Abstract
BackgroundMagnesium (Mg) released from Mg-based implants degradation is believed to be effective in improving osteogenesis, however, studies focusing on Mg-based interbody cages are limited and fusion success was never reported. As excessive Mg accumulation can inhibit new bone formation, this study is designed to explain the possible reasons for the fusion failure of Mg-based cages by analyzing the relationships between the intervertebral Mg accumulation and the resulting interbody fusion.MethodsThe experimental cage was consisted of magnesium alloy (AZ31) substrate and Silicon (Si) -containing coating. C3/C4 and C5/C6 of 24 goats were implanted with cage or autologous iliac crest bone graft (Control group), which were analyzed at 3, 6, 12, and 24 weeks post-operatively. Intervertebral Mg concentrations, Mg-related Calcium (Ca)/ Phosphorus (P) ratios, radiological evaluations and histological findings were recorded for analyzing the relationships between the three of cage corrosion, Mg accumulation, and interbody fusion.ResultsIntervertebral Mg levels were significantly increased after cage implantation, especially in the areas that were closer to the cages at 3 weeks post-operatively, and these increased concentrations could persist up to 12 weeks post-operatively, indicating a relatively rapid corrosion process. Significantly lower Mg levels were only found at 24 weeks post-operatively, but these levels were still higher than those of the control group. In addition, Mg was found to be widely distributed at the intervertebral space since high Mg concentrations could even be detected at the posterior boundary of the vertebral body. Under this Mg accumulation profile, interbody fusion was not achieved, as indicated by the decreased Ca/P ratios, low CT fusion scores and negative histological results.ConclusionsIntervertebral excessive Mg accumulation might be the primary reason for interbody fusion failure. Quantitative Mg analysis can offer insight into the association between cage degeneration and biological response.
Highlights
Magnesium (Mg) released from Mg-based implants degradation is believed to be effective in improving osteogenesis, studies focusing on Mg-based interbody cages are limited and fusion success was never reported
Though the authors attributed the fusion failure to the hindering of new bone ingrowth by the PCL coating, we considered that the failure might be due to the excessive intervertebral Mg accumulation: 1) The blood supply environment of the endplate-treated disc space after anterior cervical discectomy and fusion (ACDF) is different from that of cortical bone or cancellous bone, which is believed to influence the corrosion rates of Mg-based implants and the absorption of released Mg ions [11]
Those results indicated that though the general cage corrosion rate was not fast, the intervertebral Mg level was still relatively high, and interbody fusion was not achieved under the Mg release profile measured in our study, as indicated by the following two results: First, our study demonstrated that high Mg concentrations contributed to a steady decrease in the Ca/P ratio, indicating that Ca was partially substituted by Mg during the re-mineralization process, which is consistent with the generally accepted theory that high Mg concentrations can inhibit the precipitation of Ca-containing minerals [23]
Summary
Magnesium (Mg) released from Mg-based implants degradation is believed to be effective in improving osteogenesis, studies focusing on Mg-based interbody cages are limited and fusion success was never reported. As excessive Mg accumulation can inhibit new bone formation, this study is designed to explain the possible reasons for the fusion failure of Mg-based cages by analyzing the relationships between the intervertebral Mg accumulation and the resulting interbody fusion. Though the authors attributed the fusion failure to the hindering of new bone ingrowth by the PCL coating, we considered that the failure might be due to the excessive intervertebral Mg accumulation: 1) The blood supply environment of the endplate-treated disc space after anterior cervical discectomy and fusion (ACDF) is different from that of cortical bone or cancellous bone, which is believed to influence the corrosion rates of Mg-based implants and the absorption of released Mg ions [11]. AZ31 cages were treated with a newly designed micro-arc oxidation (MAO)-treated silicon (Si)-containing coating to increase the corrosion resistance and bone induction activity [14, 15]
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