Experimental Study On Degradation And Mechanical Properties Of Absorbable Poly-L-lactide Miniplates

Objective:The purpose of this study is to research the degradation and mechanical properties of absorbable poly-L-lactide miniplatesMethods:18 adult poochs were chosen, which were 10-15kg, female and male at random, and they were divided into 6 groups: one month group, two months group, three months group, six months group , twelve months group and fifteen months group. There were three dogs in each group. After fracture models were created artificially on both sides of the mandibles between molar teeth and premolar teeth in each dog ,the poly-L-lactide (PLLA) plates and screws were fixed on both sides and then sutured the wounds . Dogs of each group were sacrificed in the 1st, 2nd, 3rd, 6th,12th and 15th months after the operations and then plates of each group were took out. There were 6 plates in each group.After the Morphological observation and examination of weights were finished, 2 plates were used for SEM examination,and the other 4 ones were taken to measure the bend strength and molecular weight . Compare the changes of biological absorption rates ,biological degradation rates and the decline of mechanical strength ,then made a analysis .Results:Morphological observation:There were 18 dogs ,which meant 36 mandibles in all.All the mandibles healed well except 4 mandibles of 2 dogs. One month after the operations , the plates were coverd with thick fibrous membranes; three months later ,the surfaces of the plates were a little rough ,and the plates were impellucidus;six months postoperatively, the plates became soft and they could be moved on the surfaces of the bones;twelve months after the operations ,most of the plates degradated to the fragments;15 months postoperatively, the plates mostly disappeared.SEM examination:The surfaces of the normal plates were smooth without any cracks and holes. After one month,the surfaces became rough and three months later, there were some little cracks on the surfaces of the plates;six months after the operations, the cracks got deeper and there were more fragments of degradation .The changes of weight and biological absorption rate ( BAR):the BAR were2.22 % after 1 month of the operation ,2.40 % after 2 months of the operations, 3.83% after 3 months of the operations,5.17 % after 6 months of the operations.The changes of molecular weight and biological degradation rate(BDR):the BDR were 27.25% after 1 month of the operations , 39.95% after 2 months of the operations,43.65 % after 3 months of the operations, 66.67% after 6 months of the operations , 91.80% after 12 months of the operations. BDR was in advance to BAR.The changes of molecular weight were fast during the first 2 months,and got slower from the 3rd month.The decline of mechanical strength:1 month after the operations ,the plates maintained 94.29 % of the primitive strength;2 months later, the mechanical strength were 86.29% of the primitive one; from the 3rd to the 6th months, the strength went down very fast, in the 6th month , there are only 17.71% of the primitive one ,12 months after the operations,there were not any strength left .The mechanical strength of the plates declined slower during the first 3 months and got much faster from the 4th months.In the early period ,the changes of the mechanical strength were slower than the changes of molecular weight.Conclusions:Both the biological degradation rate and the speed of the decline of mechanical strength are mostly suitable .The plates can make sure that the fractures of the mandibles of dogs can cure successfully without any stress shields .If we improve the primitive mechanical strength of the plates and make it degradates slower ,it will make greater benefits for the interfixation of cranialfacial bones.