Anti-infective and osteointegration properties of silicon nitride, poly(ether ether ketone), and titanium implants

Silicon nitride (Si₃N₄) is an industrial ceramic used in spinal fusion and maxillofacial reconstruction. Maximizing bone formation and minimizing bacterial infection are desirable attributes in orthopedic implants designed to adhere to living bone. This study has compared these attributes of Si₃N₄ implants with implants made from two other orthopedic biomaterials, i.e. poly(ether ether ketone) (PEEK) and titanium (Ti). Dense implants made of Si₃N₄, PEEK, or Ti were surgically implanted into matching rat calvarial defects. Bacterial infection was induced with an injection of 1 × 10⁴ Staphylococcus epidermidis. Control animals received saline only. On 3, 7, and 14 days, and 3 months post-surgery four rats per time period and material were killed, and calvariae were examined to quantify new bone formation and the presence or absence of bacteria. Quantitative evaluation of osteointegration to adjacent bone was done by measuring the resistance to implant push-out (n = 8 rats each for Ti and PEEK, and n = 16 rats for Si₃N₄). Three months after surgery in the absence of bacterial injection new bone formation around Si₃N₄ was ∼69%, compared with 24% and 36% for PEEK and Ti, respectively. In the presence of bacteria new bone formation for Si ₃N₄, Ti, and PEEK was 41%, 26%, and 21%, respectively. Live bacteria were identified around PEEK (88%) and Ti (21%) implants, whereas none were present adjacent to Si₃N₄. Push-out strength testing demonstrated statistically superior bone growth onto Si ₃N₄ compared with Ti and PEEK. Si₃N₄ bioceramic implants demonstrated superior new bone formation and resistance to bacterial infection compared with Ti and PEEK.