**2.9.1 Types of particles**

Current data suggest that tissues adjacent to a failed joint prosthesis contain billions of particles per gram of tissue. A wide variety of particle types have been retrieved from periprosthetic tissues at the time of autopsy or revision, as well as from joint simulators. In general, these particles types may be classified as metallic, polymeric, and ceramic. The majority of reports on periprosthetic metal debris pertain to Co-Cr and titanium. Metallic particles are characteristically gray to black, and although they may appear weakly birefringent under polarized microscopy, the appearance of birefringence is an optical artifact because the particles are actually opaque. The metal particles are generally smaller than polymer particles but larger than ceramic particles.

Submicrometer metallic particles have been described as globular or irregularly shaped as well as elongated with sharp corners. Larger Co-Cr particles, in the 1- to 5-µm size range, have been described more often as needle, rod, or splinter shaped. Even larger Co-Cr particles, from 5 µm to over 1 mm, have been described as irregularly shaped or globular. These particles tend to be extracellular and may actually represent aggregates or clusters of smaller particles. The majority of titanium particles likewise range from under a micrometer to less than 5 µm in size and have been described as blackish-gray material and fine powder. Occasional titanium particles from 5 µm up to 1 mm in size have been reported. (Malviya 2010)

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 175

In certain clinical settings, relatively large numbers of particles of corrosion products have also been identified. Chromium-orthophosphate-hydrate-rich particles were noted at the modular prosthetic head-neck junction, in the UHMWPE liner, and in the pseudocapsule around the prosthesis. In fact, in certain cases, chromium orthophosphate was second only to UHMWPE in number of intracellular particles found in periprosthetic tissues. The vast majority of chromium orthophosphate particles are less than 5 µm, and they are described as noncrystalline, translucent, and colorless. In addition to particles arising from the prosthesis or cement, certain anomalous particles have also been described. Silica has been reported in interfacial membranes, most likely a remnant of the catalyst used in UHMWPE manufacturing or a remnant of the sandblasting of metal prosthetic components. These are likely sources of trace amounts of aluminum as well. Furthermore, iron- and nickelcontaining particles (stainless steel) were likely contaminants from surgical instruments. Calcium- and phosphorous-rich particles were noted and, judging from the ratio of their concentrations, were probably from bone mineral. Thus, bone particles are also present in

Osteolysis is a significant cause of aseptic loosening and is the biggest cause of revision surgery. It is due to resorption of bone, which is seen more commonly around the acetabulum than the femur. There is a link between wear and corrosive debris formation and osteolysis and is seen more commonly in patients with high wear of their implants. It is often seen as lucency around the implants on radiographs taken years after the original procedure, but can be underestimated on plain films. The primary cellular mediator is the macrophage. In the presence of debris it produces a number of cytokines and inflammatory mediators (IL-1, IL-6, IL-10, TNF-α and prostaglandins). These initiate increased osteoclast activity and also increase osteoclastic differentiation, whilst the debris particles actually have a detrimental effect on osteoblastic bone formation. (Howcroft 2008) Within osteolytic areas other cell lineages may be found including fibroblasts and lymphocytes. Both of these contribute to the ongoing osteoclastic activity and periprosthetic bone resorption.(Archibeck 2000) It is now becoming evident that the size of the wear particles is of equal importance to their number. Nanometer particulate wear debris does not appear to affect the osteoblasts significantly, and coupled with a lower wear rate longer implant survival could be predicted. As the bearing surfaces clearly have an effect on implant survival, it is important

to consider the different pairings and look at the relative merits of each. (Harris 2001)

With the aid of fluoroscopy, articular surfaces separation during the hip motion has been observed. During this hip separation there is a loss of contact area, leaving only edge contact. Although separation has been well documented, it has not been correlated to clinical complications nor has a more in-depth understanding of the cause and effect been developed. Glaser et al studied the correlation between hip separation and sound production in different bearing surfaces.(Glaser 2008) In their study, among all analyzed subjects, the patients with a metal-on-polyethylene articulating surface experienced the highest magnitude of separation. The ceramic-on-polyethylene group had, on average, relatively low separation values, and these subjects demonstrated much less jerky motion. The maximum separation for the metal-on-metal polyethylene sandwich subjects was 1.5

the periprosthetic milieu.(Jacob 2006)

