Optimization of the arthroscopic indentation instrument for the measurement of thin cartilage stiffness

Structural alterations associated with early, mostly reversible, degenerati on of articular cartilage induce tissue softening, generally preceding fibr illation and, thus, visible changes of the cartilage surface. We have alrea dy developed an indentation instrument for measuring arthroscopic stiffness of cartilage with typical thickness >2 mm. The aim of this study was to ex tend the applicability of the instrument for the measurement of thin (>2 mm ) cartilage stiffness. Variations in cartilage thickness, which will nor be known during arthroscopy, can nonetheless affect the indentation measureme nt, and therefore optimization of the indenter dimensions is necessary. Fir st, we used theoretical and finite element models to compare plane-ended an d spherical-ended indenters and, then, altered the dimensions to determine the optimal indenter for thin cartilage measurements. Finally, we experimen tally validated the optimized indenter using bovine humeral head cartilage. Reference unconfined compression measurements were carried out with a mate rial testing device. The spherical-ended indenter was more insensitive to t he alterations in cartilage thickness (20% versus 39% in the thickness rang e 1.5-5 mm) than the plane-ended indenter. For thin cartilage, the optimal dimensions for the spherical-ended indenter were 0.5 mm for diameter and 0. 1 mm for height. The experimental stiffness measurements with this indenter correlated well with the reference measurements (r = 0.811, n = 31, p < 0. 0001) in the cartilage thickness range 0.7-1.8 mm, We conclude that the opt imized indenter is reliable and well suited for the measurement of thin car tilage stiffness.