Spinal rod gripping capacity: how do 5.5/6.0-mm dual-diameter screws compare?

Study design: Biomechanical comparative study.

Objective: To evaluate pedicle screw gripping capacity from five suppliers, comparing single-diameter (S-D) systems using 5.5-mm-diameter rods to dual-diameter (D-D) systems accepting 5.5- and 6.0-mm-diameter rods with both cobalt chromium (CoCr) and titanium alloy (Ti) rods. D-D systems have become increasingly prevalent; however, these systems theoretically may compromise spinal rod gripping, particularly when a smaller-diameter rod is used within a D-D pedicle screw.

Methods: D-D pedicle screw systems from three suppliers (accepting 5.5- and 6.0-mm-diameter, Ti and CoCr rods), and S-D systems from two suppliers (accepting 5.5-mm-diameter, Ti and CoCr rods) were tested on an MTS MiniBionix machine. Axial load was applied in line with the rod to measure axial gripping capacity (AGC), and torsional load was applied to measure torsional gripping capacity (TGC) for each rod material and diameter. AGC and TGC were compared between D-D and S-D constructs, suppliers, rod diameters, and materials with subsequent classification and regression tree (CART) analysis.

Results: 5.5-mm rods within D-D screws were no weaker than 5.5-mm rods in S-D systems for AGC (dual > single, p = 0.043) and TGC (p = 0.066). As a whole, D-D systems had greater AGC than S-D systems (p = 0.01). AGC differed between suppliers (p < 0.001). No rod diameter (p = 0.227) or material (p = 0.131) effect emerged. With CART analysis, Supplier was the most significant predictor for greater AGC. As a whole, D-D systems had greater TGC than S-D systems (p = 0.008). TGC differed between suppliers (p < 0.001). Rod diameter was a significant predictor of higher TGC (6.0 > 5.5 mm, p = 0.002). CoCr rods had greater TGC than Ti (p < 0.001). CART analysis revealed that Supplier and CoCr material were significant predictors for increased TGC.

Conclusions: Despite 30%-70% variability in gripping capacity due to rod supplier and material, overall D-D pedicle screw systems had similar AGC and TGC as S-D systems.

Level of evidence: N/A.