Biomechanical Study Of Combined Cancellous Lag Screw

Part Ⅰ: The Comparison of Mamxium Compressive Force between Combined Cancellous Lag Screw and AO Cancellous Lag Screw in Different Bone Mineral DensitiesObjective: To compare maximum compressive force(MCF) between combined cancallous lag screw(CCLS) and AO cancallous lag screw(AOCLS) for different bone mineral densities(BMDs).Methods: Three synthetic cancellous bone blocks were used for this study, including0.12 g/cm3, 0.16 g/ cm3, and 0.20 g/ cm3. 24 pilot holes with 3.2 mm diameters were drilled equably in each block and were assigned randomly into two groups: CCLS group(n=12) and AOCLS group(n=12). CCLS and AOCLS were inserted through the custom-designed measuring device into a pilot hole by hand until failure, and MCF was determined.Results: 1.Among three BMDs specimens, MCF of CCLS was significantly higher than that of the AOCLS(P < 0.001). 2.MCF of two screws increased as BMD increased(P < 0.001). 3. BMD and the difference in the shank structure between CCLS and AOCLS were two influence factors which affected MCF of lag screw(P < 0.001), and the two factors had interaction(P < 0.001).Conclusions: 1.The difference in the shank structure between CCLS and AOCLS was a influence factors which affected MCF, and MCF of CCLS surpassed that of AOCLS.2.BMD was an influence factors which affected MCF, and MCF of two screws increased as BMD increased. 3.The difference of MCF between CCLS and AOCLS increased as BMD increased.Part II: The Change of Compressive Force and Pullout Strength during the Procedure before Mamxium Compressive Force Achieved for Combined Cancellous Lag Screw in a Bone Mineral DensityObjective: The aim of the current study was to investigate the change of compressive force(CF) and pullout strength(POS) during the procedure before MCF achieded when using CCLS, analyze the regular pattern of CF and POS, and test the relationships among the CF, POS and the rotational angle in this prodecure.Methods: One 0.12 g/cm3 synthetic cancellous bone blocks was used for this study.60 pilot holes with 3.2 mm diameters were drilled equably in this block and were assigned randomly into six groups according to the rotational angles: 0° group(G0,n=10), 150°group(G150,n=10), 300° group(G300,n=10), 450° group(G450,n=10), 550° group(G550,n=10), 650° group(G650,n=10). The preload was 2N in the monitor of the custom-designed measuring device. CF in a rotational angle were measured by the custom-designed rotational device and measuring device. Next, CF disappeared by rotationg the screw mannually in a counter clock-wise way in the monitor of the custom-designed measuring device, and then the pullout test was completed by the custom-designed pullout device and POS was determined.Results: 1.CFs were different in different rotational angles(P < 0.001). 2. POSs were not statistical difference in different rotational angle(p = 0.885). 3. POS was higher than CF in a rotational angle(P < 0.001). 4. CF and the rotational angle were significantly related(r = 0.944, P < 0.001), and there was not significant relationship between POS and the rotational angle(r = 0.077, P = 0.558), between POS and CF(r = 0.193,P = 0.140). 5.The cubic regression model determined that CF increased as the rotational angle increased(R2 = 0.958, P < 0.001), and POS did not change as the rotational angle increased(R2=0.03, P = 0.632). 6. The rate of mean CF per degree differed across phases, and decreased rapidly when the rotational angle increased from 550° to 650°, and the trend of increasing CF plateaued during this phase.Conclusions: 1. POS did not decrease before CF achieded the maximum when using combined cancellous lag screw for compression. 2. The rate of mean CF per degree differed across phases, and decreased rapidly when CF was adjacent to the maximum.Part III: The Change of Pullout Strength at Maximum Compressive Force when using Combined Cancellous Lag Screw in Different Bone Mineral DensitiesObjective: The aim of the current study was to determine whether POS decreased at the point of MCF during CCLS compression for different bone mineral densities.Methods: Three synthetic cancellous bone blocks were used for this study, including0.12 g/cm3, 0.16 g/cm3, and 0.20 g/cm3. 