Purpose:The aim of this study was to investigate the incidence of Z-effect after dual lag screw intramedullary nailing systems and risk factors contributing to this effect.We hypothesized that long nails provide more neck strength due to a longer lever than short nails and are therefore less likely to develop a misbalance of a higher head compressive strength than neck compressive strength.Methods:In this retrospective cohort study 103 patients treated operatively with a dual lag screw intramedullary nailing device for (sub)trochanteric hip fracture were included.We analysed patient charts regarding patient and operation characteristics.Furthermore we conducted radiologic measurements within the 2-year follow-up period to investigate the quality of fracture fixation,implant failure and predictors for Z-effect.The re-operation risk was investigated with multivariate regression analysis.Results:The incidence of(reversed) Z-effect in this study was 9％ (n =80);6 out of 7 Z-effects occurred in the short nail group,which was not significant.Patients who were treated with a long nail had a significant larger number of complications in comparison with the short nail group (median 2 vs 0.5,p =0.001).The long nail group received more often erythrocytes blood transfusions (82％ vs 31％,p ＜ 0.01) and had a longer hospital stay (13 vs 21 days,p ＜ 0.05).Migration of lag screws (p ＜0.05) and unstable fracture type (p ＜ 0.05),were risk factors for re-operation.The re-operation rate within 2 year after surgery was 21％,of which one fourth was due to a Z-effect.Conclusion:The nail length was not associated with the development of a Z-effect.Migration of lag screws after intramedullary nailing is common and a risk factor for re-operation.