[The experimental measurement of torque and axial force of 6.5 mm ASIF lag screws in fixation of femoral neck fractures (author's transl)].

An experimental study was undertaken to evaluate the torque and axial force of the 6.5 mm ASIF lag screw implanted in the femoral neck after a subcapital osteotomy. The experiments were carried out on autopsied bones of more than 60 years of age. The average torque was 204 cmN, the screws in the upper half of the femoral head showing a higher torque than those in the lower half. The axial tension was measured by strain gauge transducer. Depending on the length of the thread of the screw (16 or 32 mm) the average tension reaches 1162 N or 1960 N. By tightening the screw until the impression of optimum fit is obtained, 70% of the maximum axial force is reached. It can therefore be estimated that at the beginning an interfragmentary pressure of 4000 to 8000 N is exerted on a medial femoral neck fracture fixed by four ASIF lag screws.