The Study Of Biodegradable Calcium Phosphate/Thermosensitive PLGA-PEG-PLGA Copolymer Composite Bone Cement For Percutaneous Vertebroplasty

Severe osteoporotic low-back pain and osteoporotic vertebral compression fracture?OVCF?have become global diseases,which seriously affect the life and health of the in aged population.Percutaneous vertebroplasty?PVP?is an effective treatment for OVCF.The calcium phosphate bone cement?CPC?has a wide application prospect in bone filling biological material owing to its good biodegradability,biocompatibility,and osteoinductive activity.As filling biomaterials in load bearing bone?eg.Spine?,the development and application of CPC are limited by some bottlenecks such as low compressive strength,poor water-resisting,and injectability properties.According to the characteristics of osteoporotic vertebral biomechanics,we have synthesized the composites of calcium phosphate bone cement??-TCP-MCPM-CC?by?-TCP,Ca CO₃,and Ca HPO4 in proper proportion,aiming at improving the properties of setting time,compressive strength,and acid-base balance.At the same time,we have incorporated thermosensitive poly?lactic acid?,poly?glycolic acid?and poly?ethylene glycol??PLGA-PEG-PLGA?into?-TCP-MCPM-CC,considering its unique characteristic of phase-transition in different temperature and thixotropic rheological behavior in liquid phase.Mixing with Na H₂PO₄in proper proportion to replace the liquid phase of inorganic bone cement,we have synthesized Calcium Phosphate/Thermosensitive PLGA–PEG–PLGA Copolymer Composite Bone Cement?PLGA-PEG-PLGA/CPC?.This study has improved the properties of CPC composite bone cement with proper compressive strength comparable to that of cancellous bone,excellent resistance,injectability,and osteoinductive activity,which meets the injectable and biomechanical requirements of PVP.This research provides a rational theoretical and experimental basis for the design of bone cement for vertebral repair.Object:The purposes of this research were to elaborate the method for preparation of PLGA-PEG-PLGA/CPC composite bone cement,and to explore the effect of thermosensitive PLGA–PEG–PLGA copolymer content on the properties of composite bone cement such as setting time,compressive strength,injectability,viscosity,water-resistance,biodegradability,acid-base balance,and osteoinductive activity.We evaluated the safety and effectiveness of composite bone cement in in-vitro experiments and in vivo animal experimental models.Method:?1?The?-TCP was prepared from high-temperature calcination and ball-milling procedure using Ca HPO₄·2H₂O,Ca CO₃and Ca F₂.The powder of the CPC composite bone cement was prepared by mixing?-TCP,Ca CO₃,and Ca HPO4 in proper proportion.The PLGA-PEG-PLGA triblock polymers were prepared by ring opening polymerization and poly?ethylene glycol?was used as an initiator.It was used as the liquid phase of composite bone cement by mixing with Na H₂PO₄in proper proportion.?2?Analysis for the the physical and chemical properties of PLGA-PEG-PLGA/CPC composite bone cement:The setting time was determined according to ISO standards,which is gained through Vicat apparatus;Viscosity measurements were conducted using a rotational rheometer;Assessment of injectability was calculated as the difference between final and original bone cement mass divided by the original body mass;The compressive strength was tested using a mechanical testing machine after bone cement samples preparation.Phase analysis and surface morphology were carried out by X-ray diffraction and Scanning electron microscopy.?3?The in vitro bioactivity was evaluated by CCK8assay.Osteogenic activities were evaluated by alkaline phosphatase?ALP?and alizarin red S staining.?4?In-vivo animal model experiment:A total of 30 healthy,adult,female New Zealand white rabbits were used in the study.24 of them underwent ovariectomy to establish the osteoporosis model,then were evaluated by radiographic and morphologic evaluation.The shape and parameters of spinal specimens of other 6 rabbits were observed and measured to explore the optimal entry point,the depth and angle of injection for a rabbit model of vertebroplasty.A total of 10 rabbits were selected as an experimental group using 30wt.%PLGA-PEG-PLGA/CPC as the filling material for vertebroplasty,10of them were included as the control group using PMMA bone cement.The radiographic evaluation and morphologic evaluation were performed at 4,8,12 weeks after bone cement implantation.The position and in-vivo degradation of bone cement,as well as new bone formation,were observed by Micro-CT hematoxylin and eosin?H.E.?staining.Result:?1?Phase analysis demonstrated that hydroxyapatite?HA?is the hydration products of cement pastes.Due to the introduction of PLGA–PEG–PLGA copolymer,the micellization of the copolymer has improved the bond strength between CPC molecules followed by reducing the porosity of CPC cement,resulting in improving the compressive strength of composite bone cement.Under SEM,a dense,stacking and tightly packed microstructure was observed.The average compressive strength of 30wt.%PLGA-PEG-PLGA/CPC is 34.18±4.60 MPa on the 7^th day,which is the highest of these composite bone cement.There are significant differences in Con-CPC bone cement?p<0.05?.?2?Due to the introduction of PLGA–PEG–PLGA copolymer,the viscosity of liquid-phase has been improved.Compared with Con CPC?51.39±0.69%?,the injectability of 30wt.%PLGA-PEG-PLGA/CPC?76.17±4.37%?has been improved obviously?p<0.05?,as well as the water-resistance.?3??4?The osteoporosis rabbit models could be established by ovariectomy.Though the lumbar vertebral anatomical parameters of rabbits were different from large animals,the rabbit model of vertebroplasty can be established successfully with the optimal entry point,which was set at the bisection of a vertical line through the mammillary process and a horizontal line through the inferior margin of anapophysis.?5?Through the rabbit PVP experiment,we found that 30wt.%PLGA-PEG-PLGA/CPC bone cement can fill the bone defect.Mirco-CT and the histological examination showed that the bone cement was distributed in the cancellous bone of the rabbit vertebral body completely,and new bone trabecula wrapped with slightly degraded bone cement materials at the 12^th week post-operation.Conclusion:We have achieved the PLGA-PEG-PLGA/CPC composite bone cement via the addition of a thermosensitive,biodegradable PLGA–PEG–PLGA copolymer.The composite bone cement has proper compressive strength comparable to that of cancellous bone,excellent water-resistance,and injectability.Moreover,it keeps the osteoinductive activity of CPC and promotes osteoblast cell proliferation.Through the rabbit PVP experiment,we found that 30wt.%PLGA-PEG-PLGA/CPC bone cement has good operability,of which the biodegradation and the osteoinductive properties promote the new bone trabecula around the composite bone cement.