Calcium orthophosphates as bioceramics: state of the art

Abstract

In the late 1960s, much interest was raised in regard to biomedical applications of various ceramic materials. A little bit later, such materials were named bioceramics. This review is limited to bioceramics prepared from calcium orthophosphates only, which belong to the categories of bioactive and bioresorbable compounds. There have been a number of important advances in this field during the past 30-40 years. Namely, by structural and compositional control, it became possible to choose whether calcium orthophosphate bioceramics were biologically stable once incorporated within the skeletal structure or whether they were resorbed over time. At the turn of the millennium, a new concept of calcium orthophosphate bioceramics-which is able to promote regeneration of bones-was developed. Presently, calcium orthophosphate bioceramics are available in the form of particulates, blocks, cements, coatings, customized designs for specific applications and as injectable composites in a polymer carrier. Current biomedical applications include artificial replacements for hips, knees, teeth, tendons and ligaments, as well as repair for periodontal disease, maxillofacial reconstruction, augmentation and stabilization of the jawbone, spinal fusion and bone fillers after tumor surgery. Exploratory studies demonstrate potential applications of calcium orthophosphate bioceramics as scaffolds, drug delivery systems, as well as carriers of growth factors, bioactive peptides and/or various types of cells for tissue engineering purposes.

Figure 1

Several examples of the commercial calcium orthophosphate-based bioceramics.

Figure 2

A schematic diagram representing the changes occurring with particles under sintering.

Figure 3

Linear shrinkage of the compacted ACP powders that were converted into β-TCP, BCP (50% HA + 50% β-TCP) and HA upon heating. According to the authors: “At 1300 °C, the shrinkage reached a maximum of approximately ~25, ~30 and ~35% for the compacted ACP powders that converted into HA, BCP 50/50 and β-TCP, respectively” [224]. Reprinted from [224] with permission.

Figure 4

Photographs of a commercially available porous calcium orthophosphate bioceramic with different porosity. Horizontal field width is 20 mm.

Figure 5

β-TCP porous ceramics with different pore sizes prepared using polymethylmethacrylate balls with the diameters: (a) 100–200; (b) 300–400; (c) 500–600 and (d) 700–800 μm. Horizontal field width is 45 mm. Reprinted from [377] with permission.

Figure 6

SEM pictures of HA bioceramics sintered at (a) 1050 °C and (b) 1200 °C. Note the presence of microporosity in (a) and not in (b). Reprinted from [415] with permission.

Figure 7

A typical microstructure of a calcium orthophosphate cement after hardening. The mechanical stability is provided by the physical entanglement of crystals. Reprinted from [1] with permission.

Figure 8

Plasma-sprayed HA coating on a porous titanium (dark bars) is dependent on the implantation time and will improve the interfacial bond strength compared to uncoated porous titanium (light bars). Reprinted from [46] with permission.

Figure 9

A schematic diagram showing the arrangement of the FA/β-TCP biocomposite layers. (a) A non-symmetric functionally gradient material (FGM); (b) symmetric FGM. Reprinted from [523] with permission.

Figure 10

A sequence of interfacial reactions involved in forming a bond between tissue and bioactive ceramics. Reprinted from [46,47,48] with permission.

Figure 11

A schematic diagram representing the events taking place at the interface between bioceramics and the surrounding biological environment: (1) dissolution of bioceramics; (2) precipitation from solution into bioceramics; (3) ion exchange and structural rearrangement at the bioceramic/tissue interface; (4) interdiffusion from the surface boundary layer into the bioceramics; (5) solution-mediated effects on cellular activity; (6) deposition of either the mineral phase (a) or the organic phase (b) without integration into the bioceramic surface; (7) deposition with integration into the bioceramics; (8) chemotaxis to the bioceramic surface; (9) cell attachment and proliferation; (10) cell differentiation; (11) extracellular matrix formation. All phenomena, collectively, lead to the gradual incorporation of a bioceramic implant into developing bone tissue. Reprinted from [54] with permission.

Figure 12

A schematic diagram representing the phenomena that occur on HA surface after implantation: (1) beginning of the implant procedure, where solubilization of the HA surface starts; (2) continuation of the solubilization of the HA surface; (3) the equilibrium between the physiological solutions and the modified surface of HA has been achieved (changes in the surface composition of HA does not mean that a new phase of DCPA or DCPD forms on the surface); (4) adsorption of proteins and/or other bioorganic compounds; (5) cell adhesion; (6) cell proliferation; (7) beginning of a new bone formation; (8) new bone has been formed. Reprinted from [575] with permission.

Figure 13

A schematic view of a third generation biomaterial, in which porous calcium orthophosphate bioceramic acts as a scaffold or template for cells, growth factors, etc. Reprinted from [42,52] with permission.