Osseoconductive Surface Engineering for Orthopedic Implants: Biomaterials Engineering

By Amirhossein Goharian

Osseoconductive Surface Engineering for Orthopedic Implants provides a comprehensive overview of the state of the art of osseointegration based on surface-mediated engineering.

It offers a practical approach to the design and development of implant surface engineering, by reviewing and discussing the usability and efficacy of each processing technique. The reader can learn about the variety, characteristics, advantages, challenges, and optimum parameters for each process—enabling targeted selection of coatings and technologies to enhance long-term implant–bone integration.

• Practical and engineering notions in the field of osseoconductive surface engineering are reviewed and discussed using scientific principles and concepts.
• Engineering cases are analyzed in depth giving a thorough exploration and description of the engineering and scientific concepts for all osseoconductive surface engineering processes.
• Chapters integrate topics and are organised in such a way as to build on themes and practice.

• Language: English
• Publisher: Academic Press
• Release date: Jan 26, 2021
• ISBN: 9780128189313

Amirhossein Goharian

Amirhossein Goharian is a Senior Engineer working in the orthopedic implant industry. He earned his Bachelor’s degree from the University of Kashan in 2007, and his completed his Master’s in Mechanical Engineering in Biomechanics at the University Technology Malaysia (UTM) in 2012. He joined the R&D department of LEONIX Sdn. Bhd., Penang, Malaysia, in 2012, rising to Senior R&D Leader in Jan 2015. In 2017, he joined Isfahan Orthopedic Implant Development Co., an orthopedic implant manufacturer in Isfahan, Iran, as the QA-R&D leader until Sep 2020. Since then, he has been developing innovations in the field of orthopedic implants, based on ideas set out in his published work. Goharian has previously published three books with Elsevier, covering trauma plating systems, osseointegration and surface engineering of orthopedic implants.

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Chapter 1

General concepts of osseoconductive surface engineering

Amirhossein Goharian,    Independent Engineering Consultant in Orthopaedic Implant & Biomaterial Industry, Isfahan, Iran Independent Engineering Consultant in Orthopaedic Implant & Biomaterial IndustryIsfahanIran

Abstract

Biological integration of orthopedic implants to bone tissue, either cortical or trabecular bones, is highly crucial for successful implantation of implants during the healing or treatment period. The main strategy to promote biological integration is enhancement of the implant surfaces at the interfaces with bone tissue. Although orthopedic implants need to be inert in human body conditions, their contact surfaces with bone must be biologically active for conduction of bone formation. In fact, the osseoconductivity advantages of the implant surface promote biomechanical, biological, and clinical performance of the implant during the treatment period. Currently, two surface-engineering strategies are utilized for enhancement of osseoconductivity on the surface of orthopedic implants. One strategy is coating of bioactive materials and the other is alteration of the surface morphology. Various types of engineering methods have been developed (e.g., plasma spraying, vapor deposition, biomimetic coating, plasma electrolyte oxidation, porous structuring, etc.) for establishment of the osseoconductive surface layer. In this chapter, general concepts of osseoconductive layering are reviewed to provide sufficient prerequisite technical data for better reading of Chapters 2–7. Furthermore, the technical terms and expressions from various sciences, including biology, biomechanics, materials, manufacturing processes, and orthopedics, are explained in the book Glossary, as are the reviewed surface-engineering methods in Chapters 2–7.

Keywords

Osseointegration; osseoconductivity; biological integration; osseoconductive surface engineering; orthopedic implants; osseoconductive layer

