Aging reduces strain magnitude and heterogeneity while increasing apparent stiffness of the rat proximal tibia.

The epiphyses of long bones play a critical role in joint integrity by absorbing and redistributing joint forces. While aging induces microstructural and material changes in this anatomical site, their impact on the internal mechanical environment of the epiphysis remains poorly characterized. In this study, we quantified age-related alterations in epiphyseal strain distribution in the knee joints of adult and aged male Wistar rats using in situ mechanical testing combined with micro-computed tomography, digital volume correlation, and finite element analysis. Aging was associated with a pronounced reduction in deformation: mean compressive and tensile principal strains in aged animals decreased by approximately 63% and 67%, respectively, compared with strains in adult animals. In addition to reduced strain magnitude, aged epiphyses exhibited less heterogeneous strain distributions, altered spatial strain patterns, and increased inter-individual variability of the deformation. These mechanical changes are consistent with previously reported age-related thickening of the subchondral bone plate, trabecular coarsening, and increased tissue mineralization. Together, these findings demonstrate that aging leads to substantial stiffening of the proximal tibia and alters its behavior under compression. Such changes may affect the protective mechanical function of the epiphysis and contribute to an unfavorable mechanical environment for articular cartilage, which may predispose to joint degeneration.