| Objective The replacement of custom-made tumor prosthesis has become an important method for reconstruction after bone tumor resection.With the increasing numbers of custom-made tumor prosthesis and its prolongation of use age,complications such as prosthesis fracture are observed.The revision operation of bone-cement custom-made prosthesis is very difficult.We designed sleeve revision prosthesis for those which the extramedullary site was broken,the body part had sufficient length and the intramedullary stem was intact and fixed firmly by bone cement.In this study,we evaluated its application value and clinical effect and discussed the reasons for fracture of custom-made tumor artificial hip prosthesis.Methods The revision surgery was taken after epidural anesthesia,with an incision about 1 / 2 or 1 / 3 of the original incision,only exposed the fracture site.Remove the broken part of prosthesis,then connected the residual extramedullary part of former prosthesis to the sleeve revision prosthesis by bone cement and locked with screws,then closed the wound over suction drainage tubes.After operation,antibiotics were used for three days and subcutaneous injection of low-molecular-weight heparin 4000 IU for 7-10 days,on the second day,removed the drainage and began to functional exercises,then made follow-up.Result Made a follow-up for 31 months,there were no incision infection, no prosthesis loosening or broken,no leg-length inequality and other complications,there was no tumor recurrence and metastasis as well.The Enneking score was 1 point preoperatively,and 26 point postoperatively. The Enneking score was 25 points at the time of final follow-up.Conclusion There are many reasons for fracture of custom-made tumor prosthesis,such as the stress distribution of prosthesis,characteristics of replacement surgery,surgical procedures,complications and problems of prosthesis itself.The stress concentration site of the prosthesis should be strengthened,and its design,materials and elaboration should be more reasonable.The patients should control weight and avoid injury; while doctors should improve surgical techniques,adopt the correct procedure to;put prosthesis in right position,avoid complication as prosthetic loosening.The sleeve revision prosthesis has higher application value in its indication,but it also need further follow-up. Objective Based on CAD,CAE and digital image processing techniques,the information contained in Computed Tomography scan data is utilized to rebuild 3-D geometric shape of femur and femoral part of custom-made tumor artificial hip prosthesis,then analyze stress of femur,prosthesis' femoral part and bone cement then to provide rational suggestions for clinical treatment,and to establish 3-D FE model to prepare optimizing the sleeve revision prosthesis.Methods 1.Breaking through the traditional means that digitize CT images by scanning CT slices,instead,CT images of femur will be unbrokenly transferred into PC through network based on the standard DICOM 3.0(Digital Imaging and Communication in Medicine),so precisely data transfer can be realized.2.Utilizing Mimics image process software,contours recognition and conversion of vectors of CT images are processed to generate femur features contours.Then in terms of B-spline skin theory,with the Unigraphics software,we build femur B-surface according contour line of cross section, and complete re-establish of 3-D femur,and get geometric parameters exactly,thus 3-D hip prostheses and bone-cement mantle will be built on the basis of that.3.3-D finite element models of femur,hip prostheses,bone-cement mantle will be created with the help of analytical software Marc.Then boundary conditions of displacements and loads are applied on them,followed by material parameters setting.Finally,the stress and the displacement on the models will be analyzed.Results 1.In the femur finite element model,there are two stress concentration areas,one appeared on the superior,inferior part of the conjunction of femoral neck and shaft,the other appeared bilateral part of distal 1/3 femoral shaft.The maximum stress of femoral neck concentrated on the region above lesser trochanter and below femoral neck,the value is 23.3MP.The maximum stress of the femoral shaft concentrated on the border of the middle and distal third of the lateral shaft,the value is 61.35MP.2.In the custom-made tumor prosthesis finite element model,there are three stress concentration areas appearing on the conjunction of the femoral neck and the extramedullary stem,the conjunction the extramedullary and intramedullary stem,and the