Biointegration of Medical Implant Materials

Science and Design

Chandra P. Sharma

• Publisher: Woodhead Publishing
• Year: 2010
• eBook ISBN: 9781845699802
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

Biointegration is essential for the successful performance of implanted materials and devices within the human body. With an increasing number and wide range of implant procedures being performed, it is critical that materials scientists and engineers effectively design implant materials which will create a positive biological and mechanical response with the host tissue.Biointegration of medical implant materials provides a unique and comprehensive review of recent techniques and research into material and tissue interaction and integration. Part one discusses soft tissue biointegration with chapters on the biocompatibility of engineered stem cells, corneal tissue engineering and vascular grafts. Part two then reviews particular techniques in drug delivery including inorganic nanoparticles for targeted drug delivery and alginate based drug delivery devices. Part three covers design considerations with coverage of themes such as biocompatibility of materials and its relevance to drug delivery and tissue engineering, mechanisms of failure of medical implants during long term use and rapid prototyping in biomedical engineering.With its distinguished editor and team of international contributors, Biointegration of medical implant materials: science and design is a standard reference for medical materials scientists and engineers in industry and the academic sector. - Provides a unique and comprehensive review of recent techniques and research into material and tissue interaction and integration - Discusses soft tissue biointegration with chapters on the biocompatibility of engineered stem cells, corneal tissue engineering, vascular grafts and replacement materials for facial reconstruction - Reviews particular techniques in drug delivery featuring inorganic nanoparticles and functionalized nanoparticles for targeted drug delivery

1

Biointegration: an introduction

C.K.S. Pillai and C.P. Sharma, Sree Chitra Tirunal Institute for Medical Sciences Technology, India

Abstract:

This introductory chapter discusses the relevance of the basic aim of the book, reflecting on the importance of the interfacial concepts related to the biointegration of the implants with respect to their design and structure–function relationship. Various examples, including orthopedic implants, are discussed.

Key words

biointegration

interface

medical implants

design

biomaterials

1.1 Introduction

When biomaterials are designed, a set of properties are built in such a way as to ensure that, after implantation, they will help the body to heal itself. So it is of critical importance that these materials be integrated into organspecific repair mechanisms such as the physiologic process required for the biologization of implants (Amling et al., 2006). It should involve a direct structural and functionally stable connection between the living part and the surface of an implant. Although various materials have been developed in recent years with enhanced physical, surface and mechanical properties, the use of these materials in certain biological applications is often limited by poor tissue integration. So, the question is on how biomaterials can be converted to ‘living tissues’ after implantation. To cite an example, the bonding of hydroxyapatite to bone, which is considered as a true case of biointegration, is thought to involve a direct biochemical bond of the bone to the surface of an implant at the electron microscopic level and is independent of any mechanical interlocking mechanism (Meffert et al., 1987; Cochran, 1996). Several groups working on various aspects of the design, development and application of improved devices are concerned with how these materials become integrated with soft and hard tissues in the body and how these implanted systems have to match their physical–chemical and biological properties to those of their environment.

1.2 Biointegration of biomaterials for orthopedics

Biomaterials are defined as ‘materials intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ, or function of the body’ (Williams, 1999). Orthopedic research is developing and advancing at a rapid pace as new techniques are applied to musculoskeletal tissues. The discovery of biologic solutions to important problems, such as fracture-healing, soft-tissue repair, osteoporosis, and osteoarthritis, continues to be an important research focus. At the same time, research on biomaterials and biomechanics is critical for advances in current areas such as tissue-engineering and cytokine delivery. In orthopedic applications, there is a significant need and demand for the development of a bone substitute that is bioactive and exhibits material properties (mechanical and surface) comparable with those of natural, healthy bone. Particularly, in bone tissue engineering, nanometer-sized ceramics, polymers, metals and composites have been receiving much attention recently. This is a result of current conventional materials not invoking suitable cellular responses to promote adequate osteointegration to enable implanted devices to be successful for long periods (Balasundaram and Webster, 2006; Barrère et al., 2008).

Metallic materials are normally used for load-bearing members such as pins and plates, femoral stems, etc. Ceramics, such as alumina and zirconia, are used for wear applications in joint replacements, while hydroxyapatite is used for bone bonding applications to assist implant integration. Polymers, such as ultra high molecular weight polyethylene (UHMWP), are used as articulating surfaces against ceramic components in joint replacements. Porous alumina has also been used as a bone spacer to replace large sections of bone which have had to be removed due to disease (www.azom,2004www.azom,2004; http://academic.uprm.edu).