**2.11 Clicking and squeaking** 

**2.10 Osteolysis** 

The most common polymer particles encountered in association with joint prostheses are PMMA and UHMWPE particles. PMMA, unlike UHMWPE, is normally not birefringent. There are several potential sources of PMMA particles, including intraoperative debris, fatigue failure of cement and fretting of bone-cement and prosthesis-cement interfaces. In addition, unconsolidated or poorly mixed PMMA may release 25- to 35-µm "prepolymerized spheres". Certain elements of tissue processing for light microscopy dissolve PMMA and therefore it may be represented by histologic voids, ranging from less than 1 µm to greater than 1 mm in size. The particles or voids are irregularly shaped and have been described as multifaceted or resembling slivers of glass. Barium sulfate or zirconium oxide which have been added to most PMMA since the 1970s to permit visualization of the cement on radiographs has been reported in tissue voids left by PMMA. UHMWPE debris is translucent and strongly birefringent under polarized light microscopy. The particles from the femoral area of aseptically loose femoral components are predominantly spherical or globular in shape, ranging from 0.1 to 1 µm with a mean of 0.5 µm. Over 90% of these particles are less than 1 µm in size. The spherical particles are also associated with fibrillar attachments either singly or forming aggregates of fine particles. The fibrils ranged from 0.3 to 1.0 µm wide to 10 to 25 µm in length. Debris over 100 µm in length have been described as shredded fibers and flattened UHMWPE rolled in the articulation and resembling cigars. There has been great recent interest in the use of highly cross-linked UHMWPE in total joint arthroplasty as a means of decreasing the wear rates of the polyethylene bearing surfaces. It is known that the mechanical properties of UHMWPE are directly related to its molecular weight, crystalline ultrastructure, chemical structure and thermal history. The increased intramolecular cross-links of highly cross-linked UHMWPE are thought to better resist deformation and wear. The wear particles from highly crosslinked polyethylene are smaller (often in the submicrometer and nanometer range) than those from conventional UHMWPE. Controversy exists regarding the ability of these submicrometer particles to generate the inflammatory response that ultimately results in osteolysis. It is well known that particle morphology can strongly influence biological response. The elongated particles of UHMWPE were more biologically active than globular particles.(Gul 2003)

A resurgence of interest in ceramic articulating surfaces has been driven by various reports of their excellent wear characteristics. Still, ceramic wear debris is reported and particles are most commonly 1 to 5 µm in size. In a study of tissue retrieved at the time of revision, Hatton et al. found a bimodal distribution of sizes of ceramic wear debris. Using low resolution electron microscope, ceramic particles between 0.05 and 3.2 µm were found. Using high resolution electron microscope, particles between 5 and 90 nm were visualized. It was theorized that the larger particles were generated from microseparation and impaction of the ceramic surfaces and that the smaller particles were generated from normal articulation of the bearing surfaces.( Hatton 2002b) While often thought of as inert, ceramic wear debris may be able to generate a biologic response that leads to osteolysis. Many studies have shown that ceramic particles can incite inflammatory and cytotoxic effects. Hatton et al. demonstrated that tissues from around ceramic-ceramic hips had areas rich in macrophages, large amounts of neutrophils and lymphocytes, and areas with up to 60% necrosis. The control group, tissue from Charnley metal-on-poly hips, showed the presence of giant cells and a dense macrophage infiltrate, but there was less than 30% necrosis. There were significantly more neutrophils in the ceramic-ceramic tissues and significantly more macrophages and giant cells in the metal-on-poly group.(Hatton 2002b; Hannouche 2005)

In certain clinical settings, relatively large numbers of particles of corrosion products have also been identified. Chromium-orthophosphate-hydrate-rich particles were noted at the modular prosthetic head-neck junction, in the UHMWPE liner, and in the pseudocapsule around the prosthesis. In fact, in certain cases, chromium orthophosphate was second only to UHMWPE in number of intracellular particles found in periprosthetic tissues. The vast majority of chromium orthophosphate particles are less than 5 µm, and they are described as noncrystalline, translucent, and colorless. In addition to particles arising from the prosthesis or cement, certain anomalous particles have also been described. Silica has been reported in interfacial membranes, most likely a remnant of the catalyst used in UHMWPE manufacturing or a remnant of the sandblasting of metal prosthetic components. These are likely sources of trace amounts of aluminum as well. Furthermore, iron- and nickelcontaining particles (stainless steel) were likely contaminants from surgical instruments. Calcium- and phosphorous-rich particles were noted and, judging from the ratio of their concentrations, were probably from bone mineral. Thus, bone particles are also present in the periprosthetic milieu.(Jacob 2006)