1.1 Introduction

Orthopedic implants are extensively utilized for the treatment of severe injuries to bone tissue and connected soft tissues. The huge turnover of companies including Stryker, Zimmer Biomet, Depuy Synthes, Medtronic, Globus Medical, Wright Medical, Aesculap, etc. illustrates the importance of the orthopedic implant industry to human health and well-being. Despite the many advantages that such medical devices provide for treated patients, there are various complications that have been observed after implantation. In general, the majority of operated patients healed by orthopedic implants have a reasonable level of functionality and high level of satisfaction compared to the level of pain and functionality present before the surgical operation. It could be mentioned that, with the selection of the correct implant in accordance with the injury characteristics, followed by careful consideration of the manufacturers’ instructions for use, the success rate for implantations is increased. It is worth mentioning that patient selection is also important for treatment solutions with orthopedic implants. Furthermore, observance of the general clinical instructions by the surgical team during the operation has a significant effect on the success rate of implantation operations. However, a reported success rate of more than 90% has been reported for various types of orthopedic implants, although, the definition of clinical satisfaction is changing due to the growing numbers of young and middle-aged patients who are demanding surgical solutions with orthopedic implants. On the other hand, product developers, researchers, and innovators in the biomechanics and biomaterials sciences create many technical concepts and investigation topics that may potentially be used in the development of orthopedic implants. Thus, the new conception is continuously investigated in universities, institutes, and other research-based sectors for the creation of new breakthroughs. This conception and science creation is justifiable due to the interaction of orthopedic implants with bone tissue, with highly smart and bioactive structures from physical, chemical, mechanical, biological, and material aspects. In other words, the human musculoskeletal system is a very complicated and advanced system which requires biomechanics and biomaterial scientists to understand it and develop orthopedic implants that are more compatible. Based on that, in addition of the development of traditional types of the orthopedic implants (e.g., trauma implants, total joint replacements (TJRs), spinal implants, soft-tissue repair implants, implantable amputation implants), new types of implants with different structures and materials (e.g., tissue engineering scaffolds, bone substitutive scaffolds) have been introduced by implant developers which have not yet been marketed extensively. One of the limiting factors in the development of orthopedic implants is nonaccessibility of the user to the implant during the healing period. This creates a huge challenge for developers in how to develop an implant that is 100% reliable. Traditionally, orthopedic implants have been designed to be very rigid and stiff, to ensure mechanical safety against physiological loading conditions that may be introduced to the implant during the treatment or healing period. Such a conception would increase other complications such as undesired bone thickening or the occurrence of the stress-shielding effect. In contrast, development of implants with higher levels of flexibility might result in failure at the implant site due to insufficient strength of the implant against physiological loading conditions. A discussion of the topics related to design conception of orthopedic implants is very interesting. What we attempt to highlighted here is the boundary between rigid and flexible designing. Biomechanical designers try to design a new generation of the implant with lower rigidity and stiffness, while biomaterial researchers attempt re-alloying of the traditional materials, such as titanium, or to characterize new biomaterials to reduce the mechanical modulus of the metallic alloys while maintaining the mechanical strength with optimum mechanical flexibility conditions. In addition to such new development concepts, there is similar interest among biomechanics and biomaterial investigators in the osseointegrant surface engineering field, in which an active biological surface layer is created on the surface of orthopedic implants. This biological surface layer would enhance osseointegration of the orthopedic implants by conduction and/or induction of thde biological factors involved in bone apposition to be activated on the surface of the implant. In this book, the aim is to explore the surface-engineering methods which can be utilized for enhancement of implant bioactivity in contact with bone tissue. Osseoconduction would be more advantageous for long-term effectiveness of the osseointegrant layer, which would be more focused. In this chapter, the general concepts contemplated for the development of osseoconductive surface-engineering methods are reviewed and discussed to give a background to the discussions in Chapters 2–7Chapter 2Chapter 3Chapter 4Chapter 5Chapter 6Chapter 7 Chapters 2–7.

1.2 Various types of orthopedic implants

Orthopedic implants are utilized in the treatment of various types of bone or soft-tissue injuries. These types of medical devices are inserted through the bone tissue to be stabilized during the period of treatment or healing. The majority of orthopedic implants are implanted inside the body for a couple of months to more than 20 years. Thus, implants must be resistant to various types of mechanical, biological, physical, and chemical conditions which may be imposed or loaded onto the implant. Based on the current availability of commercialized orthopedic implants, this kind of artificial device could be categorized into six main groups of trauma implants, spinal implants, total joint replacements, soft-tissue repairs, tissue substitution implants, and osseointegrated amputation implants. Besides these six groups, there is other group, recognized as biomaterials that is delivered in the form of material-based implants in combination or individually with the other six groups of orthopedic implants. It can be added that orthopedic implants can be categorized as mechanical-based and materials-based implants.

