Age dependent mechanical properties of the infant porcine parietal bone and a correlation to the human.

An infant less than 18 months of age with a skull fracture has a one in three chance of abuse. Injury biomechanics are often used in the investigation of these cases. In addition to case-based investigations, computer modeling, and test dummies, animal model studies can aid in these investigations. This study documents age effects on the mechanical properties of parietal bone and coronal suture in porcine infants and correlates the bending properties of the bone to existing human infant data. Three beam specimens were cut from porcine specimens aged 3 days, 7 days, 10 days, 14 days, 18 days, and 21 days: one across the coronal suture and two from the parietal bone, one parallel to and one perpendicular to the coronal suture. An actuator-mounted probe applied four-point bending in displacement control at 25 mm/s until failure. Bending stiffness of bone specimens increased with age; bone-suture-bone specimens showed no change up to 14 days but increased from 14 days to 18 days. All three specimen types showed decreases in ultimate stress with age. Ultimate strain for the bone-suture-bone specimens was significantly higher than that for the bone specimens up to 14 days with no differences thereafter. There was no change in the bending modulus with age for any specimen type. Bone-suture-bone bending modulus was lower than that of the bone specimens up to 14 days with no differences thereafter. There was no change in strain energy to failure with age for the bone specimens; bone-suture-bone specimens showed no change up to 14 days but decreased from 14 days to 18 days. There was an increase in specimen porosity with age. Correlation analysis revealed a weak (-0.39) but significant and negative correlation between ultimate stress and porosity. While the mechanical properties of parietal bone and coronal suture did not change significantly with age, bone specimens showed an increase in bending stiffness with age. Bone-suture-bone specimens showed an increase in bending stiffness only between 14 days and 18 days of age. Correlation analyses using existing and new data to compute the bending rigidity of infant parietal bone specimens suggested that days of pig age may correlate with months of human age during the most common time frame of childhood abuse cases.