Biomechanical Study Of Pelvic Girdle Reconstruction After Type Ⅰ Resection Of Iliac Tumor

Background:Pelvic reconstruction after bone tumor resection is challenging for orthopedic surgeon.The reason is that the biomechanics of the resected and reconstructed pelvis is not clear.Ilium is the most commonly involved site of pelvic bone tumor.The stability and stress distribution of the pelvis after iliac subtotal resection according to Ennecking's typeⅠresection,the stability and stress distribution of the pelves after reconstruction with different methods,and whether the different reconstructive methods can afford stability for patents' earlier rehabilitation and earlier ambulation after surgery are the urgent problems to be solved in clinics.Objective:To analyze biomechanics of pelvic girdle reconstruction after iliac tumor resection and to evaiuate the stability and stress distribution of the pelves reconstructed with seven different methods compared with intact pelves,also to offer theoretical support and referring to reconstruct pelvic girdle after iliac tumor resection.Methods:biomechanics including the stability and stress/strain of the resected pelvis and reconstructed pelvis was analyzed through the experimental study and finite element method(FEM).First,six cadaveric specimens from the third lumbar vertebra to the proximal 1/3 part of femur were tested.The iliac subtotal resection models were established according to Ennecking's typeⅠresection.Then the resected pelves were reconstructed with the cement,or with double-fibula transplantation fixed with screw-rod system(double-rod or single-rod)or screw-plate system.Markers were affixed to the key region of the pelves.Axial loading from the proximal lumbar was applied by MTS in the gradient of ON to 500N,which simulated the double feet standing state.Images in front and lateral views were obtained using CCD(Charge Coupled Device).Based on the Imagej software,displacements of the inner parts of pelves were calculated in multiple views,multiple points and multiple angles, applying digital speckle image processing techniques such as digital marker tracing method using center-of-mass algorithm and digital image fringe detection method. Simultaneously,the surficial strain was detected using 6 strain gauges attached to the key regions of pelves including the site of lateral sacral mass,iliac arcuate line and pubic symphysis.The pelvic strain results were analyzed in different operating conditions.Statistical analyses of the experimental data were performed with SPSS 10.0 software(SPSS Inc,USA).Second,a set of CT images of normal 42 years old male from the third lumbar to proximal 1/3 femur were selected.Intact and impaired pelvic finite element models were swiftly constructed by means of immediately building nodes and elements with pixel of original CT image.At the same time,various reconstructed pelvic finite element models were reconstructed by the combination the impaired pelvic models with different kinds of three-dimensional finite element models of the implanting devices,which were constructed using ways of solid modeling.Relevant load was set on all pelvic models in terms of the double feet standing state.Then the magnitude of stress and displacement on all nodes and elements in the pelvic finite element models of intact and reconstructed state were figured out.Pelvic stress distribution and vertical displacement of sacrum were calculated to analyze the difference among all the reconstructed pelvic models,compared to intact pelvic model.Third,pelvic displacement and strain/stress results obtained from the experimental test and FEM analyses were compared and validated from each other.Fourth,the concept of the new method of optical measurement,digital speckle image processing techniques was illustrated,the techniques including digital marker tracing method using center-of-mass algorithm and digital image fringe diction method,which were used in our biomechanical experiment.Based on imagej software,digital 8-bit image obtained from CCD were processed.The procedure includes the recognition of digital marker,image invert,sub-pixel reconstruction, image segmentation,center-of-mass algorithm based on weighted average of pixel gray values and the marker displacement calculation.The precision and error of digital marker tracing method using center-of-mass algorithm,which was adopt in our experiment,were illustrated.Digital image fringe diction method to trace the angle displacement of the inner part of pelvis was also introduced. Results:First,biomechanical experiment showed that the intact pelvis is the most stable condition compared to the reconstructed condition according to the capability of pelvis resisting compression in axial direction;pelvic rotation on sagittal and coronal plane.Pelvis with iliac defection with or without fibular transplantation is more unstable and is supposed to strengthen the pelvic stability with internal implants.The stability is improved in some degree if the defect pelvis were reconstructed using fibular transplantation strengthened with internal implants.Double-rod fixation is more stable than the single-rod fixation and plate-screw fixation and fixation with screw-plate or