Aim. Periprosthetic joint infection (PJI) is a complication of total joint arthroplasty that typically requires revision surgery for treatment. Systemic antibiotics are usually held prior to surgery to improve yield of intraoperative cultures. However, recent studies suggest that preoperative aspirations have a high concordance with intraoperative cultures, which may allow surgeons to initiate antibiotic treatment earlier. The purpose of the study was to investigate the effect of Pre-surgical systemic antibiotic therapy on the bacterial burden within the periprosthetic space and systemic immune reaction. Method. PJI was induced with MSSA (Xen36) S. aureus in the right knee of 16-week old, female, C57BL6 mice using a previously validated murine model. Mice were randomized to three groups (n=8, each): control; Vanc, receiving systemic vancomycin (110mg/kg, SQ, twice daily); or VancRif receiving vancomycin same as in Vanc group, plus rifampin (12mg/kg dose, IV, once daily). Following 2 weeks of treatment, mice were euthanized and periprosthetic bone, soft tissue and the implant were harvested. Bacterial burden, colony forming units (CFUs), was quantified in soft tissue, tibial bone, and on the implant. Specifically, tissues were homogenized and serially plated for CFUs, while the implant was sonicated and then plated for CFUs. The host immune response was analysed through weighing inguinal and iliac lymph nodes and through measuring serum amyloid A (SAA). Non-parametric pairwise group comparisons of the three outcome measures were performed using a Mann-Whitney U test. Results. VancRif, the combined treatment significantly reduced bacterial burden in the periprosthetic soft tissue, bone, and implant compared to control (p<0.001) and Vanc alone (p<0.001). While not significant, Vanc alone did reduce bacterial load as compared to control. The ipsilateral weight of the iliac lymph nodes was significantly reduced in Vanc and VancRif mice compared to controls (p<0.001), was well as in VancRif versus Vanc alone (p<0.001). Interestingly, SAA levels did not significantly differ among all groups. During tissue harvesting, minimal purulence was observed in antibiotic treatment groups, unlike controls. Conclusions. Treating active PJI with vancomycin alone decreases periprosthetic bacterial loads and reduces the local immunological response. This effect is significantly enhanced with the combined rifampin use. These findings could suggest that when culture positive PJI is diagnosed, pre-surgical treatment with antibiotics may decrease immunosuppression and soft tissue infiltration, leading to a better chance of infection cure with subsequent surgical debridement. Histological investigations and repeat experiments involving subsequent surgical treatment are underway. Acknowledgements. Funding comes from internal institutional grants

Introduction

Osteoarthritis (OA) is a chronic degenerative disease of the entire joint leading to joint stiffness and pain (PMID:33571663). Recent evidence suggests that the sympathetic nervous system (SNS) plays a role in the pathogenesis of OA (PMID:34864169). A typical cause for long-term hyperactivity of the SNS is chronic stress. To study the contribution of increased sympathetic activity, we analyzed the progression of OA in chronically stressed mice.

Aims

This study aimed to define the histopathology of degenerated humeral head cartilage and synovial inflammation of the glenohumeral joint in patients with omarthrosis (OmA) and cuff tear arthropathy (CTA). Additionally, the potential of immunohistochemical tissue biomarkers in reflecting the degeneration status of humeral head cartilage was evaluated.

Aims

The present study investigated receptor activator of nuclear factor kappa-Β ligand (RANKL), osteoprotegerin (OPG), and Runt-related transcription factor 2 (RUNX2) gene expressions in giant cell tumour of bone (GCTB) patients in relationship with tumour recurrence. We also aimed to investigate the influence of CpG methylation on the transcriptional levels of RANKL and OPG.

Aims. Transcription factor nuclear factor kappa B (NF-κB) plays a major role in the pathogenesis of chronic inflammatory diseases in all organ systems. Despite its importance, NF-κB targeted drug therapy to mitigate chronic inflammation has had limited success in preclinical studies. We hypothesized that sex differences affect the response to NF-κB treatment during chronic inflammation in bone. This study investigated the therapeutic effects of NF-κB decoy oligodeoxynucleotides (ODN) during chronic inflammation in male and female mice. Methods. We used a murine model of chronic inflammation induced by continuous intramedullary delivery of lipopolysaccharide-contaminated polyethylene particles (cPE) using an osmotic pump. Specimens were evaluated using micro-CT and histomorphometric analyses. Sex-specific osteogenic and osteoclastic differentiation potentials were also investigated in vitro, including alkaline phosphatase, Alizarin Red, tartrate-resistant acid phosphatase staining, and gene expression using reverse transcription polymerase chain reaction (RT-PCR). Results. Local delivery of NF-κB decoy ODN in vivo increased osteogenesis in males, but not females, in the presence of chronic inflammation induced by cPE. Bone resorption activity was decreased in both sexes. In vitro osteogenic and osteoclastic differentiation assays during inflammatory conditions did not reveal differences among the groups. Receptor activator of nuclear factor kappa Β ligand (Rankl) gene expression by osteoblasts was significantly decreased only in males when treated with ODN. Conclusion. We demonstrated that NF-κB decoy ODN increased osteogenesis in male mice and decreased bone resorption activity in both sexes in preclinical models of chronic inflammation. NF-κB signalling could be a therapeutic target for chronic inflammatory diseases involving bone, especially in males. Cite this article: Bone Joint Res 2024;13(1):28–39

