**4.6 Aseptic loosening as a result of local failure of tissue homeostasis** *(Gallo)*

The term "homeostasis" was coined by Claude Bernard in 1865 to describe the constancy of the internal environment as a condition of health. Metchnikoff has introduced the concept into immunology together with the term physiological inflammation to mean active maintenance of tissue harmony, tissue harmony (Medzhitov 2010). Given that the majority of patients profit from THA, it appears that low-grade periprosthetic inflammation occurring around "healthy" THA may be a normal adaptive response to the continual burden of prosthetic particles and mechanical stresses associated with implant use. For this reason, processes leading to osteolysis and aseptic loosening can be investigated as a problem of maladaptation.

In reality, no one knows what are the histological and immunological parameters associated with long-term "physiological" tissue homeostasis around THA, a baseline for true understanding of periprosthetic pathology. We can only translate data from models of THA (animal *in vivo* or *in vitro*) to the human situation (El-Warrak et al. 2004; Ma et al. 2009) and compare these with analyses of tissues from failed THAs. Unfortunately, tissues retrieved during revision surgery reflect late stages of the process when local homeostasis has long failed in the majority of cases.

What parameters are typical for physiological equilibrium between implant derived signals and healthy status of periprosthetic tissues in terms of maintenance of at least their architecture and function? The first and inevitable condition is *stable interface between implant and bone* which protects tissues from the deleterious effects of mechanical stimuli (stable equilibrium of implant-bone interface). The ideal condition is when the bone around the implant is protected from excessive mechanical stresses and strains and at the same time can undergo bone remodelling. The next critical step is *the ability to resolve inflammation induced by surgical trauma together with prevention of biofilm formation* on the prosthetic surface. Both the inability to achieve a steady very low-inflammatory status as early as possible and the

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis 345

increased accumulation of osteoclasts around the implant. The number of terminally differentiated cells is regulated by the mitotic activity of the stem cells in a poorly

A very important role is also played by the local microenvironment which may favour development a particular cell line (Konttinen et al. 2005). In the case of THA, the inducers of inflammation can never be completely eliminated thus the tissues are under permanent activation load not only by mechanical stresses and particles but also by cytokines/ chemokines produced by cells previously activated by particles and/or microbial remnants. In fact, even under such conditions, the majority of periprosthetic tissues (patients) develop a steady state for a long time postoperatively. How is this possible? Can the tissues of successful patients develop something akin to "tolerance" (decreased responsiveness to the prosthesis-related stimuli)? Generally, to keep tissues in lowinflammatory steady state *require as low load of inducers as possible and also very fine tuning of sensors together with strict control of the quantity and activities of effector molecules/cells* (Fig. 5). On the other hand, the inability to resolve chronic high-grade inflammation can lead to irreversible tissue damage with restoration of different tissues (fibrous tissue instead of

Regarding the control of prosthesis-related inducers, most important appears to be *reduction of the particle loads* either using hard bearings or modern technologies of prosthetic surface treatment (*Part 5.1*). Of biological inflammation inducers, the most important roles are played by *cell necrosis*, *extracellular matrix damage* (both deliberating damage-associated molecular patterns, DAMPs), *reactive oxygen species (ROS), hypoxia, and activation of PRRs*. *Cell necrosis* can affect both macrophages and other cell groups. It can be a result of strong foreign particle load (particles are *per se* undigestable, cytotoxic, genotoxic), hypoxia, ROS or unfavourable mechanical conditions. These stimuli can lead separately or combined to irreparable DNA damage and eventually cell death. Molecules released from dying cells together with breakdown products of the extracelullar matrix are considered important inducers of inflammation. In contrast, ingestion of apoptotic cells is associated with release of inflammation-resolving cytokines IL-10 and TGF-β (Nathan and Ding 2010). From this, it follows that the ratio of necrotic cells to apoptotic cells around the implant could play a

significant role in relation to periprosthetic homeostasis (Landgraeber et al. 2009).

