Effect of bone mineral density and amorphous diamond coatings on insertion torque of bone screws

In this study, the potential of high-quality amorphous diamond (AD) coatings in reducing the torque and failures of bone screws was studied. Torque values were recorded for 32 stainless steel screws, 2.7 or 3.5 mm in diameter and 60 mm in length. Half of the screw sets were coated with the AD coating before installing in predrilled holes of human cadaveric femoral bone samples. The bone samples were selected from two groups of four persons with mean ages of 34 years (range 25-41 years) and 75 years (range 73-77 years), respectively. The bone mineral density (BMD) values of the samples were determined exactly at the screw insertion site by peripheral quantitative computed tomography (pQCT). In the mechanical tests, insertion and removal torques were measured. BMD had a significant effect on insertion torque; the maximum torque (adjusted with respect to the screw diameter) was significantly higher for the young bone than for the old bone (p < 0.05). By using a polished AD coating, insertion torque was decreased even up to 50% in comparison with the screws without coating. The results suggest that AD coating provides a stable, smooth surface and reduces the risk of screw failures.