20 pilot holes with 3.2 mm diameters were drilled equably in each block and were assigned randomly into two groups: CF group(n=10) and POS group(n=10). In CF group, CCLS was inserted through the custom-designed measuring device into a pilot hole by hand until failure, and MCF was determined. In POS group, CCLS was inserted through the custom-designed measuring device into a pilot hole by hand and no compression happened in the monitor of the custom-designed measuring device, and then the pullout test was completed by the custom-designed pullout device and POS was determined.Results: 1. MCF and POS were not significantly different in the three densities specimens(P = 0.581 in the 0.12 g/cm3 specimen, P = 0.650 in the 0.16 g/cm3 specimen and P = 0.107 in the 0.20 g/cm3 specimen). 2. The ratios of the mean MCF to the mean POS were very similar in the three specimens(0.98 in the 0.12 g/ cm3 specimen, 1.01 in the 0.16 g/ cm3 specimen and 0.98 in the 0.20 g/ cm3specimen).Conclusions: POS did not decrease at the point of MCF during CCLS compression for different BMDs.Part IV: The Comparison of Compressive Force and Pullout Strength after Compression between Combined Cancellous Lag Screw and AO Cancellous Lag Screw in Different Bone Mineral DensitiesObjective: To compare CF and pullout strength after screw compression(POSSC)between CCLS and AOCLS for different BMDs.Methods: Three synthetic cancellous bone blocks were used for this study, including0.12 g/cm3, 0.16 g/ cm3, and 0.20 g/ cm3. 24 pilot holes with 3.2 mm diameters were drilled equably in each block and were assigned randomly into two groups: CCLS group(n=12) and AOCLS group(n=12). CCLS and AOCLS were inserted and compressed through the custom-designed measuring device into a pilot hole by hand until the operator adjusted that the screw compression was good, and CF was determined. Next, CFdisappeared by rotationg the screw mannually in a counter clock-wise way in the monitor of the custom-designed measuring device, and then the pullout test was completed by the custom-designed pullout device and POSSC was determined.Results: 1.Among three BMDs specimens, CF of CCLS was significantly higher than that of AOCLS(P < 0.001), and coefficient of variation(CV) of CCLS was lower than that of AOCLS. CF increased as BMD increased when CCLS or AOCLS was applied(P <0.001). 2. Among three BMDs specimens, POSSC of CCLS was significantly higher than that of AOCLS(P < 0.001). POSSC increased as BMD increased when CCLS or AOCLS was applied(P < 0.001). 3. BMD and the difference in the shank structure between CCLS and AOCLS were two influence factors which affected the CF(P < 0.001), and the two factors had not interaction(P = 0.872). 4.BMD and the difference in the shank structure between CCLS and AOCLS were two influence factors which affected POSSC(P < 0.001),and the two factors had not interaction(P = 0.909).Conclusions: 1. CCLS was preferable to AOCLS in the compressive ability and mechanical ability of cancellous bone surrounding the thread after screw compression when specifications of screws, BMD and operator were identical. 2. As BMD increased,the compressive ability and mechanical ability of cancellous bone surrounding the thread after screw compression increased when screw and operator were identical. 3. Compared to AOCLS with the identical specifications in the uniform specimen, CCLS was more stable in the compressive ability when multiple compressive operations were completed by a given operator, and therefore the incidence of insufficient compression and screwstripping was lower for CCLS compression.Part V: The Comparison of Combined Cancellous Lag Screw and the AO Cancellous Lag Screw in Compressive Force and Incidence of Screwstripping in a Bone Mineral DensityObjective: To compare CF and incidence of screwstripping(ISS) between CCLS and AOCLS for different operators.Methods: Five 0.12 g/cm3 synthetic cancellous bone blocks was used for this study.48 pilot holes with 3.2 mm diameters were drilled equably in each block and 240 pilot holes were prepared in total. And 96 pilot holes were assigned randomly into eight groups(n=12). CCLS and AOCLS by four operators were inserted and compressed through the custom-designed measuring device into a pilot hole until the operator adjusted that the screw compression was good, and CF was determined. Next, CF disappeared by rotationg the screw mannually, and then the pullout test was completed by the custom-designed pullout device and POSSC was determined. 