Lecture 2.docx

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Rutgers University
Exercise Science and Sport Studies
Sara Campbell

Lecture 2: BIOMECHANICS OF BONE I. biomechanics of bone ■ bone composition: figure 2-1 ○ periosteum- where the nerve endings are ○ articular cartilage at the ends of bones protects the periosteum ■ organic and inorganic materials ■ organic: makes it flexible and resilient; made up of water ○ makes up 30% of its body weight ■ inorganic: makes it hard and rigid; made up of calcium and phosphate ○ makes up about 70% of its body weight *know this is more dont need to know % II. figure 2.2 and 2.3- picture of femur ■ medial and lateral trabecular systems are important in improving strength of the bone for wb properties and for abductor and adductor moments ■ greater trochanter ■ femoral neck ■ femoral head ■ osteoporosis causes the bone to be more porous ■ spontaneous fracture- when weak bone fractures spontaneously without any form of trauma (ex falling); can be due to cancer of the bone or severe osteoporosis III. structure ■ the combination of both organic and inorganic materials that make up the bone make it more strong than if bone was made up by solely one material or the other IV. load deformation curve (figure 2.6) ■ the load deformation curve applies to structures composed of a somewhat pliable material; bone, glass, plastic, ceramics ■ if a load is applied within the elastic region of the structure and is then released no permanent deformation will occur ■ if loading is continued past the yield point and into the structures plastic region, and then the load is released permanent deformation will occur ○ the amount of deformation that occurs depends on how far into the plastic region the load is applied ■ if loading continues eventually it will reach an ultimate failure point ■ this curve is important in determining fracture behavior and repair, the response of a structure to physical stress or the effects of various treatment ■ three parameters for determining the strength of a structure are reflected on this curve 1. the load that a structure can sustain before failing 2. the deformation it can sustain before failing 3. the energy it can store before failing III. stress and strain ■ stress is the load or force per unit area that develops on a plane surface within a structure in response to externally applied loads ■ strain is the deformation that develops within a structure in response to externally applied loads ■ stress/strain curve of metal, glass, and bone ■ figure 2.10 load deformation curve shows that metal is more pliable and glass is more brittle IV. biomechanical behavior of bone ■ what is the behavior of bone under different loading modes such as tension, compression, bending, shearing, torsion, and combined loading ○ bending is a combination of compressive and tensile forces ○ combined loading is a combination of shearing and torsional forces ■ figure 2.30- schematic representation of a small segment of bone loaded in torsion *exam ○ maximal shear stresses act on planes parallel and perpendicular to the neutral axis ○ maximal tensile and compressive stresses act on planes diagonal to this axis ■ figure 2.29- cs of a cylinder loaded in torsion ○ the magnitude of the stresses around the neutral axis is highest at the periphery of the cylinder and lowest near the neutral axis ○ important in determining where a fracture might occur - ex distal portion of the tibia is more susceptible to fracture because forces are closer to the neutral axis V. tensile fracture ■ tension- achilles attachment to calcaneus ■ strong bony protuberance on the calcaneus ■ figure 2.18- example of tensile fracture through the calcaneus produced by strong contraction of the triceps surae muscles during a tennis match ■ another name for a tensile fracture is an avulsion fracture ■ where else do we see this type of bony protuberance? ○ tibial tuberosity, greater trochanter, 5th metatarsal and distal fibula (inversion ankle sprain) VI. other types of fractures ■ displaced fracture- fracture occurs resulting in bones being moved/ displaced from anatomical position ■ nondisplaced fracture- fracture but still in anatomical position ■ open reduction internal fixation- plates and screws inserted in bones through surgery ■ open reduction external fixation ■ tibial plateau fracture from hyperextension of the knee from the collision- no ORIF is done on someone still growing with and has an epiphyseal plate ■ figure 2.49- all forces are being transferred to the plates and screws instead of the bone which causes the bone to become weak and porous, bone is also being laid down near the fixation devices where the forces are being transferred, this makes a bone susceptible to refracturing VII. compression versus shearing ■ compression: equal and opposite loads are towards the surface ○ compression fractures, tibial plateau fractures (land from high distance), calcaneal fractures, FOOSH injury, colles fracture (break radius) ■ shear: load is applied parallel to the surface of the structure, there is an angular deformation of the structure that undergoes shear loading ○ shearing of the humeral head- x ray of humeral head sheared off the humeral shaft *must be careful and check that there is adequate blood supply VIII. bending and torsion ■ bending: the bone is bending about an axis and the bone is subject to tension and compression ○ three point and four point fractures ○ ex boot top fracture in skiers- broken leg in ski boot ○ intramedullary rod patient ■ figure 2.-25 cs of a bone subject to bending shows distribution of stresses around the neutral axis ○ tensile stresses act on the superior side, and compressive stresses act on the inferior side ○ the stresses are highest at the periphery and lowest near the neutral axis ○ in asymmetrical bone the tensile and compressive stresses are unequal ■ figure 2-27- shows a boot top fracture occurring from three point bending ■ torsion: the load is applied about an axis and the torque is produced within the structure, the bone will fail first at shear with the formation of the initial crack parallel to the neutral axis of the bone ■ figure 2.28- if someone cannot bend their knee they may undergo MUA however during rehabilitation a four point bending caused the femur to refracture at its weakest point (the original fracture site) IX. combined loading ■ loads are compressive during heel strike and stance phase but tensile during toe off (tibia elongates) ; shear stress or torsional loading is higher during the late stages of the gait cycle because of ER of the tibia ○ screw home mechanism- last 20-30
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