Mechanical and corrosion properties of partially degradable bone screws made of pure iron and stainless steel 316L by friction welding

Abstract

This paper reports a series of in vitro, ex vivo and in vivo mechanical and corrosion studies of pin and screw prototype made of friction welded pure iron and 316L type stainless steel aiming to evaluate the applicability of the partially removable bone screws. Results showed that the pin possesses bending, tensile and torsional strengths of 1706 ±147, 666±7 and 0.34±0.03 MPa, respectively. The pin degraded at an average weight loss rate of 17.15×10−5 g cm−2day−1 and released Fe ions at an average concentration of 2.38 ppm. Plastic deformation induced by torsion increased the corrosion rate of the pin from 0.0014 to 0.0137 mm year−1. The maximum pull-out load of the screw prototypes was 3800 N with a calculated failure strength by shear load equal to 22.2 kN which is higher than the strength of the cortical bone. Detailed analysis of the rat’s blood cells during 60 days of the pin implantation indicated a normal response with low neutrophils/ lymphocytes ratio of 0.3‒0.5. Iron ion concentration in the rat’s blood slightly increased from 55 to 61 ppm without affecting the tissue recovering and healing phase. Histological evaluation confirmed the presence of macrophage cells as a normal response to the released iron particles around the iron section of the pin.