103 pilot holes were assigned randomly for pullout test using AOCLS to determine the reference value of POS in these specimens.Results: 1. CF of CCLS was significantly higher than that of AOCLS by a given operator(P < 0.006). 2. CF was significantly different when four operators applied the same lag screw for compression(P < 0.001). 3. CV of CCLS was lower than that of AOCLS by a given operator. 4. ISS of CCLS was lower than that of AOCLS(0 vs 25%, 16.67% vs66.67%, 0 vs 66.67%, and 33.33% vs 91.67% respectively). 5. ISS of AOCLS(25%-91.7%)varied higher that that of CCLS(0-33.3%) among surgeons. 6. CCLS was less than AOCLS in the decreasing degree of POSSC(0N vs10.73 N, 16.84 N vs 77.01 N, 0N vs43.67 N, and 36.69 N vs 97.26 N respectively). 7. The differences in operators and in the shank structure between CCLS and AOCLS were two influence factors which affected CF and ISS(P < 0.001), the two factors had interaction in CF(P < 0.001) and no interaction in ISS(P = 0.437) when excluding the impact of BMD.Conclusions: CCLS was preferable to AOCLS in the compressive ability, operative stabilty, ISS, and damaged degree of cancellous bone surrounding the thread after compression when specifications of screws, BMD and operator were identical.Part VI: The Comparison of Combined Cancellous Lag Screw and AO Cancellous Lag Screw in Maintaining Compressive Force in a Bone Mineral DensityObjective: To compare CCLS and AOCLS in maintaining CF after compression.Methods: One 0.12 g/cm3 synthetic cancellous bone blocks was used for this study.24 pilot holes with 3.2 mm diameters were drilled equably in the block and assigned randomly into two groups: CCLS group(n=12) and AOCLS group(n=12). CCLS and AOCLS were inserted through the custom-designed measuring device into a pilot hole by one operator until the operator adjusted that screw compression was good, and theprocedure was lasting 2 hours. And then, CF in 2 hours was determined.Results: 1.CF was decreasing continuously after compression, and a new banlance was achieved after 2 hours. 2.CF of CCLS 2 hours later was higher significantly than that of AOCLS when one operator applied the two lag screws for compression(P < 0.001).Conclusions: CCLS was superior to AOCLS in maintaining CF after compression.Part VII: The Comparison of the Dynamic Load Bearing Capability after Fixation by Combined Cancallous Lag Screw and AO Cancallous Lag Screw Fixation in a Bone Mineral Density.Objective: To the dynamic load bearing capability after the compressive fixation by using CCLS and AOCLS fixation.Methods: Five 0.12 g/cm3 synthetic cancellous bone blocks was used for this study to establish the fracture model. The model was compressively fixed by CCLS and AOCLS,and a dynamic load device was constructed. The Electro Force? 3510 testing machine provided cyclic dynamic load at 2 Hz, starting with a peak load of 150 N and a valley value of 100 N. The peak load increased by 10 N per 100 cycles; the valley value maintained 100 N throughout the loading test. The peak load and the number of cycles were measured when 1 mm displacement appeared.Results: 1. Compared with AOCLS, the compressive fixation by using CCLS with the bone plate was higner in the peak load of 1mm displacement when one operator completed the compression and fixation(P < 0.001). 2. CCLSwas superior to AOCLS in the cycle number of 1mm displacement when one operator completed the compression and fixation(P < 0.001).Conclusions: Compared with CCLS, AOCLS was less reliable, loosening at a lower dynamic load.