The orthopedic implant market is expected to continue growing over the next decade as the ageing population demands treatments that allow them to maintain active lifestyles. While orthopedic implants help patients regain function, clinical challenges associated with traditional metal materials, such as wear debris and difficult revision surgery, along with the desire for greater design freedom and versatile manufacturing capabilities, are driving next-generation technologies.
PEEK-Optima Natural high performance polymer was introduced as the first PEEK biomaterial by Invibio Biomaterial Solutions over 15 years ago. It has subsequently been used in over four million implants, in applications including spine, trauma, arthroscopy, hips and knees. The material properties of the PEEK implantable grade are particularly suited for orthopaedic applications. Its mechanical properties provide new opportunities to improve clinical outcome and its elastic modulus, similar to bone, promotes bone modelling and remodelling in accordance with Wolff’s law. Its compatibility with common imaging modalities and natural radiolucency of the material allows healing to be assessed, without artefacts, with MRI and CT imaging. Today’s family of PEEK-Optima products share the fundamental principles of PEEK-Optima Natural but the later additions are designed to achieve superior performance of specific features when required by a device. The new characteristics include high strength, tailored radiopacity and – as the latest offering – enhanced bone on-growth.
Ongrowth enhancement
PEEK accounts for around 70% of spinal interbody fusion devices currently on the market for the treatment of disc degenerative disease or instability of the spine (www.OrthopedicNetworkNews.com). PEEK-Optima Natural clinical outcome rates have been high but the latest surgeon-driven trend is to seek ways to increase bone apposition on the surface of the device.
Invibio Biomaterial Solutions’ PEEK-Optima HA Enhanced Polymer, which is specifically designed to enhance bone apposition, was launched in October 2013 at the EuroSpine congress. This new biomaterial combines two clinically proven advanced biomaterials: Peek-Optima Natural, and Hydroxyapatite (HA), a well-known osteoconductive material. Invibio says that HA is fully integrated into the PEEK-Optima Natural grade and provides a complete homogeneous compound, which ensures that HA will be present on all surfaces (fig 1).
Fig 1: HA distribution and availability at the surface of a generic PEEK-Optima HA Enhanced interbody fusion device. The HA is indicated by the speckling on the surface. (source: Invibio)
Testing and analysis
Peek-Optima HA Enhanced maintains similar basic mechanical properties to PEEK-Optima Natural and its performance is equivalent in application-specific testing for interbody devices. While bench testing and in vitro studies can provide some information on material performance, a pre-clinical study is ultimately needed to demonstrate efficacy in osseointegration.
A study was designed to compare the in vivo response of PEEK-Optima HA Enhanced with Peek-Optima Natural in a large animal model. The study was carried out at the Surgical & Orthopaedic Research Laboratories at the University of New South Wales under the direction of Professor Bill Walsh, following approval of the UNSW Animal Care and Ethics Committee.
Within four weeks of implantation the Peek-Optima HA Enhanced polymer demonstrated enhanced bone apposition compared to Peek-Optima Natural. With Peek-Optima Natural, some areas of direct bone contact were observed; in others, there were gaps or small regions of fibrous tissue in the intervening space. A more consistent and continuous degree of direct bone contact was observed with PEEK-Optima HA Enhanced (fig 2). Fluorochrome labelling indicated that bone was being deposited on the new material as early as ten days following implantation. Early bone apposition was not observed with PEEK-Optima Natural.
Fig 2: Analysis of direct bone contact for Peek-Optima Natural and Peek-Optima HA Enhanced at four and twelve weeks. (source: Invibio)
At four weeks following implantation, Peek-Optima HA Enhanced demonstrated increased interfacial shear strength in push-out testing, compared with PEEK-Optima Natural (fig 3), providing further evidence of increased osseointegration. Important to note is the absence of any additional surface geometry used in the implants, which may have influenced both shear stress and percentage bone-in-contact values. Surface roughness was also a consideration in evaluating osseointegration and so all implants, of both materials, were machined to have a similar surface roughness (roughness average, Ra ˜ 1µm).
Fig 3: Peek-Optima HA Enhanced demonstrates increased interfacial shear strength with bone, four weeks after implantation (source: Invibio)
Summary
PEEK is being increasingly used in medical implants because of its material properties and manufacturing versatility. There is a desire for increased bone apposition in spine interbody fusion applications and Invibio’s new PEEK-Optima HA Enhanced has been demonstrated to offer the same performance standards as PEEK-Optima Natural, along with superior solution bone apposition.
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