cross section.Material Properties: fundamental behaviors of a substance independent of its geometry. Structural Properties: the ability of an object to resist bending under torsion, axial load, or bending is a function of its shape and distribution of material around the cross section.

    3. Basic Biomechanics Force, Displacement & Stiffness Force applied to a body causes a deformation. If one plots a graph of displacement due to the force applied to a material, the slope of the curve represent the material’s stiffness.Force applied to a body causes a deformation. If one plots a graph of displacement due to the force applied to a material, the slope of the curve represent the material’s stiffness.

    4. Basic Biomechanics Stress: is the force applied per unit area (F/A). Strain: is the change in a material’s length due to an applied stress relative to its original length = ?L/ L0.Stress: is the force applied per unit area (F/A). Strain: is the change in a material’s length due to an applied stress relative to its original length = ?L/ L0.

    5. Basic Biomechanics Stress-Strain & Elastic Modulus If one graphs the resulting strain (?L/ L0) due to the applied stress (F/A), the slope of the Stress-Strain curve is the Elastic Modulus.If one graphs the resulting strain (?L/ L0) due to the applied stress (F/A), the slope of the Stress-Strain curve is the Elastic Modulus.

    6. Basic Biomechanics Common 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 Review the elastic modulus of common materials encountered in orthopaedic surgery relative to each other.Review the elastic modulus of common materials encountered in orthopaedic surgery relative to each other.

    7. Basic Biomechanics Elastic Deformation Plastic Deformation Energy Elastic Deformation: In the elastic region, the relationship of displacement to the applied force in linear. In this region, the material returns to its resting state if the force is removed (elastic), like a rubber band. Stiffness: again, is the slope of the elastic portion of the force-displacement curve. Plastic Deformation: As more force is applied, the material’s behavior becomes plastic, and permanent deformation occurs. The material will not return to its original state after the force is removed (like when you bend a plastic ice cream spoon to far it stays bent). Energy: The area under the curve represents energy absorbed into the material during the deformation process (work).Elastic Deformation: In the elastic region, the relationship of displacement to the applied force in linear. In this region, the material returns to its resting state if the force is removed (elastic), like a rubber band. Stiffness: again, is the slope of the elastic portion of the force-displacement curve. Plastic Deformation: As more force is applied, the material’s behavior becomes plastic, and permanent deformation occurs. The material will not return to its original state after the force is removed (like when you bend a plastic ice cream spoon to far it stays bent). Energy: The area under the curve represents energy absorbed into the material during the deformation process (work).

    8. Stiffness: The steeper the slope, the stiffer the material. A material with a flat slope is flexible. Yield Point: the point on the force-displacement curve where the material changes from elastic to plastic deformation is the yield point. Failure Point: At some point the material will break; this point due is the material’s Failure Point. Brittle: a material which experiences little plastic deformation before it fails is said to be brittle (glass for example). Ductile: if a material with a large plastic deformation region before it fails is said to be ductile (copper for example). Toughness: a material which can absorb more energy prior to failure (large area under the curve) is said to be tougher. Weakness: a material which can absorb little energy prior to failure (small area under the curve) is said to be weak. Stiffness: The steeper the slope, the stiffer the material. A material with a flat slope is flexible. Yield Point: the point on the force-displacement curve where the material changes from elastic to plastic deformation is the yield point. Failure Point: At some point the material will break; this point due is the material’s Failure Point. Brittle: a material which experiences little plastic deformation before it fails is said to be brittle (glass for example). Ductile: if a material with a large plastic deformation region before it fails is said to be ductile (copper for example). Toughness: a material which can absorb more energy prior to failure (large area under the curve) is said to be tougher. Weakness: a material which can absorb little energy prior to failure (small area under the curve) is said to be weak.

    11. 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. Compare the two types of failure: Load to Failure: Continuous application of force until the material breaks (failure point at the ultimate load). Bone usually fractures by load to failure. Fatigue Failure: Cyclical sub-threshold loading may result in failure due to fatigue. Load to failure testing often reported in the orthopaedic literature: however, clinically, failure fracture fixation constructs or implants often due to fatigue failure. Compare the two types of failure: Load to Failure: Continuous application of force until the material breaks (failure point at the ultimate load). Bone usually fractures by load to failure. Fatigue Failure: Cyclical sub-threshold loading may result in failure due to fatigue. Load to failure testing often reported in the orthopaedic literature: however, clinically, failure fracture fixation constructs or implants often due to fatigue failure.

