Spine interbody implants: material selection and modification, functionalization and bioactivation of surfaces to improve osseointegration

Abstract

The clinical outcome of lumbar spinal fusion is correlated with achievement of bony fusion. Improving interbody implant bone on-growth and in-growth may enhance fusion, limiting pseudoarthrosis, stress shielding, subsidence and implant failure. Polyetheretherketone (PEEK) and titanium (Ti) are commonly selected for interbody spacer construction. Although these materials have desirable biocompatibility and mechanical properties, they require further modification to support osseointegration. Reports of extensive research on this topic are available in biomaterial-centric published reports; however, there are few clinical studies concerning surface modification of interbody spinal implants. The current article focuses on surface modifications aimed at fostering osseointegration from a clinician's point of view. Surface modification of Ti by creating rougher surfaces, modifying its surface topography (macro and nano), physical and chemical treatment and creating a porous material with high interconnectivity can improve its osseointegrative potential and bioactivity. Coating the surface with osteoconductive materials like hydroxyapatite (HA) can improve osseointegration. Because PEEK spacers are relatively inert, creating a composite by adding Ti or osteoconductive materials like HA can improve osseointegration. In addition, PEEK may be coated with Ti, effectively bio-activating the coating.

Keywords: Bioactive conversion; Interbody spinal implant; Osseointegration.

© 2014 Chinese Orthopaedic Association and Wiley Publishing Asia Pty Ltd.

Figures

Figure 1

Elastic modulus of available spinal implant materials. CFR, carbon fiber‐reinforced.

Figure 2

Spinal implants with plasma‐sprayed titanium surfaces. (A) Cervical interbody implant. (B) Anterior lumbar interbody fusion implant. (C) Transforaminal lumbar interbody fusion implant. (D) Posterior lumbar interbody fusion implant (Melsungen, Germany).

Figure 3

Porous titanium lumbar interbody implant (Kasios, L'union France). (A and B) TLIF implants. (C) Scanning electron microscopy (SEM) at a magnification of 50. (D) SEM at a magnification of 500. TLIF, transforaminal lumbar interbody fusion.

Figure 4

Radiolucency at PEEK implant‐bone fusion interface persisting even at 12 months.

Figure 5

Ti composite interbody implant in which the bone‐implant surface is Ti whereas the core of the implant is PEEK (A‐Spine ASIA, Taipei, Taiwan). (A) Cervical interbody implant. (B) TLIF implant. (C) Scanning electron microscopy (SEM) of the junction of PEEK and Ti at a magnification of 50. (D) SEM of the Ti surface at a magnification of 500.

Figure 6

PEEK with Ti endplate composite cervical interbody implant: The CONSTRUX Mini PTC System (Orthofix, Lewisville, TX, USA) is composed of two endplates made of 3D porous Ti scaffold (gray) with a PEEK interior core (tan).Photographs courtesy Orthofix.

Figure 7

Trabecular tantalum implants with porosity up to 80% and average pore size of 550 μm. (A) Cervical interbody implant. (B and C) Posterior lumbar interbody fusion implants. (D) Porous architecture of trabecular metal tantalum. (E) Porous architecture of bone. Images courtesy Zimmer Spine, Minneapolis, MN, USA.