Spinal Pedicle Screw Design Retains Grip Strength After Loosening and Relocking1 (2015)

Pedicle screw fixation is the standard for posterior stabilization of patients with spinal fractures, spinal instability, or degenerative disk disease as well as correction of pathologic deformities. The success of these spinal fixation surgeries depends upon screw-to-bone purchase and screw-to-rod interconnection strength. The literature is replete with biomechanical studies examining pedicle screw purchase within bone [1,2]. Unfortunately, few studies exist which detail the strength of the interconnection of pedicle screws with rods [3,4]. Spinal fixation and deformity correction often involve the need to adjust screw-to-rod position, bend a rod, or rotate a rod to gain proper spinal alignment. These rod manipulation activities are classically performed with the rods in situ in the pedicle screws necessitating a loosening of the closure top and retightening. The effects of closure top loosening and retightening on the resultant grip strength between the screw and the rod have not been reported.The objective of this study was to perform biomechanical testing to determine the relative grip strength of the screw–rod interconnection after a single locking activity and secondary to a loosening and relocking activity. This research aims to test the hypothesis that pedicle screw design characteristics can impact the performance during loosening and relocking of a closure top as it relates to the grip strength of the pedicle screw–rod interconnection.A repeated measures factorial study design was used to evaluate two different pedicle screw designs and two tightening states (initial and after loosening). The biomechanical outcomes of this study included the axial griping capacity, the torsional gripping capacity, and the flexion/extension gripping strength in accordance with ASTM F1798-13.Two different pedicle screw and closure top designs were evaluated. One design included a deformable feature and the other design did not. The screw housing and shank were the same for both screws so that the effect of the deformable feature could be strictly evaluated in regard to its ability to retain its grip strength. The screw closure top tightening torque was held constant across all of the samples as well. Five (5) screws of each design were evaluated in each of the three grip tests in a repeated fashion being tested initially tightened and then again after a loosening and tightening cycle. Specifically, each screw was tightened on the rod and tested in one of axial grip, torsion grip, or flexion/extension grip. After testing, the closure top was loosened from the rod, the polyaxial screw re-animated, and then the screw was tightened on the rod again using the same closure top and tightened to the same torque. In a similar fashion as before, the sample was tested in the same mode of loading (axial grip, torsion grip, or flexion/extension grip) after this loosening–relocking cycle.The pedicle screw grip testing was performed per ASTM F1798-13 and included the tests illustrated in Fig. 1. The standard rates of loading, data acquisition, and data analysis were performed on each screw to reveal its grip strength for the interaction of the screw and rod. The grip strengths were normalized to the initial grip strength values and the data compared to assess the relative effects of loosening and relocking.The grip results of the screw−rod interface revealed a differential pattern in grip on the rod (axial grip and torsional grip). Design A, which included a deformable feature, exhibited a decrease in both axial grip (11%) and torsional grip (12%) with loosening and relocking of the closure top. The design without a deformable feature, design B, demonstrated an increase in axial grip of 4% and an increase in torsional grip of 8% after a cycle of loosening and relocking (Figs. 2 and 3).The flexion/extension grip strength, which is the screw's ability to resist shank angular displacement with respect to the rod, was mildly affected by the design differences. Design A displayed a 12% decrease in grip strength whereas design B had only a 3% decrease in grip strength. Since the design differences were intended to affect axial grip and torsional grip to a greater extent, this measure of flexion/extension grip was meant to reveal the design's effect on downward force provided by the closure top.All of the gripping tests resulted in slipping at interconnection surfaces and no failure or breakage of screw, closure top, or rod components. All screws re-animated appropriately and were tightened to the same tightening torque between designs and tests ±1.1%.Design A exhibited a statistically significant decrease in axial grip (p = 0.0178) and torsion grip (p = 0.0204). Design B axial grip was significantly larger (p = 0.0222) after a tightening, loosening, and tightening cycle than after the original tightening. Both designs displayed flexion/extension grip which was not distinctly different between initial and after loosening cycles (p > 0.0629).Testing the null hypothesis that design characteristics do not affect the ability of a screw–rod interconnection to retain similar or greater grip strength after a loosening and relocking activity as compared with the initial locking resulted in its rejection. Design B, without a deformable feature, displayed similar or significantly greater grip strength for all modes of loading. Thus, the stated hypothesis on loosening and relocking is viable, since design B exhibited similar or improved grip strength from initial tightening to final tightening after loosening. This design may be very advantageous for the surgical correction of deformity or fixation requiring a loosening and relocking activity to achieve the optimal fixation construct.While not classically examined in the design and evaluation of pedicle screws, the strength of its interconnection with a rod is critically important. In addition, the surgical application of closure tops to screws is essential, and this involves the initial tightening activity and any subsequent loosening and tightening activity. This research demonstrates a design which is capable of retaining the grip strength properties of the screw–rod interconnection after a loosening and relocking activity.