In osteoarthritis, chondrocytes acquire a hypertrophic phenotype that contributes to matrix degradation. Inflammation is proposed as trigger for the shift to a hypertrophic phenotype. Using in vitro culture of human chondrocytes and cartilage explants we could not find evidence for a role of inflammatory signalling activation. We found, however, that tissue repair macrophages may contribute to the onset of hypertrophy (doi: 10.1177/19476035211021907) Intra-articularly injected triamcinolone acetonide to inhibit inflammation in a murine model of collagenase-induced osteoarthritis, increased synovial macrophage numbers and osteophytosis, confirming the role of macrophages in chondrocyte hypertrophy occurring in osteophyte formation (doi: 10.1111/bph.15780). In search of targets to inhibit chondrocyte hypertrophy, we combined existing microarray data of different cartilage layers of murine growth plate and murine articular cartilage after induction of collagenase-induced osteoarthritis. We identified common differentially expressed genes and selected those known to be associated to inflammation. This revealed EPHA2, a tyrosine kinase receptor, as a new target. Using in silico, in vitro and in vivo models we demonstrated that inhibition of EPHA2 might be a promising treatment for osteoarthritis. Recently, single cell RNA-seq. has revealed detailed information about different populations of chondrocytes in articular cartilage during osteoarthritis. We re-analysed a published scRNA-seq data set of healthy and osteoarthritic cartilage to obtain the differentially expressed genes in the population of hypertrophic chondrocytes compared to the other chondrocytes, applied pathway analyses and then used drug databases to search for upstream inhibitors of these pathways. This drug repurposing approach led to the selection of 6 drugs that were screened and tested using several in vitro models with human chondrocytes and cartilage explants. In this lecture I will present this sequence of studies to highlight different approaches and models that can be used in the quest for a disease modifying drug for osteoarthritis

Chondrocytic activity is downregulated by compromised autophagy and mitochondrial dysfunction to accelerate the development of osteoarthritis (OA). Irisin is a cleaved form of fibronectin type III domain containing 5 (FNDC5) and known to regulate bone turnover and muscle homeostasis. However, little is known about the role of irisin in chondrocytes and the development of OA. This talk will shed light on FNDC5 expression by human articular chondrocytes and compare normal and osteoarthritic cells with respect to autophagosome marker LC3-II and oxidative DNA damage marker 8-hydroxydeoxyguanosine (8-OHdG). In chondrocytes in vitro, irisin improves IL-1β-mediated growth inhibition, loss of specific cartilage markers and glycosaminoglycan production. Irisin further suppressed Sirt3 and UCP- 1 to improve mitochondrial membrane potential, ATP production, and catalase. This attenuated IL-1β-mediated production of reactive oxygen species, mitochondrial fusion, mitophagy, and autophagosome formation. In a surgical murine model of destabilization of the medial meniscus (DMM) intra-articular administration of irisin alleviates symptoms like cartilage erosion and synovitis. Furthermore, gait profiles of the treated limbs improved. In chondrocytes, irisin treatment upregulates autophagy, 8-OHdG and apoptosis in cartilage of DMM limbs. Loss of FNDC5 in chondrocytes correlates with human knee OA and irisin repressed inflammation-mediated oxidative stress and deficient extracellular matrix synthesis through retaining mitochondrial biogenesis and autophagy. The talk sheds new light on the chondroprotective actions of this myokine and highlights the remedial effects of irisin during progression of OA

Aim

Infections represent a serious threat to the successful utilization of implants in modern medicine. Implant-associated infections are difficult to treat, because they involve biofilms that protect bacteria from the immune system and harbour antibiotic-tolerant persister cells.

