Background:Despite the general success of total knee arthroplasty (TKA), addressing periprosthetic joint infection (PJI) and the resulting long-term complications is a growing medical need given the aging population and the increasing demand for arthroplasty. A larger proportion of patients face revision surgery because of the long-term complication of aseptic loosening despite clearance of the infection. The pathomechanisms leading to prosthetic loosening are not understood as it has been widely assumed that the bone stock recovers after explantation revision surgery. While clinical observations suggest a reduced osteogenic potential in patients with PJI, knowledge regarding the relevant biology is sparse. In the present study, we investigated the inflammatory impact of PJI on the bone and bone marrow in the vicinity of the joint. Additionally, we evaluated changes in the local inflammatory environment in a 2-stage exchange at both explantation and reimplantation.Methods:In this study, we analyzed 75 human bone and bone-marrow specimens (obtained from 65 patients undergoing revision arthroplasty with cement for the treatment of PJI) for markers of inflammation. Samples were analyzed using hematoxylin and eosin overview staining, fluorescent immunohistochemical staining, flow cytometry, and polymerase chain reaction (PCR).Results:Leukocyte prevalence was significantly elevated at explantation (femur, +218.9%; tibia, +134.2%). While leukocyte prevalence decreased at reimplantation (femur, −49.5%; tibia, −34.2%), the number of cells remained significantly higher compared with the control group (femur, +61.2%; tibia, +54.2%). Expression of inflammatory markers interleukin (IL)-1α (femur, +2,748.7%; tibia, +1,605.9%), IL-6 (femur, +2,062.5%; tibia, +2,385.7%), IL-10 (femur, +913.7%; tibia, +897.5%), IL-12 (femur, +386.1%; tibia, +52.5%), IL-18 (femur, +805.3%; tibia, +547.7%), and tumor necrosis factor (TNF)-α (femur, +296.9%; tibia, +220.9%) was significantly elevated at prosthesis explantation in both femoral and tibial specimens. Expression remained significantly elevated at reimplantation for all inflammatory markers except IL-12 compared with the control group. Conversely, there were only limited inflammatory changes in the bone marrow environment.Conclusions:The present study demonstrated a strong and lasting upregulation of the proinflammatory environment in the joint-surrounding osseous scaffold in patients with PJI. Our data suggest that modulating the inflammatory environment has substantial potential to improve the clinical outcome in affected patients.

Background: In arthroplasty, periprosthetic joint infection (PJI) is one of the most disastrous postoperative complications with high long-term failure rates despite successful revision surgery. PJI causes a pro-inflammatory environment in the surrounding bone stock that subsequently results in compromised bone metabolism. This increases the risk for unstructured bone-resorption potentially leading to aseptic loosening. We hypothesize that PD-1-positive monocytes and monocyte-derived osteoclasts play a significant role in this pathology. In this study, we assessed the presence of PD-1-positive monocytes and their osteoclastogenic differentiation potential, as well as resulting osteoclast activity in PJI. We also investigated anti-PD-1-antibody as a potential salvage therapy. Material and Methods: Peripheral blood specimens were collected from 15 patients undergoing knee arthroplasty (ctrl) or prosthesis explantation due to PJI. Peripheral blood mononuclear cells were isolated and stained for CD33, CD11b, and CD14. Osteoclastic differentiation was induced ± PD-L1 and ± anti-PD-1-antibody. Osteoclast number and function were determined by pit assay, TRAP staining and qPCR. Results: Monocyte cell count (0.86±0.35 v 0.21±0.03/nl, p=.02) and PD-1 expression on monocytes (92.50±6.83 v 35.34±15.43%, p<.01) waselevated in patients with PJI. After differentiation, in PJI, total osteoclast surface area was larger (47.40±3.19% v 26.75±3.49%, p<.01) and increased significantly further (+64.04±1.72%, p<.01) after PD-L1 stimulation. Addition