Experimental Study Of Dexamethasone-loaded Hollow Hydroxyapatite Microspheres Applied To Direct Pulp Capping Of Rat Molars

Background:When a tooth is exposed,mechanically or due to trauma,pulp infection is likely to occur and,if left untreated,can eventually lead to pulp necrosis.The pulp tissue has nutritive,protective,and formative functions and a healthy pulp facilitates normal functioning.Therefore,the benefits of vital pulp therapy（VPT）in comparison with traditional root canal treatment are being increasingly recognized by the scientific community.Pulp capping agents play a vital role in VPT.They cover the pulp exposed to reduce the inflammatory responses,induce the migration and differentiation of deep and undamaged pulp stem cells,and promote the formation of reparative dentin.At present,as VPT is a research hotspot in the field,the search for new pulp-capping agents with high success rates and low trauma is a future trend in dental care.As a synthetic glucocorticoid,dexamethasone（DEX）plays a powerful anti-inflammatory role by inducing lymphocyte apoptosis,inhibiting pro-inflammatory transcription factor release,and stimulating anti-inflammatory gene expression.DEX also activates the classical Wnt signaling pathway to enhance osteoblast growth and differentiation.However,high concentrations of DEX inhibit osteogenesis.DEX at10^-6mol/L disrupts mitochondrial dynamics,inhibits osteogenesis of stem cells,and promotes the adipogenesis.Therefore,scientists have started to investigate more appropriate mechanisms of topical drug delivery and ways to increase its initial solubility without overloading the drug.As a major component of natural bone,hydroxyapatite（HAp）has no implant toxicity and is commonly used in bone repair given its biocompatibility,bioactivity,osteoconductivity,osseointegration,and certain osteogenesis-inducing conditions.The spherical morphology and nanoscale nature of hollow hydroxyapatite（HHAM）compared to irregular hydroxyapatite are advantageous in reducing physical damage in vivo and HHAM is well suited for loading and releasing various drug molecules owing to its high surface area,low mass density,superior cell attachment,drug loading,and drug release kinetics.Thus,HHAM not only ensures localized and sustained drug release but also promotes bone healing.Our group has successfully constructed Dexamethasone-loaded hydroxyapatite microspheres（DHHAM）with good loading and encapsulation rates.The in vitro release curve was nearly linear for 30 days,with no obvious abrupt release phenomenon.In vitro results also showed that DHHAM improved the in vitro mineralization of human dental pulp cells and elevated the expression of odontogenic genes such as DMP-1 and ALP compared with DEX and HHAM alone,which had a positive effect on the odontogenic differentiation of dental pulp stem cells.Aim:This study intend to examine the effect of DHHAM on inflammation and dentin regeneration in injured pulp tissue when used as a direct pulp capping agent.Methods:Holes were made in the first molar of Wistar rats and filled with Dycal,HHAM,and DHHAM.No drug was administered to the Control group.Rats were sacrificed at1,2,and 4 weeks postoperatively.Hematoxylin-eosin（HE）and CD45immunohistochemical（IHC）staining was performed to detect the formation of reparative dentin and pulp inflammation.IHC staining was performed to assess the expressions of DMP-1,IL-6,TNF-α,and IL-1β.Results:1.The results of HE and CD45 IHC staining showed that the degree of inflammation in the DHHAM group was less than that in the Control and HHAM groups at 1,2 and 4 weeks after capping of the rat molar teeth（p<0.01）,and there was no statistical difference with the Dycal group（p>0.05）.2.HE staining showed that the percentage of reparative dentin formed in the DHHAM group was greater than that in the Control,HHAM（p<0.001）and Dycal groups（p<0.01）at 1 and 2 weeks.At 4 weeks after pulp capping,the percentage of reparative dentin formed in the DHHAM group was significantly higher than that in the Control group（p<0.001）and the HHAM group（p<0.01）,but there was no statistical difference with the Dycal group（p>0.05）.3.IHC staining results showed a lower range and intensity of expression of IL-1β,IL-6,TNF-αand more positive expression of DMP-1 in the DHHAM group at 1,2and 4 weeks after pulp capping relative to the Control group.Conclusions:DHHAM could promote the early formation of reparative dentin in the rat accidental pulp penetration model,and its effect was superior relative to Dycal.DEX exerted a good anti-inflammatory effect in DHHAM.DHHAM also promoted the expression of DMP-1,while inhibiting those of IL-6,TNF-α,and IL-1β.