Anterior Lumbar Interbody Fusion Using Reaction Bonded Silicon Nitride Implants: Long-Term Case Series of the First Synthetic Anterior Lumbar Interbody Fusion Spacer Implanted in Humans

Abstract

Background: In this study, a historical case series is reported of reaction bonded silicon nitride (Si₃N₄) implants for anterior lumbar interbody fusion (ALIF) for a patient population of 30 and surgery levels L3/4, L4/5, and/or L5/S1. Before the study, the only work on Si₃N₄ as a biomedical material was associated preliminary work, which involved animal trials using a rabbit model. The objective was to undertake the first use of Si₃N₄ as a biomedical material for humans, as an implant for ALIF.

Methods: The Si₃N₄ implants were prepared by die-pressing silicon powder and reaction bonding in 95 N₂/5 H₂ at ∼1400°C for ∼50 hours. The surgeries involved a retroperitoneal approach for L3/4 and L4/5 levels and a transperitoneal approach for L5/S1 level. The patient follow-up involved assessment of radiologic fusion up to 30 years and clinical outcomes to 10 years.

Results: The reaction bonded Si₃N₄ implants were found to be biologically safe and to show high fusion rates with minimal subsidence, no abnormal reaction, and no other complications. The primary outcome measure, visual analog scale back pain, improved from a preoperative mean of 8.4 (range, 6-10) to a mean of 3.7 (range, 0-9) at 5 years and a mean of 4.9 (range, 0-9) at 10 years. The Oswestry Disability Index improved from a preoperative mean of 48 (range, 26-84) to a mean of 35 (range, 4-76) at 10 years.

Conclusions: This study confirms that Si₃N₄ is biologically safe in the long-term, with capacity for excellent radiologic osseointegration.

Keywords: 30 years outcome; ALIF spacer; Anterior lumbar interbody fusion; Human implantation; Intervertebral spacer; Reaction bonded Si(3)N(4); Silicon nitride.