Equine subchondral bone failure threshold under impact compression applied through articular cartilage

Subchondral bone microdamage due to high-impact loading is a key factor leading to post-traumatic knee osteoarthritis. A quantified assessment of the mechanical characteristics of subchondral bone at the tissue-level is essential to study the mechanism of impact-induced microdamage. We combined mechanical impact testing of equine cartilage-bone with µCT image-based finite element models (μFEM) of each specimen to determine subchondral bone (including calcified cartilage: CCSB) elastic tissue modulus and local stresses and strains associated with micro-fractures within the CCSB tissue. The material properties of each specimen-specific μFEM were iteratively adjusted to match the FE-predicted stress-strain curves with experimental results. Isotropic homogeneous material properties for both uncalcified cartilage (UC) and CCSB were assumed. UC large-deformation was simulated using hyperelastic material properties. Final UC shear and CCSB tissue elastic modulus of G=38±20 MPa and Et=3.3±0.7 GPa were achieved after fit procedure. The results suggested that initial failure in CCSB occurred at local tensile and compressive stresses of 29.47±5.34 MPa and 64.3±21.3 MPa, and tensile and compressive strains of 1.12±0.06% and 1.99±0.41%, respectively. Tissue-level material properties can be used in finite element modeling of diarthrodial joints under impact loading, and also in designing artificial cartilage-bone to replace the damaged tissue in the joint. Results can provide an estimate for the threshold of initial failure in subchondral bone tissue due to an impact compression transmitted through the overlying articular cartilage.