Objective　To　compare　the　effects　of　cortical　bone　trajectory　(CBT)　and　traditional　trajectory　(TT)　pedicle　screw　internal　fixation　on　the　range　of　motion　(ROM)　and　rod　system　stress　of　normal　and　osteoporotic　(OP)　spines.　Methods　The　L3-S1　finite　element　models　of　normal　and　OP　spines　were　established.　The　screw　rod　system　with　two　kinds　of　trajectory　was　used　for　internal　fixation　of　the　L4-5　segment,　so　as　to　simulate　six　physiological　loads,　namely,　flexion,　extension,　left　/　right　bending,　left　/　right　rotation.　The　effects　of　two　internal　fixation　methods　on　ROMs　and　maximum　equivalent　stress　of　screws　in　normal　and　OP　spines　were　compared.　Results　For　both　bone　conditions,　CBT　and　TT　significantly　reduced　ROM　of　the　fixed　segment　(L4-5)　and　the　entire　segment　of　lower　lumbar　spine　(L3-S1)　.　However,　the　ROM　decline　of　CBT　group　was　slightly　smaller　than　that　of　TT　group,　and　their　ROMs　were　similar　under　flexion　and　extension,　but　the　ROM　differences　were　significant　under　lateral　bending　and　axial　rotation.　In　addition,　for　both　the　normal　and　OP　spine　models,　the　maximum　equivalent　stress　of　screws　in　CBT　group　was　significantly　higher　than　that　in　TT　group.　Compared　with　TT　group,　the　screw　stress　of　CBT　group　in　normal　spine　model　under　flexion　and　extension,　lateral　bending,　axial　rotation　was　increased　by　27%,　268%　and　58%,　respectively.　However,　when　CBT　technique　was　used　at　the　same　time,　the　OP　spine　model　had　a　smaller　screw　stress　distribution　than　the　normal　spine　model.　Conclusions　Compared　with　TT　technique,　CBT　technique　can　achieve　higher　screw　stress　under　OP　condition　and　reduce　screw　stress　concentration　under　normal　bone　condition.　In　addition,　CBT　slightly　increases　ROMs　of　each　segment,　which　is　conducive　to　recovery　of　spinal　physiological　function　after　surgery.　Lateral　bending　and　axial　rotation　can　produce　negative　mechanical　effects,　and　these　two　physiological　loads　should　be　avoided.　©　2023　Shanghai　Jiaotong　University　School　of　Medicine.　All　rights　reserved.