The Design Of Static Three Dimensional Screw-Plate System (STDSP) And Comparative Biomechanical Study In Fixing Femoral Neck Fracture

Femoral neck fractures are frequently encountered injuries by theorthopaedic surgeon . More than 100 internal fixation devices have beendeveloped, each has its advantages and disadvantages. Multiple screwfixation and nail-plate system are two techniques commonly used. Theformer, which is minimally invasive and convenient to be performed, isthe most commonly used technique for the treatment of stable femoralneck fractures. Unfortunately the Pauwels Type III high-angle fracturepresents a mechanical problem that is not amenable to it. The nail-platesystem has biomechanical advantages, especially the fixed-angleimplants , not only can scatter and conduct the load, but also can supportthe body weight well. However, because only one screw is used to fix theneck, they provide little torsional stability. Besides this, it can not provideenough stability for femoral neck fracture with posterior comminution.For this reason, based on the anatomic characteristics of proximal femur,combined the biomechanical advantages of multiple screw fixation ,nail-plate system and three dimensional plate, we invent the Static ThreeDimensional Screw-Plate system(STDSP), expect to provide an effectiveinstrument for the treatment of femoral neck fractures. The currentstudy include four parts, the measurement of proximal femur, the designof static three dimensional screw-plate system , the design of the ancillaryapparatus of STDSP and the comparative biomechanical study .PartⅠ The Measurement of Proximal Femur and ItsSignificanceObjective: Measure the proximal femoral parameters and providethe basis to design internal fixed components for the treatment of femoralneck fractures.Methods: Right femurs of 54 healthy adult cadavers were measured.The maximal diameter of femoral head, head length, head-neck length of135°, the head-neck length of 135° of the top and bottom edge of neck,minimal sagittal and coronal width of femoral neck, femoral neck-shaft angle,angle between the axis of neck and lesser trochanter, and the angle of greattrochanter were obtained.Results: The maximal diameter of femoral head was 46.80±3.73mm,head length was 39.18±4.32mm,the head-neck length of 135° was 95.94±7.69mm, the head-neck length of 135° of the top edge of neck was84.31±7.06mm , of the bottom edge, 100.39±8.99mm ,minimal sagittal widthof femoral neck was 32.35±3.31mm, minimal coronal width of femoral neckwas 26.54±3.19mm,femoral neck-shaft angle was 127.04°±7.11°,anglebetween the axis of neck and lesser trochanter was 43.96°±3.03°,and theangle of great trochanter was 16.67°±3.87°。Conclusion: 1. The anatomic parameter of proximal femur ofdomestic people had great individual difference, also it differ fromforeigner, It is necessary the appropriate type of internal fixation should bechosen, and the individualized treatment should be performed for femoralneck fractures. 2. We are the first to measure the head-neck length of135°,and the head-neck length of 135° of the top and bottom edge of neck,these parameters are helpful to the selection of screws with appropriatelength;the measurement of the minimal sagittal and coronal width of femoralneck provide parameters for the dimensional arrangement of the screws.3.We are the first to measure the angle of great trochanter , provideanatomic basis for the design of anatomic plate. 4. The measurement ofthe angle between the axis of neck and lesser trochanter provide anatomicbasis for the fixation of lesser trochanter.Part Ⅱ The Design of Static Three Dimensional Screw-PlateSystem(STDSP)Objective: Design the Static Three Dimensional Screw-Platesystem(STDSP), bring a new instrument for the treatment of femoralneck fractures, which has good biomechanical features of axial loading androtation stability and can reestablish the loss of support of the posteriorneck of fracture with posterior comminution.Methods: Based on the parameters and biomechanical feature ofproximal femur, combined the biomechanical advantages of multiplescrew fixation , nail-plate system and three dimensional plate, design theSTDSP.Results: STDSP was constituted by three cancellous lag screws andone anatomic plate. The three cancellous lag screws were placed intriangular pattern. The distal screw lies against the medial cortex, theproximal screws are