Construction And Analysis Of Finite-element Model Of Patella Fracture Fixed With Tensile Force Girdle

BackgroundPatella is the maximal sesamoid bones of human body,it is flat,wide top,cuspidal underside and rough anterior surface.It connects quadriceps muscle of thigh from top and tubercle of tibia by patellar tendon.Its back is smooth articular surface which composes patellofemoral joint with patellar fossa of femur.Patella is the one of bones composing knee joint.Patella is superficial,only a skin,lamellar subcutaneous tissue and bursa synovialis above it,so it is easy to be injured by direct violence and tumble.For the past few years,with the development of science and technology and increasingly acceleration of society,bone fractures have been also increasing.Patellar fracture is frequent,it is 1.65 percent of all bone fractures.It is a ideal method that operation of open reduction and internal fixation treats patellar fracture with comparatively large dissociation,which is beneficial to early mobilization and rehabilitation of knee joint.Currently,there are many internal fixation methods for treating patellar fracture include tensile force girdle with kirschner wires,NT-PC (NiTi-Patella Concentrator),Cable-pin system,five-pointed star lattice sutures,the purse string with silk thread,etc.Each has itself advantage and disadvantage,how to select the best? Accordingly,it is very important that how to compare various kinds of internal fixation methods for selecting the best therapic method.At present,most of scholars undertake comparative studies of internal fixation methods of patellar fracture from three directions.The first direction is carried out by reviewing the different clinical cases by Bostman score,Bostman score is composed of ROM(range of Joint motion),pain degree,obstructive work degree,auantic degree of quadriceps muscle of thigh,hand crutch Assistance,quantity of hydrops articuli,strength of lower limb,ability of climbing staircase,etc.Which judges the therapeutic effects of internal fixation methods of patellar fracture by clinical bone healing and rehabilitation of knee joint,it is indirect,cann't know displaces after the same force on the postoperative patellar.The second direction is carried out by simulating different internal fixation operations on patellas of cadavers.Which is a standard biomechanical research.lt can measure results after the same force on the postoperative patellars,but it has many shortages,such as difficult cadaveric source,needing good experiment conditions include professional biomechanical engineering laboratory,costly expenses,deficient of observing deformations and offsets of inner bone and kirschner wires,metaphysical results,etc.The third direction is a theoretical biomechanical research.Which has stress analysis of numerical values by mathematical mechano-models.The finite element analysis (FEA) belongs to the theoretical biomechanical research.In contrast with experimental biomechanics,the finite element analysis has following advantages:(1)The finite element method can have simulations in different conditions,so it gets results that don't be influenced by conditons,namely,it precludes error from different experimental conditions.(2)The finite element analysis method can summarize structural shape,material efficiency and boundary condition of analogue by mathematical pattern,calculate tautologically and save experimental cost.(3)The finite element analysis method can establish three dimensional experimental analogue,then assign its biomechanical material characteristics.in the experiement of simulations,many datas can be gained which cann't in other experimental way. Owing to above advantages,the finite element analysis is made extensive use of clinical researchs of orthopedics.But on patellar fracture,it is less,most of researchs concentrate on clinical cases'reviews and biomechanics.Although Chen LP et al carried out calculus for patella fixed with tensile force girdle in planar nine nodes method in 1989,with the further development of computer and his softwares,it is more accurate,comprehensive and iconical to analyse the internal fixations of patella fracture with the finite element by many modern large scale softwares.Why to study internal fixations of patellar fracture is less in the finite element method?Because the major operative method of treating patella fracture is fixing by tensile force girdle with kirschner wires,both the kirschner wires and steel wire are small,it is very difficult to modeling them by computed tomography or magnetic resonance imaging scanning directly.So the best method is established model by large scale modeling software.There are three difficulties:Firstly,tensile force girdle is attached to surface of patella.Secondly,tensile force girdle is not attached to surface of patella,but to surfaces of two kirschner wires on quadripolar.Thirdly,steel wire tie must be satisfactorily established.On account of very irregular surface of patella,it is more difficult that tensile force gridle as above standardizations is established. Markedly,it restricts the finite element reseach on patella fracture.On this research,We have established three dimensional model of adult's normal patella,simulated bone fracture,fixed patella fracture with kirschner wires and tensile force girdles by advanced computer-assisted modeling and analytical techniques.We have overcome three difficulties,established the finite-element models of patella fracture fixed with two different tensile force girdles successfully,then have the finite element analysis,investigate feasibility and method to analyse the internal fixations of patella fracture by many modern large scale softwares,compared the two internal fixations simultaneously.Objective1.To create finite-element models of patella fracture fixed with two different tensile force girdles and have the finite element mechanical analysis on them.2.To investigate feasibility and method to analyse the internal fixations of patella fracture with the finite element by many modern large scale softwares.3.To compared the two internal fixations of patella fracture by the finite element analysis.Methods1.Establishment of the model of patellaAn adult female volunteer was selected,she was scanned at 1.25mm slice from foots to middle legs by computed tomography machine of SIMENS SENSATION 16,after backlay on the scanner platform of computed tomography machine,then 489 slices of tomoscanning images were gained.We wrote these images into R/W (readable/writable) compact disc by DICOM(Digital imaging and