Biomechanical Analysis Of Fixation For Pauwels â…¢ Femoral Neck Fracture With Three Cannulated Screws And Dhs With Anti-rotation Screw

Objective:Femoral neck fracture is a common fracture in the clinical, the incidence of this fracture accounts for3.60%of all fractures of the body. Pauwels III femoral neck fracture is a common fracture, as it withstander more shear stress. Femoral neck fractures occurred mostly in the elderly in the past. In recent years, with the development of transportation and construction industry, femoral neck fracture in the youth caused by high-energy trauma has become more and more common. Owing to the special anatomy of the femoral neck, the fracture likely to be nonunion, avascular necrosis and so on. With the development of the fixation and surgical techniques, the treatment has been greatly improved, but there are still many unresolved problems. Because of its better biomechanical stability, anti-stress and anti-rotation ability are also excellent, multiple cannulated screws have become the most popular method in the treatment of femoral neck fractures. The dynamic hip screw has the effect of dynamic and static pressure; It is also a commonly used method in the treatment of femoral neck fracture in the current clinical. However, biomechanical test confirmed that the dynamic hip screw has a worse anti-rotation effect.Therefore; some scholars have suggested that insert one cannulated screw above the nail of the dynamic hip screw for the purpose of increasing the anti-rotation effect. The purpose of this experiment is to compare the stability of three cannulated screws and DHS with anti-rotation screw in fixation for Pauwels III femoral neck fracture, provide a theoretical basis for clinical treatment.Methods:Selected8embalmed adult cadavers specimens, including5male,3female, aged35-51years with a mean age of42; remove the muscle and other soft tissue, cut off the femur from the middle of the femur, keep the femoral head, femoral neck and femoral shaft section of about25cm. None of femurs had any gross structural or radiological deformities, osteoporosis, tumors, trauma and other pathological abnormalities. No osteoporoses were found through absorptiometry line of bone mineral density measurements. The specimens were sealed with double layer plastic film, cryoprotected at-20°C and thaw at room temperature for6hours prior to test and sprayed intermittently with normal saline to keep the specimens hydrated. An osteotomy was created with a band saw, simulating a Pauwels’III (Pauwels70°) femoral neck fracture. The experimental femurs were distributed by random number table for the cannulated screw group (Group A)to left femoral neck, the right to DHS with anti-rotation screw group(Group B). Each group has8femurs.Group A:three7.3mm cannulated screws were used to fix the femoral neck, the screws were inserted parallel into the femoral head in the triangle configuration with the principle of "cortical screw support", that is the rear screw close to the cortex of the post femoral neck, the bottom screw close to the bottom cortex, the screw were5-7mm from the subchondral bone.Group B:a135-degree,4-hole DHS plate was positioned and one7.3mm Cannulated screw was inserted into the femoral neck parallelly above the central screw. The tip-to-apex distance of the central screw measured less than25mm in all8specimens.All specimens were fixed on the self-made fixture by the mixing appropriate type II denture base polymers, and then installed in the biomechanical testing machine for axial compression test, cyclical test.100N vertical preload was carried out for3times, in order to eliminate the creep effects of specimens. In axial compression test, the femoral head was loaded with vertical compressive force to700N at a rate of20N/S.The displacements were separately recorded by the computer at500N and700N. Then each specimen was loaded sinusoidally of1500N at a frequency of2HZ. The number of cycles at the end of the experiment (displacement of the osteotomy more than5mm) was measured. SPSS17.0statistical software was used for statistical analysis, with t-test and P value was set at0.05for the statistical significance.Result:1、 The bone mineral density of the specimens was checked by dual-energy X-ray absorptiometry. The mean BMD of the left femur were0.900±0.121g/cm2, the right of0.909±0.985g/cm2, No significant difference was observed between the left and right side of the specimen,(p=0.398>0.05).2、The compression test:All of the specimens were intact after axial compression test. When the pressure was500N, the cannulated screw group (Group A) displacement was0.571±0.980mm, the dynamic hip screw with anti-rotation screw group(Group B) displacement was0.404±0.320mm. The difference was statistically significant between the two groups (P<0.05). When the pressure was700N, the displacement of Group A was0.879±0.748mm, Group B was0.648±0.887mm, The difference was statistically significant between the two groups (P<0.05).3、The cyclical test:At the end of the experiment, the number of cycles for cannulated screw group is915.333±99.468, DHS with anti-rotation group is1692.667±86.775, The difference was statistically significant between the two groups (P<0.05)Conclusion:For Pauwels III femoral neck fracture, in terms of compression of biomechanical stability, the dynamic hip screw with anti-rotation screw is better than three cannulated screws.