A biomechanical analysis of an instrumented spinal fixator under torsional loads

Alkalay, Ron N.; Sharpe, Deborah; Bader, Dan L.

doi:10.1016/j.jbiomech.2004.04.029

Cited by 5 publications

(3 citation statements)

References 13 publications

(21 reference statements)

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“…The improved rotational correction, combined with the demonstrated higher torsional stiffness, should translate to improved construct performance at long-term follow-up. 1,2,39 This study, in common with many biomechanical studies, has several limitations. In the absence of a surrogate scoliosis model, we have chosen to use a calf spine model, which, although possessing an anatomy and vertebral kinematics similar to those of a normal human spine, 3,6 cannot simulate the rotational deformation and altered geometry and mechanics of the vertebrae and of the intervertebral discs that are typical of a scoliotic spine.…”

Section: Discussionmentioning

confidence: 99%

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

Lam

Groff

Alkalay

2013

SPI

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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.

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Section: Discussionmentioning

confidence: 99%

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

Lam

Groff

Alkalay

2013

SPI

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“…Rod-screw system failure through screw pullout, screw breakage and rod breakage are frequently encountered during clinical practice. Although many biomechanical studies on transpedicular screw pullout and breakage have been performed 1 2 8 10 17 22) , but it is strange that, the literature is lacking of a well-structured biomechanical study enriching the knowledge regarding the effect of rod contouring (lordotic and kyphotic curvatures) on the strength of the rod-screw construct and the impact of such different curvatures on rod fatigue and then breakage. The aim of the present biomechanical work is to study the behavior of different rod curvatures against variable stresses to detect the effect of rod contouring (straight, kyphotic and lordotic) on rod fatigue which leads later on to breakage.…”

Section: Introductionmentioning

confidence: 99%

The Mechanical Effect of Rod Contouring on Rod-Screw System Strength in Spine Fixation

Acar¹,

Karakaşlı

Karaarslan

et al. 2016

J Korean Neurosurg Soc

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ObjectiveRod-screw fixation systems are widely used for spinal instrumentation. Although many biomechanical studies on rod-screw systems have been carried out, but the effects of rod contouring on the construct strength is still not very well defined in the literature. This work examines the mechanical impact of straight, 20° kyphotic, and 20° lordotic rod contouring on rod-screw fixation systems, by forming a corpectomy model.MethodsThe corpectomy groups were prepared using ultra-high molecular weight polyethylene samples. Non-destructive loads were applied during flexion/extension and torsion testing. Spine-loading conditions were simulated by load subjections of 100 N with a velocity of 5 mm min-1, to ensure 8.4-Nm moment. For torsional loading, the corpectomy models were subjected to rotational displacement of 0.5° s-1 to an end point of 5.0°, in a torsion testing machine.ResultsUnder both flexion and extension loading conditions the stiffness values for the lordotic rod-screw system were the highest. Under torsional loading conditions, the lordotic rod-screw system exhibited the highest torsional rigidity.ConclusionWe concluded that the lordotic rod-screw system was the most rigid among the systems tested and the risk of rod and screw failure is much higher in the kyphotic rod-screw systems. Further biomechanical studies should be attempted to compare between different rod kyphotic angles to minimize the kyphotic rod failure rate and to offer a more stable and rigid rod-screw construct models for surgical application in the kyphotic vertebrae.

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“…Bone screws have been used in spinal fixation device since 1960s. The use of internal spinal fixation devices employing screws for the treatment of spinal disorders in the lumbar spine has increased since late 1980s [1]. They have been used to enhance occurrence of spinal fusion and to reduce incidence of low back pain due to involvement of adjacent normal spinal segments.…”

Section: Introductionmentioning

confidence: 99%

Analysis of Deformation and Stress Distribution of the Poly-Axial Pedicle Screw

Kim

Yang

Kim³

et al. 2008

KEM

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The effect of design features of an internal spinal fixator under loading is critical to understanding of interaction between fixator and instrumented spine. In this study, we performed finite element analysis for spinal pedicle screw installed in the lumbar spine. The purpose of this study is to model and simulate the newly designed spinal pedicle screw. The deformation and stress of the screw are analyzed for the tightening process and loading process simulating the condition when it is installed in the human body as described in the ASTM F1717 procedure. We expected this study is to derive reliable results for developing a new model by analysis of design variables and fatigue behavior.

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A biomechanical analysis of an instrumented spinal fixator under torsional loads

Cited by 5 publications

References 13 publications

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

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

The Mechanical Effect of Rod Contouring on Rod-Screw System Strength in Spine Fixation

Analysis of Deformation and Stress Distribution of the Poly-Axial Pedicle Screw

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