Effects of Polyaxial Pedicle Screws on Lumbar Construct Rigidity

Abstract

Pedicle screw constructs have been shown to increase fusion rates in the lumbar spine. Manufacturers have created pedicle screws with one or two degrees of freedom built into the screw head to allow for easier incorporation of the interlocking rod, but the effects of these screws on construct stiffness has not been tested. The purpose of this study is to compare and contrast the stiffness of lumbar pedicle screw constructs with and without the use of polyaxial pedicle screws. Nontapered, self-taping pedicle screws (6.0-mm diameter x 30-mm length, titanium) were used in the fixation of porcine spines from L3-L5. Group 1 (n = 5) contained six standard pedicle screws from one manufacturer. Group 2 (n = 5) contained six standard pedicle screws from a second manufacturer. Group 3 (n = 5) contained four standard pedicle screws placed at L3 and L5, as well as two polyaxial screws placed at L4. Group 4 (n = 5) contained six polyaxial pedicle screws. A rotational variable differential transformer was used to record angular displacement between vertebrae in the construct as it is loaded in flexion, extension, right bend, left bend, clockwise torque, and counterclockwise torque. Stiffness curves were linear throughout the range of applied force. The average r2 value for the generated stiffness graphs was 0.94 (SD = 0.06). No construct failure occurred during any of the testing. There were no significant differences (p < 0.05, two-way analysis of variance) in moment versus angle noted in any of the four groups tested. For torque tests, the all-polyaxial screw constructs showed significantly increased stiffness compared with the other groups. The current study has shown that the incorporation of polyaxial screws in pedicle screw constructs did not significantly decrease the construct stiffness. There is a suggestion that the use of all polyaxial screws may increase the resistance to torque by allowing better purchase of intervertebral rods.

Biomechanical efficacy of monoaxial or polyaxial pedicle screw and additional screw insertion at the level of fracture, in lumbar burst fracture: An experimental study

“…When using monoaxial screws, the addition of an intermediate forces the surgeon to bend the connecting rod to accommodate the additional screw. Polyaxial screws, on the other hand, facilitate the installation of the connecting rod, and their biomechanical properties have been reported in several studies 14–1623. Stanford et al 15.…”

“…pointed out that there were significant differences between the monoaxial screws and polyaxial screws in the bending stiffness, yield load, yield torque, and torsional stiffness in static tests ( P < 0.05). Shepard et al 16. suggested that polyaxial screws do not significantly decrease the stiffness of the construct.…”

“…In the past 20 years, the design and implantation techniques of the pedicle screw systems have been modified to reduce the rate of pedicle screw breakage and to facilitate easy application of the connecting rod without undue stress on the construct 13–16. The polyaxial head coupling of the pedicle screw was found to reduce the compression bending strength at the screw–rod mount, in comparison with a monoaxial screw design 1516…”

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

“…By reducing the bending stresses, the polyaxial screw design may decrease the incidence of pedicle breakage. 28,37 However, compared with spinal constructs based on fixed-axis screws, constructs based on polyaxial screws have shown decreased rotational correction in vivo. 14 This liability has been suggested to be the result of the decreased rotational control of the spine due to the movement between the screw head and the screw's inherent polyaxial design.…”

Spinal Body Reconstruction in Osteoporosis

“…Starting in the 1970s with simple systems that allowed a solid fixation between adjacent vertebrae [31], many modifications were made over the years [32]. In the 1990s polyaxial systems were developed [33,34], nowadays systems that allow a minimal invasive access surgery are the goal [35]. Those systems are useful especially for the therapy of osteoporotic vertebral body fractures, because in succession of minimal invasive approaches to the spine, less morbidity by the surgical intervention and earlier mobilization of the patients is possible.…”