06 - Bone and Hip Fractures

Question 1

Bone tissue

Answer 1

• hierarchical structure: collagen molecule -> fibril -> lamella -> trabecula -> trabecular bone -> whole bone
• multiphasic
• – organic = 35% BV, collagen => ductility
• – inorganic = 35%, CaP => stiff but brittle
• – water = 20% => plasticizer
• macroscopic structure
• – cortical bone: voids < 10% vol, 80% BM
• – cancellous bone: voids can be 95% vol, rods/arches/plates
• anisotropic
• remodeling: Wolff’s law; resorption ~ 8 days, neo-formation 30d after, need 80d
• densities
• – tissue = M_bone/V_bone
• – wet apparent density = M_bone/V_sample (2g/cm3 for cortex)
• – bone volume fraction = V_bone/V_sample

Question 2

Mechanical properties of bone

Answer 2

• measurement: direct mechcl testing OR ultrasound, nanoindentation
• stiffness = slope deformation vs applied load (sigma = E.epsilon)
• strength = load required to induce failure
• correlation to densitometric msrmt
• linear elastic range: similar stiffness in tension/compression
• non linear range (post-yield) = near elasto-plastic response (permanent deformation)
• – apparent level yield strain
• – ultimate stress = stress at max load
• – ultimate strain: ~ 2%
• – post-yield compressive response: softening and densification

Question 3

Hip fractures classification

Answer 3

• image-based: linear-elastic kd=r /OR/ non-linear models (f(d))
• explicit models (time-dpdt FEA): linear (kd=r) /OR/ non-linear
  iterative manner /!\ stability
• validation: qttv CT -> proximal femur FEM -> xpmtl validation using cadaver femurs => good correlation but no improvement compared to aBMD

Question 4

Biofidelic FEMs

Answer 4

• xpmtl testing: real-life fall vs fall simulator (with subject-specific soft tissue shape)
• validation
  — material (ballistic gel mixture)
  — component (impactor block)
  — system
  — population: fragility ratio, ∆MVS = max volumetric strain chg
  => better than aBMD /IF/ likely non-fallers are excluded (inaccuracy from non fallers with low bone strength)