*Hypoxia* seems to be a common feature in aseptic loosening. Bone-implant interface membranes can suffer from hypoxia because it is hypovascular, due to chronic hypoxiareperfusion injury of the interface membrane which is caused by implant loading. There is also increased oxygen consumption by local inflammatory cells (Santavirta et al. 1999). A hypoxic environment induces the expression of hypoxia-inducible factors (HIF-1α, HIF-2α) and some heat shock proteins that adapt the gene expression to oxygen availability within the hypoxic tissue. For this reason, decreased profileration is a fundamental physiological response to hypoxia in many cell types (Hubbi et al. 2011). Proliferation of progenitor cells is controlled by the hypoxia-sensitive Notch signalling pathway which maintains them in an undifferentiated state (Hilton et al. 2008). On the other hand, tissue macrophages are relatively well adapted to hypoxic conditions. This could at least partially explain their massive presence in periprosthetic membranes. Monocytes continue to differentiate into tissue macrophages even under severe hypoxia (0.2% O2) and macrophages increase significantly their expression of HIF-1α mRNA and pro-angiogenic cytokine vascular

understood feedback interaction with local and systemic factors.

bone, formation of granulomas etc.).

**4.6.1 Control of prosthesis-related inducers** 

Fig. 5. Components of inflammatory response to a potential set of biological and physical insults associated with total hip arthroplasty (inducers of inflammation); host sensors detect the occurrence of inducers at tissues and their recognizing triggers expression of inflammatory mediators that affect the tissues where the inducers occur (according to Medzhitov 2010)

occurrence of microbial biofilm in the joint can maintain chronic inflammation which seriously compromises the local tissue homeostasis. The third critical step is the ability of tissues *to counteract the continual delivery of soluble (ionic) and insoluble (particles) debris* liberated from the prosthetic surfaces without increased inflammation and tissue damage. Prosthetic debris can induce a multitude of interactions which greatly increase the probability of chronic inflammation and predominance of osteoclasts over the osteoblasts at the bone-implant interface (*Part 2.2*). To avoid developing particle disease, there has to be much stronger regulation ability to overcome the above inflammatory and destructive processes. If these regulatory factors were known, new preventative and regulative measures could be proposed. The fourth critical step is the *regenerative potential of the periprosthetic tissues* that can restore tissue damage regardless whether this is of mechanical, biological or combined origin. Recently for instance, it has been proposed that a part of the inter-individual variance in the risk of early prosthetic migration (i.e. late aseptic loosening development) could be explained by patient-specific differences in the regeneration processes around the implant early post-operatively (Aspenberg et al. 2008). Taken together, when these steps concur, the probability of aseptic loosening is reduced and mutatis mutandis, if any of these steps is disturbed the route to aseptic loosening is open.

Normal tissue homeostasis is regulated by *complex cross-talk between stem cells, their differentiated cells, the local microenvironment, and the whole organism* (Perez-Losada and Balmain 2003). Basically, this depends on the right cell being in the right place at the right time (Buckley 2011). In periprosthetic reality this means, having sufficient number of functional macrophages, osteoblasts, fibrocytes-fibroblasts, and lymphocytes in a configuration that prevents development of chronic inflammation, fluid oversecretion, and increased accumulation of osteoclasts around the implant. The number of terminally differentiated cells is regulated by the mitotic activity of the stem cells in a poorly understood feedback interaction with local and systemic factors.

A very important role is also played by the local microenvironment which may favour development a particular cell line (Konttinen et al. 2005). In the case of THA, the inducers of inflammation can never be completely eliminated thus the tissues are under permanent activation load not only by mechanical stresses and particles but also by cytokines/ chemokines produced by cells previously activated by particles and/or microbial remnants. In fact, even under such conditions, the majority of periprosthetic tissues (patients) develop a steady state for a long time postoperatively. How is this possible? Can the tissues of successful patients develop something akin to "tolerance" (decreased responsiveness to the prosthesis-related stimuli)? Generally, to keep tissues in lowinflammatory steady state *require as low load of inducers as possible and also very fine tuning of sensors together with strict control of the quantity and activities of effector molecules/cells* (Fig. 5). On the other hand, the inability to resolve chronic high-grade inflammation can lead to irreversible tissue damage with restoration of different tissues (fibrous tissue instead of bone, formation of granulomas etc.).

