Background:　Screw　loosening　remains　a　serious　complication　for　patients　undergoing　pedicle　screw　fixation　surgeries.　An　accurate　risk　prediction　is　significant　for　prevention　of　screw　loosening　through　preoperative　planning.　In　this　study,　we　proposed　a　novel　index,　namely　the　bone　mineral　density　surrounding　the　screw　thread　(thread　BMD),　and　tested　its　predictability　in　screw　loosening.　Methods:　86　screws　(18　loosening　and　68　non-loosening)　from　L3-L5　of　20　patients　who　experienced　pedicle　screw　loosening　were　analyzed.　The　preoperative　and　postoperative　quantitative　CT　scans　of　the　same　vertebra　were　spatially　registered　and　a　helix-based　approach　was　developed　to　extract　the　thread　BMD.　BMDs　of　the　vertebral　body,　the　pedicle　and　the　screw　trajectory　were　also　measured　from　the　preoperative　CT　scans.　Finite　element　analysis　was　conducted　to　determine　pullout　strength　and　tissue　failure　around　the　screw.　Receiver　operating　characteristic　(ROC)　curve　analysis　was　used　to　assess　the　performances　of　all　BMD　indices　and　pullout　strength　in　predicting　screw　loosening.　Linear　regression　was　used　to　examine　correlations　between　different　BMD　indices　and　screw　pullout　strength.　Results:　The　thread　BMD　had　the　greatest　value　of　area　under　the　curve　(AUC　=　0.73,　p　=　0.004)　compared　to　vertebral　BMD　(AUC　=　0.51,　p　=　0.923),　pedicle　BMD　(AUC　=　0.56,　p　=　0.474)　and　trajectory　BMD　(AUC　=　0.67,　p　=　0.020).　Also,　the　thread　BMD　showed　a　stronger　correlation　with　the　pullout　strength　(r　=　0.83,　p　G　0.001)　than　vertebral　BMD　(r　=　0.59,　p　G　0.001),　pedicle　BMD　(r　=　0.65,　p　G　0.001)　and　trajectory　BMD　(r　=　0.60,　p　G　0.001).　Conclusions:　We　developed　a　novel　approach　to　measure　a　newly-defined　thread　BMD,　which　indicates　superior　capacities　over　other　BMD　indices　in　predicting　pedicle　screw　loosening.