Relationship of forces acting on implant rods and degree of scoliosis correction

Background

Adolescent idiopathic scoliosis is a complex spinal pathology characterized as a three-dimensional spine deformity combined with vertebral rotation. Various surgical techniques for correction of severe scoliotic deformity have evolved and became more advanced in applying the corrective forces. The objective of this study was to investigate the relationship between corrective forces acting on deformed rods and degree of scoliosis correction.

Methods

Implant rod geometries of six adolescent idiopathic scoliosis patients were measured before and after surgery. An elasto-plastic finite element model of the implant rod before surgery was reconstructed for each patient. An inverse method based on Finite Element Analysis was used to apply forces to the implant rod model such that it was deformed the same after surgery. Relationship between the magnitude of corrective forces and degree of correction expressed as change of Cobb angle was evaluated. The effects of screw configuration on the corrective forces were also investigated.

Findings

Corrective forces acting on rods and degree of correction were not correlated. Increase in number of implant screws tended to decrease the magnitude of corrective forces but did not provide higher degree of correction. Although greater correction was achieved with higher screw density, the forces increased at some level.

Interpretation

The biomechanics of scoliosis correction is not only dependent to the corrective forces acting on implant rods but also associated with various parameters such as screw placement configuration and spine stiffness. Considering the magnitude of forces, increasing screw density is not guaranteed as the safest surgical strategy.     

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Ohne Zusammenfassung     

Corrective force analysis for scoliosis from implant rod deformation

Background

Scoliosis is a serious disease in which a human spine is abnormally deformed in three dimensions with vertebral rotation. Surgical treatment is attained when the scoliotic spine is corrected into its normal shape by implant rods and screws fixed into the vertebrae. The three-dimensional corrective forces acting at the screws deformed the implant rod during the surgical treatment of scoliosis. The objective of this study was to propose a method to analyze the three-dimensional forces acting at the rod using the changes of implant rod geometry before and after the surgical treatment.

Methods

An inverse method based on Finite Element Analysis is proposed. The geometries of implant rod before and after the surgical treatment were measured three dimensionally. The implant rod before the surgical treatment was reconstructed using an elasto-plastic finite element model. The three-dimensional forces were applied iteratively to the rod through the screws such that the rod is deformed the same after the surgical treatment of scoliosis.

Findings

The maximum force acting at the screw of each patient ranged from 198 N to 439 N. The magnitude of forces was clinically acceptable. The maximum forces occurred at the lowest fixation level of vertebra of each patient.

Interpretation

The three-dimensional forces distribution that deformed the rod can be evaluated using the changes of implant geometry. Although the current clinical cases are still few, this study demonstrated the feasibility of measuring the forces that deformed the implant rod after the surgical treatment of scoliosis.     

Influence of implant rod curvature on sagittal correction of scoliosis deformity

Background contextDeformation of in vivo–implanted rods could alter the scoliosis sagittal correction. To our knowledge, no previous authors have investigated the influence of implanted-rod deformation on the sagittal deformity correction during scoliosis surgery.PurposeTo analyze the changes of the implant rod's angle of curvature during surgery and establish its influence on sagittal correction of scoliosis deformity.Study designA retrospective analysis of the preoperative and postoperative implant rod geometry and angle of curvature was conducted.Patient sampleTwenty adolescent idiopathic scoliosis patients underwent surgery. Average age at the time of operation was 14 years.Outcome measuresThe preoperative and postoperative implant rod angle of curvature expressed in degrees was obtained for each patient.MethodsTwo implant rods were attached to the concave and convex side of the spinal deformity. The preoperative implant rod geometry was measured before surgical implantation. The postoperative implant rod geometry after surgery was measured by computed tomography. The implant rod angle of curvature at the sagittal plane was obtained from the implant rod geometry. The angle of curvature between the implant rod extreme ends was measured before implantation and after surgery. The sagittal curvature between the corresponding spinal levels of healthy adolescents obtained by previous studies was compared with the implant rod angle of curvature to evaluate the sagittal curve correction. The difference between the postoperative implant rod angle of curvature and normal spine sagittal curvature of the corresponding instrumented level was used to evaluate over or under correction of the sagittal deformity.ResultsThe implant rods at the concave side of deformity of all patients were significantly deformed after surgery. The average degree of rod deformation Δθ at the concave and convex sides was 15.8° and 1.6°, respectively. The average preoperative and postoperative implant rod angle of curvature at the concave side was 33.6° and 17.8°, respectively. The average preoperative and postoperative implant rod angle of curvature at the convex side was 25.5° and 23.9°, respectively. A significant relationship was found between the degree of rod deformation and preoperative implant rod angle of curvature (r=0.60, p<.005). The implant rods at the convex side of all patients did not have significant deformation. The results indicate that the postoperative sagittal outcome could be predicted from the initial rod shape.ConclusionsChanges in implant rod angle of curvature may lead to over- or undercorrection of the sagittal curve. Rod deformation at the concave side suggests that corrective forces acting on that side are greater than the convex side.