Experimental Study Of The Posterior Wall Of Acetabular Middle Column In Biomechanics And Infinite Elements Analysis

Part1Biomechanical experiments on the posterior wall ofmiddle column of acetabulumPurpose:To investigate the importance and influence of posterior wall of middlecolumn of acetabulum on biomechanical behaviors of acetabular fractures within theregion of posterior wall of middle column.Pelvic specimens were used to conductbiomechanical experiments on posterior wall of middle column of acetabulum.This paperaimed to explore the rationality of the three columns of pelvic acetabulum from theperspective of experimental biomechanics,and to provide evidence for the treatment ofacetabular fractures within the region of posterior wall of middle column.Method:Formalin-reserved pelvic specimens of twelve adult male were divided intotwo groups to simulate fracture models of posterior wall of middle column and classicposterior wall of acetabulum.TEKSCAN stress system was used to measure and analyzethe biomechanics of acetabular articular surface of intact pelvic,step displacement fracturemodel,plate fixation after restoration and ATMFS fixation,and digital image system wasused to analyze the fixation stability of the two posterior fracture models.Results:1)In paired comparison,average pressure and peak pressure of the two groupswere increased in accordance with the increase of the step height.In the same height,thepeak pressure of articular surface of0mm posterior wall of middle column group wasranged from3.03MPa~3.99MPa(3.49±0.20MPa),while the posterior wall group was3.08MPa~3.69MPa(3.40±0.17MPa),there was no significant difference between the twogroups.2)After loading,the peak pressure of articular surface of intact acetabulum wasranged from3.05MPa~3.71MPa(3.40±0.17MPa).After the middle column posteriorfracture was restored and fixed with screw and plate,the peak pressure of articular surface was ranged from3.14MPa~4.02MPa（3.60±0.29MPa）.3)Compare the stability of screwplus plate fixation of the middle column posterior wall fracture group and classic fracturegroup,the displacement of the former group was1.54±0.05mm,the latter was0.83±0.05mm,there was significant difference between them.4)The biomechanical results ofposterior wall of middle column of acetabulum fractures with different fixation method:thedisplacement of screw plus plate fixation was1.519±0.053mm;displacement of ATMFSfixation was1.552±0.040mm.No significant difference was observed.Conclusion:In loading condition,middle column posterior wall of atetabulum playedan important role in stress concentration.Step displacement of fracture in this region wouldsignificantly increase the peak contact pressure of articular surface.Both plate fixation andATMFS fixation could nearly restore the stress distribution of lunate-shaped articularsurface to the intact state,unfortunately still not fully recover.Similarly,screw plus restorelocking plate would fix middle column posterior wall fracture as well as posterior columnand wall fracture.The fixations usually meet the requirement in clinic,but the displacementof middle column posterior wall fracture was significantly larger than that of the posteriorcolumn and wall fracture.Our data indicated that middle column posterior wall was animportant part in the posterior region of acetabulum.Because the middle column posteriorwall of acetabulum bore much stress and involved the roof region,it cannot be regarded asa simple part of the posterior wall of acetabulum.Stress concentration was likely to occurin this region during fracture,so more specific and more stable fixation methods wereneeded in clinic. Part2Three-dimensional finite element analysis on theposterior wall of middle column of acetabulumPurpose:Three-dimensional finite element analysis of human pelvic including the major muscles and ligaments around the hip joint was used to establish the posterior wallof middle column and classic posterior wall fracture model.Then,analyze the stressdistribution of the lunate-shaped articular surface in the middle and posterior columnregion of human acetabulum so as to provide biomechanical reference and support for thestudy of three column theory,especially for the study on middle column posterior region.Method:1)Non-destructive model with upright position and hip flexion at60°,90°and120°as well as a1500N load was applied to the model along the femur axial to study themean pressure,peak pressure,and stress distribution of articular surface.2)Under uprightposition,articular fracture models of different regions,namely posterior wall of middlecolumns(10°-60°)fracture,the conventional posterior wall(40°-90°)fracture accompaniedwith1mm and3mm displacement,were applied in the longitude with1500N load to studythe mean pressure,peak pressure,and stress distribution of the articular surface.3)Anatomical reduction plus plate fixation were applied to the articular fracture models ofposterior wall of middle columns and posterior wall with upright position to analyze themean pressure,peak pressure,stress distribution,and fracture displacement of articularsurface.Anatomical reduction plus ATMFS was applied to the articular fracture models ofposterior wall of middle columns and posterior wall to analyze the average pressure,peakpressure,stress distribution and displacement of the fracture fragments.Result:1)When loading along the axial of the femoral head and the hip flexion was60°,the maximum contact stress of articular surface was1081N,maximum contact pressurewas4.49MPa,stress concentration located at a higher position of the back wall.2)Whenloading vertically along the L2axial,maximum contact stress of intact acetabular articularsurface was989N,maximum contact pressure was3.84MPa;the maximum contact stress ofposterior wall of middle columns(10°-60°)fracture accompanied by1mm stepsdisplacement was1242N,maximum contact pressure was6.37MPa. Maximum contactstress was1427N when step displacement increased to3mm,and maximum contact pressure was13.12MPa.3)After anatomical reduction,posterior wall of middlecolumns(10°-60°)fracture was fixed by plate,and maximum contact stress of articularsurface was1305N,maximum contact pressure was6.12MPa,the maximum displacementof the fracture fragments was2.70mm.Conclusion:In this section,three-dimensional finite element model was used tosimulate the stress conduction of acetabulum posterior fractures in different parts,and tostudy the mechanism of acetabular posterior wall fracture.Furthermore,the fracture modelsof posterior wall of middle column and classic posterior wall were established to analyzetheir impact on the stress distribution of acetabular articular surface.By comparison withthe data and conclusions from the mechanical experiment in the first part,the relativeindependence and the important role of posterior wall of middle column was highlighted. Part3Comparative study of comminuted posterior wallfracture of acetabulum treated with the AcetabularTridimensional Memory Fixation SystemPurpose：The purpose of this study was to compare outcomes between patients whoreceived fixation for comminuted posterior wall fracture using the AcetabularTridimensional Memory Fixation System(ATMFS)and patients who underwent fixationwith conventional screws and buttress plates(Plates group).Method:Between April2003and May2007,196consecutive patients who sustained acomminuted posterior wall fracture of acetabulum were treated with ATMFS orconventional screws and buttress plates.Operative time,fluoroscopy time,blood loss,andany intra-operative complications were recorded.Plain AP and lateral radiographs wereobtained at all visits(Matta’s criteria).Modified Merle d’Aubigne-Postel score,and MosSF-36score were compared between groups.Results:Fifty patients were included in the analysis with26in the ATMFS group and 24in the Plates group.The mean follow-up time was57.5months,ranging from31to69months.All patients had fully healed fractures at the final follow-up.There was nodifference in clinical outcomes or radiological evaluations between groups.Conclusion:Patients with comminuted posterior wall fractures of the acetabulumtreated with the ATMFS or conventional screws and buttress plate techniques achieve agood surgical result.Both techniques are safe,reliable,and practical.Use of the ATMFStechnique may reduce blood loss and improve rigid support to marginal boneimpaction.The use ATMFS may need additional support when fractures involve thesuperior roof.