Femur fracture biomechanics and morphology associated with torsional and bending loading conditions in an in vitro immature porcine model

Immature porcine cortical bone mechanical properties and composition change with maturation and displacement rate

Computational models of mature bone have been used to predict fracture; however, analogous study of immature diaphyseal fracture has not been conducted due to sparse experimental mechanical data. A model of immature bone fracture may be used to aid in the differentiation of accidental and non-accidental trauma fractures in young, newly ambulatory children (0-3 years). The objective of this study was to characterize the evolution of tissue-level mechanical behavior, composition, and microstructure of maturing cortical porcine bone with uniaxial tension, Raman spectroscopy, and light microscopy as a function of maturation. We asked: 1) How do the monotonic uniaxial tensile properties change with maturation and displacement rate; 2) How does the composition and microstructure change with maturation; and 3) Is there a correlation between composition and tensile properties with maturation? Elastic modulus (p < 0.001), fracture stress (p < 0.001), and energy absorption (p < 0.014) increased as a function of maturation at the quasistatic rate by 110%, 86%, and 96%, respectively. Fracture stress also increased by 90% with maturation at the faster rate (p = 0.001). Fracture stress increased as a function of increasing displacement rate by 28% (newborn p = 0.048; 1-month p = 0.004; 3-month p= < 0.001), and fracture strain decreased by 68% with increasing displacement rate (newborn p = 0.002; 1-month p = 0.036; 3-month p < 0.001). Carbonate-to-phosphate ratio was positively linearly related to elastic modulus, and fracture stress was positively related to carbonate-to-phosphate ratio and matrix maturation ratio. The results of this study support that immature bone is strain-rate dependent and becomes more brittle at faster rates, contributing to the foundation upon which a computational model can be built to evaluate immature bone fracture.

Femur morphology in healthy infants and young children

The objective of this study was to characterize femur morphology in healthy infants and young children. Anterior-posterior (AP) radiographs of the femur from children age 0-3 years with no history of bone disease were obtained from two children's hospitals and one medical examiner's office. Femur morphological measures (bone length, minimum diaphysis diameter, growth plate width, and femur radius of curvature) and sectional structural measures were determined. Measures were described and compared based on subject age and mass. Relationships between measures and age and mass were evaluated. The 169 AP femur radiographs were obtained from 99 children (59.6% males, median age = 12.0 months, IQR = 0-27.5 months, median body weight = 10.0 kg, IQR = 4.4-15.6 kg). Femur length (r_s  = 0.97, p < 0.001; r_s  = 0.89, p < 0.001), trochanter width (r_s  = 0.86, p < 0.001; r_s  = 0.85, p < 0.001), minimum diaphysis diameter (r_s  = 0.91, p < 0.001; r_s  = 0.87, p < 0.001), and growth plate width (r_s  = 0.91, p < 0.001; r_s  = 0.84, p < 0.001) increased with age and weight, respectively. Cross-sectional area (r_s  = 0.87; r_s  = 0.86; p < 0.01), polar moment of inertia (r_s  = 0.91; r_s  = 0.87; p < 0.001), moment of inertia (r_s  = 0.91; r_s  = 0.87; p < 0.001), polar modulus (r_s  = 0.91; r_s  = 0.87; p < 0.001) and medullary canal diameter (r_s  = 0.83, p < 0.001; r_s  = 0.73, p < 0.001) at the minimum diaphysis also increased with age and weight, respectively. Changes during rapid bone growth are important to understanding fracture risk in infants and young children as they transition to independent walking. Femur length, trochanter width, minimum diaphysis diameter and growth plate width increased with age and weight. Structural properties associated with fracture resistance also increased with age and weight.

Influence of Screw-Hole Defect Size on the Biomechanical Properties of Feline Femora in an Ex Vivo Model

OBJECTIVE

 The study aims to evaluate the biomechanical properties of feline femora with craniocaudal screw-hole defects of increasing diameter, subjected to three-point bending and torsion to failure at two different loading rates.

STUDY DESIGN

 Eighty femoral pairs were harvested from adult cat cadavers. For each bending and torsional experiment, there were five groups (n = 8 pairs) of increasing craniocaudal screw-hole defects (intact, 1.5 mm, 2.0 mm, 2.4 mm, 2.7mm). Mid-diaphyseal bicortical defects were created with an appropriate pilot drill-hole and tapped accordingly. Left and right femora of each pair were randomly assigned to a destructive loading protocol at low (10 mm/min; 0.5 degrees/s) or high rates (3,000 mm/min; 90 degrees/s) respectively. Stiffness, load/torque-to-failure, energy-to-failure and fracture morphology were recorded.