### **4.6.1 Control of prosthesis-related inducers**

344 Recent Advances in Arthroplasty

Fig. 5. Components of inflammatory response to a potential set of biological and physical insults associated with total hip arthroplasty (inducers of inflammation); host sensors detect

occurrence of microbial biofilm in the joint can maintain chronic inflammation which seriously compromises the local tissue homeostasis. The third critical step is the ability of tissues *to counteract the continual delivery of soluble (ionic) and insoluble (particles) debris* liberated from the prosthetic surfaces without increased inflammation and tissue damage. Prosthetic debris can induce a multitude of interactions which greatly increase the probability of chronic inflammation and predominance of osteoclasts over the osteoblasts at the bone-implant interface (*Part 2.2*). To avoid developing particle disease, there has to be much stronger regulation ability to overcome the above inflammatory and destructive processes. If these regulatory factors were known, new preventative and regulative measures could be proposed. The fourth critical step is the *regenerative potential of the periprosthetic tissues* that can restore tissue damage regardless whether this is of mechanical, biological or combined origin. Recently for instance, it has been proposed that a part of the inter-individual variance in the risk of early prosthetic migration (i.e. late aseptic loosening development) could be explained by patient-specific differences in the regeneration processes around the implant early post-operatively (Aspenberg et al. 2008). Taken together, when these steps concur, the probability of aseptic loosening is reduced and mutatis

the occurrence of inducers at tissues and their recognizing triggers expression of inflammatory mediators that affect the tissues where the inducers occur (according to

mutandis, if any of these steps is disturbed the route to aseptic loosening is open.

Normal tissue homeostasis is regulated by *complex cross-talk between stem cells, their differentiated cells, the local microenvironment, and the whole organism* (Perez-Losada and Balmain 2003). Basically, this depends on the right cell being in the right place at the right time (Buckley 2011). In periprosthetic reality this means, having sufficient number of functional macrophages, osteoblasts, fibrocytes-fibroblasts, and lymphocytes in a configuration that prevents development of chronic inflammation, fluid oversecretion, and

Medzhitov 2010)

Regarding the control of prosthesis-related inducers, most important appears to be *reduction of the particle loads* either using hard bearings or modern technologies of prosthetic surface treatment (*Part 5.1*). Of biological inflammation inducers, the most important roles are played by *cell necrosis*, *extracellular matrix damage* (both deliberating damage-associated molecular patterns, DAMPs), *reactive oxygen species (ROS), hypoxia, and activation of PRRs*.

*Cell necrosis* can affect both macrophages and other cell groups. It can be a result of strong foreign particle load (particles are *per se* undigestable, cytotoxic, genotoxic), hypoxia, ROS or unfavourable mechanical conditions. These stimuli can lead separately or combined to irreparable DNA damage and eventually cell death. Molecules released from dying cells together with breakdown products of the extracelullar matrix are considered important inducers of inflammation. In contrast, ingestion of apoptotic cells is associated with release of inflammation-resolving cytokines IL-10 and TGF-β (Nathan and Ding 2010). From this, it follows that the ratio of necrotic cells to apoptotic cells around the implant could play a significant role in relation to periprosthetic homeostasis (Landgraeber et al. 2009).

