Threshold screw insertion torque for carbon fibre-reinforced polyetheretherketone and titanium (Ti-6Al-4V) locking plate constructs

Background: Locking plate constructs are integral to modern fracture fixation, particularly in osteoporotic bone. Carbon-fibre reinforced polyetheretherketone (CFR-PEEK) plates offer a stiffness profile closer to cortical bone than titanium, potentially improving the mechanical environment for healing. However, the optimal insertion torque for locking screws in CFR-PEEK plates is unclear. This study investigated the influence of insertion torque on construct performance in CFR-PEEK plates compared to titanium alloy (Ti-6Al-4 V) plates.

Methods: Locking screws (3.5 mm, Ti-6Al-4 V) were inserted into Ti-6Al-4 V and CFR-PEEK plates at six torque levels (0.5–3.0 Nm). Construct strength was assessed via axial push-out and cantilever bending tests. Video analysis was used to correlate insertion torque with screw rotation. Additional testing was performed on two-screw CFR-PEEK constructs at three torque levels (0.5, 1.5, 2.5 Nm).

Findings: Insertion torque was linearly related to screw rotation within the tested range (0.5–3.0 Nm). In CFR-PEEK single screw constructs, higher torque (≥2 Nm) improved push-out strength (p < 0.05), while 1.5 Nm yielded the highest cantilever strength (p < 0.05). Ti-6Al-4 V constructs showed a similar trend. In two-screw CFR-PEEK constructs, no significant differences were found in performance across torques for either push-out or cantilever testing (p > 0.5).

Interpretation: Beyond an initial threshold (~0.5 Nm), increased insertion torque did not consistently enhance construct performance. These findings indicate that moderate torques (~1.5 Nm) optimise performance while reducing the risk of implant damage, support the mechanical reliability of CFR-PEEK plates.