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INTRODUCTION TO BIOMECHANICS David Malicky University of San Diego Biomechanics is (for the most part) not: “We can rebuild him. We have the technology” Biomechanics is: Research: To understand and/or repair nature Industry: Develop prosthetic joint, therapy, or treatment Biomechanics and Engineering Industrial Engr Mechanical Engr. Elec/Comp Engr. Bioengineering (Occupational Biomechanics) Bioengineering (Biomechanics) Bioengineering Types of Biomechanics Orthopaedic: • Growth, Damage, and Healing of Bones, Joints, Spine, Cartilage, Tendons, Ligaments... • Artificial Joints • Gait and Falls Cardiovascular: Cardiac Assist Devices, Blood Flow Occupational: Lower Back Pain Rehabilitation: Assistive Technology Nature: Plants, Animals Bones: Why are they hollow? B. Kosoff, Bones Bones: Why are they hollow? • Bending of a Long, Solid, Bone: Tension Stress Free in the middle Compression Bones: Why are they hollow? • Eliminate the inside to save weight but keep strength: Tension Compression Bone: An Old, High Tech Material Human Skull: Porous Center with Solid Sides. Resists Denting B. Kosoff , Bones Composite Honeycomb Sandwich Panel Bone: A Living, Responsive, Material Micro-CT scan of 1cm bone cylinder: Load FEA Unloaded Control Loaded for 12 weeks Univ. of Michigan Joints: How does cartilage work? Cartilage: 80% water in a “tight sponge” of solid matrix. Bone Cartilage Water does not squeeze out: takes most of load Cartilage Water can’t fail; little stress on solid (living) portion Cartilage Also: self-lubricating (u=0.005), ~self-repairing Bone Spine: Lower Back Pain • LBP costs $20-$50B/yr • Affects 80% of population • Hand load creates reactions in spine and back muscles Back Muscle Force Spinal Column Force 50lbs. Spine: Lower Back Pain Lever Model Fmuscle Fulcrum Fmuscle=(20/2)*50lbs = 500lbs. Fspine=Fm+50lbs 2” 20” = 550lbs. Forces up to 1000lbs Fspine 50 lb Hand Load Spine: Lower Back Pain • Computer models predict muscle and spine forces for any posture and loads. • Design safer work environments Univ. of Michigan Knee: Chondromalacia Resultant Force Bottom View Outside Excessive Pressure Knee: Chondromalacia • Normal: • Chondromalacia Pain Knee: Chondromalacia Solution: Increase Force from inner quadriceps with physical therapy Resultant Force Inner Quad Computer Models: Knee •MRI scan of knee •Reconstruct 3D Geometry •Make FEM Model of bones and cartilage •Predict Contact Forces Columbia Univ. Artificial Joints • Arthritis = worn cartilage • Corrosive, unpredictable environment • Titanium Stem/Ball and Polyethelene Socket • Wear debris immune response • Ceramic-Ceramic Tissue Engineering: Grow Your Own • Stem cells seeded onto a biodegradable scaffold/matrix • Local gene therapy instructs cells to regenerate bone, ligament, cartilage, skin, nerves, organs… • Body’s cells join in • Bone cartilage forms, scaffold is absorbed Carnegie-Mellon U. Assistive Technology iBot – Dean Kamen Nature: The Cube-Square Law • Why can fleas jump 100x their own height? • Why can’t elephants even jump ¼ x their own height? • Cube-Square Law: “Size”: “average dimension” of the animal Muscle Force ~ Cross-Sectional-Area ~ Size2 Mass ~ Volume ~ Size3 Cube-Square Law Acceleration Capacity of Animals: • F = m*a a = F/m a ~ (Size2 ) / (Size3) = Size-1 • Smaller animals can accelerate faster: - Hard to catch a fly - Football kick-returners - Soccer players The Cube-Square Law Jumping Height of Animals (1st order model): • Mass ~ Size3 • Force ~ Size2 • Work = Increase in Potential Energy F * Distance = M * g * Height change Size2 * Size ~ Size3 * Height change Size cancels on both sides: Height change of CG is not a function of Size Cube-Square Law: Jumping Height Mice, cat, dogs, human, horse… CH height change = ~3-4 feet. Cube-Square Law: Scaling Problems Iguana ? Godzilla ? Human Shoulder: Dislocation • Most Range of Motion • Most dislocated major joint • Repetitive dislocations common after first occurrence da Vinci Shoulder Instability: Dislocation Scapula (shoulder blade) Rib Cage Glenoid “socket” Humeral Head Humerus (upper arm bone) Shoulder Anatomy Humerus Glenoid Capsule Capsule: • “Membranous ligament” • Thickenings • Complex structure Scapula Shoulder Experiment • Doctors believe the capsule is stretched after the first dislocation, allowing further dislocations, but it has not been shown. • Surgical shortening of the capsule improves stability. • Shortening can restricts range of motion. Aim of my experiment: Measure the strain field in the capsule due to a dislocation Shoulder Experiment Scapula Calibration Frame Xray Cassette Dislocation Arm Humerus Total Max Principal Strain SUPERIOR G L E N O I D INFERIOR H U M E R U S Biomechanics is: Thank You