[Removal torques of small screws of steel and titanium with different surfaces]

This study was designed to investigate the relationship between metal surfaces and bone holding strength and the safety margin against breakage available in small screws at implant removal. Small cortical bone screws of 1.5 and 2 mm outer diameter (mini-screws) made of stainless steel or commercially pure titanium have been mechanically and chemically treated to give different surface conditions. They were implanted in randomized groups of four or five speciments into the proximal halves of rabbit tibiae. Tightening was performed with a standardized torque of 10 or 18 Ncm by means of an instrumented screwdriver. The animals were sacrificed after 12 weeks and screw removal torques were measured. Stainless steel screws used as reference screws were found to have loosened their seat by 27% compared with the original tightening torque. Titanium implants, in contrast, revealed improved bone contact with higher removal torques varying from 130% to 160%. No correspondence could be found between these values and the surface roughness of glass-ball-blasted versus polished screws.