    12. Basic Biomechanics Anisotropic Mechanical properties dependent upon direction of loading Viscoelastic Stress-Strain character dependent upon rate of applied strain (time dependent). Anisotropic: some materials (bone) have different mechanical properties dependent upon the type of load applied (transverse, longitudinal, shear). Viscoelastic: many biological materials to include bone reveal different stress-strain curves depending upon the speed at which the force is applied.Anisotropic: some materials (bone) have different mechanical properties dependent upon the type of load applied (transverse, longitudinal, shear). Viscoelastic: many biological materials to include bone reveal different stress-strain curves depending upon the speed at which the force is applied.

    13. 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 Bone Compression < 212 N/m2 Tension < 146 N/m2 Shear < 82 N/m2 Bone is anisotropic-its modulus is dependent upon the direction of loading. Bone is weakest in shear, then tension, then compression. This can help us understand the fracture produced (failure) from various mechanisms (applied loads). Review the values of the Ultimate Stress at Failure of cortical bone based upon the different direction of loading for cortical bone.Bone is anisotropic-its modulus is dependent upon the direction of loading. Bone is weakest in shear, then tension, then compression. This can help us understand the fracture produced (failure) from various mechanisms (applied loads). Review the values of the Ultimate Stress at Failure of cortical bone based upon the different direction of loading for cortical bone.

    14. 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. Bone is viscoelastic, the force-deformation curve will be different for different rates of loading. Trabecular bone becomes stiffer in compression the faster it is loaded. It is hypothesized that at high rates of loading the marrow elements do not have time to be push out, and act like a fluid filled shock absorber.Bone is viscoelastic, the force-deformation curve will be different for different rates of loading. Trabecular bone becomes stiffer in compression the faster it is loaded. It is hypothesized that at high rates of loading the marrow elements do not have time to be push out, and act like a fluid filled shock absorber.

    15. Bone Mechanics Bone Density Subtle density changes greatly changes strength and elastic modulus Density changes Normal aging Disease Use Disuse Bone, as a living material, can experience changes in its density. Subtle density changes can have great effects upon the material properties of the bone. Graph shows a marked change in the stress/strain curve of trabecular bone when its density is decreased by a factor of 3. These changes occur with normal aging, disease, use and disuse. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-5, pg. 101 Bone, as a living material, can experience changes in its density. Subtle density changes can have great effects upon the material properties of the bone. Graph shows a marked change in the stress/strain curve of trabecular bone when its density is decreased by a factor of 3. These changes occur with normal aging, disease, use and disuse. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-5, pg. 101

    16. Basic Biomechanics Bending Axial Loading Tension Compression Torsion These bending, torsion, tensile and compressive forces are applied to bones, and if excessive, may lead to fracture.These bending, torsion, tensile and compressive forces are applied to bones, and if excessive, may lead to fracture.

    17. Fracture Mechanics The nature of the applied force is often evident by the nature of the fracture. Tension – Transverse Bending - Butterfly Compression - Oblique Torsion - Spiral Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-9, pg. 103.   The nature of the applied force is often evident by the nature of the fracture. Tension – Transverse Bending - Butterfly Compression - Oblique Torsion - Spiral Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-9, pg. 103.  

    18. Fracture Mechanics Bending load: Compression strength greater than tensile strength Fails in tension As the bone is subjected to a bending load, one side is placed under compression, the other is placed under tension. Since the bone’s compressive strength is greater than its tensile strength, the bone fails first on the tension side. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.6 (a), (b), pg. 39. As the bone is subjected to a bending load, one side is placed under compression, the other is placed under tension. Since the bone’s compressive strength is greater than its tensile strength, the bone fails first on the tension side. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.6 (a), (b), pg. 39.