conjunction of middle and distal third of the intramedullary stem.The maximum stress values are 137.1MP,102.4 MP, 66.35MP respectively.3.The stress distribution of interface between bone cement and intramedullary stem is as below.Inferior:there is a stress-concentration peak in the proximal end,then the stress becomes lower gradually to the distal end,and becomes higher at the middle and distal 1/3,reaches the second peak value in the correlative area with distal end of intramedullary stem,the maximum is 6.78 MPa in proximal end and 4.68 MPa in distal end.Exterior:the stress increases gradually from proximal to distal end and reaches peak value in the correlative area with distal prosthesis stem, then decreases in the distal end.The maximum is 10.65 MPa in the distal end.The stress distribution of interface between bone cement and femur is as below.Interior:there is a stress-concentration peak in the proximal end,then the stress becomes lower,and becomes higher at the middle and distal 1/3,the second peak value occurs in the correlative area with distal end of intramedullary stem,the maximum is 3.68 MPa in proximal end and 2.96 MPa in distal end.Exterior:the stress increases gradually from proximal to distal end and reaches peak value in the correlative area with distal prosthesis stem,then decreases in the distal end.The maximum is 4.98 MPa in the distal end.Conclusion 1.Both the femoral neck and the femoral shaft are main stress-concentration and stress-distribution areas,it is obvious at superior and inferior part of femoral neck and at femoral shaft in the lower 1 / 3 inside and outside.The latter is higher than stress at femoral neck.2.The bone trabecula and calear femorale above lesser trochanter are the main load-bearing constructions.Load mainly spreads from femoral neck down to the distal 1/3 through compressive trabecula and calear femorale.Calear femorale plays an important role in many facts such as the stress distribution,fracture fixation,and prosthetic replacement.3.In the custom-made tumor prosthesis,stress concentration of femoral part appears in areas as below:the conjunction of the femoral neck and the extramedullary stem,the conjunction of the extramedullary and intramedullary stem,and the conjunction of middle and distal third of the intramedullary stem,the maximum occurred in the conjunction of the femoral neck and the extramedullary stem.According to the stress distribution, consideration about impact to length and diameter should be given more in the prosthesis design.In view of a abnormal stress value in femoral neck, attention must also be paid to the smooth transition from neck to the stem. The stress distribution of custom-made tumor prosthesis is similar but also different to that of ordinary prosthesis,and these can be explained by engineering principles.4.The stress of interface between bone cement and intramedullary stem of custom-made tumor prosthesis concentrate in proximal and distal part of inferior interface and distal part of exterior interface.The maximum is the last one.In these areas stess should be put down and bone cement should be reinforced.The stress of interface between bone cement and femur are low,debonding and loosening of cement-type prosthesis are unrelated to the stress basically. 5.The average values of displacement in X and Y direction are much higher than that in Z direction,so the femur deformation mainly concentrates in the XOY plane,and it is most obvious in the femoral head and the proximal femoral shaft.6.Through distribution comparison of overall stress and displacement between two models,we find that the prosthesis model has better consistency with the femoral model which indicates that the design of the prosthesis meets with biomechanical requirements. Objective The recent effect of sleeve revision prosthesis is satisfactory,but the correlative biomechanical study are still rare,it still has some defects such as no clear specifications,so we often have to depend on clinical experiences to decide the length and diameter of prosthesis.In order to evaluate the biomechanical properties of sleeve revision prosthesis and its impact to original prosthesis system,and to optimize the specifications of the prosthesis,we conducted a three dimensional limited element analysis.Methods A fracture model at the conjunction of the neck and the extramedullary stem of femoral part of a custom-made artificial hip prosthesis is established on the basis of original 3D finite element model(see Part two),a model of the sleeve revision prosthesis after application is also established,divide groups according