A long-term quasi-equivalent in vitro study of high-strength Zn alloy interference screws for anterior cruciate ligament reconstruction

Biodegradable Zn alloy interference screws are promising for anterior cruciate ligament reconstruction. However, previous in vitro studies not only predominantly focus on short-term (≤30 days) immersion, but also neglect influence of crevice corrosion in bone tunnel. This study develops a Zn-0.45Mn-0.2 Mg alloy with yield strength of 288.3 MPa, ultimate tensile strength of 325.4 MPa and elongation of 26.3 %. The alloy is fabricated into screws, inserted into polyurethane bone blocks and immersed in Hank's balanced salt solution for 180 days. The alloy degraded at a corrosion rate of 0.07 mm/y, with its yield strength and elongation eventually decreasing to 229 MPa and 4.3 %, respectively. Second phase detachment during long-term corrosion reduces alloy's surface fracture toughness (K_IC) and increases stress intensity (K_I) at corrosion pits. This causes premature fracture of the alloy (K_I > K_IC) and a larger decrease in the alloy's ductility (70 %) than that in strength (30 %). Crevice corrosion of the screws originates deep within the bone tunnel and spreads outward over time. Accumulation of Cl⁻ ions exacerbates corrosion severity in the narrow thread tops of the screws. This study provides a foundation for quasi-equivalent in vitro studies of Zn alloy implants, and highlights the necessity of developing ductility-attenuation-resistant Zn alloys.