Mechanical Evaluations of Instruments after Total en Bloc Spondylectomy

Abstract

Vertebrae are typical regions where metastasis of bone cancer is apt to occur. Total en bloc spondylectomy has been performed to remove a lumbar vertebra with a tumor. In this operation, the vertebra is replaced by an artificial vertebra fixed to higher and lower vertebrae by instruments from the back. Mechanical stress and deformation of five lumbar vertebrae are analyzed for case of a preoperation and a postoperation by using special FEM for a multilayer structure in this study. We consider two loading conditions corresponded to upright pose and stooping pose to the lumbar vertebrae models. An equivalent distributed load on the top surface of the first lumbar vertebra is giver for the upright pose. A front inclined load is given for the stooping pose. The main results obtained as follows. (i) Mechanical stability is improved in the postoperation case. (ii) A Higher stress part suggested from the FEM result agrees with a part possessing a possibility of fracturing clinically. (iii) One cause of loosening of the top screw is the stress variation with change of the pose between an upright and a stooping. (iv) The top and the bottom screws may break due to fatigue.