Effect of osteoporosis on internal fixation after spinal osteotomy: A finite element analysis

Severe kyphotic deformity can affect the quality of life of the elderly and is commonly treated by an osteotomy. Considering that the elderly often suffer from osteoporosis, the safety and efficacy of internal fixation are particularly important. The aim of this study was to analyse the effect of osteoporosis on internal fixation after spinal osteotomy.

One patient with a thoracolumbar kyphotic deformity who underwent spinal osteotomy was included. The CT images of the entire spine were used to construct a finite element model of the spine internal fixation after osteotomy. Material parameters were assigned to osteoporosis and normal bone groups, and the loads were used to simulate different working conditions, including axial compression, flexion, extension and lateral bending.

Compared with normal bone mass, the pressure on osteotomized vertebrae was reduced by 8.32%, 1.92%, 36.79% and 79.80% in mild osteoporosis model during axial compression, flexion, extension and lateral bending, respectively. The pressure on screws and rods was increased in an osteoporosis model under axial compression. During flexion and lateral bending, the pressure on screws was increased but was decreased on rods. The opposite result was found during extension. With the degree of osteoporosis increases, the change of stress is more obvious.

Under different bone mass conditions, the distribution patterns of stress in vertebrae, screws and rods were relatively similar. Collectively, the stress levels of vertebral bone were decreased and the stress levels of the screw/rod system were increased in an osteoporosis model compared to a normal bone model. Hence, osteoporosis may increase the risk of fracture and internal fixation failure.