| BackgroundAlthough the external fixator can decrease the damage to soft tissue and blood supply,the stability of the fixator is relatively poor.Moreover,the cumbersome volume of the external fixator often brings a lot of inconvenience to the daily life of patients.Since the stiffness and strength of external fixation have an important effect on the fracture healing outcome,the design of a low external fixation with sufficient stiffness and strength has been highly pursued by many researchers at home and abroad.Our research group designed a novel external fixation device for the tibial fractures,namely,the plate-type external fixator,with a lower volume compared with the external fixator commonly used in clinical practice,which poses less of an inconvenience on the daily life of patients,and the lower profile makes it more acceptable to patients.However,the biomechanical properties of the plate-type external fixator are still unclear.Therefore,the biomechanical experiments were conducted to investigate the stiffness and strength of the plate-type external fixator,thus providing a more stable,convenient and acceptable fixation method for the treatment of clinical tibial fractures.ObjectiveIn this study,we selected the the unilateral external fixator commonly used in clinical treatment as a control,the stiffness and strength in the treatment of the comminuted tibial fracture of the plate-type external fixator and the unilateral external fixator were compared through three experiments.We assumed that the plate-type external fixator could provide higher stiffness while maintaining sufficient strength.Methods 1.Design principle of the plate-type external fixatorThe plate-type external fixator,with the principle of an angle-stabilizing property,by means of the 5 mm self-tapping locking screw,consists of a proximal tibial fixation lath with a transverse slat at the proximal end and a distal tibial fixation lath with a transverse slat at the distal end.The distal end of the proximal fixation slat is equipped with a slot,and the proximal end of the distal fixation slat can be inserted into the slot and can slide along lath to adjust the length of the fixator.Moreover,the plate-type external fixator is designed to match perfectly with the shin,which could be used to treat the tibial metaphyseal fracture without cross-articular fixation,avoiding the complications,such as stiffness of the joints and lower limb atrophy.2.Experimental methods2.1 experiment one —— Investigating the stiffness on the bovine tibia modelFifteen fresh adult bovine tibias were selected and randomly divided into three groups of five specimens each,namely,the classic plate-type external fixator(CPF)group,the extended plate-type external fixator(EPF)group,and the unilateral external fixator(UEF)group.All of the selected tibias were made into middle tibia fracture models,and then stabilized either with the CPF,the EPF or the UEF.The constructs were loaded by means of Z005 electronic materials testing machine under axial compression and four-point bending.2.2 experiment two —— Investigating the stiffness on the cadaver tibia modelFifty-four fresh adult cadaver tibia models were selected and were divided into three groups by completely random grouping method,with 18 models in each group,including the CPF group,the EPF group and the UEF group.After all fracture models were completed,the 18 samples from the CPF group were randomly divided into three groups with 6 pieces in each group for axial compression,four-point bending and torsion tests,respectively.In the same way,the EPF group and the UEF group were also divided into three groups,and the stiffness test was carried out successively.2.3 experiment three —— Investigating the strength on the cadaver tibia modelThe tibial fracture model of experiment 2 was selected to be investigated in this experiment.After the construct stiffness test,the dynamic loading was carried out until the fracture model yielded.The corresponding peak load of each fracture model when yielding was observed and recorded,that is,the strength value of the model.The axial compression,four-point bending and torsion strength of fracture models were recorded,and the yield mode of each model was also observed and recorded.3.Statistical analysisThe SPSS 23.0 software was selected for statistical analysis of the collected data.One-way analysis of variance was first used for comparing the differences among the groups.If there was difference among groups,LSD test was used for pairwise comparison.A P < 0.05 was defined as statistically significant.Results 1 experiment one1.1 The average axial