    19. Fracture Mechanics Torsion The diagonal in the direction of the applied force is in tension – cracks perpendicular to this tension diagonal Spiral fracture 45ş to the long axis TORSION: The diagonal in the direction of the applied force is in tension – cracks perpendicular to this tension diagonal Spiral fracture 45ş to the long axis Figures from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.8 (a), pg. 41.   TORSION: The diagonal in the direction of the applied force is in tension – cracks perpendicular to this tension diagonal Spiral fracture 45ş to the long axis Figures from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.8 (a), pg. 41.  

    20. Fracture Mechanics Combined bending & axial load Oblique fracture Butterfly fragment Combined bending & axial loads result in oblique fractures or those with a butterfly fragment. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.7 (a), pg. 40. Combined bending & axial loads result in oblique fractures or those with a butterfly fragment. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 3.7 (a), pg. 40.

    21. 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. Resistance to bending, twisting, axial compression or tension of an object is a function of its cross-sectional shape with respect to a given axis. The relationship of resistance to applied forces to the distribution of mass (shape) with respect to an axis is related to the moment of inertia. These calculations based upon the cross-sectional shape are useful in comparing the resistance to applied forces of two different objects (solid vs. cannulated IM nails, thin vs. thick cortical bone). Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.   Resistance to bending, twisting, axial compression or tension of an object is a function of its cross-sectional shape with respect to a given axis. The relationship of resistance to applied forces to the distribution of mass (shape) with respect to an axis is related to the moment of inertia. These calculations based upon the cross-sectional shape are useful in comparing the resistance to applied forces of two different objects (solid vs. cannulated IM nails, thin vs. thick cortical bone). Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.  

    22. Fracture Mechanics Fracture Callus Moment of inertia proportional to r4 Increase in radius by callus greatly increases moment of inertia and stiffness Effect of fracture Callus: since the moment of inertia for a cylindrical tube is proportional to r4 any small increase in the radius by callus further away from the center greatly increases moment of inertia therefore the stiffness of the structure. Figures from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.   Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 4.6, pg. 62. Effect of fracture Callus: since the moment of inertia for a cylindrical tube is proportional to r4 any small increase in the radius by callus further away from the center greatly increases moment of inertia therefore the stiffness of the structure. Figures from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.   Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 4.6, pg. 62.

    23. Fracture Mechanics Time of Healing Callus increases with time Stiffness increases with time Near normal stiffness at 27 days Does not correspond to radiographs Time of Healing: Callus increases with time, therefore the stiffness increases with time. Near normal stiffness achieved at 27 days due to the callus formed further away from the axis. This increased strength does not correspond to the radiographs. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-15, pg. 109. Time of Healing: Callus increases with time, therefore the stiffness increases with time. Near normal stiffness achieved at 27 days due to the callus formed further away from the axis. This increased strength does not correspond to the radiographs. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-15, pg. 109.

    24. IM Nails Moment of Inertia Stiffness proportional to the 4th power. Recall that for a cylindrical tube, the moment of inertia is proportional to the radius to the 4th power. Small changes in the radius can mean large changes in the stiffness. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.   Recall that for a cylindrical tube, the moment of inertia is proportional to the radius to the 4th power. Small changes in the radius can mean large changes in the stiffness. Figure from: Browner B., Jupiter J., Levine A., Trafton P., Skeletal Trauma 2nd Edition, W.B. Saunders, 1998, figure 4-6, pg. 101.  

    25. IM Nail Diameter IM Nail Diameter: As the diameter of the nail increases, so does the torsional and bending moments of inertia (stiffness); a 16 mm nail is 2.5 x stiffer than a 12 mm nail. The large diameter can accommodate larger holes for larger interlocking screws. The nail size is limited by the diameter of the patient’s IM canal and the amount of reaming performed. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 9.21, pg. 263. IM Nail Diameter: As the diameter of the nail increases, so does the torsional and bending moments of inertia (stiffness); a 16 mm nail is 2.5 x stiffer than a 12 mm nail. The large diameter can accommodate larger holes for larger interlocking screws. The nail size is limited by the diameter of the patient’s IM canal and the amount of reaming performed. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 9.21, pg. 263.

