Whole Bone Modeling Using Bone Whole Bone Modeling Using Bone Mineral Data Mineral Data or or Bones Are Not Amorphous BlobsBones Are Not Amorphous BlobsThomas J. Beck, Sc.D.Thomas J. Beck, Sc.D.The Johns Hopkins University The Johns Hopkins University School of MedicineSchool of MedicineThe Fundamental problemThe Fundamental problem? ?Bones get less dense as we ageBones get less dense as we age? ?Old people with low density bones Old people with low density bones fracture easilyfracture easily? ?SpaceSpace- -flight rapidly causes bones to flight rapidly causes bones to get less denseget less dense? ?Does space flight produce the same Does space flight produce the same effects on bone as aging?effects on bone as aging?The Human FemurThe Human FemurBone loss causes Bone loss causes fragility of proximal fragility of proximal (upper) end of femur(upper) end of femurFractures are major cause Fractures are major cause of death (indirect) and of death (indirect) and disability in elderlydisability in elderlyConcern that spaceConcern that space- -flight flight might have similar might have similar consequencesconsequencesBone DynamicsBone Dynamics? ?Bone is a dynamic tissue and is constantly Bone is a dynamic tissue and is constantly being absorbed and rebuilt throughout life.being absorbed and rebuilt throughout life.? ?Because bones get less dense with age Because bones get less dense with age physicians interpret this as an imbalance physicians interpret this as an imbalance betweenbetween resorptionresorption and formation. and formation. ? ?Old people with low density bones fracture Old people with low density bones fracture easily so it is assumed that bone density is a easily so it is assumed that bone density is a measure of likelihood of fracture. measure of likelihood of fracture. Current Bone “Density”MeasurementsCurrent Bone “Density”MeasurementsDXA Scan Image of HipDXA Scan Image of HipDigital image with all soft tissues subtracted and only bone mineral present Pixel values represent areal mass in g/cm2 Areal mass is averaged over part of region containing bone (femoral neck region shown) Result approximates volumetric density and is called bone mineral density (BMD)Femoral Neck BMDFemoral Neck BMD00.20.40.60.811.21.41.620406080100Age (Years)BMD (g/cm2)Looking at the detailsLooking at the details? ?Why do bones get less dense as we age?Why do bones get less dense as we age?Does this necessarily mean that bones are Does this necessarily mean that bones are getting weaker?getting weaker? ?Why do astronauts lose bone?Why do astronauts lose bone?Are their bones getting weaker?Are their bones getting weaker? ?What factors are common between aging What factors are common between aging and and microgravitymicrogravity and how do they differ?and how do they differ?Reduced Bone Strength Reduced Bone Strength is an Engineering Problemis an Engineering ProblemPossible reasons:Possible reasons:1. The material is less able to withstand 1. The material is less able to withstand loading stressesloading stresses OrOr 2. The structure is altered to increase loading 2. The structure is altered to increase loading stressesstressesStructure vs. Material PropertiesStructure vs. Material Properties? ?Evidence exists that bone material strength declines Evidence exists that bone material strength declines with age but effect disappears when corrected for with age but effect disappears when corrected for porosityporosity? ?No good evidence that spaceNo good evidence that space- -flight influences flight influences material propertiesmaterial properties? ?(no reliable way to measure (no reliable way to measure in vivo in vivo anyway)anyway)? ?Without questionWithout question bone structure changes with age bone structure changes with age and as a result of spaceand as a result of space- -flightflightAn engineering perspectiveAn engineering perspective? ?Long bones act as inefficient levers, Long bones act as inefficient levers, with actions due to muscle forceswith actions due to muscle forces? ?Greatest mechanical stresses are in Greatest mechanical stresses are in bending and torsionbending and torsionBending Stresses in BonesBending Stresses in BonesAt a given location, stress depends on:At a given location, stress depends on:1. 1.Properties of crossProperties of cross- -sectionsection2. 2.Bending Moments (moment arm Bending Moments (moment arm lengths and force magnitudes)lengths and force magnitudes)Bending stresses in cantilever beamBending stresses in cantilever beam =MyIFydI = Cross-sectional Moment of Inertia Compressive stressTensile stressM = F x dMaximum Bending StressMaximum Bending Stressmax=yMy I=M ZSection Modulus predicts maximum stress (Section Modulus predicts maximum stress ( ) on ) on subperiosteal (outer) surfacesubperiosteal (outer) surfaceM = bending momentM = bending moment I = crossI = cross- -sectional moment of inertiasection