1. CRICOS No. 00213J Modelling the Effects of Bone Fragment Contact in Fracture Healing G. Chen, C.J. Wilson, M.E. Wullschleger, D.L.S. McElwain, S.K. Mishra, M.J. Pearcy, S.M. Perren, G.J. Pettet, B. Schmutz, R. Steck, M.A. Schütz 7 October 2006

2. CRICOS No. 00213J FE Modelling of Fracture Healing Fracture healing processes regulated by inter- fragmentary loading and motion. Finite element (FE) analysis can describe local stresses and strains in tissues under load.  “Tissue differentiation” criteria.  Iterative models: healing simulation. Enables studies of influences of fixation methods, fracture geometry etc.

3. CRICOS No. 00213J Healing Simulation With FE Model

4. CRICOS No. 00213J Healing Simulation With FE Model Gap:0.4mm0.8mm1.2mm

5. CRICOS No. 00213J Bone Fragment Contact Alters tissue deformation & load transfer in/around fracture site. Impact of assuming uniform fracture gap on model’s predictions? Bone fragment 1 Bone fragment 2 FE models to date Bone fragment 1 Bone fragment 2 Actual fracture

6. CRICOS No. 00213J “Contact Zone” Partial contact represented by introducing small volume fraction of bone into gap.  Increased stiffness.  How does this alter callus formation / evolution?

7. CRICOS No. 00213J Iterative Finite Element Model Based on Ament & Hofer, J. Biomech. 2000. FE analysis embedded in each time step; Strain energy density results  tissue changes via fuzzy “differentiation rules”; Tissue properties updated for next iteration. Tissue transformations implemented in ABAQUS FEA software via user subroutines.

8. CRICOS No. 00213J Results: Callus Development Soft1% bone10% bone Contact zone:

9. CRICOS No. 00213J Results: Load-Sharing Contact zone

10. CRICOS No. 00213J Discussion Increasing stiffness in gap / “contact zone” reduces stimulus for callus growth. –Dramatic difference suggests important to define inter-fragmentary geometry. High degree of contact maintained  minimal stimulus for healing processes in model:  Algorithm inadequate for this case. Similar problem with “absolutely stable” fixation.

11. CRICOS No. 00213J Discussion Results will be more meaningful with full range of motion (beyond axial compression): –Fragment separation, cycling, & other motions. Transverse, torsion &/or bending loads also impact on healing → 3D models. To model cases such as “absolute stability”, need to factor in underlying biology. Investigating connections between bone & fixation – load-sharing & relative movement.

12. CRICOS No. 00213J Conclusions Contact between bone fragments dramatically alters response of fracture healing model. “Contact zone” is simple way to simulate these effects on callus evolution. Demonstrates need for greater accuracy in: –Modelling inter-fragmentary geometry, –Loading applied to model, –“Tissue differentiation” algorithms  need to consider more than just mechanics.

13. CRICOS No. 00213J Acknowledgments Dr Chen funded by Queensland University of Technology Strategic Collaborative Grant Dr Wilson funded by QUT Institute of Health and Biomedical Innovation Seeding Grant Thank-you