Drill holes decrease cancellous bone strength: A comparative study of 33 paired osteoporotic human and 9 paired artificial bone samples

Abstract

This study was designed to compare compressive strength of cancellous bone retrieved from the femoral head in a specimen with and without guide wire hole, with comparison to synthetic bone samples. Femoral heads retrieved from 33 patients who sustained femoral neck fractures and underwent hip arthroplasty were cut into cuboids leaving two matching samples from the same femoral head. Similar samples were prepared from synthetic femurs. One of the matching samples was chosen at random and was drilled with a guide wire for cancellous screws. The uniaxial compression tests of bone blocks were carried out using the Zwick-Roell Z020 strength testing machine. The mean loss of sample cross section area due to drilling was 24%. The force at failure in drilled specimens was significantly smaller by 18% in human (median: 26%) and by 25% in synthetic bone (median 27%). The strength of human specimens was almost 2 times greater, and their stiffness nearly 4 times greater than in synthetic samples. The study shows that the weakening of the bone after drilling is roughly proportional to the loss of sample cross section area. The percentage decrease in strength was similar in human and artificial bone, but human samples were stronger and stiffer. The comparison shows that forces measured in biomechanical studies on artificial bone cannot be directly attributed to humans, but the relative differences in mechanical properties of synthetic samples after some damage may be accurate and resemble that of human bones.

Fig 1. The intraoperative radiographs of intertrochanteric fracture fixation.

The shapes drawn in the radiographs show the relative size, shape and direction of the cut bone samples as well as the direction of the hole drilled with the guide wire. (a) anteroposterior view. (b) axial view.

Fig 2. Bone sample fixed to custom-made inserts at Zwick-Roell Z020 testing machine.

Fig 3. Typical result of the compression test for undrilled sample.

The force at failure and stiffness parameters are shown.