Autografts are still the method of choice to substitute damaged or lost bone. The transplanted bone is largely necrotic and acts as a scaffold for the ingrowth of granulation tissue containing osteoprogenitor cells. Under the influence of local factors (cytokines, etc) these osteoprogenitor cells differentiate into osteoblasts that are responsible for new bone formation. At the same time the graft matrix is resorbed through osteoclasts. This process of graft resorption occurring parallel to bone formation was already discovered by Phemister in 1914 [3], who named it “creeping substitution”.

Bone substitute materials increasingly replace the use of autografts and can be divided into three major classes: polymers, ceramics and natural materials. Nowadays they are used either alone or in combinations called composites [4]. Regardless of their composition, once they are implanted in bone, they also undergo the process of creeping substitution, although the speed and rate of this process may vary according to their composition [5].

Synthetic polymers represent the largest class of biomaterials useful in applications in both, soft and hard tissue. They can be hydrophobic like silicone rubber (SR), polypropylene (PP), polyethylene (PE) and polymethylmetacrylate (PMMA), or water swelling or even water soluble like polyethylene glycol (PEG). Some of them are degradable, others remain almost unchanged within the body. Polymers are long chained molecules consisting of a large number of small repeating units [6]. They can be amorphous or semicrystalline and their surfaces may be modified chemically and biochemically.

Ceramics are a large family of inorganic/non-metallic compositions with a wide range of characteristics depending on the processing method used. They can be dense, porous or non-porous and resorbable like tricalcium-phosphate [24], porous, inert and lead to bone ingrowth like hydroxyapatite-coated porous metals, or dense, non-porous, surface active materials, that attach to the bone by chemical bonding like hydroxyapatite. In this chapter only the most frequently used ceramics are cited as examples.

Calcium phosphates represent a group of materials, where their properties depend on the calcium-phosphate ratio and modification of crystallinity and porosity. They are biocompatible, osseoconductive and degradable [25, 26] by extracellular dissolution of the calcium orthophosphate material [27] and by a cell mediated resorption [28] similar to the bone remodelling or bone substitution observed with autografts [3]. Their degradation time may last up to years, depending on the type of material.

Calcium phosphate ceramic blocks are brittle, highly susceptible to fatigue fractures and therefore of limited use in complex weight bearing locations [29]. Furthermore, their preset structure may render it difficult to adapt them to local defect sites. If used in blocks, their shape may not be corresponding to the original bone defect and if used as granules it may be impossible to keep them at the implantation site.

Calcium phosphate cements can overcome this problems partially as they can be administered in paste form and injected into bone defects, which makes adaptation to local requirements very easy. In addition, they can be administered through the tissues without the need of open approaches to the injection sites. They harden without elevation of temperature. Depending on the individual composition and pH the cement setting results in brushite-cement (pH ≤ 4,2) or hydroxyapatite (pH > 4,2) [30]. There are more than 100 different formulations of calcium orthophosphate cements available, which can be divided into four classes: dicalciumphosphate dihydrate, calcium and magnesium phosphates, octocalciumphosphate and non-stoichiometric apatite cements [31].

The use of ß-tricalcium phosphate (ß-TCP) is limited by its unpredictable rate of resorption and also its biocompatibility is discussed controversary. Levin [32] reported the presence of giant cells when tricalcium phosphate was implanted, whereas Jarcho [33] to the contrary stated, that there was no foreign body cell response. On the other hand, there is consense about ist osteoconductive properties [28, 33, 34].

Dicalciumphosphate is one of the most soluble of calciumphosphate phases and can be used when quick degradation is required [35]. All cements of this group are the product of an acid-base reaction and it shows good osteoconductive properties. Despite the setting pH being quite low, tissue necrosis as a response has not been detected [31].

Hydroxyapatite is used as implant coating [36], granules [37, 38] and in block structure [39]. It has a similar chemical composition as the mineral fraction of bone and attaches close to hard tissues. It is able to fill gaps between bone and implant up to 2 mm and stimulates bone ingrowth even in osteoporotic bone [40]. In contrast to ß-TCP, hydroxyapatite bone substitutes are considered non-resorbable. However, this is only partially true since also hydroxyapatite substitutes degrade in vivo, albeit much slower compared to ß-TCP or brushite compositions [30, 41, 42].

