Biomechanical studies were carried out to assess the function and performance of intramedullary (IM) nails for femoral fractures. An appropriately sized femoral IM nail with a radius of curvature of about 109 cm would most closely match the anterior bow of most human femora. A number of parameters can interact to result in bursting of the femur during insertion of the nail. These include mismatch in curvature of the nail and femur, high stiffness in bending, and poor location of the starting hole. An anatomic starting position for the IM nail is just medial to the greater trochanter and anterior to the pyriformis recess. Moving anterior to the midline of the femur significantly increases the potential for bursting the femur during insertion of the nail. Other factors can decrease the force of insertion of the IM nail in the femur. These include overreaming, shortening the axial length of the fracture component, and use of a nail of lower bending rigidity. IM-nail-fixed femoral shaft fractures with locking bolts can be expected to have about 75% the rigidity of the intact femur in bending and can support about 400% of normal body weight (= 70 kg). Slotted IM nail/femur constructs have only about 3% the rigidity of the intact femur in torsion, while an unslotted (closed) section implant produces constructs with about 50% the rigidity. The distal locking bolts increase the torsional rigidity and maximum axial load capacity of the construct, and reduce the potential for shortening and the residual deformation upon release of a torsional load. Two distal bolts reduce the toggle of the nail in the femoral shaft.(ABSTRACT TRUNCATED AT 250 WORDS)