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 5.5 mm 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 (–2 mm, –1 mm), flush (0mm), and extended positions (+1 mm, +10 mm), 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<.05. Results: AGC and TGC tests revealed significant reductions for recessed rod placements, indicating suboptimal placement. At –1 mm and –2 mm, AGC for simulated flexion decreased by 28.8% (p<.010) and 45.6% (p<.001) for the open-head design and 30.5% (p<.018) and 57.5% (p<.001) for the closed-head design, respectively, compared to the nonrecessed rod placement. TGC also showed a significant decline at –2 mm, with a 25.4% reduction compared to the +1 mm extended length (p<.001) and a 20.3% reduction compared to the –1 mm recessed length (p=.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 –2 mm 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, +10 mm) rod placements relative to the screw-tulip offer sufficient gripping capacity whereas recessed placements (-1, -2 mm) 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 provides a foundation for clinical decision-making.