The stainless steel screw as an orthopaedic implant

The study reported here dealt with the metallographical inspection and analysis of corrosion behavior of screws for mild Slipped Capital Femoral Epiphysis (SCFE), which is a serious orthopedical problem in early adolescence. The screws were fabricated from low carbon, austenitic grade stainless steel type 316L (AISI) with 1600 ppm of nitrogen (316LN2). They were clinically tested in a nearly 25-year-long study where patients in early adolescence were operated with these screws with the fixation in situ. Since the results obtained in all patients were similar, we present the results obtained in screws removed after 3 years of implantation from a 14-year-old boy. Metallographical inspection results reveal that a low content of carbon in the SS 316LN2, and annealing this material at a high temperature ensures the monophasic microstructure of austenitic grains. In the grains twin bands that appeared during mechanical deformation were observed. As the hardness measured at the stalk (262 HV) was lower than that measured at the head of the screw (293 HV) it was concluded that cold mechanical deformation of the head was much more extensive than that of the stalk. Accordingly, corrosion of the head could be more probable than that of the stalk. Corrosion behavior of the screw in simulated physiological media (0.9% NaCl solution), carried out by the electrochemical method, gave an E-corr of -0.124 V (vs. SCE), corrosion current density of 21 nA/cm(2), and corrosion rate of 0.010 +/- 0.005 mpy. This low corrosion rate indicates that SS 316LN2 expresses relatively high corrosion resistance in a saline solution. However, potentiodynamic polarization measurements, where the specimen was driven to more than 1.8 V above the E-corr, showed that in the region between -0.1 and 0.15 V the specimen corroded, while around 0.2 V a small valley appeared where passivation proceeded. The narrow passive region between 0.15 and 0.25 V indicated that chlorides prevented the formation of thick protecting films. With potentials more positive than 0.3 V the passivating film broke down and the specimen be-an to corrode. These data indicate that SS 3.16LN2 can not be effectively protected by passivation since it is not-corrosion resistant in a chloride containing environment.