# Interfaces between Cranial Bone and AISI 304 Steel after Long-Term Implantation: A Case Study of Cranial Screws

## TL;DR

This study examines how cranial bone interacts with steel screws over 42 years, revealing a corrosion process linked to bone resorption.

## Contribution

The paper introduces a novel understanding of long-term corrosion and bone turnover at cranial implant interfaces.

## Key findings

- Peri-implant bone tissue is generally younger than distant cranial bone.
- An 80 nm thick steel surface layer enriched with oxygen indicates corrosion.
- Released iron ions stimulate osteoclast activity, accelerating bone resorption.

## Abstract

Interfaces between
AISI 304 stainless steel screws and
cranial
bone were investigated after long-term implantation lasting for 42
years. Samples containing the interface regions were analyzed using
state-of-the-art analytical techniques including secondary ion mass,
Fourier-transform infrared, Raman, and X-ray photoelectron spectroscopies.
Local samples for scanning transmission electron microscopy were cut
from the interface regions using the focused ion beam technique. A
chemical composition across the interface was recorded in length scales
covering micrometric and nanometric resolutions and relevant differences
were found between peri-implant and the distant cranial bone, indicating
generally younger bone tissue in the peri-implant area. Furthermore,
the energy dispersive spectroscopy revealed an 80 nm thick steel surface
layer enriched by oxygen suggesting that the AISI 304 material undergoes
a corrosion attack. The attack is associated with transport of metallic
ions, namely, ferrous and ferric iron, into the bone layer adjacent
to the implant. The results comply with an anticipated interplay between
released iron ions and osteoclast proliferation. The interplay gives
rise to an autocatalytic process in which the iron ions stimulate
the osteoclast activity while a formation of fresh bone resorption
sites boosts the corrosion process through interactions between acidic
osteoclast extracellular compartments and the implant surface. The
autocatalytic process thus may account for an accelerated turnover
of the peri-implant bone.