Various different interlocking intramedullary nail designs are now available for the tibia the designs resulting from a different approaches to achieving stability for fracture fixation. During recent years small-diameter unreamed interlocking nails have been developed. So far there are no data on stiffness of different nail designs that can justify the use of certain nail types. We compared different nail types in the bone-implant complex (BIC) with reference to stiffness under axial load, bending and torsion of the BIC. We simulated comminuted mid-shaft fractures by a 2-cm defect osteotomy in paired human cadaver tibiae. We fixed each tibia with one of nine different interlocking nails (AO Unreamed Tibial Nail 9 mm [UTN9], AO Unreamed Tibial Nail 8 mm [UTN8], Russell & Taylor Delta Tibial Nail 9 mm [RTD], Russell & Taylor Reconstruction Tibial Nail 11 mm [RTR], Brooker & Wills Nail 11 mm [B-W], Grosse & Kempf Tibial Nail 11 mm [G-K], AO Universal Tibial Nail 11 mm [AOU], Klemm & Schellmann Tibial Nail 11 mm [K-S], Börner & Mattheck Tibial Nail 11 mm [B-M]) according to the manufacturers' recommendations. Each BIC was tested mechanically using a Zwick universal testing machine. Torque testing was performed with a maximum moment of 5 Nm in both directions. Maximum axial load was 1100 N. Four-point bending was done up to a moment of 66 Nm. For each implant bending and torque stiffness of the nail were calculated and compared with the test results obtained in the cadaver study. A P-value less than 0.05 (least significance difference test, ANOVA) was considered significant.(ABSTRACT TRUNCATED AT 250 WORDS)