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

Contractile mitral annular forces are reduced with ischemic mitral regurgitation

ObjectiveForces acting on mitral annular devices in the setting of ischemic mitral regurgitation are currently unknown. The aim of this study was to quantify the cyclic forces that result from mitral annular contraction in a chronic ischemic mitral regurgitation ovine model and compare them with forces measured previously in healthy animals.MethodsA novel force transducer was implanted in the mitral annulus of 6 ovine subjects 8 weeks after an inferior left ventricle infarction that produced progressive, severe chronic ischemic mitral regurgitation. Septal–lateral and transverse forces were measured continuously for cardiac cycles reaching a peak left ventricular pressure of 90, 125, 150, 175, and 200 mm Hg. Cyclic forces and their rate of change during isovolumetric contraction were quantified and compared with those measured in healthy animals.ResultsAnimals with chronic ischemic mitral regurgitation exhibited a mean mitral regurgitation grade of 2.3 ± 0.5. Ischemic mitral regurgitation was observed to decrease significantly septal–lateral forces at each level of left ventricular pressure (P < .01). Transverse forces were consistently lower in the ischemic mitral regurgitation group despite not reaching statistical significance. The rate of change of these forces during isovolumetric contraction was found to increase significantly with peak left ventricular pressure (P < .005), but did not differ significantly between animal groups.ConclusionsMitral annular forces were measured for the first time in a chronic ischemic mitral regurgitation animal model. Our findings demonstrated an inferior left ventricular infarct to decrease significantly cyclic septal–lateral forces while modestly lowering those in the transverse. The measurement of these forces and their variation with left ventricular pressure contributes significantly to the development of mitral annular ischemic mitral regurgitation devices