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Background:Reports on the efficacy of modifications to the thread design of pedicle screws are scarce.The aim of the study was to investigate initial and early fixation of pedicle screws with a plasma-sprayed titanium coating and dual pitch in the pedicle region (dual pitch titanium-coated pedicle screw [DPTCPS]) in a polyetheretherketone (PEEK) rod semi-rigid fixation system.Methods:Fifty-four sheep spine specimens and 64 sheep were used to investigate initial (0-week controls) and early (post-operative 6 months) fixation,respectively.Sheep were divided into dual pitch pedicle screw (DPPS),standard pitch pedicle screw (SPPS),DPTCPS,and standard pitch titanium-coated pedicle screw (SPTCPS) groups.Specimens/sheep were instrumented with four screws and two rods.Biomechanical evaluations were performed,and histology at the implant-bone interfacewas investigated.Results:At 0-week,mean axial pull-out strength was significantly higher for the DPTCPS and SPTCPS than the SPPS (557.0 ± 25.2 vs.459.1 ± 19.1 N,t =3.61,P ＜ 0.05;622.6 ± 25.2 vs.459.1 ± 19.1 N,t =3.43,P ＜ 0.05).On toggle-testing,the DPTCPS was significantly more resistant than the SPPS and SPTCPS (343.4 ± 16.5 vs.237.5 ± 12.9 N,t =3.52,P ＜ 0.05;343.4 ± 16.5 vs.289.9 ± 12.8 N,t =3.12,P ＜ 0.05;124.7 ± 13.5 vs.41.9 ± 4.3 cycles,t =2.18,P ＜ 0.05;124.7 ± 13.5 vs.79.5 ± 11.8 cycles,t =2.76,P ＜ 0.05).On cyclic loading,maximum displacement was significantly lower for the DPTCPS than the SPPS and SPTCPS (1.8 ± 0.13 vs.3.76 ± 0.19 mm,t =2.29,P ＜ 0.05;1.8 ± 0.13 vs.2.46 ± 10.20 mm,t =2.69,P ＜ 0.05).At post-operative 6 months,mean axial pull-out strength was significantly higher for the DPTCPS and SPTCPS than the SPPS (908.4 ± 33.6 vs.646.5 ± 59.4 N,t =3.34,P ＜ 0.05;925.9 ± 53.9 vs.646.5 ± 59.4 N,t=3.37,P ＜ 0.05).On toggle-testing,the DPTCPS was significantly more resistant than the SPPS and SPTCPS (496.9 ± 17.9 vs.370.3 ± 16.4 N,t =2.86,P ＜ 0.05;496.9 ± 17.9 vs.414.1 ± 12.8 N,t =2.74,P ＜ 0.05;249.1 ± 11.0 vs.149.9 ± 11.1 cycles,t =2.54,P ＜ 0.05;249.1 ± 11.0 vs.199.8 ± 7.2 cycles,t =2.61,P ＜ 0.05).On cyclic loading,maximum displacement was significantly lower for the DPTCPS than the SPPS and SPTCPS (0.96 ± 0.11 vs.2.39 ± 0.14 mm,t =2.57,P ＜ 0.05;0.96 ± 0.11 vs.1.82 ± 0.12 mm,t =2.73,P ＜ 0.05).Resistance to toggle testing (370.3 ± 16.4 vs.414.1 ± 12.8 N,t=3.29,P ＜ 0.05;149.9 ± 11.1 vs.199.8 ± 7.2 cycles,t=2.97,P ＜ 0.05) was significantly lower and maximum displacement in cyclic loading (2.39 ± 0.14 vs.1.82 ± 0.12 mm;t=3.06,P ＜ 0.05) was significantly higher for the SPTCPS than the DPTCPS.Bone-to-implant contact was significantly increased for the DPTCPS compared to the SPPS (58.3％ ± 7.0％ vs.36.5％ ± 4.4％,t =2.74,P ＜ 0.05);there was no inflammatory reaction or degradation of coated particles.Conclusion:DPTCPSs might have stronger initial and early fixation in a PEEK rod semi-rigid fixation system.