At present, strong requirements in orthopaedics are still to be met, both in bone and joint substitution and in the repair and regeneration of bone defects. In this framework, tremendous advances in the biomaterials field have been made in the last 50 years where materials intended for biomedical purposes have evolved through three different generations, namely first generation (bioinert materials), second generation (bioactive and biodegradable materials) and third generation (materials designed to stimulate specific responses at the molecular level). In this review, the evolution of different metals, ceramics and polymers most commonly used in orthopaedic applications is discussed, as well as the different approaches used to fulfil the challenges faced by this medical field.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2008.0151

Biomaterials in orthopaedics.

Bioceramics of calcium orthophosphates

The clinical use of plasma-sprayed hydroxyapatite (HA) coatings on metal implants has aroused as many controversies as interests over the last decade. Although faster and stronger fixation and more bone growth have been revealed, the performance of HA-coated implants has been doubted. This article will initially address the fundamentals of the material selection, design, and processing of the HA coating and show how the coating microstructure and properties can be a good predictor of the expected behavior in the body. Further discussion will clarify the major concerns with the clinical use of HA coatings and introduce a comprehensive review concerning the outcomes experienced with respect to clinical practice over the past 5 years. A reflection on the results indicates that HA coatings can promote earlier and stronger fixation but exhibit a durability that can be related to the coating quality. Specific relationships between coating quality and clinical performance are being established as characterization methods disclose more information about the coating.

Material fundamentals and clinical performance of plasma-sprayed hydroxyapatite coatings: A review

Scaffold-based bone tissue engineering aims to repair/regenerate bone defects. Such a treatment concept involves seeding autologous osteogenic cells throughout a biodegradable scaffold to create a scaffold-cell hybrid that may be called a tissue-engineered construct (TEC). A variety of materials and scaffolding fabrication techniques for bone tissue engineering have been investigated over the past two decades. This review aims to discuss the advances in bone engineering from a scaffold material point of view. In the first part the reader is introduced to the basic principles of bone engineering. The important properties of the biomaterials and the scaffold design in the making of tissue engineered bone constructs are discussed in detail, with special emphasis placed on the new material developments, namely composites made of synthetic polymers and calcium phosphates. Advantages and limitations of these materials are analysed along with various architectural parameters of scaffolds important for bone tissue engineering, e.g. porosity, pore size, interconnectivity and pore-wall microstructures.

State of the art and future directions of scaffold-based bone engineering from a biomaterials perspective.

https://www.cell.com/trends/biotechnology/pdf/S0167-7799(04)00292-6.pdf

Rapid prototyping in tissue engineering: challenges and potential

Influence of fluorine in the synthesis of apatites. Synthesis of solid solutions of hydroxy-fluorapatite.

One of the most important concerns with the clinical use of plasma-sprayed hydroxyapatite (HA) coatings is the resorption of the coating, and dissolution at neutral pH is one of the two major resorption mechanisms. In this study, highly crystalline pure HA powders were atmospherically plasma sprayed using various parameters. Dissolution of both HA powders and coatings was measured using a calcium ion meter. Surface characteristics, including phase, morphology, and roughness, were compared for the coatings before and after dissolution. Pulverized HA coatings exhibited significantly higher dissolution compared with the same quantity of feedstock HA powders because of the decreased crystallinity and fine crystal size of the coating. Furthermore, the dissolution decreased with the crystallinity of the coating. Dissolution of HA coatings did not show much difference with respect to the coatings in the initial stage of immersion (4 h). However, dissolution of all coatings reached saturation in a fresh physiological solution. The saturation values were much lower compared with their counterparts in the form of powders, which may imply the stability of HA coatings in long-term use. In addition to crystallinity, the particle melting status in the coatings, i.e., the volume of nanocrystals, and porosity, was found to be another important factor for the dissolution of the HA coating. X-ray diffraction patterns of HA coatings indicated the complete dissolution of impurity phases and amorphous phase after the coatings were immersed in the solution for 4 days. Coatings sprayed at lower power (27.5 kW) exhibited a pattern of crystalline HA whereas coatings sprayed at higher power (42 kW) exhibited a pattern of bone apatite. Surface morphologies showed preferential dissolution of amorphous phase in all coatings accompanied with precipitation of bone apatite observable for coatings sprayed at higher power. Surface roughness measured after the dissolution studies increased for the two coatings sprayed at lower power level but decreased for coatings sprayed at higher power level. This decrease is attributed to the better match in solubility characteristics between the fine crystals and the amorphous calcium phosphate within the coating.

Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating.

https://personalpages.manchester.ac.uk/staff/j.gough/lectures/te/7_8_bone/BONE_ENG/composite/bone_eng15.pdf

Biodegradable composite scaffolds with an interconnected spherical network for bone tissue engineering.

The field of biomedical materials has grown rapidly over the past 20 years and offers solutions to repair defects, correct deformities, replace damaged tissue and provide therapy. This has contributed to the increase in the average lifetime of individuals in developed countries. The market value for biomaterials is of the order of billions of dollars per annum worldwide and is growing as new products offer improved performance or provide new solutions health problems. Apatites are playing a key role in biomedical implants.

In developing materials used for implantation consideration must be given to both the influence of the implanted material on the body, and how the body affects the integrity of the material. The body will treat implants as inert, bioactive, or resorbable materials. Generally “inert” materials will evoke a physiological response to form a fibrous capsule, thus, isolating the material from the body. Calcium phosphates fall into the categories of bioactive and resorbable materials. A bioactive material will dissolve slightly, but promote the formation of an apatite layer before interfacing directly with the tissue at the atomic level. Such an implant will provide good stabilization for materials that are subject to mechanical loading. A bioresorbable material will, however, dissolve and allow tissue to grow into any surface irregularities but may not necessarily interface directly with the material (Neo et al. 1992).

The first use of calcium phosphate as an implanted biomaterial provided accelerated bone healing in surgically created defects in rabbits (Albee and Morrison 1920). Interest in apatite specifically started in the 1960s and initial studies principally involved the synthesis and analysis of hydroxylapatites in an attempt to better understand biological apatites (Le Geros 1965, McConnell 1965, Nancollas and Mohan 1970, Selvig et al. 1970). Hydroxylapatite is a specific form of apatite with a chemical …

Karlis Agris Gross

Papers

Material fundamentals and clinical performance of plasma-sprayed hydroxyapatite coatings: A review

Influence of fluorine in the synthesis of apatites. Synthesis of solid solutions of hydroxy-fluorapatite.

Surface characteristics and dissolution behavior of plasma-sprayed hydroxyapatite coating.

Biodegradable composite scaffolds with an interconnected spherical network for bone tissue engineering.

Biomedical Application of Apatites