Finite Element Analysis Of The Biomechanics Of The Pelvis And Acetabulum During Normal Gait And Acetabular Three Dimensional Memory Fixation System In Treating Acetabular Fracture

Estimation of the hip joint load area and pressure distribution during activities ofnormal gait is important in predicting joint degeneration mechanism. The abnormalmechanism in the joint cartilage is one of the key factors which lead to ostearthritis,usually the pathology mechanism and local stress in the hip joint cartilage result in it.At the same time walking is the most often behavior during activities of daily living,therefore it is necessary to estimate the hip joint load area and pressure distribution.For once it is known how the acetabulum behaves under normal loading conditions,we will be able to develop a better understanding of normal and abnormal mechanismin the hip joint. In addition we also have evaluated the basic mechanics of the naturalpelvic bone, such data is critical for accurately understand possible differences due topelvic fractures, deformity and acetabular reconstructions.The another objective of this study is to further verify the rapid bone healingwith high quality under the internal fixation of acetabular three dimensional memoryfixation system (ATMFS), to explore the related mechanism of ATMFS on promotingfracture healing and to evaluate the stress distribution on the fracture surface, and toestimate the effect on the contacting relationship in the hip joint under gait loadingconditions, such data are very important to use ATMFS in the clinic.Section 1 Three-dimensional finite element analysis of thebiomechanics of the acetabulum during normal gaitExperimental 1 Establishment of a three—dimensional finite elementmodel of actabulum with DICOM data.PURPOSE: To introduce an easier and more precise method for construction of three-dimensional(3D) finite element model of actabulum and investigate itssignificance for the continued research. METHODS. A normal acetabular from amale adult was scanned by CT and the CT images were analyzed and managed withDICOM data and Mimics software at the same time for getting a 3D modeldirectly, which then was analyzed with PATRAN software. RESULTS.. A moreprecise 3D finite element model of actabulum was established with DICOM data,which objectively reflected the real acetabulum anatomy and biomechanical behavior.The result of the biomechanical study was better correlated with the availableexperimental data. CONCLUSION: The method of this study can construct an easierand more precise 3D finite elemetnt model of actabulum with DICOM data andMimics software, it also provides a reasonably and effective model for biomechanicalanalysisExperimental 2 The three-dimensional finite analysis of hip contactstress and loading area during normal gaitPURPOSE: To explore the hip joint loading area and stess distribution duringdifferent gait phases, which is important in predicting joint degeneration mechanism.METHODS: CT scanning and computer image processing system were used toestablish the 3-D acetabular finite model to simulate the conditions of acetabularcartilage for 32 phases during the gait. A finite element solves was used to calculatestress and loading area. RESULTS: (1) The stress distribution within the acetabularcartilage was obtained and regions with elevated stress at 32 phases were located.The stress demonstrated the bimodal shape of a pronounced beginning left singlesupport phase and a pronounced end left single support phase which were seen in trialgait cycle (3.3 Mpa; 4.1 Mpa), accordingly the peak value (1470mm²) of loading areawas also seen at the end left single support phase, obviously the value of the loadingarea during stance phases was higher than one during period of swing phases. (2) Thestress distributions of 32 phases were significantly different, during complete stancephases (from heel-strike to toe-off), the elevated Von raises stress was located at the acetabular roof, and during complete swing phases (from beginning left swing phaseto end left swing phase) the Von mises stress was located at the medial roof of theacetabular cartilage. (3) During a gait cycle the persistent stress-transfer area waslocated on the roof (318mm²), the no stress-transfer area was located was locatedanterior horn and posterior horn (895 mm²), the occasional stress-transfer area was1695mm², the whole cartilage surface was 2908mm². CONCLUSION: These resultscan be used to rationalize rehabilitation protocols, functional restrictions aftercomplex acetabular reconstructions, the regions of elevated stress and contact area ofacetabulum are important, which provide an insight into the factors contribution to thearthritis.Section 2 The pelvic biomechanics study under normal gaitExperimental 1 Construction of three-dimensional finite element pelvicmodel with pelvic muscles and hip joint force during gaitPURPOSE: To construct three dimensional (3D) finite element pelvic modelwhich includes both three-dimensional pelvic