S1R(single rod fixation to the S1 vertebra and iliac neck)method is more stable than the fixation with L5S1VR(vertical rod fixed to lateral L5/S1 vertebral arches and iliac neck fixation)method and L5R(rod fixed to bilateral L5 vertebral arches and iliac neck fixation)method.The stability of the single compressed rod/plate is correlated to the length and primary bending angle of the rod/plate.It is less stable if the rod/plate is too long or the primary bending angle is too big.Pelvic reconstruction with cement is better in the capability of anti-rotation in sagittal plane than any other reconstructive method.But it is little worse than the fibular transplantation fixed with double-rods method.The results of strain gauge detection showed as following:it is more evenly distribute in the intact pelvis according the different sites of the pelves.The strain value is much letter than that in any other reconstructed pelvis.After the subtotal ilium resected the remarkable change is the strain value on the site of symphysis,compressed strain altering to tension strain.In addition the strain values on the sites of lateral mass and iliac arcuate line are enlarged,which are the sites of stress concentration in the resected pelves.After fibular transplantation,the pelvis is closed and the function of pelvic stress distribution is resumed.But stress concentration in normal lateral site of pelvis is not changed.When the implant device is introduced to strength the stability of the reconstructed pelvis,stress concentration in normal lateral site of pelvis is improved in some extent.But it can not recover to the normal stain/stress condition in intact pelvis.In the fibular transplantation methods with internal fixation,pelvic reconstruction with double-rod system and plate system is better than that with single-rod system according to the strain value on the sites of normal lateral pelvis (p＜0.05).There has no statistical difference(p＞0.05)in strain values between the method of cement reconstruction and the intact pelvis.Second,A three-dimensional finite element model of normal human pelvis was establish,including lumbar,sacrum,acetabulum and proximal femur.The finite element model of normal human pelvis could be divided into 207248 nodes and 721820 units.The number of the nodes and units exceeded that in the other former pelvic finite element models.The FEM analyses showed as following:From various factors,such as stress distribution of reconstructed pelvis,Von-mise stress of the internal implants,the total capability of pelvis resisting compression in axial direction, etc,we can find that reconstruction of pelvis with the method of cement and the method of fibular transplantation strengthened by two-rod screw system are closer to intact pelvis.The reconstructive method with double rods is better than single rod or plate method,while reconstruction only using fibula have the remarkable difference toward the intact pelvis.Third,pelvic displacement and strain/stress results obtained from the experimental test and FEM analyses are comparable and matchable from each other.Fourth,the results of the digital speckle image processing techniques showed as following,according to pelvic displacement measurement using digital marker tracing method with center-of- mass algorithm in our biomechanical experiment,there is an excellent correlation between the sample speckle image and target speckle image. The correlation coefficient is about 0.983.The precision of the displacement detected in our experiment is about 0.018 pixels.The load-displacement curves obtained from the method of digital marker tracing and digital image fringe detection matched up to the results in clinics.Conclusion:This research primarily illustrated the biomechanical mechanism of the pelvic girdle reconstruction after iliac tumor resection,answered the questions why the pelvic girdle should be reconstructed,how to select the appropriate reconstructive method and gave the tips that should be paid attention after subtotal resection of ilium. Also the research provided theoretical support and referring in biomechanical aspect. After resection of the subtotal ilium due to the iliac tumor,stability and stress distribution of the pelvis alter to worse.Reconstruction should be introduced to maintain the normal stress transmitting function of the pelvis.According to the distribution and the stability of the reconstructed pelvis,reconstruction of pelvis with the method of cement and the method of fibular transplantation strengthened by two-rod screw system are the good choices.Double-rod internal fixation system is better than single-rod system and plate-screw system,which can afford the stability for patents' earlier rehabilitation and earlier ambulation after surgery.To some extent,the three-dimensional finite element models of intact and Various reconstructed pelvis set up in the project have reliability and availability. Experimental test and FEM analyses are comparable and complementary to each other.Digital speckle image processing techniques have good perspective for application in biomechanical research,which can avoid the complex process of computing the correlation coefficient in digital speckle correlation method.