stiffness of the CPF group,the EPF group,and UEF group were 244.82 N/mm,166.69 N/mm,and 133.63 N/mm,respectively.One-way analysis of variance showed that the differences were statistically significant.The stiffness of the CPF group was 83.21% higher than the UEF group,and the stiffness of the EPF group was 24.74% higher than the UEF group.1.2 The four-point bending stiffness values of the CPF group,the EPF group,and the UEF group were 9.74 Nm/deg,8.53 Nm/deg,and 7.55 Nm/deg,and the differences were statistically significant.The stiffness of the CPF group is 29.01% higher than the UEF group,and the stiffness of the EPF group is 12.98% higher than the UEF group.2 experiment two2.1 The average axial compression stiffness of the CPF group,the EPF group,and the UEF group were 1898.8 N/mm,1715.8 N/mm,and 1157.8 N/mm,respectively,and the differences were statistically significant.The stiffness value of the CPF group was 64% higher than that of the UEF group,and the stiffness value of the EPF group was 48% higher than that of the UEF group.2.2 The average four-point bending stiffness among the three construct groups were 26.7 Nm/deg,24.1 Nm/deg,and 15.0 Nm/deg.The bending stiffness value of the CPF group was 78% higher than that of the UEF group,and the bending stiffness value of the EPF group was 61% higher than that of the UEF group.2.3 The average torsional stiffness among the three groups,namely,the CPF group,the EPF group,and the UEF group were 3.0 Nm/deg,2.6 Nm/deg,and 1.3 Nm/deg,respectively.The torsional stiffness value of the CPF group is 131% higher than that of the UEF group,and the EPF group is 100% higher than the UEF group.3 experiment three 3.1 The average axial compression strength among the three groups,namely,the CPF group,the EPF group,and the UEF group,were 2792.2 N,2560.5 N,and 1769.0 N,respectively,and the differences were statistically significant.The strength value of the CPF group was 58% higher than that of the UEF group,and the strength value of the EPF group was 45% higher than that of the UEF group.3.2 The average four-point bending strength values of the CPF group,the EPF group,and the UEF group,were 58.2 Nm,56.4 Nm,and 47.9 Nm,respectively.One-way ANOVA revealed that the differences were statistically significant.The four-point bending strength value of the CPF group was 22% higher than that of the UEF group,and the bending strength value of the EPF group was 18% higher than that of the UEF group.3.3 The average torsional strength values among the three groups were 34.2 Nm,30.0 Nm,and 24.2 Nm,respectively.The difference in torsional strength values was statistically significant.The torsional strength value of the CPF group was 41% higher than that of the UEF group,and the strength value of the EPF group was 24% higher than that of the UEF group.3.4 Both the CPF and the EPF groups failed by catastrophic fracture.After fracture,the CPF groups in one specimen displayed screw bending,and screw breakage occured in one specimen.Among the EPF groups,screws and steel plates bending occured in three models,and screw breakage occurred in two models.The UEF groups failed as a result of fracture of the diaphysis in one specimen and nonrecoverable fracture gap closure due to model bending in five specimens.3.5 Both the plate-type external fixator and the UEF groups failed by new fracture.After fracture,none of the CPF groups displayed implant hardware failure.The EPF groups showed screw bending in three specimens,and the UEF groups showed half-pin and rod bending in four specimens and half-pin and rod breakage in two specimens.3.6 The CPF groups failed by screw and plate bending in four specimens,screw breakage in one specimen and spiral fracture in one specimen.The EPF groups failed as a result of screw and plate bending in two specimens,screw breakage and plate bending in two specimens and spiral fracture in two specimens.The UEF groups showed oblique fracture in four specimens and half-pin and rod bending in two specimens,resulting in nonrecoverable deformation in the region of fracture.Conclusions1.The plate-type external fixator could provide higher axial compression stiffness and four-point bending stiffness in the bovine tibial fracture model.2.The plate-type external fixator could provide higher stiffness in the cadaver tibial fracture model,and the stiffness value is closer to the optimal value.3.The plate-type external fixator could provide a higher strength value.Moreover,the plate-type external fixator is firmer in structure and harder to be damaged during progressive dynamic loading to failure. |
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