Abstract
Introduction
The ability to manufacture implants at the point-of-care has become a desire for clinicians wanting to provide efficient patient-specific treatment. While some hospitals have adopted extrusion-based 3D printing (fused filament fabrication; FFF) for creating non-implantable instruments with low-temperature plastics, recent innovations have allowed for the printing of high-temperature polymers such as polyetheretherketone (PEEK). Due to its low modulus of elasticity, high yield strength, and radiolucency, PEEK is an attractive biomaterial for implantable devices. Though concerns exist regarding PEEK for orthopaedic implants due to its bioinertness, the creation of porous networks has shown promising results for bone ingrowth.
In this study, we endeavor to manufacture porous PEEK constructs via clinically-used FFF. We assess the effect of porous geometry on cell response and hypothesize that porous PEEK will exhibit greater preosteoblast viability and activity compared to solid PEEK. The work represents an innovative approach to advancing point-of-care 3D printing, cementless fixation for total joint arthroplasty, and additional applications typically reserved for porous metal.
Methods
Three porous constructs – a rectilinear pattern and two triply period minimal surface (TPMSs) - were designed to mimic the morphology of trabecular bone. The structures, along with solid PEEK samples for use as a control, were manufactured via FFF using PEEK. The samples were mCT scanned to determine the resulting pore size and porosity. The PEEK constructs were then seeded with pre-osteoblast cells for 7 and 14 days. Cell proliferation and alkaline phosphatase activity (ALP) were evaluated at each time point, and the samples were imaged via SEM.
Results
mCT imaging showed the pores in the PEEK constructs to be open and interconnected. The average pore size was 535 ± 92 µm for the rectilinear, 484 ± 237 µm for the diamond, and 669 ± 216 µm for the gyroid. Porosity was 71% for the rectilinear, 76% for the diamond, and 68% for the gyroid. The average error between the theoretical and actual values was −37.3 µm for pore size and −2.3 % for porosity.
Normalized ALP activity of the three porous PEEK samples at 7 days were found to be significantly greater than the solid sample (p < 0.05 rectilinear, p < 0.005 gyroid, p < 0.001 diamond). At 14 days, the same relationships were observed (p < 0.001 for all three designs). No difference between the three geometries was found.
SEM imaging revealed cells with flat, elongated morphology attached to the surface of the PEEK. The 14-day samples appeared to have proliferated well and spread along the PEEK pores. Extensions of filopodia and lamellipodia were observed along with large blankets of cells covering the PEEK surface.
Discussion
We demonstrated the ability of FFF printed porous PEEK surfaces to promote cellular processes necessary for bone-implant fixation. While all porous structures showed promising results, more investigation into their material characteristics and osteogenic potential are necessary to determine which geometry may be suitable for orthopaedic use. Our work offers an innovative approach to advancing point-of-care 3D printing, cementless implant fixation, and additional applications typically reserved for porous metal.