In this work, we developed an antibody-drug conjugate (ADC) containing the anti-neoplastic drug mitomycin C (MMC) as a novel treatment paradigm for implant-associated infections. MMC was chosen as it is a potent antimicrobial against biofilms and its synthesis into an ADC was chosen to alleviate toxicity. Following development and synthesis of the ADC, stability and release of MMC was measured. We then used the ADC to kill bacteria in suspension and in biofilms, in vitro and in vivo.

Aim. Osteomyelitis (OM) is a debilitating infection of the bone that originates from hematogenous spreading of microbes or contamination after surgery/fracture. OM is mainly caused by the opportunistic bacterium Staphylococcus aureus (SA), which can evade the host immune response, acquire antibiotic resistance and chronically colonize the musculoskeletal tissue . 1,2. , yet the underlying molecular and cellular processes are largely unclear. This study aimed to characterize the pathogenetic mechanisms of SA-OM with a focus on the long pentraxin 3 (PTX3), a soluble pattern recognition molecule and bone tissue component that is emerging as a new player in osteoimmunology . 3. and a diagnostic marker of periprosthetic joint infections, a common form of OM. 4. . Method. A murine model of OM based on intra-bone injection of SA was developed that closely mimicked surgery/trauma-related OM in humans and allowed addressing the role of PTX3 in gene-modified (Ptx3-/-) animals. Local and systemic infection and inflammation were assessed via microbiology, flow cytometry, histochemistry and microCT techniques. Results. SA-injected mice developed chronic infection with measurable levels of viable bone-resident bacteria up until 30 days from microbial challenge. The infection was confined to the treated limbs only and accompanied by extensive tissue remodelling. The bacterial load was higher in WT than Ptx3. -/-. animals at 6 and 14 days from SA injection. Accordingly, WT mice had enhanced systemic inflammation with expanded innate immune compartment in the spleen and increased serum levels of inflammatory cytokines and chemokines. PTX3 levels were higher in SA- than vehicle (PBS)-injected WT animals both in the serum and bone tissue. Furthermore, administration of a PTX3-targeting antibody reduced the bacterial burden in the bones of SA-injected WT mice. Conclusions. In a mouse model of SA-OM, genetic deficiency of PTX3 protected from infection and inflammation, pointing to this pentraxin as a crucial player in OM pathogenesis and a novel therapeutic target in bone infections. The study was approved by the Italian Ministry of Health (approval n. 520/2019-PR issued on 19/07/2019) and supported by Fondazione Beppe and Nuccy Angiolini

Title. Longitudinal Intravital Imaging to Quantify the “Race for the Surface” Between Host Immune Cell and Bacteria for Orthopaedic Implants with S. aureus Colonization in a Murine Model. Aim. To assess S. aureus vs. host cell colonization of contaminated implants vis intravital multiphoton laser scanning microscopy (IV-MLSM) in a murine model. Method. All animal experiments were approved by IACUC. A flat stainless steel or titanium L-shaped pin was contaminated with 10. 5. CFU of a red fluorescent protein (RFP) expressing strain of USA300LAC, and surgically implanted through the femur of global GFP-transgenic mice. IV-MLSM was performed at 2, 4, and 6 hours post-op. Parallel cross-sectional CFU studies were performed to quantify the bacteria load on the implant at 2,4,6,12,18 and 24 hours. Results. 1) We developed a high-fidelity reproducible IV-MLSM system to quantify S. aureus and host cell colonization of a bone implant in the mouse femur. Proper placement of all implants were confirmed with in vivo X-rays, and ex vivo photos. We empirically derive the ROI during each imaging session by aggregating the imaged volume which ranges from (636.4um × 636.4um × 151um) = 0.625 +/- 0.014 mm. 3. of bone marrow in a global GFP-transgenic mouse. 2) IV-MLSM imaging acquisition of the “race for the surface”.In vitro MPLSM images of implants partially coated with USA300LAC (RFP-MRSA) were verified by SEM image. Results from IV-MLSM of RFP-MRSA and GFP. +. host cell colonization of the contaminated implants illustrated the mutually exclusive surface coating at 3hrs, which to our knowledge is the first demonstration of “the race for the surface” between bacteria and host cells via intravital microscopy. 3) Quantifying the “race for the surface” with CFU verification of S. aureus on the implant. 3D volumetric rendering of the GFP. +. voxels and RFP+ voxels within the ROI were generated in Imaris. The voxel numbers suggeste that the fight for the surface concludes ∼3hrs post-infection, and then transitions to an aggressive MRSA proliferation phase. The results of WT control demonstrate a significant increase in CFU by 12hrs post-op for both stainless steel (P<0.01) and titanium (P<0.01). Conclusions. We developed IV-MLSM to quantify the “Race for the Surface” between host cells and contaminating S. aureus in a murine femur implant model. This race is remarkably fast, as the implant surface is completely covered with 3hrs, peak bacterial growth on the implant occurs between 2 and 12 hours and is complete by 12hrs

