In Vivo Bone Formation and Bacterial Infection Between Silicon Nitride Ceramic, PEEK, and Titanium
Objective Superior bone ingrowth and resistance to bacterial infection are ideal for orthopaedic implants. We compared new bone formation, strength of bone bonding, and infection rates between silicon nitride ceramic (Si3N4; abbreviated SiN), medical-grade PEEK (PEEK), and titanium (Ti) in rat calvariae. PEEK and Ti are used in spinal and arthroplasty implants respectively, while SiN is a non-oxide ceramic used in spinal implants for the past 4 years. Methods Specimens of 10 mm × 10 mm by 1.75 mm size were implanted into experimental calvarial defects in 2-year old Wistar rats using standard surgical techniques (n9s: SiN=48; PEEK=24; Ti=24). One group of animals was immediately inoculated with 1 × 104 Staphylococcus epidermidis ; control animals received saline only. After sacrifice at 3 days, 7 days, 14 days, and 3 months post-inoculation (n=4 rats per time period), one calvarial sample each for PEEK and Ti, and two samples for SiN (per bacterial condition and time point) were retrieved for histology; remaining samples were used for sample push-out testing with a Micro Tester 5848 (Instron) with a 1-kN load cell, using published techniques. New bone formation was measured with tetracycline double-labeling at 11 and 4 days before the 14-day and 3-month time periods. Results Of the materials tested, 3-month bone ingrowth and periprosthetic infection rates were most favorable for SiN (Fig. 1). At 90 days post-implantation without bacteria, new bone growth in calvariae with SiN was ∼69% compared with 24% and 36% for PEEK and Ti, respectively. With bacterial inoculation, new bone growth was 21%, 26% and 41% for PEEK, Ti, and SiN, respectively. At 3-months, live bacteria were observed for PEEK (88%) and Ti (21%), while no bacteria were present around SiN (Fig. 2). Push-out strength was greater for SiN when compared to Ti and PEEK (Fig. 3). Conclusions Superior bone formation, bone ingrowth, and bacterial resistance were associated with SiN when compared to Ti and PEEK. SiN proved effective in inhibiting bacteria and promoting osteogenesis in experimental osseous defects, when compared to Ti and PEEK. These observations are most likely related to the hydrophilic properties of SiN that contributed to the adsorption of proteins known to decrease bacteria attachment and growth (vitronectin and fibronectin). We conclude that SiN may be a superior biomaterial for the development of orthopaedic implants.