Intramedullary osteosynthesis is preferred for shaft fractures of the long bones of the lower extremities because it generally results in early weight-bearing stability, allowing full function of the leg and rapid rehabilitation. Only transverse and short oblique fractures near the middle of the medullary cavity can be sufficiently stabilised using intramedullary nails alone. Additional stabilising aids such as cerclages or interlocking nails must be used for all other types of shaft fracture. There are no previous studies of stability that make a comparison between conventional intramedullary nails and interlocking bolts with or without cerclages. This paper set out to clarify therapeutically relevant questions in the light of a comparative study of stability. To this end a comparative experimental investigation was carried out using femora from human cadavers to determine whether or not either method of intramedullary osteosynthesis attained the stability of the intact bone. The study also set out to establish the relevance of the type of fracture, localisation and mode of fitting to the resulting stability of the osteosynthesis. A comparison was also made of the stability in the bone-implant complex of two different commercially available interlocking nails. Finally, the question was raised as to the clinical conclusions that can be drawn from the present investigation. The bone-implant complex represented by a fracture of the femur secured by an interlocking nail is a complex mechanical system. An optical measuring arrangement was developed so as to ensure that this system was not subject to any interference resulting from the method of measurement. This consisted of a laser light source which projected a beam of light parallel to the axis of the femur shaft across the fracture onto a mirror system attached to the bone. The rays of light reflected from the mirrors were recorded as dots of light on measuring screens. A load of up to 1000 N was gradually applied along the bearing axis, and the resulting changes in the position of the parts of the osteosynthesised fracture relative to one another indicated by the deviation of the beams of light. The axial tilt and rotation of the pieces of bone could be determined from the coordinates of the dots of light. The stability of nailed transverse femoral fractures (n = 6), short oblique fractures (n = 6), long oblique fractures (n = 6) and comminuted fractures (n = 6) was determined and compared with the deformation of intact femora. The stability of all types of osteosynthesis was several times less than that of the intact bones.(ABSTRACT TRUNCATED AT 400 WORDS)