Evaluation of an Injectable Composite Bone Cement with Engineered Micro-Architecture Matthew A Wettergreen, Mark D Timmer, Jeremy J Lemoine, Antonios G Mikos, Michael AK Liebschner 13 th GRIBOI March 14th, 2003 Rice University Computational and Experimental Biomechanics Laboratory

Background Vertebroplasty as prophylactic treatment Use of bone cements in vertebroplasty Positive effects Restoration of strength Negative effects Increased stiffness Non-osteointegrative

Vertebroplasty as prophylactic treatment

Use of bone cements in vertebroplasty

Positive effects

Restoration of strength

Negative effects

Increased stiffness

Non-osteointegrative

Incorporation of Solid Phase Previous studies attempted to increase mechanical properties UHMWPE + MMA, Yang et al., 1997 Titanium + PMMA, Topoleski, 1995 Paradigm shift - Solid phase for therapeutic purposes Compensate for strength weakening in material Remove solid to create porosity Incorporate therapeutic drugs

Previous studies attempted to increase mechanical properties

UHMWPE + MMA, Yang et al., 1997

Titanium + PMMA, Topoleski, 1995

Paradigm shift - Solid phase for therapeutic purposes

Compensate for strength weakening in material

Remove solid to create porosity

Incorporate therapeutic drugs

Motivation Use of biologically active material Deliver solid material with injection Create engineered structure with injectable materials Attempt to mimic porosity of bone architecture

Use of biologically active material

Deliver solid material with injection

Create engineered structure with injectable materials

Attempt to mimic porosity of bone architecture

Hypothesis and Goal Hypothesis – use of solid phase with engineered architecture will improve permeability of the composite Goal – Use Rapid Prototyping to make solid particles for use in injectable material

Hypothesis – use of solid phase with engineered architecture will improve permeability of the composite

Goal – Use Rapid Prototyping to make solid particles for use in injectable material

Methodology Generation of micro-architecture 2. Building of engineered architecture 3. Creation of porous scaffolds 5. Evaluation of permeability of scaffolds 4. Evaluation of porosity of scaffolds

Generation of micro-architecture

Process similar to creation of porosity with implantable bone scaffolds Size of generated architecture on order of NaCl particles Bounding box of micro-architecture small enough for use in syringe Geometry must promote connections to improve permeability Design of Micro-Architecture

Process similar to creation of porosity with implantable bone scaffolds

Size of generated architecture on order of NaCl particles

Bounding box of micro-architecture small enough for use in syringe

Geometry must promote connections to improve permeability

Generation of Y-Shapes Avg. Volume = .221194 mm 3 Avg. Volume = .221445 mm 3 Bounding box = 1.03mm x 1.58mm Surface Area = 2.8948 mm 2 Surface Area = 2.19615 mm 2 .984 .394 .288 .334 .45 .500-.710

Building of 3-D architecture

Creation of Porous Scaffolds 3168 Y’s, ~3161NaCl used per porosity value 50, 60, 70% porosity by volume created in acrylic resin Majority of studies utilize weight percentage Material packed into GPC vials to form scaffolds 6mm diameter, 12mm length NaCl leached with H 2 O, Y7 leached with EtOH

3168 Y’s, ~3161NaCl used per porosity value

50, 60, 70% porosity by volume created in acrylic resin

Majority of studies utilize weight percentage

Material packed into GPC vials to form scaffolds

6mm diameter, 12mm length

NaCl leached with H 2 O, Y7 leached with EtOH

Evaluation of Permeability Importance of permeability of material Dependence of permeability on surface area Q A = (k/u)( Δ P/ Δ L) Q A = Flow Rate (m 3 /s) μ = Viscosity (Ns/m 2 ) Δ P = Pressure (N/m 2 ) Δ L = Scaffold Length (m) Darcy’s Law of permeability

Importance of permeability of material

Dependence of permeability on surface area

Permeability Results Permeability of NaCl scaffolds illustrates ↑ porosity ≠ ↑permeability Engineered architecture promotes permeability even with lower volume compared to NaCl 3 2 2 5 n 1.71E-04 50% Vol. Y7 2 not permeable 70% Vol. NaCl 1.92E-05, 1 not permeable 60% Vol. NaCl 9.19E-06 , 4 not permeable 50% Vol. NaCl Permeability, k (m 2 ) Porosity value

Permeability of NaCl scaffolds illustrates

↑ porosity ≠ ↑permeability

Engineered architecture promotes permeability even with lower volume compared to NaCl

Evaluation with uCT Samples scanned with Scanco80 μ CT Porosity evaluated by thresholding by density Connectivity of scaffolds evaluated 30.17% 50% Y7 45.18% 70% NaCl 23.49% 60% NaCl Porosity Sample

Samples scanned with Scanco80 μ CT

Porosity evaluated by thresholding by density

Connectivity of scaffolds evaluated

Error in build processes Dimensions of Y-shapes measured Print width range 90-115 μ m Surface features evaluated on Y shapes

Dimensions of Y-shapes measured

Print width range 90-115 μ m

Surface features evaluated on Y shapes

Results Architecture Resolution of parts within acceptable error RP unable to produce part as originally designed Porosity Porosity lower than originally calculated due to shape change Permeability Higher than with the random generated scaffolds Higher surface area contributed to permeability difference Inability to build scaffolds with high porosity when using Y’s due to permeability

Architecture

Resolution of parts within acceptable error

RP unable to produce part as originally designed

Porosity

Porosity lower than originally calculated due to shape change

Permeability

Higher than with the random generated scaffolds

Higher surface area contributed to permeability difference

Inability to build scaffolds with high porosity when using Y’s due to permeability

Future Directions Explore additional architectures FEA of geometry for optimal permeability Correlate porosity with permeability

Explore additional architectures

FEA of geometry for optimal permeability

Correlate porosity with permeability

Acknowledgements Computational and Experimental Biomechanics Laboratory Mikos Research Group Funding Source Texas ATP Grant

Computational and Experimental Biomechanics Laboratory

Mikos Research Group

Funding Source

Texas ATP Grant

Thank You!!

Scratch