    26. Slotting SLOTTING: Placing a longitudinal slot in a nail makes the nail moderately more flexible in bending (especially if slotted on the compression side), and significantly more flexible in torsion. This slot allows for radial compression of the nail during insertion into the femur. The increased bending flexibility aids in allowing an easier insertion and canal fit, but also decreases the fatigue resistance of the nail (does not seem to be a clinical problem). Figures from: Nail cross sections: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 9.20, pg 262.   Stress diagrams: Rockwood and Green’s 4th ed.   SLOTTING: Placing a longitudinal slot in a nail makes the nail moderately more flexible in bending (especially if slotted on the compression side), and significantly more flexible in torsion. This slot allows for radial compression of the nail during insertion into the femur. The increased bending flexibility aids in allowing an easier insertion and canal fit, but also decreases the fatigue resistance of the nail (does not seem to be a clinical problem). Figures from: Nail cross sections: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 9.20, pg 262.   Stress diagrams: Rockwood and Green’s 4th ed.  

    27. Slotting-Torsion Slotting-Torsion: A slotted nail is about 2% as stiff as an intact femur in torsion, while a solid-section nail is about 50% as stiff (Tencer, Tech Orthop 1988). Figures from: Bar graph: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippencott, 1994, figure 9.22, pg. 263.   Slotting-Torsion: A slotted nail is about 2% as stiff as an intact femur in torsion, while a solid-section nail is about 50% as stiff (Tencer, Tech Orthop 1988). Figures from: Bar graph: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippencott, 1994, figure 9.22, pg. 263.  

    28. 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 INTERLOCKING SCREWS: Control torsion and axial loads. ADVANTAGES: The use of interlocking bolts expanded the use of IM nails to more proximal, more distal, and unstable fractures (highly comminuted, segmental), as well as adds rotational stability with the non-reamed technique. DISADVANTAGES: Requires some technical skill to place. The holes in the nail act as stress risers (the weakest part of the nail to fatigue is at or just proximal to the most proximal distal locking screw). There is an increased rate of nail breakage if the fracture is within 5 cm of these screws (Bucholz, JBJS 1987), or if the screw hole closest to the fracture is left unfilled (Hahn, Injury 1996). LOCATION: Screw holes closer to the end of the nail allow for the fixation of more proximal or distal fractures, but at the expense of stability of the construct. INTERLOCKING SCREWS: Control torsion and axial loads. ADVANTAGES: The use of interlocking bolts expanded the use of IM nails to more proximal, more distal, and unstable fractures (highly comminuted, segmental), as well as adds rotational stability with the non-reamed technique. DISADVANTAGES: Requires some technical skill to place. The holes in the nail act as stress risers (the weakest part of the nail to fatigue is at or just proximal to the most proximal distal locking screw). There is an increased rate of nail breakage if the fracture is within 5 cm of these screws (Bucholz, JBJS 1987), or if the screw hole closest to the fracture is left unfilled (Hahn, Injury 1996). LOCATION: Screw holes closer to the end of the nail allow for the fixation of more proximal or distal fractures, but at the expense of stability of the construct.

    29. Biomechanics of Internal Fixation

    30. Biomechanics of Internal Fixation Screw Anatomy Inner diameter Outer diameter Pitch Screw Anatomy: review screw terminology. Inner diameter: the inner, “core” or “root” diameter determines the screw’s strength. Outer diameter: the difference between the inner core diameter and the outer diameter of the threads is the thread width. Increasing the thread width increases the pull-out strength. Pitch: threads per inch. Increasing pitch increases the pull out strength of the screw. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 6.19 (a), pg. 133.   Screw Anatomy: review screw terminology. Inner diameter: the inner, “core” or “root” diameter determines the screw’s strength. Outer diameter: the difference between the inner core diameter and the outer diameter of the threads is the thread width. Increasing the thread width increases the pull-out strength. Pitch: threads per inch. Increasing pitch increases the pull out strength of the screw. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 6.19 (a), pg. 133.  

