ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

operative situation where the implant-bone interface was considered in contact with
friction; and a medium-long term situation presuming a rigid fixation of the implantbone
interface. In a second phase of the study, experimental tests were performed with
synthetic femurs. The experimental tests (five) allowed assessing the strain-shielding
effect at the proximal cortex through strain gauges. The principal strains before and
after stem insertion (Fig. 1) were measured. Finally, a fatigue test (one million of cycles
for a load of 3 times body weight) was performed in order to evaluate the cortex bone
strain changes and relate it with loosening and stability of the implant. CoCr femoral
stems were manufactured using CAD/CAM technology.
Fig. 1 a) implanted CAD model, b) stem model with cavities filled with cement, c)
strain gauges in proximal cortex.
3. RESULTS
The peak of minimal principal cancellous bone strains occurs in the medial side (-
7500µstrain). The peak of the von Mises cement stresses was 3.84 MPa and occurred at
the proximal cavities. The maximum von Mises stem stress was 191 MPa. For the
immediate post-operative scenario, the highest micromovement at the implant-bone
interface was 35µm and occurred at the medial side. The standard deviation was less
than 12% of the respective mean principal strains for all strain gauges. A good
correlation between the numerical and experimental cortex strains results was
determined (R2=0.97). The highest strain shielding effect was observed at the medial
proximal strain gauge, where a 3 times reduction was measured relatively to the intact
femur. After one million fatigue cycles, the implant did not reveal any permanent
subsidence or signals of loosening. The development of complementary studies is
necessary to prove the viability of the concept for clinical purposes.
4. ACKNOWLEDGEMENTS
The authors wish to thank to the Program COMPETE funding through the projects
PTDC/EME-PME/103578/2008, PTDC/EME-PME/111305/2009 and PTDC/EME-
TME/113039/2009.
5. REFERENCES
1. Gruen, T. A., McNeice, G. M., and Amstutz, H. C., Modes of Failure of Cemented
Stem-Type Femoral Components: A Radiographic Analysis of Loosening, Clin.
Orthop. Relat. Res., 1979, Vol. 141, 17–21.