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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 October 09,Basic Biomechanics,Material Properties Elastic-Plastic Yield point Brittle-Ductile Toughness Independent of Shape!,Structural Properties
2、 Bending Stiffness Torsional Stiffness Axial Stiffness Depends on Shape and Material!,Basic Biomechanics Force, Displacement & Stiffness,Force,Displacement,Slope = Stiffness = Force/Displacement,Basic Biomechanics,Stress = Force/Area,Strain Change Height (L) / Original Height(L0),Force,Area,L,Basic
3、Biomechanics Stress-Strain & Elastic Modulus,Stress = Force/Area,Strain = Change in Length/Original Length (L/ L0),Slope = Elastic Modulus = Stress/Strain,Basic Biomechanics Common Materials in Orthopaedics,Elastic Modulus (GPa),Stainless Steel 200 Titanium 100 Cortical Bone 7-21 Bone Cement 2.5-3.5
4、 Cancellous Bone 0.7-4.9 UHMW-PE 1.4-4.2,Stress,Strain,Basic Biomechanics,Elastic Deformation Plastic Deformation Energy,Energy Absorbed,Force,Displacement,Plastic,Elastic,Basic Biomechanics,Stiffness-Flexibility Yield Point Failure Point Brittle-Ductile Toughness-Weakness,Force,Displacement,Plastic
5、,Elastic,Failure,Yield,Stiffness,Stiff Ductile Tough Strong,Stiff Brittle Strong,Ductile Weak,Brittle Weak,Strain,Stress,Flexible Ductile Tough Strong,Flexible Brittle Strong,Flexible Ductile Weak,Flexible Brittle Weak,Strain,Stress,Basic Biomechanics,Load to Failure Continuous application of force
6、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.
7、,Basic Biomechanics,Anisotropic Mechanical properties dependent upon direction of loading,Viscoelastic Stress-Strain character dependent upon rate of applied strain (time dependent).,Bone Biomechanics,Bone is anisotropic - its modulus is dependent upon the direction of loading. Bone is weakest in sh
8、ear, then tension, then compression. Ultimate Stress at Failure Cortical Bone Compression 212 N/m2 Tension 146 N/m2 Shear 82 N/m2,Bone Biomechanics,Bone is viscoelastic: its force-deformation characteristics are dependent upon the rate of loading. Trabecular bone becomes stiffer in compression the f
9、aster it is loaded.,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.,Basic Biomechanics,Bending Axial Loadi
10、ng Tension Compression Torsion,Bending Compression Torsion,Fracture Mechanics,Figure from: Browner et al: Skeletal Trauma 2nd Ed, Saunders, 1998.,Fracture Mechanics,Bending load: Compression strength greater than tensile strength Fails in tension,Figure from: Tencer. Biomechanics in Orthopaedic Trau
11、ma, Lippincott, 1994.,Fracture Mechanics,Torsion The diagonal in the direction of the applied force is in tension cracks perpendicular to this tension diagonal Spiral fracture 45 to the long axis,Figures from: Tencer. Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,Fracture Mechanics,Combined
12、bending & axial load Oblique fracture Butterfly fragment,Figure from: Tencer. Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,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
13、 (shape) with respect to an axis.,Figure from: Browner et al, Skeletal Trauma 2nd Ed, Saunders, 1998.,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 weaker,Figure from: Browne
14、r et al, Skeletal Trauma 2nd Ed, Saunders, 1998.,Figure from: Tencer et al: Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,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 fro
15、m: Browner et al, Skeletal Trauma, 2nd Ed, Saunders, 1998.,IM Nails Moment of Inertia,Stiffness proportional to the 4th power.,Figure from: Browner et al, Skeletal Trauma, 2nd Ed, Saunders, 1998.,IM Nail Diameter,Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,Slotti
16、ng,Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,Figure from Rockwood and Greens, 4th Ed,Allows more flexibility In bending Decreases torsional strength,Slotting-Torsion,Figure from: Tencer et al, Biomechanics in Orthopaedic Trauma, Lippincott, 1994.,Interlocking Screws,Controls torsion and axial loads Advantages Axial and rotatio
