2.5.1 Histology and organization of healthy synovium
The synovium forms an enclosed environment for the joint. It consists of a synovial membrane that attaches to the bone surfaces. The functions of the synovium include lubrication of the joint and defense against pathogens. The synovial membrane can further be divided into intimal and subintimal layers. The intimal layer, also termed the synovial lining, is the innermost layer. Intimal cells are known as synoviocytes and can be divided into two major types: A and B. Type A synoviocytes are macrophage-like and form an immunological barrier. Type B synoviocytes resemble fibroblasts and produce important components of synovial fluid. Under the intima lays the subintima, which consists of the extracellular matrix and a few cells. Occasional inflammatory cells, such as macrophages and lymphocytes, are also observed. (Smith 2011). A photomicrograph of a histological section of hip joint synovium is shown in Figure 6.
Figure 6. A photomicrograph of a H&E stained section of healthy hip joint synovium. The thick arrow points to intact synovial lining. The thin arrow points to synoviocytes. The intermediate arrow shows subintimal macrophages and fibroblasts. Photomicrograph captured with Nikon Eclipse 50i fitted with 20x objective (total magnification 200x).
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2.5.2 Overview of the histopathology of ARMD
Several studies have been conducted regarding the histopathology of ARMD. In these studies, samples, such as synovial capsule and pseudotumor tissue, have been obtained from the periprosthetic tissues and graded using light microscopy. Some studies have used flow cytometry to determine the number of inflammatory cells present.
Findings observed in periprosthetic tissue samples obtained from patients with ARMD include necrosis, inflammatory cell infiltrates consisting of variable amounts of neutrophils, macrophages, plasma cells, T- and B-lymphocytes, germinal centers, sarcoid-like granulomas and vascular damage (Davies et al. 2005, Willert et al. 2005, Campbell et al. 2010, Natu et al. 2012, Grammatopoulos et al.
2013). Most of the inflammatory cells are either macrophages or lymphocytes.
Necrosis may be coagulative or fibrinoid (Krenn et al. 2014). In addition, there appears to be distinct patterns of histopathological findings. At least three different pathological responses have been suggested: 1. Foreign-body macrophage response, 2. Cytotoxic response and 3. ALVAL response (Davies et al. 2005, Willert et al. 2005, Mahendra et al. 2009, Campbell et al. 2010, Natu et al. 2012, Berstock et al. 2014). It should be noted, however, that these responses may overlap considerably and may present simultaneously (Berstock et al. 2014, Ricciardi et al. 2016). Several different scoring systems for tissue samples have been used of which the ALVAL score is likely the most popular (Campbell et al. 2010, 2018b). ALVAL scoring and Natu scoring are explained in more detail in the Methods section, chapter 4.2.4.
2.5.3 Foreign-body macrophage response
It is well recognized that the generation of polyethylene particles in patients with MoP implants induces an inflammatory macrophage response which can, over time, lead to osteolysis and aseptic loosening of the implant. The degree of osteolysis is related to the high volume of polyethylene wear debris produced.
(Harris 1994). Although MoM implants are substantially lower wearing, metal particles also appear to evoke a macrophage response in the synovial tissue in some patients, although milder than that seen with MoP implants (Doorn et al. 1996, Willert et al. 1996, 2005). Macrophages have three major roles in tissue inflammation: phagocytosis of foreign particles or not viable tissues, antigen presentation and modulation of the immune response via numerous cytokines and
growth factors (Fujiwara and Kobayashi 2005). Histopathologically, this type of response in the synovium is characterized by the presence of superficially located macrophages with or without foreign-body giant cells and granulomas (Figure 7) and preservation of the underlying tissue architecture (Campbell et al. 2010, Berstock et al. 2014). Fine metal particles or “metal dust” are often seen in the cytoplasm of macrophages as a sign of phagocytosis (Nawabi et al. 2014) (Figure 8). Immunologically, metal particles are recognized by macrophages as foreign material, which activates numerous signaling pathways to gather more macrophages to the site to help clear the metal debris. Diffuse lymphocytes may also be present in minor quantities. This type of macrophage-mediated response is termed innate or non-specific immune response. (Athanasou 2016). For a microscope image of the characteristic histological features, please see Figure 11, page 100.
Figure 7. Photomicrograph of an H&E stained section of synovial tissue removed from patient revised for ARMD. The two-headed arrow shows the whole granuloma with metallic debris encapsulated inside. One-headed arrow points to a multinucleated giant cell.
Photomicrograph captured with Nikon Eclipse 50i fitted with 20x objective (total magnification 200x).
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Figure 8. Photomicrograph of an H&E stained section of synovial tissue removed from patient revised for ARMD. The arrow points to a macrophage with phagocytized fine metal debris inside. Photomicrograph captured with Nikon Eclipse 50i fitted with 40x objective (total magnification 400x).
2.5.4 Cytotoxic response
In some patients, the histological picture of ARMD comprises substantial tissue necrosis in addition to macrophage infiltration. It has been suggested that in these patients the underlying cause of necrosis is the direct cytotoxic effect of cobalt-chromium particles and ions (Mahendra et al. 2009). The presumed mechanism is the following: metal particles are phagocytized by periprosthetic cells and contained in phagolysosomes, the acidic environment of lysosomes leads to degradation of the metal particles into ions, which then escape the lysosome and lead to apoptosis and necrosis of the affected cells (Salvati et al. 1993, Xia et al.
