title: Do Surface Porosity and Pore Size Influence Mechanical Properties and Cellular Response to PEEK?
abstract: BACKGROUND
  Despite its widespread use in orthopaedic implants such as soft tissue fasteners and spinal intervertebral implants, polyetheretherketone (PEEK) often suffers from poor osseointegration. Introducing porosity can overcome this limitation by encouraging bone ingrowth; however, the corresponding decrease in implant strength can potentially reduce the implant's ability to bear physiologic loads. We have previously shown, using a single pore size, that limiting porosity to the surface of PEEK implants preserves strength while supporting in vivo osseointegration. However, additional work is needed to investigate the effect of pore size on both the mechanical properties and cellular response to PEEK.
  
  
  OBJECTIVE
  (1) Can surface porous PEEK (PEEK-SP) microstructure be reliably controlled? (2) What is the effect of pore size on the mechanical properties of PEEK-SP? (3) Do surface porosity and pore size influence the cellular response to PEEK?
  
  
  METHODS
  PEEK-SP was created by extruding PEEK through NaCl crystals of three controlled ranges: 200 to 312, 312 to 425, and 425 to 508 µm. Micro-CT was used to characterize the microstructure of PEEK-SP. Tensile, fatigue, and interfacial shear tests were performed to compare the mechanical properties of PEEK-SP with injection-molded PEEK (PEEK-IM). The cellular response to PEEK-SP, assessed by proliferation, alkaline phosphatase activity, vascular endothelial growth factor production, and calcium content of osteoblast, mesenchymal stem cell, and preosteoblast (MC3T3-E1) cultures, was compared with that of machined smooth PEEK and Ti6Al4V.
  
  
  RESULTS
  Micro-CT analysis showed that PEEK-SP layers possessed pores that were 284 ± 35 µm, 341 ± 49 µm, and 416 ± 54 µm for each pore size group. Porosity and pore layer depth ranged from 61% to 69% and 303 to 391 µm, respectively. Mechanical testing revealed tensile strengths > 67 MPa and interfacial shear strengths > 20 MPa for all three pore size groups. All PEEK-SP groups exhibited > 50% decrease in ductility compared with PEEK-IM and demonstrated fatigue strength > 38 MPa at one million cycles. All PEEK-SP groups also supported greater proliferation and cell-mediated mineralization compared with smooth PEEK and Ti6Al4V.
  
  
  CONCLUSIONS
  The PEEK-SP formulations evaluated in this study maintained favorable mechanical properties that merit further investigation into their use in load-bearing orthopaedic applications and supported greater in vitro osteogenic differentiation compared with smooth PEEK and Ti6Al4V. These results are independent of pore sizes ranging 200 µm to 508 µm.
  
  
  CONCLUSIONS
  PEEK-SP may provide enhanced osseointegration compared with current implants while maintaining the structural integrity to be considered for several load-bearing orthopaedic applications such as spinal fusion or soft tissue repair.
authors.affiliation: Mechanical Engineering, Guldberg Musculoskeletal Research Laboratory, Georgia Institute of Technology, 315 Ferst Drive NW, Atlanta, GA, 30332, USA. brennan@gatech.edu.
authors.forename: F Brennan
authors.initials: FB
authors.lastname: Torstrick
authors.affiliation: Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
authors.forename: Nathan T
authors.initials: NT
authors.lastname: Evans
authors.affiliation: Mechanical Engineering, Guldberg Musculoskeletal Research Laboratory, Georgia Institute of Technology, 315 Ferst Drive NW, Atlanta, GA, 30332, USA.
authors.forename: Hazel Y
authors.initials: HY
authors.lastname: Stevens
authors.affiliation: Mechanical Engineering and Materials Science, Duke University, Durham, NC, USA.
authors.forename: Ken
authors.initials: K
authors.lastname: Gall
authors.affiliation: Mechanical Engineering, Guldberg Musculoskeletal Research Laboratory, Georgia Institute of Technology, 315 Ferst Drive NW, Atlanta, GA, 30332, USA.
authors.forename: Robert E
authors.initials: RE
authors.lastname: Guldberg
mesh_terms: D016475:3T3 Cells; D000497:Alloys; D000818:Animals; D001577:Benzophenones; D015415:Biomarkers / Q000378:metabolism; D049109:Cell Proliferation; D055119:Elastic Modulus; D019544:Equipment Failure Analysis; D006801:Humans; D007659:Ketones / Q000737:chemistry*; D008422:Materials Testing; D059630:Mesenchymal Stem Cells / Q000378:metabolism / Q000502:physiology*; D051379:Mice; D019637:Orthopedic Procedures / Q000295:instrumentation*; D016348:Osseointegration; D010006:Osteoblasts / Q000378:metabolism / Q000502:physiology*; D010012:Osteogenesis; D011092:Polyethylene Glycols / Q000737:chemistry*; D011108:Polymers; D016062:Porosity; D019736:Prostheses and Implants; D011474:Prosthesis Design; D011475:Prosthesis Failure; D019919:Prosthesis Implantation / Q000295:instrumentation*; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D013997:Time Factors; D014025:Titanium / Q000737:chemistry; D055114:X-Ray Microtomography
chemical_list: D000497:Alloys; D001577:Benzophenones; D015415:Biomarkers; D007659:Ketones; D011108:Polymers; C031462:titanium alloy (TiAl6V4); C063834:polyetheretherketone; D011092:Polyethylene Glycols; D014025:Titanium