*Hypoxia* seems to be a common feature in aseptic loosening. Bone-implant interface membranes can suffer from hypoxia because it is hypovascular, due to chronic hypoxiareperfusion injury of the interface membrane which is caused by implant loading. There is also increased oxygen consumption by local inflammatory cells (Santavirta et al. 1999). A hypoxic environment induces the expression of hypoxia-inducible factors (HIF-1α, HIF-2α) and some heat shock proteins that adapt the gene expression to oxygen availability within the hypoxic tissue. For this reason, decreased profileration is a fundamental physiological response to hypoxia in many cell types (Hubbi et al. 2011). Proliferation of progenitor cells is controlled by the hypoxia-sensitive Notch signalling pathway which maintains them in an undifferentiated state (Hilton et al. 2008). On the other hand, tissue macrophages are relatively well adapted to hypoxic conditions. This could at least partially explain their massive presence in periprosthetic membranes. Monocytes continue to differentiate into tissue macrophages even under severe hypoxia (0.2% O2) and macrophages increase significantly their expression of HIF-1α mRNA and pro-angiogenic cytokine vascular

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis 347

mechanisms underlying the tolerance to chronic unsolvable signalling. Theoretically, there are at least two ways leading to deactivation of macrophages. The first is associated with decrease in stimulation signalling level (e.g. interferon gamma and other cytokines, lipopolysaccharides and other bacterial stimuli). The second could be associated with activity of tissue-resident cells inducing negative modulation of macrophage pro-

Activated fibroblasts perpetuate inflammation via inappropriate expression of survival molecules leading to the retention of activated cells in affected tissues or ectopic secretion of chemokines supporting recruitment of new cells as a fuel for continuation of inflammation (Buckley 2011). They also express several bone-resorbing metalloproteinases and "osteoclastogenic" cytokines in periprosthetic membranes including M-CSF, VEGF or RANKL contributing together with suppression of osteoblast function, to functional predominance of bone resorption over osteogenesis (Koreny et al. 2006). Based on these observations, it seems inevitable that deactivated and quiescent fibroblasts can, like macrophages significantly contribute to the resolution of particle-induced inflammation. Recent studies have revealed that inflammation is not generic but contextual. Therefore, *tissue-resident fibroblasts are able to switch the inflammation to the resolution*. In this connection, it is important to know which mechanisms induce fibroblast anti-inflammatory and regeneration activities. Firstly, fibroblasts could act as a source of anti-inflammatory and regenerative cytokines such as IL-4, IL-10, FGF and several others. Secondly, fibroblasts can provide anti-inflammatory stromal microenvironment in the periprosthetic interface membrane involving a plethora of cell-to-cell

*Lymphocytes* together with other cells enhance osteoclast differentiation; stimulate formation of foreign body giant cells and exhibit many other activities contributing to tissue damage. On the other hand, they have several pathways by which they *can "shift" the immune response in favour of TH2 and TH3* instead of TH1 and TH17 responses or delayed type of

Until now, we have assumed that the immune system alone dictates the direction and fate of periprosthetic tissues after stimulation by prosthetic derivates. However, this concept does not reflect the reality of the situation. Currently discussed is the concept of *tissue-appropriate immunity*, stressing the role of tissues in control of the effector mechanisms that prevent selfdestruction (Matzinger and Kamala 2011). On this basis, tissue-resident cells might influence the response to stimulus in order to maintain the health of affected tissues by modifying cell activities and via soluble anti-inflammatory factors. Unfortunately, no such strategy is available currently in clinical practice to modify the above processes and induce/maintain

**4.7 Individual susceptibility to aseptic loosening/ development of periprosthetic** 

High variability in survival of THAs, aseptic loosening/size of periprosthetic osteolysis has been observed between individuals with similar polyethylene wear rates. For this reason, the question is how to explain such a degree of inter-patient variability? This could be caused for example, by differences in implants (e.g. type of bearing material, sterilisation method, shape of prosthesis, surface technology etc.), variations in surgical technique (orientation of implants, final quality of bone-implant interface, protection of bone bed from joint fluid etc.), and variations due to patient-related factors (e.g. age, co-morbidities, level of activity and differences in mechanical loading). In addition, genetic factors can contribute to

inflammatory activities and macrophage apoptosis (Valledor et al. 2010).

and perhaps also paracrine interactions (Buckley 2011).

hypersensitivity (Matzinger and Kamala 2011).

localized tissue homeostasis around THA.

