Introduction: Bony on-growth is an important requirement for interbody fusion devices. PEEK spinal fusion implants have demonstrated to develop a fibrous tissue layer around the implant which compromises its attachment strength to bone. Clinically, this raises the concern of graft dislodgement as well as pseudoarthrosis. While titanium implants have demonstrated greater osseous integration, clinical assessment of bone integration is limited due to the imaging characteristics of the material. Silicon nitride (Si3N4) is a novel biomaterial with favorable imaging characteristics, utilized in spinal interbody fusion devices. An in vivo comparison of bone integration between silicon nitride, PEEK, and titanium has not been performed to date.
Methods: Uniform samples of Si3N4, PEEK, and Ti were implanted into the calvaria (skull) of Wistar rats (2 years old) using standard techniques. Rats were sacrificed at 3, 7, and 14 days. Resected samples of bone and implants were subjected to a mechanical push-out test to assess the bony integration strength on a Micro Tester 5848 (Instron) with a l-kN load cell.
Results: Push-out forces for bone bonded to PEEK samples were 4.0, 5.0 and 5.7 N after 3, 7, and 14 day implantations. For Ti samples, push-out forces were 8.8, 10.5 and 10.5 N, respectively. For Si3N4, push-out forces were 10.1,12.0 and 22.1N, respectively. Push-out strengths for Titanium were nearly twice that of PEEK, while Si3N4 samples had strengths higher than Ti at all-time points. At 3 and 7 days, Si3N4 exhibited a 14-20% increase in push-out strength and at 14 days nearly twice that of Titanium.
Conclusions: At 14 days, Si3N4 implants provided substantially higher bony integration compared to Titanium. Both Si3N4 and Ti demonstrated greater push-out strengths than PEEK at all time points. Early bone integration onto implanted devices is favorable to devices not as "bone-friendly". Further analysis radiographically would provide useful clinical data.
Patient Care: Enhancing bony on-growth surrounding a device would be beneficial when fusion is desired. This material would appear to be superior to existing materials commonly used in this laboratory model.
Learning Objectives: By the conclusion of this session, participants should describe the differences in bone integration with silicon nitride, PEEK, and titanium material samples implanted in an in vivo model.