Hydroxyapatite-coated implants integrate well with the bone healing process [39, 43]. This characteristic, called “osteophilic” [36], provides a good substrate for osteoblasts. The use of hydroxyapatite as a coating of implants, therefore, is quite common. However, some authors found out, that it can lead to osteolysis when it is exposed to bone marrow and soft tissues [44]. There, the hydroxyapatite wear debris is thought as the main cause for implant failure [45] as its phagocytosis stimulates the release of cytokines. Subsequently these products are held responsible for (granulomatous) inflammation, disturbance in bone remodelling and local osteolysis.

derive from the mononuclear phagocyte system. All members of that system arise from a common stem cell in the bone marrow, possess lysosomes and are capable of phagozytosis. In the first few days after a fracture has occurred or a bone defect has been produced, granulation tissue grows in between the fracture ends or edges respectively. As mentioned above, neutrophil polymorphs are the type of cells which can be found most frequently in this period. The macrophage derived interleukin-1 may cause neutrophil infiltration, induce angiogenesis and antibody production and lymphokine synthesis [82]. Later more chronic inflammatory cells like macrophages are found, which remove red cells, necrotic fat and tissue debris [76]. These are transformed monocytes which arrive at the implantation site via a complex pathway of chemotactic and chemokinetic agents like mast cells/histamine [54], and Tumor Necrosis Factor (TNF) -alpha [83] release, complement factors, lymphokines, chemokines, platelet factors, leukotrienes and eventually bacterial fragments [84]. They adhere to the biomaterials via several adhesion ligand-receptor superfamilies [80], such as with an integrin binding [68]. Certain biomaterial-adsorbed proteins promote monocyte adhesion. The most active in this regard are fibrinogen, fibronectin and immunglobulin G [85-87].

After adhesion they transform into macrophages which are characterized by cell enlargement and an increased secretion of inflammatory mediators (cytokines and chemokines), an increased expression of membrane proteins (e.g. integrins) [88, 89] and the expression of angiogenic and other growth factors [80]. The released factors attract different cell types like additional macrophages, neutrophils, fibroblasts and other cells [89-92].

Macrophages are cells that secrete factors to promote physiological wound healing. Some of the macrophages may also function as accessory antigen-presenting cells [93]. On the other hand, they can be the central cellular mediators of the chronic inflammatory response to foreign materials [56] by secreting monocyte chemoattractant protein-1 (MCP-1) contributing to the development of the foreign body reaction [94], disturbing wound healing and ultimately contributing to implant failure.

Macrophages phagocytose damaged cells, cellular debris and foreign substances and digest the ingested material by hydrolytic enzymes in their lysosomes. While this mechanism functions for several biomaterials in the same way, their enzymatic apparatus is not able to degrade synthetic polymers [95]. Some authors consider the presence of macrophages around or near biomaterials to be part of a chronic inflammatory reaction, whereas others relate to them as part of the normal degradation behaviour, at least in case of degradable materials in bone [27, 62, 96, 97] (Fig. 4).

Fig. (4). Biocompatibility study of a biomaterial as bone substitute in sheep bone. Macrophages (arrows) with ingested foreign material. (a: Kossa 20x, b: Kossa 40x).

Macrophages also modulate the tissue reaction through production of interleukins, growth factors and other bioactive agents and most importantly they are the precursors of osteoclasts and foreign body giant cells. The role of RANKL and what triggers the fusion and further differentiation into osteoclasts or foreign body giant cells is not entirely clear [96, 97].

Mechanical wear of implants (the smaller the more) [45] activate macrophages to phagocytosis which in turn induces secretion of TNF-α, IL-1ß, IL-6 and prostaglandin (PGE)₂ [45, 98] that stimulates differentiation of osteoclast precursors into mature osteoclasts [99]. This effect leads to bone resorption and in excess can induce implant failure [100]. While in debate whether osteoclasts contain foreign material in their cytoplasm, it also has been shown, that macrophages, which have phagocytosed particles, are capable of osteoclast differentiation [101].

Interaction of macrophages and lymphocytes are complementary. It is likely that the adhesion of macrophages to a surface also is the initial signal to activate the lymphocytes which in turn release molecules that furthermore influence macrophage activity [77] and fusion [81, 102-104]. Therefore, the presence and activity of lymphocytes may be a determining factor in excessive resorption behaviour or ultimate biocompatibility questions.