muscle and acetabular contact forcesduring gait. METHODS: A normal pelvic from a adult was scanned by CT andimages of every cross-section pelvic layer were obtained, and an pelvic modelincluding both three-dimensional pelvic muscle and acetabular contact forces duringgait was constructed with PATRAN 2005R2 software. RESULTS: (1) The modelwas divided into 113 028 nodes and 137 524 units, the constructed 3D finite elementpelvic model clearly reflected the real pelvic anatomy. (2) The simulation ofloading conditions reflected the real biomechanical behavior, the hip joint force andthe forces of 22 muscles attached to the pelvic bone were constructed, especially thedirections of the muscles forces were found by subtracting the coordinates of theirdistal and proximal insertions and the relative position of the pelvic bone and thefemur changes during walking were also taken into account. CONCLUSION: Theconstruction of pelvic 3D finite element model provides basic data which are criticalfor accurately modeling either normal loads or stresses and strains, or the effects of abnormal conditions.Experimental 2 An investigation for the biomechanies of the pelvisduring normal gaitPURPOSE: To investigate the basic load transfer and stress distributions of thepelvis under normal gait loading conditions with more precise finite element analyses.METHODS: A normal pelvis from a male adult was scanned by CT and imagesof every cross-section pelvic layer were obtained, and an pelvic model including boththree-dimensional pelvic muscle and acetabular contact forces during four gait phases(Heel-Strike phase; Beginning left single support phase; Halfway left single supportphase; End left single support phase)was constructed with PATRAN 2005R2software. Which was used to investigate its basic load transfer and stress distributionsunder normal gait loading conditions. RESULTS: (1) The analyses showed that themajor part of the load was transferred through the cortical shell, stresses in the corticalshell were about 40 times higher than in the underlying trabecular bone (8.9～18MPa, 0.14～0.45 MPa). Highest intraarticular pressures were found to occur at theEnd left single support phase and measured about 4.1 MPa and 18 MPa in thecartilage and subchondral bone respectively, the primary load transfer was directedalong the axis from the sacro-iliac joint to the pubic symphysis. (2) The primaryareas of load transfer to be found in the acetabular cartilage, but in subchondral cortexthe principal stress was divided into two directions: from superior acetabular rim andmedial acetabular rim to the incisura ischiadaca region respectively,and to a lesserextent, the pubic bone. (3) Muscles forces are important with respect to the overallloading of pelvic bone, yet unlike the hip joint force, they had little effect on theacetabular load transfer. CONCLUSION: the pelvic bone behaves like a sandwichconstruction. The muscles forces have a stabilizing effect on the pelvic load transfer.Within the acetabulum itself, the stress distributions arc more directly dependent onthe magnitude of the hip joint force, while Within the subchondral cortex, they arealso affected largely by anatomy characteristic. Section 3 Three-dimensional finite element analysis of thebiomechanics of ATMFS treating acetabular fractureExperimental 1 Three dimensional element analysis for fixation of atransverse acetabular fracture with ATMFSPURPOSE: To study the biomechanical basis of Acetabular TridimensionalMemoryalloy-Fixation System (ATMFS) for treating transverse acetabular fracture,and the facilitating effects of dynamic memorial stress of ATMFS for bone healing.METHODS: By computer emulation and three dimensional (3D) finite elementanalysis, the biomechanical behavior of ATMFS was emulated and analyzed. Thefinite element Baâ…¡model of ATMFS was divided into 7946 units, the number ofnodes was 2520. The finite element Câ…¢model was divided into 1667units,thenumber of nodes was 6332. The element was a unit with 20 nodes, 4 faces. The finiteelement model of acetabulum was divided into 5 783 units, the number of nodes was9 863. And the finite element was a 3D unit with 10 nodes, 4 faces. RESULTS: (1)When the connector fixated acetabulum, in Baâ…¡model the maximum first structuralmajor stress of metamorphosed compressive part was 228 MPa and -24.5 MPa, In Câ…¢model it was 198MPa and -9.48MPa, both far less than its utmost stress andfatigue limit. The initiative memorial bone holding force in Baâ…¡model formaintaining axial stability was 196.02N, and the longitudinal initiative memorialcompression force In Câ…¢model was 125.05N. (2) The stress distribution in fixedacetabulum and the medial surface of fracture was even, the stress in most nodes waspositive stress, only the one at the edge and anterior horn of the acetabulum wastensile stress. (3) on the surface of the bone fracture, the stress in most nodes wascompressive stress, which was transferred principally through the cortex bone(about1.46 MPa), it is near 6 