of PD-1 inhibitor reduced osteoclast surface area in PJI (36.51±3.95 v 64.04±1.72%, p<.01). Expression of osteoclast differentiation marker Nfatc1 and Ctsk were higher in the PJI group ( Nfatc1: 1.09±0.20 v 0.45±0.06, p<.01; Ctsk: 2.27±1.00 v 0.73±0.23, p=.02), but did not differ in Mmp9 and Acp5 expression. In PJI, the expression of all four genes were upregulated after PD-L1 stimulation, while PD-1 inhibitor treatment reversed this effect. PD-L1 stimulation led to increased osteoclast bone resorption compared to without stimulation (7.65±3.15 v 2.49±0.67%, p=.05), while addition of PD-1 inhibitor showed no influence in the ctrl group. In PJI, osteoclast surface resorption was increased after PD-L1 stimulation (20.69±6.54 v 8.06±1.54, p=.01). Conversely, osteoclast function was reduced after treatment with PD-1 inhibitor (4.23±0.55 v 8.06±1.54, p=.03). Conclusion: Elevated numbers of monocytic osteoclast-progenitor cells and increased osteoclastic differentiation and function may contribute to decreased bone volume and impact prosthesis osseointegration in patients with PJI. Our results show PD-L1 can stimulate osteoclast generation and function and that this can be reversed by PD-1 inhibitor. This effect is particularly prominent in PJI. Targeting PD-1 can be a potential therapy to inhibit monocyte-mediated osteoclastogenesis while retaining anti-microbial activity of macrophages to limit the impact of PJI on the bone stock. Dr. Arne Kienzle is participant in the BIH-Charité Junior Clinician Scientist Program funded by the Charité - Universitätsmedizin Berlin and the Berlin Institute of Health. The authors wish to acknowledge the support of the non-profit German Arthritis Society (Deutsche Arthrose-Hilfe e.V.) and its president Helmut H. Huberti, MD by grant P482. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

After periprosthetic joint infection (PJI)‐dependent revision surgery, a significantly elevated number of patients suffer from prosthesis failure due to aseptic loosening and require additional revision surgery despite clearance of the initial infection. The mechanisms underlying this pathology are not well understood, as it has been assumed that the bone stock recovers after revision surgery. Despite clinical evidence suggesting decreased osteogenic potential in PJI, understanding of the underlying biology remains limited. In this study, we investigated the impact of PJI on bone homeostasis in a two‐stage exchange approach at explantation and reimplantation.64 human tibial and femoral specimens (20 control, 20 PJI septic explantation, and 24 PJI prosthesis reimplantation samples) were analyzed for their bone microstructure, cellular composition, and expression of relevant genetic markers. Samples were analyzed using X‐ray microtomography, Alcian blue and TRAP staining, and RT‐qPCR.In patients with PJI, bone volume (BV/TV; 0.173 ± 0.026; p < 0.001), trabecular thickness (164.262 ± 18.841 μm; p < 0.001), and bone mineral density (0.824 ± 0.017 g/cm2; p = 0.049) were reduced; trabecular separation (1833.939 ± 178.501 μm; p = 0.005) was increased. While prevalence of osteoclasts was elevated (N.Oc/BS: 0.663 ± 0.102, p < 0.001), osteoblast cell numbers were lower at explantation (N.Ob/BS: 0.149 ± 0.021; p = 0.047). Mean expression of bone homeostasis markers osteocalcin, osteopontin, Runx2, TSG‐6, and FGF‐2 was significantly reduced at prosthesis explantation. Despite partial recovery, all analyzed parameters were still significantly impacted at reimplantation. In contrast, mean expression of osteoclastogenesis‐stimulating cytokine IL‐17a was significantly increased at both explantation and reimplantation.In this study, we found a strong and lasting impact of PJI on the bone homeostasis on a molecular, cellular, and microstructural level. These changes may be responsible for the increased risk of prosthesis failure due to aseptic loosening. Our data suggest there is significant potential in modulating bone homeostasis to improve prosthesis fixation and long‐term clinical outcome in affected patients.This article is protected by copyright. All rights reserved.