Macrophages also modulate in the process of tissue repair. Since they derive from the vascular system, a good vascularity, therefore, is one of the important factors [105]. Animals depleted of macrophages or having received antimacrophage monoclonal antibodies show deficient wound healing [106].

are present in cases of bone defect healing with or without autologous bone grafts and play a significant role if biomaterials are applied. In case of biomaterials and bone controversy exists in whether they are part of normal bone healing or material degradation, or whether they play a significant role in issues of bioincompatibility.

Under chronic inflammatory conditions and if the matter to be disposed is very large (bigger than the diameter of a macrophage [27], than 80 µm [95], 5µm [44], 12 µm [35]), or indigestible for osteoclasts [107, 108], several macrophages fuse to form a foreign body giant cell [109]. Anderson [69], in contrary, has the opinion that „the presence of mononuclear cells, including lymphocytes and plasma cells, is considered chronic inflammation, whereas the foreign body reaction with the development of granulation tissue is considered the normal wound healing response to implanted biomaterials“. His view in regard to foreign bodies is similar to Lassus et al. who considers macrophages to be part of the normal degradation process [96, 97].

The mechanism of cellular fusion of macrophages to FBGC is similar to phagocytosis and is mediated by several mediators [110, 111], but little is known regarding the biological responses which are considered to influence the transition to FBGC development [69].

It can lead to very large cells (up to 1 mm²) with hundreds of nuclei [112, 113]. FBGCs are also generally observed in granulomas induced by bacterial pathogens, such as in tuberculosis or trichinellosis, which is probably the main reason for the negative association with their presence in tissue.

Cytokines like interleukin-4 and 13 are known as potent inductors of macrophage fusion into FBGC [103, 114, 115]. They also play a central role in cellular reactions which cause bone lysis around implants [116] as they modulate the balance between osteoblasts and osteoclasts [117]. The injection of anti-interleukin-4-antibody significantly decreased FBGC density on polyetherurethane ureas in vivo [102].

Foreign body giant cells act to concentrate phagocytic and degradative activities at the host-implant interface. They ingest and dissolute implanted material intracellularly or by the release of degradative agents like lysosomal enzymes and reactive oxygen intermediates (ROIs) at the ventral cell surface [118] in response to certain stimuli, whereby their phagocytic capacity can be as effective as macrophages [119]. They also can directly contribute to osteolysis by differentiation into TRAP-positive osteoclast-like cells [120-122].

FBGC rapidly differentiate after the implantation and progressively decrease with time [123]. Their size depends on the intensity of the inflammatory response [124]. They resemble osteoclasts morphologically, such that both are multinucleated, are found near implant/bone contact and have a cytoplasm with many vacuoles (Fig. 5). FBGCs contain great numbers of mitochondria of various size and oval or round nuclei. Rough endoplasmatic reticulum is found throughout the cytoplasm [125]. The osteoclast on the other hand is defined as possessing a resorbing apparatus consisting of ruffled border and clear zone, expression of the tartrat–resistant acid phosphatase and the expression of calcitonin-receptors [125]. However, it has to be kept in mind that other authors question whether the TRAP-epitopes as well as other markers (such as calcitonin receptors) are specific for osteoclasts [73].

Fig. (5). Foreign body giant cell (arrow) close to implanted material. (Kossa 40x).

One of the most important aspects in the evaluation of biomaterials is the degradation resistance, with the exception of the class of biodegradable polymers, that rely on enzymes, acid or ROIs for degradation of the polymer matrix [126]. The discussion about the role of FBGCs is controversary with the main question being whether the presence of FBGCs near an implant is just a sign of biodegradability [126], therefore a part of normal bone healing and resorption after the implantation of a bone substitute, or a sign of insufficient biocompatibility, inflammation and implant failure? Both sides have valuable arguments ready which are outlined below.