Orthopedic implants are manufactured from various types of materials with good biocompatibility characteristics for use in the human body. Initially, the implants were developed from stainless steel, and gradually pure titanium and titanium alloys were introduced in the 1940s as a substitute material. Titanium alloys and pure titanium are still utilized in the development of a major portion of trauma implants and spinal implants. Other materials such as cobalt–chromium (CoCr) alloys and ultrahigh-molecular-weight polyethylene (UHMWPE) are extensively used in the fabrication of prosthesis components for joint replacements. Polyetheretherketone (PEEK) is another polymeric material that is used in the fabrication of orthopedic implants. Currently, PEEK polymer is utilized for fabrication of intervertebral fusion cages and soft-tissue anchors. This material can be partially incorporated for manufacturing of trauma plating systems and intramedullary nails. Likewise, PEEK or PEEK composites (e.g., carbon fiber reinforced polyetheretherketone - CFRPEEK) are utilized for the development of spinal rods, spinal pedicle screws, and vertebral body replacements. There are also other metallic, ceramic, and polymeric materials such as zirconium (Zr) alloy, hydroxyapatite (HA), alumina (aluminum oxide), nitinol, polylactic acid (PLA), and polylactic glycolic acid (PLGA), which are used in the development of orthopedic implants, but not as extensively as stainless steel, titanium and titanium alloy, UHMWPE, PEEK, and CoCr alloy.

1.2.1 Trauma implants

These types of orthopedic implants are widely used nowadays in the treatment of bone fractures caused by accidents, falling, or any high-energy loading conditions on the body skeletal bones. Almost all fractured bones can be healed by trauma implants. The fundamental concept of trauma implants in the treatment of bone fractures is fixing of the bone fragments at the anatomical position through utilization of screws, plates, nails, and rods. In fact, in the first place, the bone fragments are returned or reduced to their normal position and then a fixator (e.g., plate, nail, external fixator) is fixed to the reduced bone fragments by screws to hold the bone–fixator construct until completion of bone fracture healing at the fracture gaps. Trauma implants can be categorized as plating system, nail intramedullary system, external fixator system, and cannulated screw system. Trauma plating systems have the largest share of the trauma implant market and are internally implanted to the bone. Currently, plating systems are made of stainless steel and titanium alloys to provide fracture fixation with a high level of rigidity and stability. This author has written a technical reference book in the topic of Trauma Plating Systems published by Elsevier publisher, which explains various aspects of these types of implants from biomechanical, biological, material, clinical, and design perspectives. Based on the Arbeitsgemeinschaft für Osteosynthesefragen (AO) principles [1], the fracture fixation should be managed with consideration of four major principles to achieve successful treatment of bone fractures. These four principles are: early and safe mobilization, preservation of blood supply (vascularization), fixation stability, and restoring of the anatomical positioning. Early and safe mobilization is achieved when the bone–implant construct is effectively managed to the extent that movement of the body limb at the fracture site results in no pain and there is functional confinement. Preservation of the blood supply is ensured by consideration of the limited contact surface between the implant and bone surface, which would be a major design parameter in modeling of the trauma plate. Regarding fixation stability, the integrated construct of the implant and bone is crucial to be maintained without any debonding effect at the interface zones. In fact, stabilized positioning of the bone fragments and implants must be kept constant until completion of fracture healing. As the final principle, the bone fragments are essentially fixed in an anatomical position as healthy bone for effective functioning of the attached soft tissues (e.g., tendons, ligaments, muscles) to the bone fragments. Readers are invited to explore more technical details by reading Trauma Plating Systems referred to