THE DEVELOPMENT OF AN IN-VITRO IMMATURE ANIMAL MODEL FOR PREDICTING PEDIATRIC FEMUR FRACTURE STRENGTH

Fractures are the second most common presentation of child abuse, soft tissue injury being the most common. Femurs are the most common long-bone fractured in inflicted injury. When a child presents to the emergency department, a clinician must judge if the child's fracture matches the account provided by the caregiver. An objective tool is needed to aid in the assessment of injury plausibility. Predicting femur fracture strength is key to developing this tool. Immature porcine femurs are widely used to model pediatric human femurs. This study investigated immature porcine femur fracture load, energy to failure and stiffness in three-point bending, torsion and axial compression, with and without soft tissue intact and at different displacement rates.Significant differences exist between three point bending with soft tissue intact (n=6) and devoid of soft tissue (n=6) for stiffness (means=1607.9 lbf/in. and 1981.9 lbf/in, respectively, p=0.046) and energy to failure (means=36.9 in-lbf and 25.0 in-lbf, respectively, p=0.046). Torsion tests show significant differences in the fracture torque between groups tested at 0.167 degrees/sec (n=7) and 90 degrees/sec (n=7, means=30.69 in-lbf and 46.13 in-lbf, respectively, p=0.018). Axial compression experiments at 0.04 in/sec (n=5) resulted in fracture load, energy to failure and stiffness of 273.4 lbf, 70.7 in-lbf and 829.4 lbf/in, respectively, while axial compression experiments at 2 in/sec (n=2) resulted in higher fracture loads, energy to failure and stiffness (441 lbf, 154.2 in-lbf and 1894 lbf/in, respectively). Three-point bending tests resulted in oblique or transverse fractures, torsion and axial compression tests resulted in spiral and growth plate fractures, respectively. Correlations between bone mineral density and structure geometry showed promise as a predictive model for femur fracture response in all loading mechanisms. Multivariable regression modeling resulted in high R2 values (0.62 - 0.74) for femurs tested with soft tissue intact in three-point bending, but low values (0.22 - 0.29) for femurs tested devoid of soft tissue in three-point bending; relatively high R2 values (0.66 - 0.78) for fracture torque in torsion and low R2 values (0.22 - 0.47) for energy to failure in torsion. Further investigation with a larger sample is needed to reliably predict immature femur fracture response