spread out and placed anteriorly and posteriorly. Allthe lag screws have an angle of 125°with the superior part of plate, andfix the femoral neck with an angle of 135° with the femoral shaft. Theposterior screw is stable connected to the plate. The plate is anatomical,and was affixed crosswise to the lateral and anterior lateral side bybicortical screws.Conclusion: STDSP is a fixation with good biomechanical featurefor the treatment of femoral neck fractures.Part Ⅲ The Design of the Ancillary Apparatus of StaticThree Dimensional Screw-Plate System(STDSP)Objective: Design the ancillary apparatus of STDSP, provideconvenience for the manipulation of STDSP.Methods: Based on the parameters of the plate and screw of STDSP,design screw guides, drill guides, fix rods, screwdrivers and nutdrivers.Results: 1. The screw guide has a same undersurface as the plate ofSTDSP .In the guide, there are three 6.5mm screw holes which are consistentto the holes of the plate and six hole for 2mm leading pin. All the holes areparallel with each other, and have an angle of 135°with the inferior part ofplate, guarantee the screws are parallel and fix the femoral neck with anangle of 135° with the femoral shaft. 2. The outside diameter of thedrillguide is 6.5mm, the inner diameter is 4.5mm,it can be inserted intothe screw hole of the screw guide and can prevent the swing of the drillwhen drilling. 3. The fix rod is 100mm in length, the front part is 4.5mmround and 85mm in length, and the back part is 5mm round. It can beinserted into the drillguide and can maintain the position of the fractureand the screw guide. 4. There are three kind of screwdrivers and two kindof nutdrivers, they can fit well to the tails of the screws or of the nuts.Conclusion: The ancillary apparatus of STDSP have simpleconfiguration, depending on them, STDSP can be installed accurately,conveniently and rapidly.Part Ⅳ Comparative Biomechanical Study of Static ThreeDimensional Screw-Plate System(STDSP) in Fixing FemoralNeck FractureObjective: Undertake a comparative biomechanical study of thestability of the Static Three Dimensional Screw-Plate system (STDSP),and evaluate the biomechanical capability of STDSP.Methods: Twenty three matched pairs of human cadaveric femurswas divided randomly into two groups, the fracture group(17 pairs) andintact group(6 pairs).To the fracture group, vertically oriented femoralneck osteotomies (Pauwel's angle 70° ) and posterior comminution of theproximal part of fracture were created, and then divided into two groupsagain, the rotation test group(8 pairs) and the axial loading test group(9pairs). One femur of each pair was randomly selected to be stabilized bySTDSP, and the other was fixed by the three cancellous lag screws(TCS)or DHS. Then the construction were tested with axial loading or rotationto failure. The intact group were fixed and then taken out with the threemethods, then tested with axial loading. For axial loading, the bone andimplant constructs were secured in a vise at 15°in the coronal plane andneutral in the sagittal plane;For rotational loading, they were secured in avise at 25°in the coronal plane and also neutral in the sagittal plane. Thefracture displacement, load and the failure circumstances of bone andimplant constructs were obtained.Results: 1. Rotation test: Compare the loads of the same rotation angleand the loads of the same width the anterior gap of the osteotomy open,STDSP group is bigger than TCS and DHS group(P﹤0.05). 2. Axialloading test: Compare the loads of the same inferior femoral head displacementand the ultimate yield load, STDSP group﹥TCS group (P﹤0.05);theinferior femoral head displacement of the same loads and the width of superiorgap open of the same load ,STDSP group﹤TCS group (P﹤0.05). Thereare no significance between STDSP group and DHS group when comparethe above four items(P﹥0.05). 3. Axial loading test of the healed model:The ultimate load to failure of STDSP group is bigger than TCS group by2376N,and bigger than DHS group by 3075N (P﹤0.05).Conclusion: 1. To fix femoral neck fracture models, STDSP is sameto DHS and superior to TCS in the axial loading stability. in rotation stability;STDSP is superior to both DHS and TCS. In the capability of boneconstruction strength retention, STDSP is superior to TCS and DHS. 2.STDSP is a fixation with good biomechanical feature for the treatment offemoral neck fracture.