Abstract 5

TOWARDS BETTER CUTTING TOOLS FOR CANCELLOUS BONE AUTOGRAFT

I.A. Anderson, S.F.F. Malak

Bioengineering Institute, University of Auckland, New Zealand

The harvesting of autogenous cancellous bone graft often leads to trauma at the donor site, resulting in chronic pain. Heat generation and excessive bone fragmentation during cutting will also increase cell mortality within the graft. These complications can be mitigated by improving cutter design. Cancellous bone graft is commonly harvested using rotational cutting tools. However it is not easy to visualize the cutting process during rotational cutting.

To obtain a better understanding of the mechanics of cancellous bone cutting we have performed orthogonal cutting experiments, using an instrumented laboratory milling machine, on bone samples taken from fresh, frozen bovine femur and patella. Measurements were performed with tool rake angles of 23º, 45º and 60º, and depths of cut in the range 0.1-3mm. Horizontal and vertical cutting forces were measured, and the chip formation process visually recorded.

Continuous and discontinuous chip formation was observed for cutting depths larger than 0.5mm. Chip formation at depths above 1mm was accompanied by bone marrow extruding out of the bone free surface and away from the rake face. This might be beneficial as the bone cells contained in the marrow get carried away from the hot region of cutting. The specific cutting energy was generally low for larger rake angles, and larger depths of cut.

Our results suggest a good choice of rake angle to be about 45 degrees with a depth of cut of not less than 1mm, to ensure continuous chip formation (avoiding excessive fragmentation), bone marrow extrusion and low specific cutting energies.

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