Vergleichende Stabilitätsuntersuchungen an DHS- und DHN-Osteosynthesen subkapitaler Pauwels-III-Osteotomien als Modell höhergradig instabiler medialer Schenkelhalsfrakturen

Abstract

High failure rates due to instability and redisplacement are reported using the dynamic hip screw-fixation of femoral neck fractures in the elderly. This fact was the reason for the development of the dynamic hip nail (DHN) which has a TU cross-section profile and should provide more resistance against cutting through the femoral head trabecular bone than the commonly used DHS. For a comparing stability investigation fixation of vertical femoral neck osteotomies in eight pairs of macroscopically normal cadaver femora was performed with either the DHS supplemented proximally with a parallel 6.5 mm cancellous screw or the DHN. Fixed in a testing machine at an adduction angle of 9° the specimens were vertically loaded from 1 kN up to 4.5 kN with a stepwise increase of 0.5 kN and 500 load cycles within each load level. The applied loads were increased until a distal displacement of the femoral heads of 2 mm was measured. From the recorded loads and number of cycles at the 2 mm distal head displacement the overall load bearing capacity of the DHS- and DHN-osteosyntheses were calculated in kN. The test results indicated that the DHN provided a much more stronger support for the femoral head against dynamic vertical compression loads than the DHS. In three of the eight DHN-osteosyntheses the 2 mm distal head displacement did not occur after 500 cycles at the maximal load of 4.5 kN. The mean overall load bearing capacity of the DHS-osteosyntheses at the 2 mm distal head displacement was 4301,4 kN and that of the DHN-osteosyntheses was 8332,3 kN. Especially in the pairs of femora with weak bone the DHN provided a much more higher resistance against cutting through than the DHS. From the biomechanical standpoint the results indicate that the DHN seems to be a more saver device for the fixation of femoral neck fractures than the DHS.