    31. 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. Increasing the screws root diameter a factor of two allows it to withstand torques eight times greater. The more volume of bone caught between threads, the greater the pull out strength. This can be achieved by increasing the thread width of the screw, or increasing the number of threads in contact with the bone. Density of the bone also affect pull out strength (young> than old bone, cortical bone > cancellous bone).Increasing the screws root diameter a factor of two allows it to withstand torques eight times greater. The more volume of bone caught between threads, the greater the pull out strength. This can be achieved by increasing the thread width of the screw, or increasing the number of threads in contact with the bone. Density of the bone also affect pull out strength (young> than old bone, cortical bone > cancellous bone).

    32. Biomechanics of Screw Fixation Cannulated Screws Increased inner diameter required Relatively smaller thread width results in lower pull out strength Screw strength minimally affected (a r4outer core - r4inner core ) Cannulated Screws Increased inner root diameter of a cannulated screw is large than a comparable non-cannulated screw. This extra material is required to compensate for the cannulation (mechanical effects of converting a solid cylinder to a tube). This results in a relatively smaller thread width and lower pull out strength. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 6.30 (b), pg. 140.   Cannulated Screws Increased inner root diameter of a cannulated screw is large than a comparable non-cannulated screw. This extra material is required to compensate for the cannulation (mechanical effects of converting a solid cylinder to a tube). This results in a relatively smaller thread width and lower pull out strength. Figure from: Tencer A., Johnson K., Biomechanics in Orthopaedic Trauma, J.P. Lippincott, 1994, figure 6.30 (b), pg. 140.  

    33. Biomechanics of Plate Fixation Plates: Bending stiffness proportional to the thickness (h) of the plate to the 3rd power. Bending stiffness proportional to the thickness of the plate to the 3rd power, and directly to the elastic modulus. Therefore, changing the plate thickness has more effect upon stiffness than changing the material. This is expressed as the base (b) times the height or thickness cubed over 12.Bending stiffness proportional to the thickness of the plate to the 3rd power, and directly to the elastic modulus. Therefore, changing the plate thickness has more effect upon stiffness than changing the material. This is expressed as the base (b) times the height or thickness cubed over 12.

    34. Biomechanics of Plate Fixation Function of the plate Internal splint Compression “The bone protects the plate” The primary function of the plate is to maintain alignment as an internal splint, and to create compression between the fracture ends such that bone can transfer some of the applied loads itself. A compression plate, tension band, or a lag screw does this by generating compression across the fracture. In fixation constructs in which the plate-bone system can carry load, the compressed fractured bone carries a major part of the load. “The bone, therefore, protects the implant” (B.G. Weber). The primary function of the plate is to maintain alignment as an internal splint, and to create compression between the fracture ends such that bone can transfer some of the applied loads itself. A compression plate, tension band, or a lag screw does this by generating compression across the fracture. In fixation constructs in which the plate-bone system can carry load, the compressed fractured bone carries a major part of the load. “The bone, therefore, protects the implant” (B.G. Weber).

    35. Biomechanics of Plate Fixation Unstable constructs Severe comminution Bone loss Poor quality bone Poor screw technique Unstable constructs: A construct in which compression cannot be achieved across the fracture is unstable and at risk for failure. Plates alone cannot tolerate the functional loads of a limb, and if the fracture does not heal, the construct may eventually fail due to fatigue under cyclical loading. This situation may be due to: Severe comminution: consider bridge plating techniques. Bone loss Poor quality bone Poor screw technique Improper choice of implantsUnstable constructs: A construct in which compression cannot be achieved across the fracture is unstable and at risk for failure. Plates alone cannot tolerate the functional loads of a limb, and if the fracture does not heal, the construct may eventually fail due to fatigue under cyclical loading. This situation may be due to: Severe comminution: consider bridge plating techniques. Bone loss Poor quality bone Poor screw technique Improper choice of implants

    36. Biomechanics of Plate Fixation Fracture Gap /Comminution Allows bending of plate with applied loads Fatigue failure A gap at the fracture site such that there is no fracture compression with applied loads allows bending or torsion of the plate with the fulcrum at the level of the fracture. All of the forces are transmitted through the plate alone instead of being shared by the bone and the plate.A gap at the fracture site such that there is no fracture compression with applied loads allows bending or torsion of the plate with the fulcrum at the level of the fracture. All of the forces are transmitted through the plate alone instead of being shared by the bone and the plate.

