The effect of screw head design on rod derotation in the correction of thoracolumbar spinal deformity

Abstract

Object. The use of fixed-axis pedicle screws for correction of thoracolumbar deformity in adult surgery is demanding because of the challenge of assembling the bent rod to the screw in order to achieve curve correction. Polyaxial screw designs, providing increased degrees of freedom at the screw-rod interface, were reported to be insufficient in achieving correction of thoracic deformity in the axial plane. Using a multisegment bovine calf spine model, this study investigated the ability of a new uniplanar screw design to achieve derotation correction of the vertebrae and maintain a degree of correction comparable to that of fixed-axis and polyaxial screw designs.Methods. Eighteen calf thoracolumbar spine segments from T-6 to L-1 (n = 6 per screw design) underwent bilateral facetectomies at the T9-11 levels and were instrumented bilaterally with pedicle screws and rods. To assess the efficacy of each screw design in imparting rotational correction, each instrumented level was tested under applied torsional moments designed to simulate the motion applied during derotation surgery. Once rotation was achieved, the whole spine was tested to assess the overall stiffness of the construct.Results. The fixed-axis construct showed increased efficacy in imparting rotation compared with the uniplanar (115% increase, p > 0.05) and polyaxial (210% increase, p < 0.05) constructs. Uniplanar screws showed a 21% increase in torsional stiffness compared with the polyaxial screws, but this difference was not statistically significant.Conclusions. The design of screw heads plays a significant role in affecting the rotation of the vertebrae during the derotation procedure. Uniplanar screws may have the advantage of maintaining construct stiffness after derotation.

Efficacy and Radiographic Analysis of Minimally Invasive Posterior Mono-Axial Pedicle Screw Fixation in Treating Thoracolumbar Burst Fractures

“…Many biomechanical studies demonstrated that the designs of the screw head play a significant role in the correction of spinal deformity and have different effects on the stiffness in three dimensions (coronal, sagittal, and axial plane) [17]. An experimental study reported by Wang showed that the mono-axial pedicle screw with no motion between the screw head and shaft can significantly increase the stiffness in the axial direction compared with the poly-axial pedicle screw and can reduce the risks of correction loss [32].…”

“…Thus, a transpedicular screw is popularly used to treat thoracolumbar burst fractures. Many biomechanical studies demonstrate that the designs of the screw head play a significant role in the correction of spinal deformity and have different effects on the stiffness in three dimensions (coronal, sagittal, and axial plane) [17].…”

Efficacy and Radiographic Analysis of Posterior Mono Axial Pedicle Screw Fixation in Treating Thoracolumbar Burst Fractures

“…11 Many biomechanical studies demonstrate that the designs of screw head play a signi cant role in the correction of spinal deformity and have different effects on the stiffness in three dimensions (coronal, sagittal, and axial plane). 16 An experimental study has reported by Wang that the mono axial pedicle screw with no motion between screw head and shaft can signi cantly increase the stiffness in axial direction compared with poly axial pedicle screw, and reduce the risks of correction loss. 29 Mono axial pedicle screws can be a better xation instrumentation for thoracolumbar burst fractures in theory.…”

“…Many biomechanical studies demonstrate that the designs of screw head play a signi cant role in the correction of spinal deformity and have different effects on the stiffness in three dimensions (coronal, sagittal, and axial plane). 16 According to biomechanical theory, the mono axial pedicle screw could, more effectively than the poly axial pedicle screws, restore the vertebral height, correct the kyphosis, reduce the postoperative kyphosis loss, and maintain fracture reduction.…”