In applications involving the loading phase, the best material has been titanium and its alloys, whereas calcium phosphate seems to be the best material to be used in joint replacement and osseointegration (the degree to which bone will grow next to or integrate into the implant). Titanium is used primarily for the loading faces, which include the pin structure, and the fabrication of plates and femoral stems. The integration of a biomaterial to bone involves, essentially, two processes: interlocking with bone tissue and chemical interactions with bone constituents. The direct bonding of orthopedic biomaterials with collagen is rarely considered; however, several non-collagenic proteins have been shown to adhere to biomaterial surfaces (Rey, 1998).

Many studies have been reported on the biointegration of orthopedic devices. Hydroxyapatite (HA) films have been widely recognized for their biocompatibility and utility in promoting biointegration of implants in both osseous and soft tissue. In a study on hydroxyapatite-coated (by electroplating) cp-titanium implants, Badr and El Hadary (2007) showed the formation of recognizable osseointegration of bone regeneration with more and denser bone trabeculae, and concluded that electroplating provided a thin and uniform pure crystalline hydroxyapatite coating. The characterization of the precipitated film is promising for clinically successful long-term bone fixation. An AFM analysis of roughness on seven materials widely used in bone reconstruction carried out by Covani et al. showed that the biointegration properties of bioactive glasses can also give an answer in terms of surface structures in which chemical composition can influence directly the biological system (e.g. with chemical exchanges and development of specific surface electrical charge) and indirectly, via the properties induced on tribological behavior that expresses itself during the smoothing of the surfaces (Covani et al., 2007).

The biological behavior of an implant, such as osseointegration, depends on both the chemical composition and the morphology of the surface of the implant. Irradiation with laser light (Nd:YAG (λ = 1064 nm, τ = 100 ns)) is used for the surface modification of Ti-6Al-4 V – which is widely used in implantation to enhance biointegration (Mirhosseini et al., 2007). Conventional sputtering techniques have shown some advantages over the commercially available plasma spraying method for generating hydroxyapatite (HA) films on metallic substrates; however, the as-sputtered films are usually amorphous, which can cause some serious adhesion problems when post-deposition heat treatment is necessitated. Nearly stoichiometric, highly crystalline HA films strongly bound to the substrate were obtained by an opposing radio frequency (RF) magnetron sputtering approach. HA films have been widely recognized for their biocompatibility and utility in promoting biointegration of implants in both osseous and soft tissue (Hong et al., 2007). Oudadesse et al. (2007) studied the in vitro behavior of compounds in contact with simulated body fluid (SBF) and in vivo experiments in a rabbit’s thigh bones. The inductively coupled plasma–optical emission spectroscopy (ICP-OES) method was used to study the eventual release of Al from composites to SBF and to evaluate the chemical stability of composites characterized by the succession of SiO₄ and AlO₄ tetrahedra. The results obtained show the chemical stability of composites. At the bone–implant interface, the intimate links revealed the high quality of the biointegration and the bioconsolidation between composites and bony matrix. Histological studies confirmed good bony bonding and highlighted the total absence of inflammation or fibrous tissues, indicating good biointegration (Oudadesse et al., 2007). Zanotti and Verlicchi (2006) proposed a bioglass–alumina spacer that could perform an excellent arthrodesis by a mechanical stabilization (primary one) and a biological bio-mimetic stabilization (biointeraction, biointegration and biostimulation). Other advantages are easy use, even in the low somatic interspaces (C7-D1 type), reduction of convalescence, and reduced costs of this type of device. That makes it interesting, even in societies with low economic well-being (Zanotti and Verlicchi, 2006).

The process by which these materials are integrated into the organspecific repair cascade is named ‘bone remodeling’, which is the concerted interplay of two cellular activities: osteoclastic bone resorption and osteoblastic bone formation. The latter physiologic process not only maintains bone mass, skeletal integrity and skeletal function but is also the cellular process that determines structural and functional integration of bone substitutes. A molecular understanding of this process is therefore of paramount importance for almost all aspects of the body’s reaction to biomaterials and might help us to understand, at least in part, the success or the failure of various materials used as bone substitutes. Recent genetic studies have demonstrated that there is no tight cross-control of bone formation and bone resorption in vivo and that there is also a central axis controlling bone remodeling, radically enhancing our understanding of this process. Amling et al. (2006) have shown how an understanding of bone remodeling – the physiologic process required for the biologization of bone substitutes – has evolved during recent years, providing a platform for the design, development and application of improved biomaterials.

Roughness was evaluated by measuring root mean square (RMS) values and RMS/average height (AH) ratio, in different...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Chapter 1: Biointegration: an introduction
• Part I: Soft tissue biointegration
• Part II: Drug delivery
• Part III: Design considerations
• Index