    37. Biomechanics of Plate Fixation Fatigue Failure Even stable constructs may fail from fatigue if the fracture does not heal due to biological reasons. Fatigue Failure: Even stable constructs may fail from fatigue if the fracture does not heal due to biological reasons (infection, soft tissue loss, periosteal stripping, smoking). The fracture surgeon must learn to balance the desire for rigid fixation with the need to preserve the soft tissue (biological fixation). Fatigue Failure: Even stable constructs may fail from fatigue if the fracture does not heal due to biological reasons (infection, soft tissue loss, periosteal stripping, smoking). The fracture surgeon must learn to balance the desire for rigid fixation with the need to preserve the soft tissue (biological fixation).

    38. Biomechanics of Plate Fixation Bone-Screw-Plate Relationship Bone via compression Plate via bone-plate friction Screw via resistance to bending and pull out. Screw-Plate Relationship: When load is applied to the bone, a portion of the load is transferred across the fracture line if the construct allows compression. A portion of the load is also transferred to the plate by bone-plate friction. A portion is supported by the screw resisting bending and pull out. Lag Screw: adding a compression lag screw across the fracture site greatly increases the load transfer to the bone, further protecting the screws and plate. Screw-Plate Relationship: When load is applied to the bone, a portion of the load is transferred across the fracture line if the construct allows compression. A portion of the load is also transferred to the plate by bone-plate friction. A portion is supported by the screw resisting bending and pull out. Lag Screw: adding a compression lag screw across the fracture site greatly increases the load transfer to the bone, further protecting the screws and plate.

    39. 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. The screws closest to the fracture see the most forces. The screws further away experience progressively less force. The construct rigidity decreases as the distance between the innermost screws increases. A longer plate can increase the rigidity of the construct. The screws closest to the fracture see the most forces. The screws further away experience progressively less force. The construct rigidity decreases as the distance between the innermost screws increases. A longer plate can increase the rigidity of the construct.

    40. 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) Through experience with failure the recommended strength of plate and number of screws in each fragment has been determined. These are general guidelines and more may be required with poor quality bone. Anatomic constraints may limit also the construct. Also it is important to note that the increased spread of screws is more important than the number of screws (provided each screw has equal purchase) for construct stability. Through experience with failure the recommended strength of plate and number of screws in each fragment has been determined. These are general guidelines and more may be required with poor quality bone. Anatomic constraints may limit also the construct. Also it is important to note that the increased spread of screws is more important than the number of screws (provided each screw has equal purchase) for construct stability.

    41. Biomechanics of External Fixation It is uncertain how rigid an external fixation frame should be; too rigid may be detrimental to fracture healing.It is uncertain how rigid an external fixation frame should be; too rigid may be detrimental to fracture healing.

    42. Biomechanics of External Fixation Pin Size {Radius}4 Most significant factor in frame stability SIZE OF PINS: RADIUS: Stiffness proportional to {radius}4, so small changes in pin diameter greatly increases stiffness. **This is the most significant factor affecting stability of the fixator. Larger pins provide more rigid fixator, providing less bending stress at the bone-pin interface and lower rate of loosening. However, the hole size acts as a stress riser; if the pin diameter is greater than 30% the size of the bone the risk of fracture greatly increases (McBroom, Orthop Res 88). SIZE OF PINS: RADIUS: Stiffness proportional to {radius}4, so small changes in pin diameter greatly increases stiffness. **This is the most significant factor affecting stability of the fixator. Larger pins provide more rigid fixator, providing less bending stress at the bone-pin interface and lower rate of loosening. However, the hole size acts as a stress riser; if the pin diameter is greater than 30% the size of the bone the risk of fracture greatly increases (McBroom, Orthop Res 88).

