Study On Difference Of Mechanical Properties Of Skull Material

Objective:The difference of morphological structure and material mechanical properties of parietal bone in different age,different parts,different levels of skull and different partitions were discus through that detection of morphological structure and material mechanical parameters of skull.Methods:(1)intercept skull slices of different part(Left frontal bone(LF),Left parietal bone(LP),and Coronal suture(CS))of human carcasses of different ages(A total of49 cadaveric samples were divided into 5 groups: 0-1-year-old,2-12-year-old,31-50-year-old,51-70-year-old and 71-90-year-old)and parietal bones of different partition((A total of 11 cadaveric samples were selected from the left upper parietal,lateral parietal and posterior parietal,respectively)measuring that thickness of the bone slices by using a vernier caliper,detecting/calculating material mechanical parameters such as ultimate load,deformation under the ultimate load,bending strength and elastic modulus after a three-point bending experiment is performed by using an electronic universal material testing machine,analyzing the differences of the thickness and the material mechanical parameters of the human carcasses of different ages and different parts and the parietal bones of different partitions and discussing the change correlation between the thickness and the material mechanical parameters;The skull slides of different parts(LF and CS)of human carcasses of different ages(5 cases in total,aged 17 days,3 years,8 years,51 years and 77 years,respectively)were intercepted,and the experimental samples at different levels(inner plate,diploe and outer plate)were prepared by epoxy resin embedding.The Indentation hardness(HIT)and Elastic modulus(E)of each sample were measured using a nanoindenter,and differences in the mechanical parameters of the bone fragment samples were analyzed.(2)A rat head compression mold of different ages was prepared,and a single-axis longitudinal compression experiment was performed on the rat head using an electronic universal material testing machine to detect/calculate various material mechanical parameters such as limit load,compression strength and compression modulus,and further analyze the correlation between the material mechanical parameters of the rat head and age changes.The Micro-CT was used to scan and detect bone parameters such as bone density,bone volume,and trabecular thickness on the undamaged side,and analyze the correlation between the material mechanical parameters and changes.Results:(1)The thickness of LF,LP and CS in human cadaveric skull increases firstly and then decreases with age.The limit loads of LF,LP and CS in adult group were higher than those in juvenile group.LF,LP and CS had statistically significant differences in thickness,ultimate load,deformation under ultimate load,and strain and elastic modulus under ultimate load(P < 0.05).The skull thicknesses of LP and CS were lower than LF’s,and LP had the highest deformation under ultimate load,with LF >LP > CS as the ultimate load and elastic modulus.LF,LP and CS all show a positive correlation between ultimate load and bending strength and thickness,while the deformation under ultimate load has a negative correlation with thickness(P < 0.05).The strain under limit load has a positive correlation with the thickness only in LF(P< 0.05).There was a positive correlation between the elastic modulus and thickness only in CS(P < 0.05),but no correlation in other parts(P > 0.05).The experimental results of human parietal bone in each division showed that the thickness of the upper,lateral and posterior were in the order of upper > posterior > lateral,the ultimate load was in the order of upper > posterior > lateral,the bending strength and elasticmodulus were in the order of lateral > posterior > upper,and the deformation under the ultimate load was in the order of lateral > upper(P < 0.05).The ultimate load of each section of parietal bone was positively correlated with the thickness,and the flexural strength and elastic modulus were negatively correlated with the thickness(P< 0.05).The nanoindentation results showed that HIT and E in human cadaveric skull samples from different age groups were significantly different(P < 0.01).HIT and E tended to increase before 51 years old and decrease slightly after 51 years old.Diploe E was higher than inner and outer plates(P < 0.05),while HIT showed no significant difference among different levels.HIT and E in LF were significantly higher than those in CS by locations.(2)The differences of ultimate load,compressive strength and compressive modulus of rat skull among different age groups were statistically significant(P < 0.05).The ultimate load,compressive strength and compressive modulus all increased before 26 weeks,showing a significant positive correlation with age(P < 0.05).The differences of skull thickness,bone density,bone volume and trabecular thickness among different age groups were statistically significant(P <0.05).With the age,the bone density,bone volume and trabecular thickness increased in the 2–52 week group(the trabecular thickness decreased slightly in the 26 week group)and decreased slightly after 52 weeks.All bone parameters were positively correlated with age in 52 weeks(P < 0.05).There were differences in skull fracture type and morphology of rats among different age groups.Only cranial sutures were separated in the 2–4 week group and comminuted fractures occurred in the 6–104week group.The ultimate load,compressive strength,and compressive modulus of all rats were positively correlated with bone density,bone volume,trabecular thickness,and cranial thickness.Conclusions:(1)With respect to ages difference,the thickness,ultimate load,bending strength and elastic modulus of infant skull are significantly lower than those of adults.The microstructures and mechanical parameters of rat skull at different ages show aging-related changes,and there is a correlation between them,suggesting that the structural parameters and mechanical parameters of material of skull can be used for age estimation,and also suggesting that the material parameters should be assigned separately when establishing the finite element model of head/skull for people of different ages;(2)With respect to parts difference,the thickness of the skull of the parietal and coronal sutures is lower than that of the frontal bone,and the deformation of the parietal under ultimate load is higher than that of the frontal and coronal sutures.The ultimate load and elastic modulus are shown as: frontal bone > parietal bone > coronal suture.The ultimate loads in different zones of parietal bone were as follows: left upper > left posterior > left lateral,and the bending strength and elastic modulus were as follows: left lateral > left posterior > left upper;The elastic modulus of different layers of skull was as follows: diploe > outer plate > inner plate;It was suggested that the values of different parts and different levels of the skull as well as different partitions of the parietal bone should be respectively assigned when the finite element model of human skull was constructed,and the effect of the differences in mechanical properties of the parts and levels on the fracture should be considered when analyzing the biomechanical mechanism of skull fracture.