times than that on the fracture surface of the cancellous bone.CONCLUSION: The ATMFS has good anti-fatigue and reuse characters. Thedynamic memorial compressive stress field is good for the stability of fixation andenhancement of bone healing. Fixation of transverse acetabular fracture with ATMFShas good stability and good fitness and stress distribution of the repaired acetabulum. Experimental 2 Three dimensional element analysis for fixation of aposterior wall fracture of the acetabulum with ATMFSPURPOSE: To study the biomechanical basis of Acetabular TridimensionalMemoryalloy-Fixation System (ATMFS) for treating posterior wall fracture of theacetabulum, and the facilitating effects of dynamic memorial stress of ATMFS forbone healing. METHODS: By computer emulation and three dimensional (3D) finiteelement analysis, the biomechanical behavior of ATMFS was emulated and analyzed.The finite element Baâ…¡model of ATMFS was divided into 7946 units. The numberof nodes was 2 520. The finite element Bbâ… model was divided into 1787units.Thenumber of nodes was 623. The element was a unit with 20 nodes, 4 faces. RESULTS:(1) When the connector fixated acetabulum, In Baâ…¡model the maximum firststructural major stress of metamorphosed compressive part was 228 MPa and - 24.5MPa, In Bbâ… model the maximum first structural major stress of metamorphosedcompressive part was 108M Pa and -4.58 M Pa, both far less than its utmost stress andfatigue limit. The initiative memorial bone holding force in Baâ…¡model formaintaining axial stability was 196.02N, and the longitudinal initiative memorialcompression force In Bbâ… model was 41.83N. (2) The stress distribution in fixedacetabulum and the medial surface of fracture was even, the stress in most nodes waspositive stress. The stress near the memory targeting hole was higher, which wascompressive stress(6.68～1.15 MPa), only that at the edge of the acetabulum wastensile stress. CONCLUSION: The ATMFS has good anti-fatigue and reusecharacters. The dynamic memorial compressive stress field is good for the stability offixation and enhancement of bone healing. Fixation of posterior wall of theacetabulum with ATMFS has good stability and good fitness and stress distribution ofthe femoral head and the repaired acetabulumExperimental 3 how acetabular tridimensional temoryalloy fixationsystem (ATMFS) treating acetabuluar posterior wall fracture affects the contactrelationship of hip joint PURPOSE: Finite element method was used to analyze the stress distributionthe mobility of fracture segment,contact area and contact stress with ATMFSfixation under four kinds of different gait conditions (Heel-Strike phase; Beginningleft single support phase; Halfway left single support phase; End left single supportphase), so as to provide reference for ATMFS internal fixation in clinic. METHODS:Through the models of acetabuluar posterior wall fracture and the fracture withATMFS fixation, the regularity of stress distribution of acetabuluar posterior wallfracture and the mobility of fracture segment under different four kinds of gaitconditions were analyzed by PATRAN software with investigation of the effect of theATMFS fixation. RESULTS: (1) After the posterior wall of the acetabulum fracturwas replaced and fixed by ATMFS, the contact area was increase largely Under thefour kinds of gait conditions, at the same time the peak stress in the acetabularcartilage was decreased accordingly. At the gait 4, the contact area was the highest,124.37% of the normal contact area, at the same time the peak stress was 0.82 timesof the normal gait. When the fracture was not fixed, the contact area was the least atthe gait 1, only 40.94 % of the normal contact area, at the same time the peak stresswas 4. 04 times of the normal gait, and there was a obvious stress concentration at thefracture line. (2) the acetabulum took place contracting deformation(1.89^*10^-5～4.55^*10^-12m), when it was fixated by ATMFS. which can decrease thecontact stress and increase the contact area. (3) Stress concentration took place at thefracture line, according to the direction of force from posterior to anterior, it wasincline to alleviate. The stress concentration not only was affected by the value of thehip joint force, but also was affected more largely by the direction of the hip jointforce.(4) Under the 4 gaits, when fixed with ATMFS, the mobility of fracturesegment accordingly was 0.0319 mm,0.0477 mm,0.0372 mm,0.0289 mm, all ofthem were in safety margin. (5) During the gait, principally the stress of fracture partwas compressing stress and only near the acetabular margin it was tensile stress, all ofthe stress was close to physiological loading. CONCLUSION: Fixation of posteriorwall of the acetabulum with ATMFS has good fitness and stress distribution of therepaired acetabulum, which can decrease the contact stress and increase the contact area effectively, and the memory dynamic compressive stress on the fracture surfacecan also effectively facilitate bone healing.