**osteolysis** *(Gallo, Petrek)* 

endothelial growth factor (VEGF) as a response to chronic hypoxia. Hypoxia stimulates osteoclast differentiation, viability and activity, the latter predominantly via increased activity of hydrolytic enzymes including cathepsin K and in this way it could directly contribute to the extension of bone resorption around the THA. On the other hand, this effect depends on time and O2 concentration with reduction in number and activity of osteoclasts as a function of longevity and severity of hypoxia (Knowles and Athanasou 2009). Osteoblastic cell lines can exhibit decreased growth, differentiation and mineralization capacity under hypoxic conditions. Moreover, these cells predominantly induce angiogenesis via increased secretion of VEGF (Knowles and Athanasou 2009). An important role in the transduction of hypoxic signal to bone-implant interface may be played by osteocytes via increased expression of HIF and osteopontin leading eventually to osteocyte-induced osteoclastic bone resorption (Gross et al. 2005).

*Activation of PRRs* by occurrence of PAMPs in periprosthetic tissues can distort local tissue homeostasis at any time postoperatively (Greenfield et al. 2010). PRRs have a strong potential to trigger inflammation via several pathways including differentiation of more aggressive M1 and M17 macrophages, multinuclear foreign body giant cells, osteoclasts and granulomas (Anderson et al. 2008). In addition, it has been shown that polymeric alkane structures released during UHMWPE breakdown can directly activate PRRs (TLR-1, 2 signalling pathway) while UHMWPE particles phagocytosed by periprosthetic cells induce endosomal destabilization and inflammasome activation (Maitra et al. 2009).

### **4.6.2 Tuning of inflammatory sensors and their response to prosthetic-related stimuli**

Currently there is no evidence supporting the concept of "adaptation of inflammatory sensors" to chronic stimuli associated with prosthesis. The following is therefore simply translated from knowledge of the response of macrophages to microbial stimuli (Medzhitov and Horng 2009).

After several hours or days of prosthetic-related stimulation, the expression of several hundreds of genes could be induced by macrophages, fibroblasts and other cell groups. These gene expressions determine cell response to the stimuli. Simultaneously, a set of regulators is co-activated to control intensity and extension of the local inflammatory response (negative regulators of inflammation). These can be distinguished as *signal-specific regulators and gene-specific regulators*. The first category consists of regulators that inhibit signal transduction by PRRs and other inflammatory pathways (e.g. IL-1R-associated kinase M, IRAKM; suppressor of cytokine signalling, SOCS – proteins). The second category comprises transcriptional repressors (basal repressors and inducible repressors) or other negative regulators that modulate gene expression (e.g. microRNAs, long-non coding RNAs). Although there is no evidence that currently supports the existence of "adaptors" on prosthetic stimuli, we believe they must exist. Assuming their role, it may be postulated that in the absence of a key part of the negative feedback loop the proinflammatory cytokines/chemokines develop severe inflammatory microenvironment associated especially with the predominance of activated osteoclasts over functional osteoblasts , and vice versa, effective negative regulation of sensors could protect from development of osteolysis and aseptic loosening (*Parts 4.2, 4.3*).

### **4.6.3 Control of the effectors**

Generally, macrophages lie at the centre of the process called particle disease. Therefore there is a question *whether activated macrophages can be deactivated* and what are the

endothelial growth factor (VEGF) as a response to chronic hypoxia. Hypoxia stimulates osteoclast differentiation, viability and activity, the latter predominantly via increased activity of hydrolytic enzymes including cathepsin K and in this way it could directly contribute to the extension of bone resorption around the THA. On the other hand, this effect depends on time and O2 concentration with reduction in number and activity of osteoclasts as a function of longevity and severity of hypoxia (Knowles and Athanasou 2009). Osteoblastic cell lines can exhibit decreased growth, differentiation and mineralization capacity under hypoxic conditions. Moreover, these cells predominantly induce angiogenesis via increased secretion of VEGF (Knowles and Athanasou 2009). An important role in the transduction of hypoxic signal to bone-implant interface may be played by osteocytes via increased expression of HIF and osteopontin leading eventually to

