Balloon Inlfating And Cement Filling In Treatment Of Avascular Necrosis Of Femoral Head: An Experimental Study

Background:Avascular necrosis of femoral head refers to the pathological process of boneactive ingredient loss due to destruction in blood supply or bone cells degeneration. Bothtraumatic and none traumatic causes share the same basic pathological process of necrosis ofcancellous bone and subchondral bone, fibrous repaired followed by mechanical strengthdeclination. At first, there's no obvious bone structure and mechanical property changing, butthe mechanical property of the area of necrosis is poor, cannot load and transfer stress, leadingto the collapse of femoral head. The femoral head would collapse in80%patients during1to3years if not effectively treated. Some severed patients would have to be operated by total hipreplacement. The complications of THR occurred in young patients are much higher thanothers and the long-term effect is poor. The femoral head reserved surgery can prevent or retaincollapse, avoid or delay the artificial joint replacement.There are many different head reserved surgeries. Blood supply can be improved in earlystages by decreasing the pressure inside the femoral head, self-repairing in the femoral necroticarea. Studies suggest that the mechanical property of the necrotic area in the femoral headdecreasing would be the main reason for the collapse. It's assumed that transfer of bone graft tofemoral subchondral area can support the head to prevent or retain collapse. But bone graftalone can't support femoral head immediately due to poor mechanical properties. Transfer ofvascular pedicle bone flap needs high microsurgical technique and the complications at donorarea are high. Porous tantalum rod implantation is not effective for large and multiple lesions,hormones induced necrosis or ANFH combined with chronic diseases. In brief, all kinds ofhead reserved surgeries have different deficiencies.Bone cement or bone graft substitutes filling in the defects of osteonecrosis area is a kindof treatment. Fundamental techniques includes debridement of necrotic tissue, filling with bone cement or substitutes and reduction of cartilage surface collapse.Reconstruction of thefemoral head spherical contour and increasing the intensity of the necrotic area are assumed toavoid collapse again.The mechanism of the treatment is unclear yet. Kyphoplasty canreducethe heigh of compressed vertebral body by balloon inflating percutaneously and fill thecave with bone cement under low pressure. It is widely accepted and used for it's amini-invasive process which can relief pain safely and easy to manipulate. It is assumedkyphoplasty technique can be used to treat ANFH. Balloon can be inserted into necrotic area ofthe femoral head to reduce collapsed head by inflating and cause local cave. Then bone cementor bone graft substitutes can be filled into the cave to support subchondral bone effectively.It's assumed that mechanical strength of the necrotic area may increase and mechanicalproperties may be improved so that the collapse of the femoral head may be avoided orpostponed.Objective: To evaluate the effect of balloon inflating and cement filling in treatment ofavauscular necrosis of femoral head. Establish a three-dimensional computer simulated modeland use finite element analysis techniques to simulate load and calculate the changes of stresspre-and postoperation. To verify finite element analysis by biomechanical testing of femoralhead specimens.Methods:1. Establish a three-dimensional finite element analysis model of the balloon inflatingand bone cement filling procedure in the treatment of avascular necrosis. Select freshspecimens of femoral head necrosis of human, simulate percutaneous puncture and ballooninfalting procedure under C-arm fluoroscopy, after entering the necrotic area, inflate theballoon and fill in the bone cement. Use dual sources spiral CT thin-layer scan and collect allscanned images, apply computer aid technique to establish balloon inflating and bone cementfilling procedure and avascular necrosis three-dimensional finite element model.2. Analyze physiological load using three-dimensional finite element model, simulatingload and calculating stress, and applying the peak load force of the hip during flat groundwalking. Apply finite element analysis on the avascular necrosis model and balloon augmentbone cement filling model to measure the Von-Mises force of the top, neck and the weight bearing area of the femoral head, in order to compare the alternation of force before and afterthe filling of bone cement.3. Use biomechanical tester to record the displacement of the femoral head specimen atthe loads100N,200N,300N,400N,500N,600N and700N, compare the displacement ofthe two models (necrosis model and bone cement model) under control study. Analyze thestiffness chages of the femoral head and verify the result of the finite element analysis.The study applies non-parametric rank sum examination in the inspection of the load resulton different femoral head areas collected by finite element analysis model. Paired Texamination is applied in the inspection of biomechanic testing results of the femoral headstiffness. Statistical software uses SPSS17.0in data analysis, p <0.05represents statisticaldifference of the two groups, p <0.01represents significant difference.Results:1. After bone cement filled in the necrosis area, the load reduced significantly on theweight bearing area of the femoral head. The reduction in the load alternated the mechanicalstructure of the femoral head that promotes repair of surrounding bone tissue. This lowers thecollapse rate of the femoral head and provides theoretical support for the balloon augment bonecement filling procedure in the treatment of avascular necrosis.2. The filled bone cement shares higher stiffness than the surrounding cancellous bone, sothe loading force concentrates on the bone cement portion. The bone cement is proven toeffectively bear load, the support force of the femoral head is reinforced by the load bearing ofthe bone cement.3. The maximum movement range of the femoral head increased after the filling of bonecement that proves the incensement effective load bearing area. The reduction of stress per unitarea could also be responsible for the load declination of the weight bearing area.4. The standard deviation of the bone cement model and the stress load of the femoral neckare significantly lower that the necrosis model that implies the bone cement model shares lessstress dispersion degree and more stress uniformity than the necrosis model. Test the stressload of the femoral neck of the bone cement model and the necrosis model in4divisions. Theresult reflected less stress on the inner side of the femoral neck and more stress on the outer side of the femoral neck. Hence, the load distribution became more uniformed on the femoralneck after bone cement filling that reduces the force concentration on the femoral neck.5. Both the bone cement model and the necrosis model display similar linear change underload-displacement examination of the femoral head. Comparing the results of finite elementanalysis and bio-mechanical testing, conclusion can be drawn that both methods could displaythe similar linear change of the bone cement model and the necrosis model under various loads.Also, the study discovered the bone cement model shares less displacement that the necrosismodel under the same load condition. This proves that the stiffness and the anti-deformationability of the necrosis femoral head increases after bone cement filling. On the other hand, thesimilarity of the linear changes verifyed that finite element model could highly simulatebiomechanical examination of human specimens.Conclusions:1. Percutaneous balloon inflating and bone cement filling of the necrosis area in femoralhead can carry load effectively, reduce load of the weight-loading area, disperse the stress onthe surface, improve supporting capacity and prevent collapse progressing.2. Percutaneous balloon inflating and bone cement filling in the necrosis area of femoralhead can diverse the stress on femoral neck surface, reduce risks of neck fracture complicationsin theoretically.3. Percutaneous balloon inflating and bone cement filling in the necrosis area of femoralhead can increase stiffness of femoral head, increase the support capacity under load.4. Finite element model could simulate biomechanical changes of percutaneous ballooninflating and bone cement filling procedure accurately.In brief, percutaneous balloon inflating and bone cement filling in the necrosis area offemoral head can change the biomechanics mechanism of femoral head and neck, improvesupporting capacity under load, resist and prevent head collapse progressing.