Effects of growth on residual stress distribution along the radial depth of cortical cylinders from bovine femurs.

Residual stress is defined as the stress that remains in bone tissue without any external forces. This study investigated the effects of growth on residual stress distributions from the surface to deeper regions of cortical cylinders obtained from less-than-one-month-old (Group Y) and two-year-old (Group M) bovine femurs. In these experiments, five diaphysis specimens from each group were used. Residual stress was measured using a high-energy synchrotron white X-ray beam to penetrate X-rays into the deeper region of the bone specimens. The measurements in the cortical cylinders from Groups Y and M were performed at 0.5- and 1-mm intervals, respectively, from the outer surface to the deeper region of the diaphysis specimens at four positions: anterior, posterior, lateral, and medial. The residual stress was calculated on the basis of variation in the interplanar spacing of hydroxyapatite crystals in the bone tissue. According to the results, the diaphysis specimens from Group Y were not subjected to large residual stresses (average -1.2 MPa and 2.4 MPa at the surface region and 1.5mm depth, respectively). In Group M, the surface region of the diaphysis specimens was subjected to tensile residual stresses (average 6.7 MPa) and the deeper region was subjected to compressive stresses (average -8.2 MPa at 3mm depth). There was a strong significant difference between both these regions. The value of residual stresses at the surface region of the diaphysis specimens in both the groups had a positive statistical correlation with the cortical thickness at the measured locations.