*Activation of PRRs* by occurrence of PAMPs in periprosthetic tissues can distort local tissue homeostasis at any time postoperatively (Greenfield et al. 2010). PRRs have a strong potential to trigger inflammation via several pathways including differentiation of more aggressive M1 and M17 macrophages, multinuclear foreign body giant cells, osteoclasts and granulomas (Anderson et al. 2008). In addition, it has been shown that polymeric alkane structures released during UHMWPE breakdown can directly activate PRRs (TLR-1, 2 signalling pathway) while UHMWPE particles phagocytosed by periprosthetic cells induce

**4.6.2 Tuning of inflammatory sensors and their response to prosthetic-related stimuli**  Currently there is no evidence supporting the concept of "adaptation of inflammatory sensors" to chronic stimuli associated with prosthesis. The following is therefore simply translated from knowledge of the response of macrophages to microbial stimuli (Medzhitov

After several hours or days of prosthetic-related stimulation, the expression of several hundreds of genes could be induced by macrophages, fibroblasts and other cell groups. These gene expressions determine cell response to the stimuli. Simultaneously, a set of regulators is co-activated to control intensity and extension of the local inflammatory response (negative regulators of inflammation). These can be distinguished as *signal-specific regulators and gene-specific regulators*. The first category consists of regulators that inhibit signal transduction by PRRs and other inflammatory pathways (e.g. IL-1R-associated kinase M, IRAKM; suppressor of cytokine signalling, SOCS – proteins). The second category comprises transcriptional repressors (basal repressors and inducible repressors) or other negative regulators that modulate gene expression (e.g. microRNAs, long-non coding RNAs). Although there is no evidence that currently supports the existence of "adaptors" on prosthetic stimuli, we believe they must exist. Assuming their role, it may be postulated that in the absence of a key part of the negative feedback loop the proinflammatory cytokines/chemokines develop severe inflammatory microenvironment associated especially with the predominance of activated osteoclasts over functional osteoblasts , and vice versa, effective negative regulation of sensors could protect from development of

Generally, macrophages lie at the centre of the process called particle disease. Therefore there is a question *whether activated macrophages can be deactivated* and what are the

osteocyte-induced osteoclastic bone resorption (Gross et al. 2005).

and Horng 2009).

osteolysis and aseptic loosening (*Parts 4.2, 4.3*).

**4.6.3 Control of the effectors** 

endosomal destabilization and inflammasome activation (Maitra et al. 2009).

mechanisms underlying the tolerance to chronic unsolvable signalling. Theoretically, there are at least two ways leading to deactivation of macrophages. The first is associated with decrease in stimulation signalling level (e.g. interferon gamma and other cytokines, lipopolysaccharides and other bacterial stimuli). The second could be associated with activity of tissue-resident cells inducing negative modulation of macrophage proinflammatory activities and macrophage apoptosis (Valledor et al. 2010).

Activated fibroblasts perpetuate inflammation via inappropriate expression of survival molecules leading to the retention of activated cells in affected tissues or ectopic secretion of chemokines supporting recruitment of new cells as a fuel for continuation of inflammation (Buckley 2011). They also express several bone-resorbing metalloproteinases and "osteoclastogenic" cytokines in periprosthetic membranes including M-CSF, VEGF or RANKL contributing together with suppression of osteoblast function, to functional predominance of bone resorption over osteogenesis (Koreny et al. 2006). Based on these observations, it seems inevitable that deactivated and quiescent fibroblasts can, like macrophages significantly contribute to the resolution of particle-induced inflammation. Recent studies have revealed that inflammation is not generic but contextual. Therefore, *tissue-resident fibroblasts are able to switch the inflammation to the resolution*. In this connection, it is important to know which mechanisms induce fibroblast anti-inflammatory and regeneration activities. Firstly, fibroblasts could act as a source of anti-inflammatory and regenerative cytokines such as IL-4, IL-10, FGF and several others. Secondly, fibroblasts can provide anti-inflammatory stromal microenvironment in the periprosthetic interface membrane involving a plethora of cell-to-cell and perhaps also paracrine interactions (Buckley 2011).

