Mechanical Evaluation And Collapse Prediction Of Onsteonecrosis Of The Femoral Head

Background: Osteonecrosis of the femoral head(ONFH)is a severe bone disease that may induce bone collapse.Early-stage ONFH(ARCO I-II)is commonly suggested for conservative treatment.With progression of the disease,at advanced stages(ACRO III or IV),collapse of the femoral head is occurred and total hip replacement is inevitable.However,joint prostheses seriously affect hip joint function and they are not durable.The effective treatment for joint preservation in the early stages is very important in improving the development of the disease as well as keeping the motility of the hip joint;however,the effective evaluation of the treatment effect as well as the accurate prediction of the risk of collapse are crucial in the early treatment.Currently,evaluation of the treatment effects and prediction of collapse mainly rely on medical scans,which can provide morphologic information of the bone,the perceptions of patients and the experience of the doctors.Thus,the outcomes are partly subjective.In fact,it is difficult to obtain the information which is represented the bone abilities directly.Thus,in the present study,the biomechanical properties of the necrotic bone is studied and the mechanical parameters that can help evaluating the effectiveness of medical treatment and predicting the risk of collapse are proposed.Methods: Patient-specific models on pre-(initial presentation)and post-(6-8 months follow-up)medical treatment are established based on medical scans.The cases are classified by the follow-up outcomes and the stages and then studied to assess the medical treatment effect and to predict the risk of collapse.Patient-specific models are established based on medical scans;then the models are discretized to obtain the finite element models.The density and mechanics properties of the bone are assigned based on spatial-variant HU value.The boundary condition of each model is applied according to the individual dates.The statical models are calculated by finite element analysis and the distributions of the mechanical parameters are obtained.The corresponding relations of mechanical results and the clinical practice are studied to propose the mechanical parameters that are able to assess the medical treatment effect and predict the risk of collapse preliminarily.Results:(1)The evaluation of medical treatment effect: The variation in morphology and mechanics are studied pre-and post-medical treatment.The morphological parameter,such as the volume ratio of the necrotic region,is not consistent with the 1-year follow-up results;while some mechanical parameters,such as the equivalent stress variation at the region close to the Ward triangle,show consistence to the 1-year outcome.Moreover,the relative stress index,which is defined as the ratio between the mean stress index of volume and the mean equivalent stress of volume,is proposed.The stress index is relevant to the bone compressive strength and in the cases,its variation is consistent to the follow-up results.(2)The prediction of the risk of collaspe: The mechanical analysis is conducted in each model pre-and post-collapse.The results show that there is a relative high equivalent stress region on the superior surface of the femur neck in each model of the non-collapse group suggesting a higher risk of fracture across the femoral neck.Moreover,a relative stress index(SD),defined as the ratio between equivalent stress and total deformation,is proposed.By calculating and comparing its difference in non-collapse and collapse models,it is found that in the non-collapse group,SD of each case is within 0.80,even blow 0.30 in the case with anteromedial-located necrosis(out of the weigh-bearing area).However,in the collapse group,the maximum value of SD of each case exceeds 0.90.Thus,we can preliminary conclude that SD=0.80 could be regarded as the reference value in collapse prediction preliminarily.Conclusions: This study implicates that to evaluate the effectiveness of the medical treatment,morphologic change should be considered but mechanical capability of the bone,which is related to the size,density and position of the necrosis,may play a more important role.Moreover,the morphologic change of the necrotic region may be unconspicuous between non-collapse and collapse cases while its mechanical behavior could be different,which could help us predicting the risk of collapse.The relative index and the ratio between the equivalent stress and total deformation is proposed as the mechanical parameters to assess the medical treatment effect and predict the risk of collapse respectively.