Effect Of Freezing Time On Biomechanical Properties Of Dental Tissue: An In-vitro Study

Objective:Autogenous tooth transplantation(ATT)has been widely used in clinical practice,but in most cases,the time of missing teeth and the time of extraction of healthy non-functional teeth could not be matched.In order to make better use of healthy teeth,scholars have conducted a large number of studies on the changes in the viability of periodontal membranes and pulp cells after freezing.However,there are few reports on the biomechanical properties of teeth in different age groups after cryopreservation.This study is to research the changes of biomechanical properties of frozen teeth under controled age groups at controlled freezing periods.Methods: The third molars were obtained from 84 patients and the samples of dentin,cementum and enamel were prepared.The experiment was divided into three parts.The effects of freezing time on the biomechanical properties of dentin,the microstructure of cementum,and the biomechanical properties of enamel were tested respectively.Design three experimental groups for each section,including the youth group aged 18-23,the middle-aged group aged 33-43,and the elderly group aged 58-63;A control group,with the same age grouping criteria as the experimental group.The first two parts of the experiment will receive the medium-low temperature preservation scheme of-196℃ cryoprotectant solution,with the storage period of seven days,30 days and 90 days,respectively.The control group will not undergo freezing treatment,with n=7 for each group.In the third part of the experiment,the experimental group had the same freezing plan as before,stored for 90 days,and the control group had the same treatment as before,with n=2 for each group.The biomechanical properties of each part of the dental tissue were tested,and the microstructure and integrity changes were observed under scanning electron microscopy(SEM).Results: 1.There was no significant difference in the compressive strength of dentin between the experimental groups and the normal temperature group after cryopreservation(P>0.05).The elastic modulus of dentin decreased with the increase of freezing time and age(P<0.05).There was no interaction between age and freezing time(P>0.05).In the cross section of the scanning electron microscope,the dentine tubules become narrow and partially occluded,which makes it easier to attach impurities,especially in the long freezing time and the elderly group.In the longitudinal section,with the increase of freezing time and age,the inner wall of dentinal tubules became rough,especially in the aging group which was frozen for 90 days.No obvious microcracks were found in any longitudinal section of dentin.2.The cementum deposits and thickens with age,and the surface becomes irregular,with visible spiny protrusions and cracks.With the increase of freezing time,the surface of cementum tends to deposit and thicken.The youth group only showed deposition thickening,and the middle-aged and old group only showed deposition thickening within 30 days of freezing.The change of cracks was not obvious,but after 90 days of freezing,cracks increased,cracks deepened,and holes increased more significantly.3.The compressive strength and elastic modulus of enamel gradually decreased with age(P<0.05).After cryopreservation,the compressive strength of enamel in all age groups showed an upward trend,and the strength change in the elderly group was the most obvious(P<0.05).There was no significant change in the elastic modulus of enamel before and after cryopreservation(P>0.05).Conclusion: The biological properties of the hard tissues of teeth,including enamel,dentin and cementum,are gradually decreased with age.The performance of frozen teeth is different.Nevertheless,the biological properties of the frozen teeth reached the requirements of restoring masticatory function as a whole.SEM observation showed that dentin and cementum thickened with the prolongation of freezing time and were easier to adhere to impurities.Therefore,it can be speculated that the longer the cryopreservation time and the older the age,the higher the risk of dental pulp and periapical diseases of the transplanted teeth after cryopreservation.Especially for the elderly patients who choose frozen tooth transplantation,the requirements for aseptic operation during freezing and operation are higher,and more careful maintenance is required after operation.