*Lymphocytes* together with other cells enhance osteoclast differentiation; stimulate formation of foreign body giant cells and exhibit many other activities contributing to tissue damage. On the other hand, they have several pathways by which they *can "shift" the immune response in favour of TH2 and TH3* instead of TH1 and TH17 responses or delayed type of hypersensitivity (Matzinger and Kamala 2011).

Until now, we have assumed that the immune system alone dictates the direction and fate of periprosthetic tissues after stimulation by prosthetic derivates. However, this concept does not reflect the reality of the situation. Currently discussed is the concept of *tissue-appropriate immunity*, stressing the role of tissues in control of the effector mechanisms that prevent selfdestruction (Matzinger and Kamala 2011). On this basis, tissue-resident cells might influence the response to stimulus in order to maintain the health of affected tissues by modifying cell activities and via soluble anti-inflammatory factors. Unfortunately, no such strategy is available currently in clinical practice to modify the above processes and induce/maintain localized tissue homeostasis around THA.

### **4.7 Individual susceptibility to aseptic loosening/ development of periprosthetic osteolysis** *(Gallo, Petrek)*

High variability in survival of THAs, aseptic loosening/size of periprosthetic osteolysis has been observed between individuals with similar polyethylene wear rates. For this reason, the question is how to explain such a degree of inter-patient variability? This could be caused for example, by differences in implants (e.g. type of bearing material, sterilisation method, shape of prosthesis, surface technology etc.), variations in surgical technique (orientation of implants, final quality of bone-implant interface, protection of bone bed from joint fluid etc.), and variations due to patient-related factors (e.g. age, co-morbidities, level of activity and differences in mechanical loading). In addition, genetic factors can contribute to

Aseptic Loosening of Total Hip Arthroplasty as a Result of Local Failure of Tissue Homeostasis 349

Genetic variation in patient-specific response to THA can be determined by several methods. For example, *genetic association studies* investigate possible association between the occurrences of phenotypic traits and candidate genetic variants using special rules and computational algorithms. In the case of *genome-wide association studies* the effects of up to 500,000 SNPs are analysed simultaneously, giving rise to an initial set of promising

When dealing with a genetic – environmental interaction it is necessary *to test simultaneously cases, controls exposed to the same key pathogenetic factors as the cases but do not exhibit the risk phenotype, and the population control*. In the case of aseptic loosening/osteolysis it is however rather difficult to recruit completely comparable cases and controls making it hard to address the question of individual susceptibility to these complications. In addition, identifying and characterizing the targets (mechanisms/molecules) for a genetic-association study is a key prerequisite for genetic investigation, and vice versa, inaccuracy in understanding the true pathophysiology of aseptic loosening/osteolysis compromises their outcomes. Therefore, the initial step of any genetic-association study should be *choice of potentially valuable candidate genes*

Wilkinson et al were the first to publish a study on the association between polymorphisms in the gene encoding for TNF alpha and risk of periprosthetic osteolysis in THA (Wilkinson et al. 2003). After this introductory work several papers were published nominating other molecules as candidates involved in the processes of aseptic loosening/ osteolysis. Structurally and functionally, these include receptors, intracellular mediators, enzymes, cytokines and other proteins. A recently published systematic review on genetically determined susceptibility to aseptic loosening of THA, revealed several areas of potential agreement (SNPs of TNF-238A allele, IL1RA +2018C allele, polymorphisms in genes for IL-6, MMP-1 etc.), but also several sources of heterogeneity between studies, showing the need for large multi-centre prospective studies that should provide stronger evidence for genetic predisposition to THA premature failure (Del Buono et al. 2011). As a result, further progress in the field of genetic risk of aseptic loosening/ osteolysis will require both sophisticated research strategy and statistics (e.g. improved risk-analysis models) to overcome a series of challenges. Fortunately, as experience accumulates, there is increased

susceptibility genes that are further tested in a separate replication study.

followed by their validation in a candidate-gene study (Chanock et al. 2007).

interest of orthopaedic specialty in the outcomes of such research.

resistance against osteolytic membrane and joint fluid expansion.

