J Fernandez , T Besier, P Pivonka, P Hunter, V Shim

1. Exploring the effects of different walking strategies on bone remodelling mechanisms EUROPEAN CONGRESS ON COMPUTATIONAL METHODS IN APPLIED SCIENCES AND ENGINEERING (ECCOMAS 2012) J Fernandez, T Besier, P Pivonka, P Hunter, V Shim

2. Motivation • Acetabulum of the hip fails from low loads in 10-20% of cases. Did the hip fail before or after the fall ? • Is a history of remodelling playing a role? Is the loading stimulus contributing to this condition? • What is the loading pattern from healthy subjects? • What are suitable boundary conditions for understanding bone remodelling ? Is it simply walking or a combination of tasks ?

3. Previous work Remodelling at the knee bone-cartilage interface *V Shim, P Hunter, P Pivonka and J Fernandez (2011), ‘A multiscale framework based on the Physiome markup languages for exploring the initiation of osteoarthritis at the bone cartilage interface.’ IEEE Trans Biomed Eng. 58(12):3532-6.

4. Previous work Remodelling of the femoral neck *J Fernandez, R Das, P Cleary, P Hunter, CDL Thomas and JG Clement (2012), ‘Using smooth particle hydrodynamics to evaluate the influence of femoral cortical bone architecture on bone remodelling’, International Journal for Numerical Methods in Biomedical Engineering. In Press DOI: 10.1002/cnm.

5. The IUPS Physiome Project .. is a modelling framework for understanding biologicalstructure and function from proteins to whole organisms. Genes mRNA Proteins Lipids Carbohydrates Cellstructure-function Tissuestructure-function Organstructure-function Clinical medicine … Physiome Project HGP/Transcriptome/Metabolome/Proteome 4 tissue types 25,000+ genes 100,000+proteins 200+ cell types 12 organsystems 1 body Hunter, PJ and Borg, TK. Integration from proteins to organs: The Physiome Project. Nature Reviews Molec & Cell Biol. 4:237-243, 2003

7. Multiscale Framework

8. Stair rise & walking tasks

9. Methodology • Linear elastic mechanics – CMISS (www.cmiss.org) • Frost remodelling rules (osteocyte density of 150/mm2)A • Material properties derived from Hounsfield units of a cadaver CTB • Muscle paths derived from a Somso model & literature (with the assistance of a hip surgeon). • Muscle forces computed using OpenSim and combined with data from Bergman. • (A) Power J et al. Bone. 2002, Osteocyte density in aging subjects is enhanced in bone adjacent to remodelinghaversiansystems. Jun;30(6):859-65. • (B) Shim, V. B. et al., 2008, "Development and validation of patient-specific finite element models of the hemipelvis generated from a sparse CT data set," J BiomechEng, 130(5)

10. Muscle force evaluation • Muscle forces evaluated against EMG and an instrumented knee* * Kim HJ, Fernandez JW, Akbarshahiet al. Evaluation of predicted knee-joint muscle forces during gait using an instrumented knee implant. J Orthop Res. 2009 Oct;27(10):1326-31.

11. Hip model validation * Shim, V., Boehme, J., Vaitl, P., Klima, S., Josten, C., and Anderson, I., "Finite element analysis of acetabular fractures -Development and validation with a synthetic pelvis," Journal of biomechanics, 43(8), pp. 1635-1639.

12. Hip muscle paths Gluteus Maximus Psoas Iliacus Gluteus Medius & Minimus Deep Ext Rotators Pectineus Sartorius TFL Biceps Femoris Adductor Brevis/Longus /Magnus Rectus Femoris Semitendinosus Semimembranosus

13. Initial material properties Cortical Trabecular E=200MPa to 2.2GPa, ʋ = 0.2 E=16.7GPa, ʋ = 0.3

14. Healthy state of stress (1.4 m/s) Superior 70% Walking Anterior Posterior 30% Stair ascent Inferior From 1MPa to 30MPa,

15. Walking (0.8 m/s) with gradients Remodelling Strain Stimulus Difference between current & healthy state converted to strain. 0 <500<1000< 2000<3000με Blue bone is maintained; 1000 μεto3000 με bone material is degraded

16. Walking (0.8 m/s)with no gradients Remodelling Strain Stimulus Difference between current & healthy state converted to strain. 0 <500<1000< 2000<3000με Blue bone is maintained; 1000 μεto3000 με bone material is degraded

17. Assisted walking Remodelling Strain Stimulus Difference between current & healthy state converted to strain. 0 <500<1000< 2000<3000με Blue bone is maintained; 1000 μεto3000 με bone material is degraded

18. Concluding Remarks • Acetabulum stress pattern was consistent with previous works1,2 where the load is primarily superior to the cup and along the ridge. • Gradient-based tasks are important and when removed led to increased remodelling inferiorly and outside the pelvic cup. • This is important as the inferior wall of the acetabulum is thinner. • Tasks that mimic gradients like chair rising may be useful substitutes. DalstraM, Huiskes R. Load transfer across the pelvic bone. Journal of biomechanics 1995;28(6):715–24. 2. Phillips A, Pankaj P, Howie C, Usmani A, Simpson A. Finite element modelling of the pelvis: inclusion of muscular and ligamentous boundary conditions. Medical engineering & physics 2007;29(7):739–48.

19. What’s next • Validation of bone remodelling. To address this challenge we have started to look at time-lapsed in vivo bone remodelling in mice (with Prof Ralph Muller’s group at the ETH, Zurich) • Evaluate the full range of dynamic tasks including shuffle gait • Improved description of spatial osteocyte density – Variability with old age influences remodelling. • Recompute muscle forces using EMG driven muscle force method (Lloyd & Besier, J Biomech, 2003) • Sensitivity of acetabulum strain pattern to individual muscle perturbations. Which muscles are most influential ?

20. Acknowledgements • Raj Das (Mechanical) • Jillian Cornish (Medicine) • Dr Jacob Munro (ABI/Med) • David Thomas (Dentistry) • John Clement (Dentistry) • Helen Davies (Veterinary Science) • Paul Cleary • Matt Sinnott