4. A comparison of two implant materials in an in vivo rabbit model

Abstract

BACKGROUND CONTEXT

The study of the relationship between the musculoskeletal system and the immune system has become an increasingly important consideration in biomaterials research. Investigation of the immune response provides insight on the efficacy of current design materials in spinal implant devices and also offers a potential clinical understanding of complications. The potential for adverse events, including a fibrotic response, presents challenges that should be addressed. Studies involving the pro-inflammatory cytokine response inform on the mechanism of fibrous development pathway which has been reported with PEEK spinal interbody implants. With bone grafting materials, the probability of arthrodesis and bone apposition increases due to the selected materials. It is hypothesized that bone contact with PEEK devices would be different compared to titanium and the various surface finishes that can be applied.

PURPOSE

The purpose of this study was to compare the in vivo bone response to PEEK as an implant material compared to titanium with different surface roughness as an implant material. Specifically, microCT data was used to quantify new bone formation within the peri-prosthetic region, and histology was used to evaluate bone apposition and the presence of soft tissue at the bone-implant interface

STUDY DESIGN/SETTING

A rabbit distal femur model with two distinct material cohorts was conducted to investigate the immune response and bone growth surrounding PEEK dowels compared to titanium dowels.

PATIENT SAMPLE

N/A

OUTCOME MEASURES

N/A

METHODS

A rabbit distal femoral condyle bone defect model with two separate materials was conducted. PEEK and titanium dowels (4.5mm diameter x 8mm) with a central graft window were implanted into skeletally mature New Zealand White rabbits (3-4 kg). Implants did not contain graft material in the graft chamber and were randomized to either the left or right distal femoral condyle. Test subjects were randomized into either 4- or 8-week cohorts and histological or immunological analyses (4 subjects per cohort). Samples retrieved at necropsy were fixed, and radiographic imaging completed using a SkyScan 1172 desktop MicroCT (16 µm isotropic voxel size). All scans were reconstructed with a constant attenuation coefficient to enable comparison of bone mineral density and volume (CTAn, Bruker).

RESULTS

When observing inflammatory cytokine production, our ELISA data showed a significant decrease in TNFα and IL-1ß in PEEK implants. We also observed a difference in IL-1ß production from MSCs in culture titanium when compared to PEEK alone on Day 7, and a significant difference by Day 14. Mineralization data measured from microCT on harvested distal femurs of implanted rabbits was analyzed using a one-way ANOVA with multiple comparisons. Similarly, higher levels of mineralized bone were evident on the roughened titanium cohort for the layer closest to the implant surface. The 0-136 micron surface showed statistically increased new bone formation when compared to the PEEK cohort. Other titanium surfaces also demonstrated an increase in bone mineralization.

CONCLUSIONS

Biomedical implant development has traditionally been a strong area of research due to the ongoing necessity for immediate stabilization via fixation. The materials considered in device design should incorporate strong, durable, immunomodulatory material characteristics specifically for the acute condition following surgical procedures. PEEK has recently been developed and used in the field of orthopedics for both material and radiolucent properties. Although PEEK is a ubiquitous orthopedic spinal implant material, the inert, hydrophobic surface limits cell adhesion, attachment, and growth, which has the potential to lead to a persistent inflammatory response, fibrosis, and implant failure. The inclusion of traditional titanium implants has mitigated risks associated with the material evident in both the in vitro and short-term animal results. The bone mineralization immediately adjacent to the implant measured in microCT supports the layer at the bone-implant interface. The results suggest the use of titanium surfaces results in increased bone production, potentially leading to more bone apposition to the implant surface due to material and surface finish properties. The preliminary data suggest titanium implants could be advantageous for the suppression of early onset and a potentially sustained inflammatory response, and thus, resulting in bony on-growth with increased chances of arthrodesis.

FDA Device/Drug Status

This abstract does not discuss or include any applicable devices or drugs.