A finite element model based on medical image for evaluating biomechanical stability of percutaneous vertebroplasty

Xiang CHEN; Haiyun LI; Xinjian YANG

ABSTRACT

BACKGROUND:

Researches on vitodynamics present that percutaneous vertebroplasty (PVP) can strengthen hardness and intension of single osteoporosis vertebral body after injection of bone cement; however, the infused volume and site of bone cement for maintaining mechanical stability of vertebral body should be further studied.

OBJECTIVE:

A numerical calculation method on finite element models (FEM) for biomechanical analysis has been developed, while a boundary condition describing the relative L1 -L2 displacement is imposed on the FEM to account for three-dimensional physiological states.

DESIGN:

Non-randomized control study.SEITING Beijing Neurosurgical Institute.MATERIALS One female patient aged 54 years with compressibility fracture at lumbar vertebrae L1-L2 induced by osteoporosis was diagnosed with CT examination, and the patient was told the fact. Based on CT scanning before and after PVP, 90 serial sections with the thickness of 1 mm were obtained and the size of each pixei was 0.33 mm.

METHODS:

The experiment was carried out Beijing Neurosurgery Institute from October 2005 to June 2006. ①Establishment of three-dimensional FEM Integrating the anatomical structure from the spine CT and MRI image of a patient, a novel three-dimensional geometric model of lumbar functional spinal units (FSUs) has been built. Meanwhile, 90 serial sections were obtained to exchange data of CT sections and divide imagings. Based on the geometric model, two kinds of three-dimensional FEM of L1-L2 segments for preoperative and postoperative vertebrae were created. ② Model evaluation The lcad of (500 N, 1 000 N, 1 500 N, 2 000 N, 2 500 N) axial compression were applied to the superior surface of the model in the form of a uniformly concentrated lcad over all L1 superior surface nodes respectively. We could observe the stress distribution of L1-L2 segment by applying the load and clue on the high stress concentration region as the most likely areas fracture.MAIN OUTCOME

MEASURES:

① Changes of displacement, stress and strain distributions of FEM at L1-L2 segment of lumbar vertebrae under various loads; ② Effect of increase of bone cement volume on displacement, stress and strain distribution.

RESULTS:

① The increase in displacement, stress and strain of FE model with the increase of loading in the postoperarive cases. The tendency was approximately linear which also illustrates the spine have flexible biomechanical characteristics. The region was a common place for injures due to loading. The magnitude of stress in the intervertebral disc depended on the proportion of load applied to the superior surface of the motion segment. The heavier the lcad was, the stronger the stress was. ② With the increase of stress on intervertebral disc, the increasing volume of bone cement could induce transfusion of load of nearby vertebral body.

CONCLUSION:

Building three-dimensional FEM of L1-L2 segments for preoperative and postoperative PVP can explain three-dimensional physiological status of vertebral body based on calculating relative shift of L1-L2 segment of lumbar vertebrae. Meanwhile, simulative calculation can clearly express distribution of stain and stress and preoperative and postoperative deformity of vertebral body.