2011). In keeping with this hypothesis are in vitro studies which show that cobalt and chromium ions can cause dose-dependent necrosis and apoptosis in macrophages (Catelas et al. 2001, 2005, Kwon et al. 2009). Several authors have
suggested that the necrosis induced by metal ions leads to a cycle in which macrophages are first recruited to clear the cellular and metallic debris but end up undergoing cell death themselves, which then leads to more macrophages being recruited and a worsening of the situation (Salvati et al. 1993, Mahendra et al. 2009, Grammatopoulos et al. 2013). Further, there is evidence that the release of metal ions and particles from dead macrophages leads to the cell death of adjacent fibroblasts as well (Xia et al. 2011). It is not understood why some patients develop necrosis in addition to macrophage inflammation, while some do not (Eltit et al.
2019). In conclusion, the cytotoxic response is likely a combination of the direct cytotoxic effect of metal debris and the activation of the innate immune response which leads to the histopathological findings of tissue necrosis and heavy macrophage infiltration. For the characteristic features of this response, please see Figure 12, page 101.
2.5.5 ALVAL response
Willert et al. and Davies et al. were the first to report that in some patients with early onset of pain the capsular tissues display prominent lymphocytic infiltration, necrosis, fibrin exudation, vascular wall changes, occasional plasma cells and variable amounts of macrophages (Davies et al. 2005, Willert et al. 2005).
Lymphocytes were present diffusely in the superficial layer and as perivascular cuffs in the intermediate layer of the capsule. The authors suggested the presence of lymphocyte-dominated type IV hypersensitivity response to metal debris and described it as Aseptic Lymphocyte-dominated Vasculitis-Associated Lesion (ALVAL). Perivascular lymphocytic cuffing of the capillaries, swelling of the vascular walls and necrosis are characteristics for vasculitis, and thus the findings were described as vasculitis-associated lesion. Davies et al. noted, however, that it is unclear whether these findings represent active vasculitis or a novel form of immunological response with unknown consequences.
Since the pioneering work by Davies et al. and Willert et al., numerous studies have described mostly similar findings (Witzleb et al. 2007, Huber et al. 2008, Pandit et al. 2008b, Mahendra et al. 2009, Campbell et al. 2010, Natu et al. 2012, Grammatopoulos et al. 2013, Berstock et al. 2014, Langton et al. 2016, Ricciardi et al. 2016). The role of vascular wall changes in the development of tissue necrosis has been questioned (Mahendra et al. 2009, Natu et al. 2012). Natu et al. suggested that necrosis is likely due to pronounced lymphocytic inflammation but may also
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be due to vascular wall changes leading to local ischemia and necrosis. The authors stated that it is still not understood whether the vascular wall changes are a consequence of lymphocytes transiting through the wall or true vasculitis. In a study by Langton et al., it was shown that the thickness of lymphocytic cuffing correlated with the degree of necrosis (Langton et al. 2011b). Further, T-killer lymphocytes, which can cause necrosis, have been found in tissues with suggested type IV response and may be related to the common finding of tissue necrosis in ALVAL response (Hasegawa et al. 2016). In some patients with lymphocytic tissue responses, germinal centers containing B- and T-lymphocytes have also been observed (Natu et al. 2012, Langton et al. 2013b, Mittal et al. 2013). A microscope image of germinal center is shown in Figure 9. Natu et al. suggested that these are the end stage of lymphocytic inflammatory response. However, Mittal et al.
concluded that these germinal centers, or tertiary lymphoid organs, form their own distinct pathological subset of ARMD. The role of these germinal centers is not fully understood (Hasegawa et al. 2016). Altogether, studies have supported the hypothesis that an adaptive immune response, leading to lymphocyte-dominated inflammation and subsequent necrosis, is the cause of failure in some patients. For a microscope image of characteristic histological features, please see Figure 13, page 102.
Figure 9. Photomicrograph of an H&E stained section of synovial tissue removed from patient revised for ARMD. The arrow shows a germinal center. Photomicrograph captured with Nikon Eclipse 50i fitted with 4x objective (total magnification 40x).
2.5.6 Histopathology of pseudotumors
The term pseudotumor is often used interchangeably with the term ARMD.
However, by definition, pseudotumors are cystic or solid masses that connect with the joint space (Pandit et al. 2008a). It is not understood why pseudotumors form in only a subset of patients who develop ARMD. Pseudotumors can be associated with all of the responses described above – Foreign-body, cytotoxic and ALVAL response (Mahendra et al. 2009, Campbell et al. 2010, Grammatopoulos et al.
2017a). Further, pseudotumors do not seem to be specific to MoM implants as they are also observed in patients with other types of bearing couples (Carli et al.
2011). In patients with MoP hip implants, corrosion debris from the trunnion most likely causes the formation of pseudotumors (Cooper et al. 2012, 2013). However, pseudotumors may also form in response to polyethylene wear (Murgatroyd 2012).
The histology of these lesions appears to be solely granulomatous macrophage
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inflammation and no lymphocytes have been observed (Carli et al. 2011). This is in contrast to the common finding of heavy lymphocytic infiltration and necrosis in MoM pseudotumors (Mahendra et al. 2009, Campbell et al. 2010).