In situ microindentation for determining local subchondral bone compressive modulus

Abstract

Alterations to joint tissues, including subchondral bone, occur with osteoarthritis. A microindentation technique was developed to determine the local compressive modulus of subchondral bone. This test, in conjunction with a cartilage indentation test at the same location, could evaluate changes of these material properties in both tissues. The accuracy of the technique was determined by applying it to materials of known moduli. The technique was then applied to rat tibial plateaus to characterize the local moduli of the subchondral bone. An established nanoindentation method was adopted to determine the modulus of subchondral bone following penetration of the overlying articular cartilage. Three cycles of repeated loadings were applied (2.452 N, 30 s hold). The slope of the load-displacement response during the unloading portion of the third cycle was used to measure the stiffness. Indentation tests were performed on two polyurethane foams and polymethyl-methacrylate for validation (n=15). Regression analysis was used to compare the moduli with reference values. Subchondral bone moduli of tibial plateaus from Sprague-Dawley rats (n=5) were measured for central and posterior locations of medial and lateral compartments. An analysis of variance was used to analyze the effects of compartment and test location. The measured moduli of the validation materials correlated with the reference values (R(2)=0.993, p=0.05). In rat tibial plateaus, the modulus of the posterior location was significantly greater than the center location (4.03+/-1.00 GPa and 3.35+/-1.16 GPa respectively, p=0.03). The medial compartment was not different from the lateral compartment. This method for measuring the subchondral bone in the same location as articular cartilage allows studies of the changes in these material properties with the onset and progression of osteoarthritis.

Figure 1

(a) Indentation loads showing the initial penetration of the cartilage with three more cycles with 30 s holds. Also shown are the loads at which the depth of penetration is measured and the range for fitting the elastic unloading. (b) Indentation displacements with the fit of the creep in the last half of the last hold cycle and the depth of penetration.

Figure 2

Load-displacement response of a rat tibia plateau to cyclic indentation. Horizontal dashed lines show the range of loads for fitting the elastic unloading stiffness. The solid line shows the linear fit to the elastic unloading.

Figure 3

Axial view of a rat tibia showing the four test sites at the central and posterior locations of the medial and lateral compartments (MC-medial central, MP-medial posterior, LC-lateral central, LP- lateral posterior).

Figure 4

Compressive modulus measured by indentation (E_indentation) with reference values(E_reference) for two densities of rigid polyurethane foams and a dental polymethyl-methacrylate (PMMA).