| Objective: With the development of modern society, the prevalence of femoral neck fractures as well as demographic projections of an increasingly aged population is higher, the tendency is rising. Because of the special anatomic site of femoral neck fracture, the treatment of femoral neck fracture is till a big challenge. To this day many questions are still insolubility 1n the treatment and result of femoral neck fracture. Recently bone non-healing and necrosis of femoral head are the main difficulties in the treatment of femoral neck fracture .After femoral neck fracture, incidences of ischemic necrosis and bone non-healing are closely related to biomechanical characteristics of internal fixation devices. Now frequently used internal fixation devices are Smith-petersen nail, cannulated compression screw and DHS, etc. In 1980'annulated compression screw gradually replace others and used generally . Three screws can consist a stereoscopic structure with bone tissue, and the structure is coincidence with biomechanical principles and characterized by high stress, curved strength and torsibility. Intensity or rigid degree of internal fixation device is designed reasonably; in addition, position, angle and distribution of internal device, different position, angle and distribution of internal device, different position or distributions can cause different compression strengths, tensile forces and stabilities. This study was designed to compare the biomechanical characteristics of the two geometric configuration of three cannulated compression screws in the fixation for femoral neck fracture and to provide a theoretical foundation in the treatment of femoral neck fracture.Methods: Eight cadavers including five males and three females with similar bone mineral density were provided by Department of Anatomy, Hebei Medical University. The mean age was 41 years, range 30-65years. X-ray photographs proved that all the cadavers did have rheumatism, tuberculosis, tumor, fracture or deformity. Femur was separated from each corpse sample, stripped out the attached soft tissues, cut at 1/3 crossed site of middle-inferior segment, and obtained superior segment for experiment. Femurs in the two groups were used to establish femoral neck fracture models at Pauwels 70°.Femurs were fixed with three cannulated compression screws (7.3mm) based on surgical demands. the experimental group: the first screw was placed into anterior-superior position of femoral neck and approach to anterior-superior cortex; the second screw was placed into inferior position of femoral neck and approach to inferior cortex; the third screw was placed into posterior position of femoral neck and approach to posterior cortex; the control group: the position of the first screw is same to the experimental group; the position of the second screw is 1.0mm on inferior cortex; the position of the third screw is 1.0mm before posterior cortex ;an Femoral neck fracture was reduced and drilled with three Steinmann pins(2.0mm) along aiming tunnel to lateral cortex of femur. And then, cannulated compression screws(7.3mm) were tightly fixed to make sure the compact conjugation between fracture part and dissection-reduction site. Top of cannulated compression screws was allocated below 0.5-1.0cm of cortex of femoral head and end was slightly fallen into lateral cortex of femur.Torsibility test: The finished femur was unloaded to make sure the coincidence of femoral neck axis with torsion of Biomechanical meter. And then, the femur was tightly fixed to measure torsibility at the anticlockwise speed of 0.2°/s and record the torque moments at 2°,4°and6°. Compression strength test: Loading status of human body was simulated at the phase of monoploidy standing in this study. During monoploidy standing, shaft of femur was angled with longitudinal axis of body at 15°. Self-made fixturn was used to maintain monoploidy standing of femur. And then, the fixed femur was put at the center of platform of biomechanical meter. Femoral head was vertically loaded at the speed of 20N/s, and the finally vertical loading was 800N. Displacement of femoral head was recorded at 100N, 200N, 400N, 600N and 800N vertical loading. Maximal vertical loading test: After Compression strength test , the maximal vertical loading was measured with biomechanical meter till the failure of internal fixation. Failing criteria: Samples had fracture, disruption of femoral neck and femoral head and maintenance or descent of displacement curves.Statistical analysis: Data were dealt with SPSS 13.0 software and t test by the first author in this study. P<0.05 was regarded as the significant difference.Results: Torsibility test: The torque-moments of the experimental group were 5.127±0.627NM,10.406±1.515NM and 15.328±1.471NM at 2°, 4°and 6°torsibility. The torque-moments of the control group were 2.270±0.104NM, 6.628±1.163NM and 9.072±1.570NM at 2°, 4°and 6°torsibility. There was significant difference between the two groups(P<0.01). Compression strength test: The displacements of femoral head of the experimental group were 0.132±0.048mm, 0.231±0.070mm, 0.457±0.106mm, 0.677±0.135mm and 0.907±0.132mm at 100N, 200N, 400N, 600N and 800N vertical loading. The displacements of femoral head of the control group was 0.359±0.038mm, 0.506±0.046mm, 0.674±0.130mm, 0.899±0.160mm and 1.202±0.152mm at 100N-800N vertical loading. There was significant difference between the two groups in compression strength at the 100N, 200N, 400N, 600N and 800N vertical loading.(P<0.05).Maximal vertical loading test: The maximal vertical loading of the experimental group was 2782±228N。The maximal vertical loading of the control group was 1950±281N.There was significant difference between the two groups at the maximal vertical loading (P<0.01)。Conclusion: Based on the data mentioned above, we can come to the following conclusion: The torque-moments of the experimental group was larger than that of the control group; The femoral head displacement of the experimental group was shorter than that of the control group; The maximal vertical loading of the experimental group was higher than that of the control group. This study was designed to compare the biomechanical characteristics of the two geometric configuration of three cannulated compression screws in the fixation for femoral neck fracture and to provide a theoretical foundation in the treatment of femoral neck fracture. |
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