Porous PEEK Fusion Device Shows High Strength and Facilitates Bone Ingrowth

The surface-porous COHERE® Cervical Interbody Fusion Device has demonstrated high strength, fatigue resistance, and interfacial shear strength in recent studies. The porosity of the all-PEEK device has been demonstrated to facilitate bone ingrowth.

“Porous PEEK (PEEK Scoria®) retains the mechanical properties necessary for intradiscal spinal applications, while potentially accelerating bone cell proliferation and differentiation,” said J. Kenneth Burkus, MD, who has used the device in more than 20 patients undergoing anterior cervical discectomy and fusion surgery. “Importantly, the mean interfacial shear strength of PEEK Scoria is higher than the published values for vertebral trabecular,” said Dr. Burkus, who is an orthopaedic surgeon at The Hughston Clinic in Columbus, Georgia.

PEEK Implants: Coated Versus Porous

Dr. Burkus noted that a complication of standard PEEK-based implants relates to poor integration with the surrounding bone, producing a “PEEK-halo” effect.

“Coating the surface with osteoconductive materials like hydroxyapatite (HA) can improve osseointegration,” Dr. Burkus explained. “Similarly, new composite implants, created by adding Ti to PEEK, have been recently introduced and known as titanium-PEEK or Ti-PEEK. However, these composite titanium-coated PEEK implants are susceptible to wear and delamination of the coating. Importantly, titanium cages with subtractive surface etching (no coating) are less susceptible to such failure but can still potentially stress shield and produce imaging artifacts.”

“The latest approach to enhance PEEK osseointegration has focused both on surface modification and porosity of the material,” Dr. Burkus said. “PEEK Scoria is an innovative PEEK surface modification technology where the porous architecture is created using a patented process. Instead of using additive processes, the pores are grown directly out of the underlying solid material, creating a seamless transition between porosity and solid PEEK. The depth of surface porosity and the size of the pores can be precisely controlled by extruding PEEK through a porous template under heat and pressure. Varying the template geometry during this process can reliably control surface pore morphology.”

Clinical Research Shows Advantages of Porous PEEK Implant

In vivo research by Evans et al demonstrated that limiting porosity to the surface of PEEK implants preserved the structural integrity of the implant while promoting bony ingrowth.

“Substantial bone formation within the pore architecture was confirmed via micro-CT and histology in a rat femur segmental defect study,” Dr. Burkus explained. The pore sizes in this study ranged from 200-312 mm.

A more recent in vitro study by Torstrick et al demonstrated the ability of the COHERE implant to facilitate bone cell proliferation and differentiation, Dr Burkus told SpineUniverse. “Laboratory testing showed higher levels of proliferation and cell-mediated mineralization compared with smooth PEEK and titanium implants.”

“The surface pore size that is effective in facilitating bone ingrowth and vascularization does not significantly affect the strength, interfacial shear strength, and ductility of PEEK Scoria when compared with standard PEEK used in implants,” Dr. Burkus said, referring to the findings by Torstrick et al. “The low cycle fatigue strength of porous PEEK was unaffected by the porosity grown on the surface while the high cycle fatigue strength was only slightly decreased.  There are no differences in the modulus between PEEK Scoria and standard PEEK. Importantly, the mean interfacial shear strength of porous PEEK is higher than trabecular bone.”

Device Reduces Stress Riser Risk

In addition, because the COHERE is made of all one material, there is a smooth transition from the PEEK structure to the bony ingrowth to the vertebrae, with minimal stress and shear at these transition points. Thus, Dr. Burkus said the COHERE device may be more advantageous than devices that use superficial bonding between polymer particles, which may cause stress and delamination.

Dr. Burkus said that he uses the COHERE device for all patients requiring anterior cervical fusion surgery, and in particular, for challenging cases, including revision surgeries and patients with adjacent segment degeneration. Examples of successful cases are shown in Figures 1 and 2.

preoperative lateral x-ray, 61-year-old female with adjacent segment degeneration at C3-C4Figure 1A. Preoperative lateral radiograph from a 61-year-old female with adjacent segment degeneration at the C3-C4 level, disc space narrowing, radial osteophytes, and retrolisthesis. The patient had prior cervical fusions at C4-C5, C5-C6 and C6-C7. Reprinted with permission from The Hughston Foundation.

postoperative lateral x-ray at 4 months, disc space height restored with traveculated mature bone graftFigure 1B. Postoperative lateral radiograph at 4 months following surgery shows that disc space height has been restored with trabeculated mature bone graft uniting the adjacent vertebral bodies. There are no radial lucencies around the COHERE cage. Reprinted with permission from The Hughston Foundation.

9 month postoperative lateral x-ray, 55-year-old femaleFigure 2A. Nine-month postoperative lateral radiograph from a 55-year-old female nine months status post two-level anterior cervical interbody fusion using standard PEEK cage. Lucencies surround the PEEK vertebral body interface. There is minimal bone growth within the standard PEEK cage. Reprinted with permission from The Hughston Foundation.

CT scan sagittal reconstruction demonstrates lucencies surrounding standard PEEK implantsFigure 2B. CT scan of sagittal reconstructions demonstrate the lucencies surrounding the standard PEEK implants and failure of formation of fusion across the instrumented disc space. Reprinted with permission from The Hughston Foundation.

COHERE is the first porous all-PEEK interbody system approved by the U.S. Food and Drug Administration.

Disclosure: Dr. Burkus is a consultant for Vertera Spine and a stockholder.

Updated on: 06/11/19
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