Paired Biomechanical Comparison of Cortical and Traditional Pedicle Screw Constructs Under Cyclic Loading in a Severely Destabilized Lumbar Spine Model.

Study Design: Cadaveric biomechanical analysis.

Objective: To compare the fatigue resistance and failure behavior of cortical bone trajectory (CBT) and traditional pedicle screw (TPS) constructs in severely destabilized lumbar segments under cyclic flexion-compression loading.

Summary Of Background Data: CBT screw fixation is increasingly used for lumbar stabilization and offers theoretical biomechanical advantages over TPS, including higher insertional torque and increased cortical engagement. However, its fatigue behavior in unstable constructs remains poorly characterized, despite clinical reports of early failures following interbody reduction maneuvers.

Methods: Fourteen lumbar motion segments (L2-L3 and L4-L5) from seven cadaveric spines were randomized to CBT or TPS fixation (n=7 per group) using a paired design. Following subtotal discectomy and bilateral facetectomy, constructs were cyclically loaded (200-750 N, 2 Hz, 50,000 cycles). Segment stiffness, flexion range of motion (ROM), and survival were compared. Bone mineral density (BMD) was quantified via dual-energy X-ray absorptiometry.

Results: In the intact state, CBT and TPS constructs showed no significant differences in stiffness or ROM. After destabilization and instrumentation, CBT constructs demonstrated reduced flexion ROM (P=0.026), with similar end-range stiffness (P=0.219). BMD significantly correlated with ROM and stiffness in the destabilized state (ρ=-0.72, P=0.007; ρ=0.65, P=0.014). Three of seven constructs in each group survived cyclic loading. While CBT constructs withstood more cycles on average (19,500±13,026 vs. 5,625±6,419), the difference was not statistically significant (P=0.710).

Conclusions: In a highly destabilized flexion-compression model, CBT and TPS constructs demonstrated similar fatigue performance. CBT screws offered greater resistance to ROM but no significant advantage in construct stiffness or survival. These results suggest that observed clinical failures of CBT may be attributable to surgical technique or placement sensitivity rather than inferior fatigue behavior.