RESULTS

 Defect size to bone diameter ratio was significantly different between defect groups within bending and torsional experiments respectively (intact [0%; 0%], 1.5 mm [17.8%; 17.1%], 2.0 mm [22.8%; 23.5%], 2.4 mm [27.8%; 27.6%], 2.7 mm [31.1%; 32.4%]) (p < 0.001). No significant differences in stiffness and load/torque-to-failure were noted with increasing deficit sizes in all loading conditions. Screw-hole (2.7 mm) defects up to 33% bone diameter had a maximum of 20% reduction in bending and torsional strength compared with intact bone at both loading rates. Stiffness and load/torque-to-failure in both bending and torsion were increased in bones subjected to higher loading rates (p < 0.001).

CONCLUSION

 Screw-hole defects up to 2.7 mm did not significantly reduce feline bone failure properties in this ex vivo femoral study. These findings support current screw-size selection guidelines of up to 33% bone diameter as appropriate for use in feline fracture osteosynthesis.

The effect of the hip impact configuration on the energy absorption provided by the femoral soft tissue during sideways falls

The femoral soft tissue (i.e., skin, muscle, fat) may play a key role in preventing hip fractures during a fall by absorbing the impact energy. We measured the femoral soft tissue deformation and associated compressive force during simulated sideways falls to estimate the energy absorbed by the soft tissue, and then examined how this was affected by the hip impact configuration and gender. Eighteen young adults (9 males and 9 females) participated in the pelvis release experiment. The pelvis was raised through a rope attached to an electromagnet on the ceiling, so the skin surface barely touches the ultrasound probe, which flush to a Plexiglas plate placed on a force plate. The electromagnet was turned off to cause a fall while the soft tissue deformation and associated compressive force were being recorded. Trials were acquired with three hip impact configurations. An outcome variable included the energy absorbed by the femoral soft tissue during a fall. The energy absorbed by the femoral soft tissue ranged from 0.03 to 3.05 J. Furthermore, the energy absorption was associated with the hip impact configuration (F = 4.69, p = 0.016). On average, the absorbed energy was 62% greater in posteriolateral than anteriolateral impact (0.92 versus 0.57 J). However, the energy absorption did not differ between male and female (F = 0.91, p = 0.36). The force-deflection behavior of the femoral soft tissue during a fall has been recorded, providing insights on the potential protective benefits of the soft tissue covering during a fall.

Effects of age and rate of twist on torsional fracture patterns in infant porcine femora

OBJECTIVE

Long bone fractures are a common injury in the pediatric population. Differentiation between abusive, or non-accidental trauma, and accidental trauma in children remains challenging for forensic practitioners. A recent clinical-based study was able to separate pediatric abusive from accidental trauma based on femoral fracture pattern using the ratio of fracture length over bone diameter (fracture ratio), as determined from radiographic analysis of this fractured bone. The forensic literature indicates more cases of abuse in younger pediatric victims than accidental cases. While this was the case in the clinical study, the effect was not shown to be statistically significant. Furthermore, while speed of trauma was not considered in the clinical study, a laboratory study with an immature bovine model indicates rotational speed influences fracture pattern, but specimen age was not varied in that study. Therefore, the objective of the current study was to use immature porcine femora to investigate the effects of age and rate of twist on a modified version of this fracture ratio parameter.

METHODS

Fifteen pairs of porcine femora with various ages were twisted until observable failure using a custom-built torsional fixture. The left femur of each pair was twisted to failure at a rate of 3 deg/s, while the right femur was twisted at a rate of 90 deg/s. The torque and angle of rotation were recorded at a sampling rate of 10,000 Hz. Fracture ratio was defined as total fracture length divided by bone diameter.

RESULTS

Fracture ratio increased with specimen age, with specimens under the low rate of twist yielding a consistently lower fracture ratio than those from specimens under the high rate of twist. The results showed that both specimen age and rate of twist were significant factors influencing fracture ratio.

CONCLUSION

The determination of abusive from accidental trauma in criminal cases, based on the pattern of long bone fracture alone, may need to include additional data on the specific age of the pediatric victim and the potential speed of the traumatic event.