**5. Prevention of aseptic loosening/ periprosthetic osteolysis** *(Gallo,* 

Based on the pathophysiology of periprosthetic osteolysis and aseptic loosening, there is clear that advances in biomaterials can improve at least partially the survival of THA by reducing the extension and intensity of particle disease. Here we focus only on bearing surfaces, although the material of implant and surface technology also influence the

The wear of ultra high molecular weight polyethylene (UHMWPE) is a major contributor to premature failure of THAs. For this reason, a number of years ago, it was questioned

**4.7.2 Current status of the research** 

*Goodman, Konttinen)*  **5.1 Biomaterial solutions** 

**5.1.1 Advances in polyethylene** 

the risk for aseptic loosening (Fig. 6). Engh et al estimated that both wear and patient propensity to osteolysis might together account for 53% of the variance in total area of osteolysis (Engh et al. 2011).

Fig. 6. Functional variation in genes for cytokines (other molecules) could influence the rate of aseptic loosening via contribution to severity of bone resorption around the implant

### **4.7.1 Methods for investigating genetic susceptibility to aseptic loosening/ periprosthetic osteolysis**

The question is which genes (genotype) could influence the fate of implant in terms of premature aseptic loosening/ periprosthetic osteolysis (phenotype). A gene is traditionally defined as a segment of DNA encoding a protein. Single nucleotide polymorphism (SNP) is a common form of variation in the human genome implicating that a single base change in the DNA sequence could influence the amount/functionality of secreted proteins. With the sequencing of the human genome, it has been recognized that SNPs occur about once every 1000 base pairs (Dupuis and O'Donnell 2007). The key problem is *how to distinguish the gold (functional polymorphisms) from fool´s gold (unimportant variants)*. By genotyping a large number of SNPs in a large number of patients there is a good chance of detecting those that are significantly associated with the target disease. Unfortunately, epigenetic factors and further DNA sequence variants such as copy number variants create other sources of variation between individuals. In addition, there are several non-coding small regulatory molecules (e.g. miRNAs, siRNAs) that significantly influence the process of translation of genes into effector proteins.

According to the number of genes participating in the pathophysiology of a disease and the size of their effect, *polygenic predisposition* assuming small/ marginal effects of many genes fits much better to the model of THA failure than the oligogenic model with two or more major gene (locus) effects. In this line, the odds ratios of 1.2 to 1.6 are required for differentiating true effects of individual genetic variants on a complex disease from the potential impact of bias (Ioannidis et al. 2006). Studies reporting weaker individual size effects are unlikely to be used for predicting a target condition; on the other hand, a combination of several even weaker genetic variants (especially at multiple loci) can increase the chance for reasonable prediction.

Genetic variation in patient-specific response to THA can be determined by several methods. For example, *genetic association studies* investigate possible association between the occurrences of phenotypic traits and candidate genetic variants using special rules and computational algorithms. In the case of *genome-wide association studies* the effects of up to 500,000 SNPs are analysed simultaneously, giving rise to an initial set of promising susceptibility genes that are further tested in a separate replication study.

When dealing with a genetic – environmental interaction it is necessary *to test simultaneously cases, controls exposed to the same key pathogenetic factors as the cases but do not exhibit the risk phenotype, and the population control*. In the case of aseptic loosening/osteolysis it is however rather difficult to recruit completely comparable cases and controls making it hard to address the question of individual susceptibility to these complications. In addition, identifying and characterizing the targets (mechanisms/molecules) for a genetic-association study is a key prerequisite for genetic investigation, and vice versa, inaccuracy in understanding the true pathophysiology of aseptic loosening/osteolysis compromises their outcomes. Therefore, the initial step of any genetic-association study should be *choice of potentially valuable candidate genes* followed by their validation in a candidate-gene study (Chanock et al. 2007).