    43. Biomechanics of External Fixation Number of Pins Two per segment Third pin NUMBER OF PINS: 1] Two pins per fragment to prevent rotation, usually in same plane. 2] A third pin in the fragment adds slightly to the resistance to bending and axial deviations, but little to rigidity. A third pin does allow the subsequent removal of a loose or infected pin without sacrificing the construct. NUMBER OF PINS: 1] Two pins per fragment to prevent rotation, usually in same plane. 2] A third pin in the fragment adds slightly to the resistance to bending and axial deviations, but little to rigidity. A third pin does allow the subsequent removal of a loose or infected pin without sacrificing the construct.

    44. Biomechanics of External Fixation Deformation perpendicular to the plane of two pins is best controlled with a third pin in that perpendicular plane. A third pin (C) out of plane of two other pins (A & B) stabilizes that segment. Deformation perpendicular to the plane of two pins is best controlled with a third pin in that perpendicular plane. A third pin (C) out of plane of two other pins (A & B) stabilizes that segment.

    45. 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 90ş PIN LOCATION: 1] Pin out of zone of injury if possible (avoid infecting fracture): a] For stability, place pins as close to the fracture site as possible (limited by anatomy and soft tissue injury). b] For sterility place away location of incision for any future, delayed ORIF, bone grafting, soft tissue coverage. 2] More stable if the second pin in a fragment is spread as far from the first pin to best resist motion in the perpendicular plane. 3] Wire constructs most stable when at 90? to each other. PIN LOCATION: 1] Pin out of zone of injury if possible (avoid infecting fracture): a] For stability, place pins as close to the fracture site as possible (limited by anatomy and soft tissue injury). b] For sterility place away location of incision for any future, delayed ORIF, bone grafting, soft tissue coverage. 2] More stable if the second pin in a fragment is spread as far from the first pin to best resist motion in the perpendicular plane. 3] Wire constructs most stable when at 90? to each other.

    46. Biomechanics of External Fixation Pin Bending Preload Bending preload not recommended Radial preload (predrill w/ drill < inner diameter or tapered pin) may decrease loosening and increase fixation PIN PRELOAD: BENDING PRELOAD: Previously thought that bending (preloading) the pins would decrease pin loosening. However, the pressure necrosis on the compression side of the pin leads to earlier loosening (Hydahl, JOT 91). RADIAL PRELOAD: Can radial preload the pin by using a pilot hole slightly smaller than the pin’s root diameter or a tapered pin. This is thought to decrease loosening and increase fixation. PIN PRELOAD: BENDING PRELOAD: Previously thought that bending (preloading) the pins would decrease pin loosening. However, the pressure necrosis on the compression side of the pin leads to earlier loosening (Hydahl, JOT 91). RADIAL PRELOAD: Can radial preload the pin by using a pilot hole slightly smaller than the pin’s root diameter or a tapered pin. This is thought to decrease loosening and increase fixation.

    47. Biomechanics of External Fixation Bone-Frame Distance Rods Rings Dynamization BONE-FRAME DISTANCE: 1] ROD: The closer the frame is to the bone the more stable is the construct. Allow room for edema and wound care (2-3 cm). 2] RING: Decreasing the span that the wire travels increases the overall rigidity. 3] DYNAMIZATION: Can “dynamize” the frame easily by moving the rod further away from the bone. BONE-FRAME DISTANCE: 1] ROD: The closer the frame is to the bone the more stable is the construct. Allow room for edema and wound care (2-3 cm). 2] RING: Decreasing the span that the wire travels increases the overall rigidity. 3] DYNAMIZATION: Can “dynamize” the frame easily by moving the rod further away from the bone.

    48. 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.

    49. Biomechanics of Locked Plating

    50. Conventional Plate Fixation In conventional plate fixation, as the screws are tightened, and the screws displace the bone into compression, a compressive force is generated between the plate and the bone. This compressive force must be sufficient in order to generate enough friction between the plate and the bone to maintain stability of the fracture. As long as the resultant load from body weight and muscles does not exceed what the frictional interface can support, the construct will remain stable and prevent collapse.In conventional plate fixation, as the screws are tightened, and the screws displace the bone into compression, a compressive force is generated between the plate and the bone. This compressive force must be sufficient in order to generate enough friction between the plate and the bone to maintain stability of the fracture. As long as the resultant load from body weight and muscles does not exceed what the frictional interface can support, the construct will remain stable and prevent collapse.

