Fatigue of Small Bone Fragment Fixation Plates Made from Low-Nickel Steel

Today's implant quality stainless steels contain up to 16 wt% nickel although nickel ions are the most widespread skin contact allergens. Previously sensitized persons may develop allergic reactions when nickel is released from stainless steel medical implants. New low-nickel stainless steels combine the benefits of excellent mechanical properties with virtual absence of nickel.

Miniature bone plates and corresponding 2.0 mm screws for the fixation of small bone fragments were produced of a low-nickel stainless steel. The implants were tested in a static reverse-bending setup and under dynamic conditions, and compared to commercially pure (CP) titanium and standard 316L implant steel counterparts. The low-nickel plate could withstand over 200 cycles of bending, whereas the titanium plate broke at 26 cycles. This confirms the higher tolerance of the low-nickel plate to multiple contouring during surgery.

Nevertheless, high cycle fatigue tests under physiologic conditions showed that the low-nickel steel plates exhibit lower resistance to cyclic loads than titanium and 316L plates. SEM investigations of the fatigue fractures confirmed that the cracks preferentially propagate along grain boundaries leading to intergranular fracture of the low-nickel steel. It is suggested that the intergranular crack initiation facilitates the early failure under high cycle fatigue conditions, whereas plastic bending properties are not affected. The tendency to intergranular crack initiation in the low-nickel steel could stem from surface deformation (work hardening) introduced during machining and related embrittlement in the surface zone.