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.
Subject(s)
Molecular Weight , Polyethylene , SpineABSTRACT
One of various decompression methods in treatment of spinal stenosis is the indirect instrumental decompression. Theoretically, the distraction of the disc space can widen the intervertebral foramen of the stenotic segment and even increase the canal diameter by distracting the posterior annulus as well as reduce the extent of decompressive laminectomy site. The indirect instrumental decompression, however, was not guaranteed to maintain the restored discal height because of the loss of fixation strength between rod and screw, viscoelasticity of vertebra itself, bone density, type of screw and rod, and operative technique. As well the magnitude of the stresses on the instrumentation particularly at the rod-screw interface may depend on rod-contouring in order to make mormal sagittal curvature of the lumbar spines. Therefore, the aim of this experimental study was to evaluate the effect of different rod-contour on the axial sliding strength in Compact Cotrel-Dubousset (CCD) instrumentation. Axial sliding strength was tested by Universal Test Machine (Instron). Test was performed for 3 groups of different rodcontouring on the biomechanical axial strength: straight rod (no contour), 10 and 20 contouring rod. The length of contact surface between rod and screw was measured with Fuji pressure sensitive film. The study was performed using 6.5 mm open body screws and 7 mm rods of CCD instrumentation. Axial sliding strength of straight rod was 2518.6N, 1871.8N in 10 and 1528.8N in 20 contouring rod. The length of contact surface between rod and screw significantly decreased according to degree of rod contouring; 9.88mm in straight rod, 9.08mm in 10 and 8.57mm in 20 contouring rod. There was a statistically significant linear correlation (R=0.96) between failure load and length of contact surface. Therefore, this study has shown that excessive contour of the rod in order to make normal sagittal curvature of the lumbar spine using CCD instrumentation cannot provide sufficient axial sliding strength. That may be a cause of loss of restored disc space height after surgery.