A biomechanical investigation of the retentive force of pedicle screw structures for different screw tulip designs

Impact of Rod Placement and Tulip Design on Screw-Rod Gripping Capacity in Spinopelvic Fixation: Evaluation Across a Spectrum of Recessed to Extended Lengths

BACKGROUND CONTEXT

High rates of pelvic instrumentation failure (4.5-38%) have been reported, often attributed to issues within the screw-tulip-rod connection. While previous research has explored various aspects of this connection, the influence of tulip design and relative rod placement on mechanical failure remains unclear.

PURPOSE

This study aims to investigate how screw-tulip design and variations in rod placement relative to the tulip affect the integrity of the screw-tulip-rod connection, utilizing axial and torsional gripping capacity tests to evaluate mechanical stability.

STUDY DESIGN/SETTING

Biomechanical METHODS: Mechanical testing was conducted following ASTM F1798-21 to assess the interconnection mechanisms in pelvic fixation constructs. Using 5.5mm Cobalt Chromium rods with porous fusion/fixation (PFFS) screws, axial gripping capacity (AGC) tests measured the axial load before translatory slippage of the rod, while torsional gripping capacity (TGC) tests assessed the torque required to induce rotational slippage. Variations in rod placement at the tulip head were tested in recessed (-2mm, -1mm), flush (0mm), and extended positions (+1mm, +10mm), simulating failure during flexion, extension, and rotation for both open and closed tulip-head designs. ANOVA was used to evaluate the effects of rod placement on connection failure, with significance set at p<0.05.

RESULTS

AGC and TGC tests revealed significant reductions for recessed rod placements, indicating suboptimal placement. At -1mm and -2mm, AGC for simulated flexion decreased by 28.8% (p<0.010) and 45.6% (p<0.001) for the open-head design and 30.5% (p<0.018) and 57.5% (p<0.001) for the closed-head design, respectively, compared to the non-recessed rod placement. TGC also showed a significant decline at -2mm, with a 25.4% reduction compared to the +1mm extended length (p<0.001) and a 20.3% reduction compared to the -1mm recessed length (p=0.005), irrespective of head design. The open and closed-head designs exhibited similar trends; however, the closed-head design was shown to better resist structural failure at recessed lengths. At -2mm simulating extension, the closed-head design was 54.8% greater than the open-head design for AGC (p<.001) and 28.3% greater for TGC.

CONCLUSION

Our findings underscore that both flush (0mm) and extended (+1, +10mm) rod placements relative to the screw-tulip offer sufficient gripping capacity whereas recessed placements (-1, -2mm) have substantial reductions. The closed-head design was shown to better resist structural failure at recessed placements.

CLINICAL SIGNIFICANCE

Rod placement relative to the most distal pelvic screw during spinopelvic fixation varries in surgical practice - whether flush to, extended past, or recessed into the screw-head. Biomechanical evaluating of the axial and torsion gripping capacities at these positions provies a foundation for clinical decision-making.

Novel uniplanar pedicle screw systems applied to thoracolumbar fractures: a biomechanical study

Objective: In this study, the advantages of the internal fixation configuration composed of uniplanar pedicle screws in the treatment of thoracolumbar fractures were verified by biomechanical experimental methods, which provided the basis for subsequent clinical experiments and clinical applications. Methods: A total of 24 fresh cadaveric spine specimens (T12-L2) were utilized to conduct biomechanical experiments. Two different internal fixation configurations, namely, the 6-screw configuration and the 4-screw/2-NIS (new intermediate screws) configuration, were tested using fixed-axis pedicle screws (FAPS), uniplanar pedicle screws (UPPS), and polyaxial pedicle screws (PAPS) respectively. The spine specimens were uniformly loaded with 8NM pure force couples in the directions of anteflexion, extension, left bending, right bending, left rotation, and right rotation, and the range of motion (ROM) of the T12-L1 and L1-L2 segments of the spine was measured and recorded to access biomechanical stability. Results: No structural damage such as ligament rupture or fracture occurred during all experimental tests. In the 6-screw configuration, the ROM of the specimens in the UPPS group was significantly better than that of the PAPS group but weaker than those of the FAPS group (p < 0.01). In the 4-screw/2-NIS configuration, the results were identical to the biomechanical test results for the 6-screw configuration (p < 0.01). Conclusion: Biomechanical test results show that the internal fixation configuration with UPPS can maintain the stability of the spine well, and the results are better than that of PAPS. UPPS has both the biomechanical advantages of FAPS and the superiority of easy operation of PAPS. We believe it is an optional internal fixation device for minimally invasive treatment of thoracolumbar fractures.

Deviating from the Recommended Torque on Set Screws Can Reduce the Stability and Fatigue Life of Pedicle Screw Fixation Devices

Background and Objectives: Using an appropriate torque to tighten set screws ensures the long-term stability of spinal posterior fixation devices. However, the recommended torque often varies between different devices and some devices do not state a recommended torque level. The purpose of this study is to evaluate the effect of set screw torque on the overall construct stability and fatigue life. Materials and Methods: Two commercial pedicle screw systems with different designs for the contact interface between the set screw and rod (Group A: plane contact, Group B: line contact) were assembled using torque wrenches provided with the devices to insert the set screws and tighten to the device specifications. The axial gipping capacity and dynamic mechanical stability of each bilateral construct were assessed in accordance with ASTM F1798 and ASTM F1717. Results: Increasing or decreasing the torque on the set screw by 1 Nm from the recommended level did not have a significant effect on the axial gripping capacity or fatigue strength of Group A (p > 0.05). For Group B, over-tightening the set screw by 1 Nm did cause a significant reduction in the fatigue strength. Conclusions: Excessive torque can damage the rod surface and cause premature failure. When insertion using a manual driver is preferred, a plane contact interface between the set screw and rod can reduce damage to the rod surface when the set screw is over-torqued.