ABSTRACTObjectiveEvaluate the stability provided by two flexible intramedullary nails (FINs) in a simulation of fractures at the proximal levels in pediatric femur models.MethodsTwo FINs were inserted in 18 synthetic models of pediatric femurs. Fractures were simulated at one of three levels, and the models were divided into the following groups (n=6): diaphysis (control), subtrochanteric and trochanteric. Flex-compression tests were performed with force up to 85 N. Relative stiffness and the average deformation was obtained. Torsion tests were performed by rotating the proximal fragment until 20°, to obtain the average torque.ResultsAt flex-compression, the set’s average relative stiffness and average deformations were: 54.360x103N/m and 1.645 mm in the control group, respectively. In the subtrochanteric group, the relative stiffness was 31.415x103N/m (-42.2%) and the deformation was 2.424 mm (+47.3%) (p<0.05). For the trochanteric group, the relative stiffness was 30.912x103N/m (+43.1%) and the deformation was 2.508 mm (+52.4%) (p<0.05). In torsion, the average torque was 1.410 Nm in the control group; 1.116 Nm in the subtrochanteric group (-20.8%), and 2.194 Nm in the trochanteric group (+55.6%) (p<0.05).ConclusionFINs do not seem to be biomechanically competent for the treatment of proximal femoral fractures. Level of Evidence I; Therapeutic Studies - Investigating the results of treatment.