Plasma-sprayed titanium coating to polyetheretherketone improves the bone-implant interface

Abstract

Background context

Rapid and stable fixation at the bone-implant interface would be regarded as one of the primary goals to achieve clinical efficacy, regardless of the surgical site. Although mechanical and physical properties of polyetheretherketone (PEEK) provide advantages for implant devices, the hydrophobic nature and the lack of direct bone contact remains a limitation.

Purpose

To examine the effects of a plasma-sprayed titanium coated PEEK on the mechanical and histologic properties at the bone-implant interface.

Study setting

A preclinical laboratory study.

Methods

Polyetheretherketone and plasma-sprayed titanium coated PEEK implants (Ti-bond; Spinal Elements, Carlsbad, CA, USA) were placed in a line-to-line manner in cortical bone and in a press-fit manner in cancellous bone of adult sheep using an established ovine model. Shear strength was assessed in the cortical sites at 4 and 12 weeks, whereas histology was performed in cortical and cancellous sites at both time points.

Results

The titanium coating dramatically improved the shear strength at the bone-implant interface at 4 weeks and continued to improve with time compared with PEEK. Direct bone ongrowth in cancellous and cortical sites can be achieved using a plasma-sprayed titanium coating on PEEK.

Conclusions

Direct bone to implant bonding can be achieved on PEEK in spite of its hydrophobic nature using a plasma-sprayed titanium coating. The plasma-sprayed titanium coating improved mechanical properties in the cortical sites and the histology in cortical and cancellous sites.

Introduction

The definition and understanding of biomaterials used in medical devices continues to be refined and clarified. Professor David Williams refined the biomaterial paradigm and redefined the word “biomaterial” that should be considered for all implantable materials; “a biomaterial is a substance that has been engineered to take a form which, alone or as part of a complex system, is used to direct, by control of interactions with components of living systems, the course of any therapeutic or diagnostic procedure, in human or veterinary medicine” [1]. It is within the context of the complex in vivo environment that one must consider any implantable material.

Interbody bony fusion is an established treatment for a variety of degenerative conditions of the spine. Biomechanically, the interbody spacer and graft material combine to provide anterior column support and means to bridge the levels with a biological fusion. The ability to assess fusion status with simple radiographic means as well as clinical issues of delayed union, loss of correction, and device subsidence remain a significant concern for surgeons. Design and development of biomaterials used for interbody fusion have undergone a rapid evolution [2], [3] since the days of autograft and allograft spacers [3], [4] in attempts to address the above concerns. Developments starting from stainless steel cages [5], [6] to carbon fiber implants [7], [8] and degradable polylactides [9], [10] have provided some improvements and increased understanding of the complexity of interbody fusion. Polyetheretherketone (PEEK) [2], [11], [12], [13] has clear benefits in terms of reduced modulus and radiolucency to assist in confirmation of fusion status, whereas the lack of direct bone apposition remains a potential concern.

Early stable fixation and ultimately a biological bridge (fusion) rely on the mechanical and the biological environment to work in concert. Perhaps the ideal interbody fusion device would be one that provides a mechanical environment that can stabilize not only axial loads but off-axis loading in all planes and assist in controlling local micromotion. Direct bone contact to the implant and a robust bone-implant interface appears to be required to achieve all this mechanical stabilization.

Rapid and stable fixation at the bone-implant interface would be regarded as one of the primary goals to achieve clinical efficacy regardless of the surgical site. This has been proven to be true in the traditional orthopedic realm in fixation of uncemented total joint replacements, where bony ongrowth or ingrowth to the implants provides a biological means to achieve implant fixation [14], [15] and dental implants [16], [17], [18]. The application of surface topography or coatings to metal substrates has been shown to improve biological implant fixation [18] through bony ongrowth or ingrowth in preclinical [15], [19], [20], [21], [22], [23], [24] and clinical studies [14], [15], [25]. Direct bone attachment and integration at the implant interface can play a vital role in stress transfer and micromotion as well as influence healing and mechanical performance of the implant.

Although many studies have demonstrated PEEK as biocompatible, [26], [27], [28], [29] preclinical studies and many human studies demonstrate that PEEK does not directly bond to the bone [12], [13], [30], [31], [32]. The present study examined the effects of plasma-spraying titanium on PEEK to present an implant interface to bone that would support early bone integration and long-term stability of the implant-bone interface using an established bone implantation model [21], [22], [23], [24], [33], [34], [35]. We hypothesized that the application of a plasma-sprayed titanium layer to PEEK would encourage early bony fixation and improve the mechanical properties at the bone-implant interface. Early bone fixation of the implant surface has the potential additional benefits of reducing implant complications, such as subsidence, expulsion, or nonunion.