1. Biomaterials Classifications
When a synthetic material is placed within the human body, tissue reacts towards the implant in a variety of ways depending on the material type. The mechanism of tissue interaction (if any) depends on the tissue response to the implant surface. In general, there are three terms in which a biomaterial may be described in or classified into representing the tissues responses. These are bioinert, bioresorbable, and bioactive, which are well covered in range of excellent review papers [1]
1.1 Bioinert Biomaterials
The term bioinert refers to any material that once placed in the human body has minimal interaction with its surrounding tissue, examples of these are stainless steel, titanium, alumina, partially stabilised zirconia, and ultra high molecular weight polyethylene. Generally a fibrous capsule might form around bioinert implants hence its biofunctionality relies on tissue integration through the implant (Figure 1a). [1]
 |
|
Figure 1. Classification of biomaterials according to their bioactivity (a) bioinert alumina dental implant, (b) bioactive hydroxyapatite [Ca10(PO4)6(OH)2] coating on a metallic dental implant, (c) surface active bioglass and (d) bioresorbable tricalcium phosphate ([Ca3(PO4)2] implant. |
1.2 Bioactive Biomaterials
Bioactive refers to a material, which upon being placed within the human body interacts with the surrounding bone and in some cases, even soft tissue. This occurs through a time – dependent kinetic modification of the surface, triggered by their implantation within the living bone. An ion – exchange reaction between the bioactive implant and surrounding body fluids – results in the formation of a biologically active carbonate apatite (CHAp) layer on the implant that is chemically and crystallographically equivalent to the mineral phase in bone. Prime examples of these materials are synthetic hydroxyapatite [Ca10(PO4)6(OH)2], glass ceramic A-W and bioglass® (Figure 1b and c)). [1]
1.3 Bioresorbable Biomaterials
Bioresorbable refers to a material that upon placement within the human body starts to dissolve (resorbed) and slowly replaced by advancing tissue (such as bone). Common examples of bioresorbable materials are tricalcium phosphate [Ca3(PO4)2] and polylactic–polyglycolic acid copolymers. Calcium oxide, calcium carbonate and gypsum are other common materials that have been utilised during the last three decades (Figure 1d). [1]
2. Bone Grafting
The principles involved in successful bone grafts include osteoconduction (guiding the reparative growth of the natural bone), osteoinduction (encouraging undifferentiated cells to become active osteoblasts), and osteogenesis (living bone cells in the graft material contribute to bone remodeling). Osteogenesis only occurs with autografts. [5] Further detail of mentioned phenomena is as follows
2.1 Osteoconduction
This term means that bone grows on a surface. An osteoconductive surface is one that permits bone growth on its surface or down into pores, channels or pipes. Wilson-Hench has suggested that osteoconduction is the process by which bone is directed so as to conform to a material’s surface. However, Glantz has pointed out that this way of looking at bone conduction is somewhat restricted, since the original definition bears little or no relation to biomaterials. [2]
Osteoblasts from the margin of the defect that is being grafted utilize the bone graft material as a framework upon which to spread and generate new bone.In the very least, a bone graft material should be osteoconductive. [5]
Currently used bone graft substitutes that primarily offer osteoconductive properties include coralline hydroxyapatite, collagen-based matrices, calcium phosphate, calcium sulfate, and tricalcium phosphate. These products vary considerably in chemical composition, structural strength, and resorption or remodeling rates. Understanding these differences is important in selecting a bone graft substitute with the properties desired for a specific clinical situation. [3]
2.2 Osteoinduction
This term means that primitive, undifferentiated and pluripotent cells are somehow stimulated to develop into the bone-forming cell lineage. One proposed definition is the process by which osteogenesis is induced [2]
A bone graft material that is osteoconductive and osteoinductive will not only serve as a scaffold for currently existing osteoblasts but will also trigger the formation of new osteoblasts, theoretically promoting faster integration of the graft. The most widely studied type of osteoinductive cell mediators are bone morphogenetic proteins (BMPs). [5]
2.3 Osteoproduction
Osteoproduction is used to describe bone proliferation resulting from the combined properties (i.e. osteoinductive, osteoconductive, and/or osteogenic properties) of a grafting material. This term has also been used to describe the properties of class A bioactive materials, such as bioactive glass, that enhance both the proliferation and the differentiation of progenitor cells (Hench 1998, Wilson & Low 1992). As opposed to class B bioactive materials, such as synthetic hydroxyapatite, the properties of which are limited to osteoconduction. [4]
References
[1] G. Heness and B. Ben-Nissan, Innovative Bioceramics, Materials Forum VOL. 27 (2004) 104 – 114
[2] T. Albrektsson, C. Johansson, Osteoinduction, osteoconduction and osseointegration, Eur Spine J (2001) 10 :S96–S101 DOI 10.1007/ s00586010028
[3] David. J. Hak, The Use of Osteoconductive bone graft Substitutes in Orthopedic Trauma, J Am Acad Orthop Surg, Vol 15, No. 9, September 2007, 525 – 536
[4] Dominique J Griffon, Evaluation of Osteoproductive Biomaterials: Allograft, Bone Inducing agent, bioactive glass, and ceramics, Academic Dissertation, Department of Clinical Veterinary Sciences, Division of Surgery Faculty of Veterinary Medicine, University of Helsinki, Finland, Helsinki University Printing House, Helsinki (2002)
[5] Wikipedia, the free encyclopedia
Muhammad Musaddique Ali Rafique 