The virtues of polyetheretherketone (PEEK) as a material for spinal implants are well documented. PEEK’s radiolucency along with its similar strength and stiffness to bone have led to the widespread use of the material in implant manufacture. However, PEEK is a hydrophobic substance, with a number of studies associating it with fibrous encapsulation. Titanium is growing in popularity as an alternative implant material for its osseointegrative properties. Titanium, however, produces imaging artefacts that can limit a surgeons’ visualisation of the surgery site. A new study presented at the annual meeting of the International Society for the Advancement of Spinal Surgery (ISASS17; 12-14 April, Boca Raton, USA) has demonstrated that, when manufactured with a porous surface, PEEK can actually lead to greater osseointegration than micro-textured titanium, with none of the imaging drawbacks.
J Kenneth Burkus (Hughston Clinic, Spine Service, Columbus, USA) and colleagues used in vitro and in vivo testing to discover how bone would respond to smooth-surface PEEK, porous PEEK (300μm pore size) and micro-textured titanium plasma-coated PEEK.
The team grew MC3T3—an osteoblast precursor cell line derived from mouse calvaria—in an osteogenic media on each of the three materials, harvesting cells after 14 days. “Osteogenic markers studied included osteocalcin, calcium and vascular endothelial growth factor (VEGF),” Burkus—who presented the data—told the audience.
Osteocalcin and calcium mark “osteogenic differentiation” and are “associated with cell culture mineralisation”, while the angiogenic marker VEGF offers an “indication of vascular tissue formation,” the presentation noted.
Statistically significantly (p<0.01) greater amounts of all three markers were observed in the porous PEEK cultures compared to the titanium-coated PEEK and the smooth-surface PEEK. This translates to “greater osteoblast maturation, better mineralisation and advanced osteogenesis.”
Results from in vivo testing were similarly positive for the porous PEEK. Cylindrical implants of each material were placed in an established rat tibial defect model. The researchers analysed bone-implant interface with micro computed tomography scanning
(μCT) at eight weeks, as well as pullout testing.
“ΜCT scanning revealed only a thin layer of mineralised tissue near the surface of the smooth implants. In contrast, substantial mineralisation is seen here in the porous architecture of the porous PEEK implants,” Burkus reported. Metal artefact on the μCT scan of the titanium-coated implant made mineralised tissue analysis impossible.
Significantly (p<0.05) greater pullout failure loads were observed for the porous PEEK
implants in comparison with the other two implants.
These results revealed that not only did porous PEEK result in a more differentiated bone cell phenotype, but it showed greater implant fixation than microtextured titanium plasma-coated PEEK. The smooth PEEK and titanium-coated PEEK “performed similarly in vitro and in vivo,” the presentation noted.
Perhaps most importantly, this study revealed that PEEK implants “do not necessarily generate a fibrous response.” Conventional wisdom that PEEK inhibits osseointegration de facto may be false. “Porous PEEK,” Burkus asserted, “may become a clinically viable alternative for improving osseointegration of intradiscal interbody implants.”
“Burkus noted that, rather than suggesting that porous PEEK (in this study, porosity: 67% and interconnectivity: >99%) might offer a superior platform for osseointegration than titanium in general, it could be the surface porosity itself which promotes such a marked osteogenic response. “Surface topography may play a greater role than implant chemistry, with three-dimensional macro-porous features exhibiting improved osseointegration compared to smooth and two-dimensional micro-scaled textured surfaces,” he told the audience.
Richard Guyer (Texas Back Institute, Plano, USA) gave a talk immediately prior to Burkus’, offering evidence that a deeply porous scaffold (Forticore, Nanovis; thickness 750μm, pore size 500μm, porosity 70%) comprised of layers of titanium “bonded together and injection-moulded with PEEK” may lead to significantly greater (p<0.05) osseointegration than titanium plasma-coated PEEK. Guyer and colleagues recorded shear pushout strength of 10.2MPa for the porous scaffold in comparison to 5.7MPa for the titanium-coated PEEK, suggesting starkly greater osseointegration.
“I think it is more of a surface issue, that is, surface topography,” Guyer noted after the talks, when asked what might make the best material for spinal implants. “I would never have thought you could make porous PEEK bioactive, but I must say that I am impressed.”
“Moving forward, we are really going to have to study looking at macro-size of the pores and interconnectivity…What is overall porosity?” Burkus concluded. “As surgeons and clinicians, we are going to have to make sure that we understand what we are talking about.”
“The primary focus of current studies is to investigate the relative importance of implant chemistry and surface topography in determining implant osseointegration,” Burkus told Spinal News International, commenting on the next steps for this research. “It has been shown that bone grows into the pores and provides strong fixation. Now the question being investigated is why.”