Identification and analysis of microplastics in human lower limb joints
January 5, 2024 | ScienceDirect
Abstract
Microplastics (MPs) have been detected in various human tissues, including the liver, placenta, and blood. However, studies about MPs in the human locomotor system are limited. This study evaluated the presence of MPs in the synovium of 45 patients undergoing hip or knee arthroplasty using micro-Fourier transform infrared spectroscopy, scanning electron microscopy, and Raman microscopy and investigated their association with clinical indicators and local cellular responses. A total of 343 MPs of nine common types were identified, with a mean abundance of 5.24 ± 2.07 particles/g and ranging from 1.16 to 10.77 particles/g. Although there was no clear correlation between MP abundance and demographics, MP abundance was higher in hip samples than in knee samples. In addition, a potential association was observed between MP abundance and specific clinical diagnoses. Transcriptomic analysis revealed that a three-fold increase in MP abundance corresponded to enhanced local cellular stress responses, particularly heat shock protein reactions. Our findings demonstrate the presence of MPs in human joints and suggest that further studies are needed to explore the intricate associations between MPs and anatomical location, clinical diagnosis, and local cellular responses.
Introduction
As the pace of industrial production accelerates and diverse synthetic materials are promoted, plastic particles are rapidly accumulating in the environment [56]. Microplastics (MPs), defined as plastic particles < 5 mm in size, have emerged as crucial pollutants that contaminate nearly all ecosystem compartments [14], [35]. In addition to their pervasive presence in diverse environments, such as oceans [5], urban runoff [17], sediments [55], atmosphere [50], and inside buildings [23], MPs have infiltrated the human food chain via seafood [11], [22], sugar [1], canned foods [2], and drinking water [24], leading to exposure in humans. In addition, the COVID-19 pandemic resulted in an unprecedented level of personal protective equipment-associated plastic contamination [13], [34], [7]. Existing estimates of human intake of MPs are up to 120,000 particles per year, highlighting the unavoidable nature of direct MP intake [10].
Several studies have investigated the distribution of MPs in the humans. MP accumulation has been identified at 15 sites in humans, including the lungs, skin, placenta, and liver, at variable levels [25]. Furthermore, the detection of plastic particles in human blood indicates their widespread dissemination through the circulatory system [26]. However, reports on MPs in the human locomotor systems are scarce. In a recent study on human joint synovial fluid, approximately a quarter of the samples tested positive for MPs [32]. Another study identified MPs in enclosed human body fluids; however, their concentration was relatively low, with only a single MP particle detected in synovial fluid samples, which may be due to their diffusion being impeded by biological barriers and membranes [16]. Moreover, the permeability of the biological barrier and secretory function of the surrounding tissues can be drastically altered under pathological conditions, resulting in large amounts of fluid entering the joint cavity [8], thereby reducing MP abundance. Thus, the MP abundance in the joints was likely underestimated in past studies and the levels in the synovium may be more representative of the actual accumulation of MP in the joints; however, relevant data are lacking.
The potential association between MP exposure and human health remains unclear. A recent investigation found significantly higher MP concentrations in the stools of patients with inflammatory bowel disease than in those of healthy individuals [54]. Similarly, a cadaveric study confirmed elevated MP levels in patients with cirrhosis [19]. At the microscopic level, several studies have shown that MPs can induce oxidative stress, inflammation, genotoxicity, and metabolic impairment in animal models and cell lines [36], [49]. Patients undergoing prosthesis implantation may experience substantial accumulation of polyethylene (PE) or polyethylene terephthalate (PET) debris in periprosthetic tissues, which may cause inflammatory responses and macrophage buildup [46], [48]. In addition, macrophage infiltration engulfing abundant PE particles has been observed in the lymph nodes after arthroplasty, leading to heightened immune activation and cytokine reactions [18], [31], a potential incipient phase in eliciting periprosthetic osteolysis responses. These data provide preliminary insights into the potential effects of the accumulation of large numbers of MPs in joints. However, there is still a dearth of data on the human health implications at concentrations that reflect environmental exposure in the absence of prosthetics [6]. Further exploration of the association between the natural MP exposure levels and local cellular responses in human specimens could improve our understanding of how MPs affect human health.
Therefore, this study aimed to identify and analyze MPs within the synovium to ascertain their actual accumulation and the connection with clinical data or local cellular responses in vivo, thus providing additional insights into the association between actual accumulated MPs and human health.
