J Korean Neurosurg Soc.  2016 Sep;59(5):425-429. 10.3340/jkns.2016.59.5.425.

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

• Acar N
• Karakasli
• Karaarslan AA
• Ozcanhan M
• Ertem F
• Erduran M

• KMID: 2351708
• DOI: http://doi.org/10.3340/jkns.2016.59.5.425

Abstract

OBJECTIVE
Rod-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.
METHODS
The 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.
RESULTS
Under 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.
CONCLUSION
We 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.

Keyword

Biomechanics; Rod contouring; Rod failure

MeSH Terms

Molecular Weight
Polyethylene
Spine*
Polyethylene

Figure

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  Fig. 1 Rod-screw system used in the corpectomy model. Universal test machine used for flexion/extension and deformation testing. a-f : UHMWPE blocks, b : poly-axial screws, c : strain-gauge, d-e : fixation apparatus.

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  Fig. 2 Schematic illustration of the lordotic and kyphotic rod-screw systems. D1 and D2 show the difference of the moment arm; D2>D1.

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  Fig. 3 A corpectomy model in the torsion testing machine.

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  Fig. 4 The stiffness values of different rod models.

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  Fig. 5 Rod strain values during flexion and extension.

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  Fig. 6 Screws strain values of different rod curvatures during flexion and extension.

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  Fig. 7 Rod-screw systems stiffness values under torsional load.

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