Biomechanical analysis of correction force and Cobb angle in a simple model of scoliotic spine fixation

Scoliosis is a medical condition in which a person’s spine has a sideways curve. Treatment to reduce the scoliosis depends on the degree of curve, location, and causes. Surgery is commonly recommended by orthopedists for curves with a high progression by installing instruments that consist of pedicle screws, rods, and connectors. However, many cases of failure both in the implant instruments and the interface of bone and pedicle screw were found caused by high corrective force. The bigger Cobb angle directly means the increase of correction force, which acts on bone-implant interface during scoliosis surgery. In this paper, estimation of corrective forces during scoliosis fixation are investigated using Finite-element analysis (FEA). The research is carried out by modeling a normal and a scoliotic spine with specific Cobb 50.43 degrees. The forces are applied in various numbers of pedicles screws that implanted in thoracic spine, i.e single, three and five pairs of screws. It is found in numerical simulation that the total forces that are needed to fix the scoliotic spine are almost equal for five, three, and five pedicle screws in thoracic spine. However, the maximum force for each screw will increase significantly by reducing the number of screws. The biggest correction force for 5 screws is 54.5 N in the apical section, while it is 218 N for single screws. The higher force applied to a pedicle screw, the higher possibility to get failure and to be pulled out from the bone. It is needed to find the optimal number of using pedicle screws based of the working force, stress and implant cost.