the length of sleeve revision prosthesis into 50 mm,70mm,90mm and 100mm;divide groups according the bone cement between the sleeve revision prosthesis and the extramedullary stem into 3mm,5mm and 7mm,then combined the length and cement each other.Then boundary conditions of displacements and loads are applied on them,followed by material parameters setting.Finally,the stress and the displacement on the models will be analyzed.Result That are the key points we observed in the study:the stress distribution of the exterior and interior interface between bone cement and extramedullary stem of the original prosthesis,and between bone cement and sleeve revision prosthesis.The results are as below:1.The stress distribution of the interior interface of bone cement and extramedullary site is:there is a moderate stress-concentration region in the proximal end,then the stress becomes lower,and the stress becomes higher at the conjunction of the middle 1 / 3 and distal 1 / 3,the highest stress occurs in the distal end,when the length is 50 mm,70mm and 90mm correlatively,the peak value are 34.4943 MPa,30.5537MPa,17.263MPa respectively with the thickness of bone cement 3mm,and 26.5352 MPa,20.3719MPa,18.0981MPa with bone cement 5 mm,and 33.08 MPa,25.897MPa,22.739MPa with bone cement 7mm.When the length of sleeve revision prosthesis is 100mm,there is no peak value;the values in the distal end are lower than moderate stress-concentration values in the proximal end,all about 10 MPa.2.The stress distribution of the exterior interface of bone cement and extramedullary site is:when the length are 50 mm,70mm and 90mm respectively, the stress increases from proximal to distal end and reaches peak value in the distal end.The highest value are 33.4261MPa,15.3311MPa,8.22237MPa correlatively with bone cement 3mm;20.2884MPa,19.3152MPa,12.7398MPa with bone cement 5mm; and 20.1455MPa,18.5169MPa,16.4955MPa with bone cement 7mm。The stress distribution of prosthesis with 100mm length is stable,there is no obvious peak value,and all its values are below 6 MPa.3.The stress distribution of the interior interface of bone cement and sleeve revision prosthesis is:the stress reaches peak value in the proximal end.When the length are 50mm,70mm,90mm,100mm respectively,the highest value are 23.2085MPa,21.3257MPa,34.9706MPa,26.8445MPa correlatively with bone cement 3mm;24.8927MPa,35.995MPa,25.0992MPa,22.2198MPa with bone cement 5mm; 30.3749MPa,26.6358MPa,26.5053MPa,27.837MPa with bone cement 7mm,then stress became lower and reached a minimum in the distal end.4.The stress distribution of the exterior interface of bone cement and sleeve revision prosthesis is relatively gentle from proximal to the distal end,and all the values are less than 8 MPa.It is more significant with bone cement thickness of 5 mm.5.The stress concentration occurs at the conjunction of neck and body of the prosthesis,mainly concentrates above and below the neck part,the maximum is below the neck,135.376 MPa.The overall stress of body is low. 6.The peak stress of lateral femur and intramedullary stem were lower than preoperation.The rest stress distribution of femur and the original prosthesis has no significant changes.Conclusion 1.The sleeve revision prosthesis also has stress concentration as other femoral prosthesis because of the deformation of junction of neck and body. It also need increased reverse arc in order to avoid fracture.The overall stress of the body is low.2.The application of sleeve revision prosthesis diminished peak stess of lateral femur and intramedullary stem and reduce probability of fracture of fumer and prosthesis.Bone cement between extramedullary stem and sleeve revision prosthesis buffering the bend stress may cause this phenomenon,medial stess of femur and intramedullary stem of original femoral prosthesis had no significant change.This is related to these facts as below:sleeve revision prosthesis dose not change general form of the entire femoral prosthesis system,its own weight is much lower than the strength of loading.3.Taken the minimum stress between prosthesis and bone cement as standard,the combination of length 100mm and bone cement thickness 5mm is the best specifications of prosthesis.At the same time,as there are still problems that stress maybe higher than fatigue tolerance of bone cement,so using lock screws to scatter stress is necessary.In the region surpassing fatigue tolerance of bone cement,we can increase its strength by other means such as bone cement enhancer,and reduce the risk of fragmentation and debond.
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