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The lag screw osteosynthesis of medial femoral neck fractures is a straightforward procedure with little danger of complications. A conclusion may be drawn from the results of several authors that this method may even have a favourable influence in reducing the frequency of pseudarthrosis. In order to come to a final conclusion, however, the results of large treatment programmes would have to be studied. Fractures which are technically accurately treated allow probably full weight bearing.

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.

In postmortem examinations 26 hip joints of 23 adult individuals were filled with Ringer's solution in order to furnish data on the relationship between intra-articular pressure, capsular content, and joint position. A tube system was connected to an 18-gauge needle and to a pressure transducer. After instillation of 2.5, 5.0, 7.5, and 10.0 ml of Ringer's solution, intra-articular pressure was recorded in six different joint positions (extension, 45° and 90° flexion, 40° internal and 40° external rotation, and 45° abduction in extension). Intra-articular pressure depends mainly on the position of the joint, with highest values in external and internal rotation and lowest values in 45° flexion. In this last position, the pressure was zero in two of four cases after injection of 10 ml, whereas with external rotation and instillation of 2.5 ml almost 60 kPa was recorded. In extension and after filling with 10 ml the pressure exceeded the level of normal systolic blood pressure in all cases. Under pathologic conditions intra-articular pressure of the hip joint can rise well above arterial pressure and thus eventually compromise vascular supply to the femoral head, even in the presence of only minimal intra-articular effusion.

An experimental study in rats was done to investigate the bone-regenerating properties of collagen apatite (Collapat) and to compare it with osteoinduction dependent on osteogenin-containing gelatine (OCG). The test substances were implanted orthotopically (calvarial defect — 7 mm in diameter) and heterotopically (paravertebral muscles, abdominal muscles). The results were evaluated histologically and enzymatically (alkaline phosphatase). Collapat caused neither osteoinduction in the heterotopic site nor healing of the bone defects. Foreign body reaction without new bone formation was encountered. OCG implantation leads to new bone formation in the muscles within 3 weeks, associated with a significant increase in alkaline phosphatase activity, and to extensive new bone formation in the calvarial defect within 4 weeks. The defects did not heal if left empty. The value of clinical application of Collapat appears to be doubtful. Osteoinduction with OCG requires further experimental investigation.