communications in medicine) format,and imported into the software of Mimics 10.01,selected right region patellaris by Region growing,extracted 40 slices of polylines of right patella,then imported into the software of Geomagic studio 10.0 by IGES (Initial graphics exchange specification) format,reduced noise,wraped polygons,smoothed surfaces,lastly established geometrical model of patella.2.Establishment of the models of AO and 8 font style tensile force gridleThe geometrical model of patella was imported into the software of Pro/ENGINEER Wildfire 2.0 by IGES format,by measure we divided it into quartering by axis of abscissa and trisection by axis of ordinate,firstly selected front of 1/3,then on the plane,selected outside of 1/4 and inside of 1/4 as axises of kirschner wires,generated two bended kirschner wires by mixed-sweeping function,their diameter was 1.5 millimeter.Finally,curves of steel wires were drawed respectively by winding ways of AO and 8 font style tensile force gridles,generated two bended steel wires by mixed-sweeping function,their diameter was 0.6 millimeter.3.Establishment of the models of patella fractureWe selected middle point of axis of ordinate,extended a vertical plane (Z direction),then divided the patella into two fractured blocks by the software of Pro/ENGINEER Wildfire 2.0.4.Establishment of models of patella fracture fixed with AO and 8 font style tensile force girdlesWe assembled two patella fractured blocks,two kirschner wires and two bended steel wires into two models of patella fracture fixed with AO and 8 font style tensile force girdles by the software of Pro/ENGINEER Wildfire 2.0.5.Establishment of ANSYS models of patella fracture fixed with AO and 8 font style tensile force girdlesWe activated interface modular of Connection for ProEngineer in the setting options of the soft of ANSYS 10.0,directly imported the documents of Pro/ENGINEER Wildfire 2.0 into the software of ANSYS 10.0,because patella is composed of cortical bone and cancellous bone,we established two sections,offseted inward 1 millimeter,generated model of cancellous patella,by boolean we generated model of cortical patella,then glued them,established two integrated patella fractured blocks.Lastly imported two kirschner wires and two bended steel wires into the software of ANSYS 10.0,assembled them into ANSYS models of patella fracture fixed with AO and 8 font style tensile force girdles.6.Establishment of finite-element models of patella fracture fixed with AO and 8 font style tensile force girdlesWe assigned elemental type and materical parameter to each entity by following table,meshing them,then seted up contact pairs,finally we get finite-element models of patella fracture fixed with AO and 8 font style tensile force girdles.The finite-element model of patella fracture fixed with AO tensile force girdles has 404343 nodes,307719 element units;the finite-element model of patella fracture fixed with 8 font style tensile force girdles has 401853 nodes,296078 element units.Table of elemental types and materical features in the finite element models of patella fracture fixed with tensile force girdles 7. Settings of boundary condition and loadingBecause contiguous articular surface of patella is varied with the range of patella motion,it is the top of patella when the range of patella motion is 90°,we selected and exerted force of 600N from norientation on the section,after wholly constrainted attachment regions of patellar ligament in the two polar of patella,analyzed displacement and deformation features simulating patellofemoral joint under 600 N loading at 90°knee bending.Results1.Fractured blocks of patella offsets from post to fore(z norientation) and offset's absolute values gradually diminish after exerted force,accordingly,tensile force girdles have an evident pressure on Z-direction.(positive displace of Z-direction is from post to fore)2.Fractured blocks of patella wherever above or below two kirschner wires offsets mainly downward(Y-negative direction)after exerted force,accordingly,tensile force girdles have an evident effect against drawing upward from quadriceps muscle of thigh.(positive displace of Y-direction is from below to above)3.Although Fractured blocks of patella above two kirschner wires have different offset's nephograms from below after exerted force,both of their offset's absolute values (Y-negative direction) gradually diminish from center to environment of bone fracture,accordingly,tensile force girdles have an evident pressure from environment to center.4.Offset's absolute values of fractured blocks of patella below two kirschner wires from right to left gradually diminish to zero then gradually increase after exerted force,namely,have an evident dispersion from center to two sides.But above two kirschner wires their Offset's absolute values from right to left gradually diminish to zero then gradually increase,namely,have an evident dispersion from center to two sides,simultaneously two sides form two center,absolute values gradually diminish from center to environment,namely,tensile force girdles have an evident pressure from environment to two center,which can prevent two-side dispersion.(positive displace of X-direction is from right to left)5.Two kirschner wires have marked deformations which gradually diminish from center to environment of bone fracture,two kirschner wires have an role of elastic fixation,furthermore,their deformations are dissymmetrical.6.Maximum displaces of both kirschner and steer wires concentrate mainly on bone fracture.7.Maximum displaces of kirschner wires is bigger than of steer wires.Therefore,tensile force girdles make good use of better kirschner wires' bending efficiency than steer wires'8.Displace of patella fracture fixed with 8 font style tensile force girdles is smaller than with AO's under the same loading.Conclusions1.AO/8 font style tensile force girdles can pressurizing on bone fracture from many directions and convert patellofemoral force against drawing upward from quadriceps muscle of thigh.2.8 font style tensile force girdles is better than AO fixed on transverse fracture of patella.3.Results of the finite element analysis is coincident with of biomechanics,which confirms that finite-element models of patella fracture fixed with AO and 8 font style tensile force girdles are effective,it is feasible to analyse the internal fixations of patella fracture with the finite element by many modern large scale softwares,we can compared different internal fixations by the finite element analysis and provide clinical instruction.