Introduction

Magnesium (Mg)-based implants constitute a promising alternative to gold-standard materials in the orthopedic field, due to their favorable mechanical and osteoconductive properties. Moreover, implant removal surgeries are obviated, due to their ability to degrade in the body. However, controlling the degradation rate of Mg-alloys remains challenging. Rapid Mg-degradation is associated with excessive hydrogen gas formation, which may compromise bone-to-implant-contact (BIC) and screw stability in the bone. To enable constant replacement of the degraded implant material with newly formed tissue, slow and homogeneous implant degradation is crucial. Various factors influence the degradation behavior of Mg-based implants, such as added alloying elements, microstructure, and surface treatments. Here, we evaluated the influence of a plasma-electrolytic-oxidation (PEO) surface treatment on the degradation behavior and osseointegration of Mg-Zinc-Calcium (ZX00) screws in a big animal model.

Material and methods

A total of 37 juvenile sheep was used for this study. We performed implantation of up to three screws into each tibia shaft of the sheep, using ZX00-, PEO-ZX00 and titanium (Ti) screws. The animals were euthanized 0, 3, 6, 12 or 24 weeks post-surgery. Tibiae were harvested and scanned with a micro-computed tomography (µCT) device. The obtained µCT data were used to compute volume loss, gas evolution and degradation rate of ZX00- and PEO-ZX00 screws at the given time points. Bone histology was performed on embedded bone samples to gain information on the bone quality in the peri-implant area. Furthermore, biomechanical pullout tests were performed to obtain additional information on osseointegration of the different screw materials.

Results

The µCT data revealed a slow and homogeneous degradation of both, ZX00- and PEO-ZX00 screws until 24 weeks after implantation. The PEO-treatment successfully decreased the degradation rate of the ZX00-screws from the 6-week time point on. Furthermore, enhanced BIC was observed around PEO-ZX00 screws compared to ZX00-screws in the histology and higher pullout forces were measured.

Conclusion

The observed degradation rate deceleration and enhanced osseointegration of PEO-treated screws indicates effectiveness of this technique. Especially for patients with extended fracture healing durations the use of PEO-treated Mg-screws is highly promising. Further investigations with fractures are needed to verify, that fracture healing is promoted when using PEO-treated implants for fracture stabilization.