The arguments in favour of the FBGC are that i) these cells are part of the normal wound healing response to implanted inert or biodegradable biomaterials [56, 127], ii) their presence does not impair bone formation [31], iii) the presence of FBGCs indicates a low degradability of the implanted substance [125, 128, 129], iv) FBGC in the absence of other inflammatory cells show a good biocompatibility [107], and most importantly v) macrophages and FBGCs mediate material resorption and fragmentation of biodegradable implants [125, 129-131]. The latter is supported through the fact that resorbed material could be seen in intracytoplasmatic vacuoles by transmission electron microscopy [132]. Furthermore, the implantation bed of resorbable poly-L-lactide (PLLA) plates and screws showed a “foreign body reaction without signs of inflammation”: only a few polymorph nuclear leucocytes were present and the remnants of the plates and screws were surrounded by connective tissue with macrophages, foreign body giant cells and fibrocytes. The foreign body reaction was thought to be evoked by very small particles (22 µm) of disintegrated PLLA plates and screws [61] and the amount of the degraded material seems to influence the intensity of the foreign body reaction [133]. This view, that FBGC are responsible for the degradation of biomaterials, is further supported by the analyses of retrieved implants showing material surface cracks directly under adherent FBGC [113]. If macrophages and foreign body cells are part of the normal wound healing process, the foreign body cells may persist for the lifetime of the implant, it is not known if activated or quiescent [69]. Biocompatibility may be in jeopardy, if the presence of a large mass of disintegrated material seems to “exceed the local tissue tolerance” [61]. Here, the question arises where the line has to be drawn for local tissue tolerance?

The arguments pointing towards FBGC being a a bad sign for tissue tolerance are that i) the fusion of specialized macrophages is induced by poorly tolerated foreign bodies [78, 112, 134], ii) avoiding monocyte or macrophage adhesion and FBGC formation, inflammatory degradation could be minimized [135], iii) the presence of macrophages and FBGC is associated with structural and functional failure of the implant [136], and iv) FBGC seem to concentrate the phagocytic and degradative activities at the tissue-material interface and therefore are responsible for the damage and failure of the implant. For the latter the authors tested coating the surface of an implant with a material that promotes programmed cell death to inhibit the adhesion and fusion of macrophages into FBGCs [137].

close or around an implanted biomaterial is frequently seen. The reasoning behind it is that if the implanted biomaterial cannot be ingested by macrophages and FBGCs (“frustrated phagocytosis”) the next best protection for the host seems to be the isolation of the foreign object. This can be achieved best by a layer of FBGC in a fibrous, quite avascular capsule limiting further interaction between host and implanted device. The capsule type depends on the secreted cytokines [95, 138], the extent of injury or defect created and the amount of provisional matrix [69].

On one hand this fibrous capsule may indeed downsize the inflammatory reaction, but on the other hand in osseointegration processes can lead to device failure and restricted nutrient supply. Poor tissue device contact can lead to infection. A fibrous layer (so called interface membrane) between methylmethacrylate and an implant has shown to be morphologically synovial-like [55]. In well fixated or coated [139] implants the interfacial membrane is thin with only some macrophagic aggregates. The quantity and quality of the fibrous scar tissue depends on the implantation technique and the prosthetic material used [11-13, 55].

There are two different models representing different principles to test novel materials in bone: a wound model, where spontaneous healing will occur in a short time, provides information about osteocompatibility and inflammatory potential of a given material; and a model of large osseous defects [163], that do not heal spontaneously.

The correct placement of an implant is of utmost importance, as post implantation mobility leads to chronic inflammation with the development of a thick fibrous capsule with all its disadvantages [11-13].

Bone is subjected to high strain [164, 165]. In its function of load bearing continuous microdamage to bone occurs [166] that constantly demands structure remodelling. As described earlier this remodelling is mostly coordinated by bone forming cells (osteoblast, osteocytes and periostal cells), monocyte-derived osteoclasts and multinuclear cells. Extensive micromotion between implant and host bone leads to an intermediate layer between them. One has to keep in mind that this intermediate layer, also called interface membrane, cannot be distinguished according to its primary cause such as biocompatibility and/or mechanical problems. Tissue composition and cellular components are more less identical in both cases and again, this thick and fibrous layer severely disturbs osseointegration of any implant.

Biomaterials inserted into bone normally elicit local and very seldom systemic responses. Therefore, the influence of systemic hormones may be negligible in most cases. Nevertheless, hormones do influence the overall reaction of the immune system of a patient or an experimental animal which consequently may even have a – albeit relatively minor-local effect on immune response. Hormon concentrations may be directly related to the intensity of the immune response and as a result also to the tissue-implant response [148, 167]. High doses of steroid hormones could have significant down regulating effects on immune response [168]. High testosteron levels and low estrogen levels seem to depress immune function [169]. Estrogens are enhancers of humoral immunity [105].