Experimental Study Of Biodegradation And Mechanical Properties Of New Absorbable Material PLLA/PLLA-gHA

Fracture is a common desease in orthopedics and dentofacial surgery.The basic principle in treatment of fracture are reposition, fixation, functional exercise early and any one is necessary. Accurate reposition and stable fixation are basic factors which are important in healing course of bone fracture. Internal fixation of fractures is classically performed using metallic devices. Although these are claimed to be inert, complications may occur as a result of their permanent presence in the body, such as migration, growth disturbances, stress shielding, infection, osteoporosis and secondary removal operation. To resolve all these problems, absorbable devices have been developed. Polylactide acid (PLA) is most favorite materail in all kinds of absorbable materials, because it has such good properties. (1) excellent biocompatibility, non-toxicity, non-antigenicity. Polylactide acid was certifide as a safe material by FDA. (2) Mechanic power is decline gradually during degradation. Stress force transmits to fracture line slowly which can stimulate healing of bone fracture and avoid stress shielding. (3) Complete absorption. Degradation product can be absorbed or expeled by body without second operation to taking it out.Comercial products of PLA existed and were used in clinic more than twenty-year,especially were used in cancellous bone fracture and non-loading bone fracture fixation whicn obtain excellent results.Though obtained excellent results, the bioabsorbable devices are not yet an ideal internal fixation system and many problems should be resolved. (1) Mechanic power is not strong enough strong for cortical bone repairing and no compression, so just fit to cancellous bone repairing. (2) There is still some kind of inflammatory reaction during the degradation course of the polymers. In order to improve the PLA material's properties, hydroxyapatite (HA) was mixed into PLA materiaLs to produce HA/PLA composite. HA either raise mechanic strength or buffer the acid of degradation product of PLA. Osteoinductive of HA provides proper circumstance of osteocyte growth.There are many methods that produce HA/PLA composite including chemical method and physical method.The strategy of physical method is physical mixture of HA particle and PLA matrix. Interfacial adhesion between the HP particles and the PLLA matrix is not strong enough.After transplanted to humanbody the interfacial layer between the filler and the polymer matrix was destroyedfirstly, thus HA particles may easily disengage from the organic matrix, resulting in a sharp decrease of mechanical properties in a short time,so its mechanical properties are low and its load-bearing applications are limited.To reshape the surface of HA particle is the key points of chemical method. Grafting polymerization of LLactide on the surface of hydroxyapatite nano-crystals change the properties of HA in order to increase interfacial adhesion between HA particles and the PLLA matrix.Because of chemical binding,so it needs stronger force whicn can depart the HA particles from PLLA matrix.It shows increasing of mechanical intensity.It is a optimal method.A new type PLLA/PLLA-gHA composite which is pocessing self- intellectual property rights wsa produces by Changchun institute of applied chemistry Chinese Academy of sciences.A noveL method to modify the surface of n-HA particles and grafting Poly-L-Lactic acid (PLLA) to synthesis composite which was then mixed with PLLA matrix to form HA/PLLA composite.The first stage we study the biocompatibility and the results indicates that PLLA /PLLA-gHA composite has excellent biocompatibility.The mechanical propeties and rate of degradation are unknown.Whether it can fixed fracture effectiveLy and whetther it can avoid stress shielding and osteoporosis unknown either.So we test the mechanical propeties and degradable propeties of the new composite by fixing the mandible fractures of beagle dogs using PLLA/PLLA-gHA composite plate and compare with PLLA plate.We hope to provide the effective data. The experiment was divided into three parts:1,An introduction of the preparation of PLLA/PLLA-gHA and their properties. PLLA was directly grafted onto the hydroxyl group of the surface of n-HA particles by ring-opening polymerization of L-Lactide in the presence of stannous octanoate (Sn (Oct) 2) as cataLyst.And the PLLA-gHA was further blended with PLLA matrix to form PLLA/PLLA-gHA.2,Test velocity of degradation of PLLA/PLLA-gHA and PLLA.Put the PLLA/ PLLA-gHA plate and PLLA plate into madible fracture area, muscle and PBS solution, Taking out them at corresponding time to test absorption rate and degrad- ation rate of two materials. Transection of material was scanning by electronm- icroscope. The date were compared with statistics method.3,Mechanical properties of PLLA/PLLA-gHA and PLLA were tested. To study the influence of fracture healing course by two plates. To establish animal modles of mandible fracture .They were fixed by PLLA/PLLA-gHA pLate and PLLA plate either and taken X-RAYS and then execute them at 1st, 2send, 3th, 6th and 12th month postoperatively.Taken the plates out from the mandible, the plates were tested on the CCS-44100 type of electron omnipotent mechanics apparatus to get bending strength of two kind plates and mandible bone. Bony callus were taken hematoxylin and eosin stain (HE stain) to make histological test. The date were compared with statistics method.Main Results are as follows:1,In the study, we synthesized a new bone repairing material PLLA/PLLA-gHA, grafting with PLLA successfully modified the surface of HA nano-particles. The PLLA-gHA particles could be more uniformly dispersed into chloroform than the non-grafted one. When they are blended with PLLA they can be uniformly dispersed in the PLLA matrix and show improved adhesion with PLLA matrix.The PLLA/PLLA-gHA composites exhibit better mechanical properties than simple PLLA/HA blend.2,The sequence of degradation rate was bone fracture>muscle>vitro situation, The pLates were not absorbed absolutely at 12th month. The degradation rate and absorption rate of PLLA/PLLA-gHA composite was slower than that of PLLA material.There was evident difference between two kinds in statistics after 6th month.3,Mechanical properties and ductility of PLLA/PLLA-gHA composite was stronger than that of PLLA material.The mechanical decLine rate of PLLA/PLLA-gHA composite was slower than that of PLLA material. The difference was evident. Observation of bone fracture healing, bony callus, X-rays and histological test showed that the cure of bone fractures which were fixed by PLLA/PLLA-gHA plate was better than that of PLLA plate, but the difference was not evident in statistics.ConcLusionThe PLLA/PLLA-gHA composite showed good mechanical properties and proper degradation rate and decrease the rate of inflammatory complication.It meets requirement of fixing mandible fracture. It is good substitute material of bone fracture fixtion system.