    51. Locked Plate and Screw Fixation Here is the initial stress in the bone due to tightening (roughly 0). When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture. Here is the initial stress in the bone due to tightening (roughly 0). When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture.

    52. Stress in the Bone If we compare the stresses in the bone immediately after implantation, the bone around the bi-cortical screws is already stressed from tightening whereas the bone around the locked screw is not subjected to any shear stresses. When the patient starts to weight bear, the bone in the bi-cortical screws is more highly stressed than the locked screws. If we compare the stresses in the bone immediately after implantation, the bone around the bi-cortical screws is already stressed from tightening whereas the bone around the locked screw is not subjected to any shear stresses. When the patient starts to weight bear, the bone in the bi-cortical screws is more highly stressed than the locked screws.

    53. Standard versus Locked Loading Here is the initial stress in the bone due to tightening. When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture. As the load is increased, and the angle between the screw and the plate starts to change, the resultant loading vector on the screw changes from pure compression to a compressive and a tensile component. It is this additional tensile component which, when the elastic limit of the bone is exceeded, will cause toggling and eventual collapse across the fracture gap.Here is the initial stress in the bone due to tightening. When the bone is loaded along the axes of the bone, the bone around the screw is compressed on the load side of the fracture. As the load is increased, and the angle between the screw and the plate starts to change, the resultant loading vector on the screw changes from pure compression to a compressive and a tensile component. It is this additional tensile component which, when the elastic limit of the bone is exceeded, will cause toggling and eventual collapse across the fracture gap.

    54. Once the angular stability of one screw is lost [by pulling thru 2nd cortex], the cycle begins and the inherent angular stability of the construct is compromised.Once the angular stability of one screw is lost [by pulling thru 2nd cortex], the cycle begins and the inherent angular stability of the construct is compromised.

    55. Resistance of larger area decreases this form of pullout. Rather the next form is necessary [see next slide]. Resistance of larger area decreases this form of pullout. Rather the next form is necessary [see next slide].

    56. 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 Through experience with failure the recommended strength of plate and number of screws in each fragment has been determined. These are general guidelines and more may be required with poor quality bone. Anatomic constraints may limit also the construct. Also it is important to note that the increased spread of screws is more important than the number of screws (provided each screw has equal purchase) for construct stability. Through experience with failure the recommended strength of plate and number of screws in each fragment has been determined. These are general guidelines and more may be required with poor quality bone. Anatomic constraints may limit also the construct. Also it is important to note that the increased spread of screws is more important than the number of screws (provided each screw has equal purchase) for construct stability.

    57. Preservation of Blood Supply Plate Design

    58. Preservation of Blood Supply Less bone pre-stress

    59. Vorteil einer winkelstabilen VerschraubungVorteil einer winkelstabilen Verschraubung

    60. Mechanisch gesehen entspricht das LIF einem Externen Fixateur, mit dem Vorteil, dass er sehr nahe am Knochen liegt und dadurch stabiler ist. Es ist jedoch wichtig, dass der Längsträger (Platte), wie beim externen Fixateur lange gewählt wirdMechanisch gesehen entspricht das LIF einem Externen Fixateur, mit dem Vorteil, dass er sehr nahe am Knochen liegt und dadurch stabiler ist. Es ist jedoch wichtig, dass der Längsträger (Platte), wie beim externen Fixateur lange gewählt wird

    61. Surgical Technique Compression Plating

    62. Surgical Technique Reduction

    63. Surgical Technique Reduction

    64. Surgical Technique Reduction 1. Contour of plate is important to maintain anatomic reduction.

    65. Surgical Technique Reduction with Combination Plate

    66. Surgical Technique Reduction with Combination Hole Plate

    68. Locked Screws Understand that the position of the plate and the bone will be “locked in” when a locked screw is utilized Conical screws usually utilized first to bring the “plate to the bone” and then locked with locking screws Lag before Lock

    69. 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. Haidukewych GJ, “Innovations in Locking Plate Technology,” JAAOS 12(4), 205-212 review.

    70. Questions?

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