A finite element study on intra-operative corrective forces and evaluation of screw density in scoliosis surgeries

Abstract

Scoliosis is an abnormal sideways curvature of the spine and rib cage, which may need surgical treatments. Most of the corrective maneuvers in scoliosis surgeries are based on surgeon's experience; hence, there is great interest of understanding how the correction ratio can be influenced by the magnitude of forces and moments. Therefore, the objective of this study was to develop and validate a detailed finite element model of the thoracolumbar which can be used to simulate the scoliosis surgeries based on patient-specific clinical images. The validated models of five patients were carefully developed, and the surgery procedures were simulated and the corrective forces were estimated using inverse finite element analysis during the surgery. Furthermore, parametric studies including the influences of the corrective force magnitude and screw density were evaluated. The results showed that the maximum estimated correction force and moment were 173 (655.43) N and 10.67 (62.02) N m, respectively, which were aligned with measured clinical observations. The sensitivity analysis on the magnitude of applied force to the screws showed that correction ratio was slightly increased in level 1 (i.e. F B = 1.3 3 F) but decreased in level 2 (i.e. F B = 1.6 3 F). In addition, the parametric study on increasing the number of pedicle screws showed that there was no significant difference between lower and higher screw density. However, the stress distribution was significantly greater using higher screw density during correction maneuvers. In conclusion, this study shows a direct relationship between the applied force/moment and screw density and the correction ratio up to a border line which should be defined accurately. This detailed computational modeling can be used in clinic in hope of achieving the optimum outcome of scoliosis surgery using individual patient-specific characterization.

Computational modelling of the scoliotic spine: A literature review

“…In addition to the effect density has, the choice of the level at which to attach the screws 96,98,146 and the anterior–posterior position 71,106 played an important role in the stresses and corrections. Increases in correction force was shown to lead to greater Cobb angle correction, 95 until a certain point, when adjacent end plate come into contact 84,86 …”

“…Several studies 69 , 70 , 86 , 98 , 139 , 146 have simulated the surgical correction of scoliosis to investigate the effect of increasing screw density, concluding that increasing the number of screws did not necessarily improve the correction. The effect of screw density depended on the side of the curve to which it is applied.…”

“…Increases in correction force was shown to lead to greater Cobb angle correction, 95 until a certain point, when adjacent end plate come into contact. 84 , 86 …”

Finite Element Method for the Evaluation of the Human Spine: A Literature Overview

“…Scoliosis is a pathological lateral curvature of the spine, while vertebral bodies are in rotation in the transverse plane (Figure 3). Most scoliosis cases are characterized as idiopathic, while others can be caused by neuromuscular diseases, trauma or congenital conditions [10]. Scoliosis is defined as a curvature of the spine in the coronal plane of more than 10 • [11].…”

The biomechanical study of cervical spine: A Finite Element Analysis

“…18–21 Also, the FE models helps to find the intervertebral disc mechanics, injury mechanism and contributes in the diagnosis of cervical spine problems and treatment methodologies. 22–25…”