µCT images are commonly analysed to assess changes in bone density and architecture in preclinical murine models. Several platforms provide automated analysis of bone architecture parameters from volumetric regions of interest (ROI). However, segmentation of the regions of subchondral bone to create the volumetric ROIs remains a manual and time-consuming task. This study aimed to develop and evaluate automated pipelines for trabecular bone architecture analysis of mouse proximal tibia subchondral bone. A segmented dataset involving 62 knees (healthy and arthritic) from 10-week male C57BL/6 mice were used to train a U-Net type architecture, with µCT scans (downsampled) input that output segmentation and bone volume density (BV/TV) of the subchondral trabecular bone. Segmentations were upsampled and used in tandem with the original scans (10µ) as input for architecture analysis along with the thresholded trabecular bone. The analysis considered the manually and U-Net segmented ROIs using two available pipelines: the ITKBoneMorphometry library and CTan (SKYSCAN). The analyses included: bone volume (BV), total volume (TV), BV/TV, trabecular number (TbN), trabecular thickness (TbTh), trabecular separation (TbSp), and bone surface density (BSBV). There was good agreement for bone measures between the manual and U-Net pipelines utilizing ITK (R=0.88-0.98) and CTan (R=0.91-0.98). ITK and CTan showed good agreement for BV, TV, BV/TV, TbTh and BSBV (R=0.9-0.98). However, a limited agreement was seen between TbN (R=0.73) and TbSb (R=0.59) due to methodological differences in how spacing is evaluated. This U-Net/ITK pipeline seamlessly automated both segmentation and quantification of the proximal tibia subchondral bone. This automated pipeline allows the analysis of large volumes of data, and its open-source nature may enable the standardization of stereologic analysis of trabecular bone across different research groups

Osteochondral injuries are a recognised factor in the development of osteoarthritis (OA). Mesenchymal stromal cells (MSCs) represent a promising biological therapeutic option as an OA-modifying treatment, and they also secrete factors that may have an anti-catabolic effect and/or encourage endogenous repair. We aim to study the effects of (i) intra-articular injection of human bone-marrow-derived MSCs and (ii) their secretome on recovery in a murine knee osteochondral injury model.

The MSC secretome was generated by stimulating human bone-marrow-derived MSCs with tumour necrosis factor alpha (TNFα). Mice (n=48) were injected with i) MSC secretome, ii) MSCs or iii) cell culture medium (control). Pain was assessed by activity monitoring, and cartilage repair, subchondral bone volume and synovial inflammation were evaluated using histology and microCT.

Both MSC- and MSC-secretome-injected mice showed significant pain reduction at day 7 when compared to control mice, but only the MSC-injected mice maintained a significant improvement over the controls at day 28. Cartilage repair was significantly improved in MSC-injected mice. No significant effects were observed with regards to synovial inflammation or subchondral bone volume.

The MSC secretome demonstrates regenerative effects but this does not appear to be as sustained as a MSC cell therapy. Further studies are required to investigate if this can be overcome using different dosing regiments for injection of the MSC secretome. As we further understand the regenerative properties of the MSC secretome, we may be able to enhance the clinical translatability of these therapies. Direct intra-articular injection of MSCs for the treatment of OA also appears promising as a potential future strategy for OA management.

Acknowledgements: MS is supported by a grant from the Wellcome Trust (PhD Programme for Clinicians)

Aims

Glucose-insulin-potassium (GIK) is protective following cardiac myocyte ischaemia-reperfusion (IR) injury, however the role of GIK in protecting skeletal muscle from IR injury has not been evaluated. Given the similar mechanisms by which cardiac and skeletal muscle sustain an IR injury, we hypothesized that GIK would similarly protect skeletal muscle viability.

Undifferentiated pleomorphic sarcoma (UPS) is one of the most common and aggressive adult soft tissue sarcomas (STS). Once metastatic, UPS is rapidly fatal. Most STS, including UPS, are resistant to conventional immunotherapies as these tumours have low numbers of spontaneous tumour infiltrating lymphocytes (TILs) and are densely populated with immune suppressive macrophages. Intra-tumoural activation of the STimulator of INterferon Genes (STING) pathway is a novel immunotherapeutic strategy to recruit anti-tumour TILs into the tumour microenvironment. In a murine model of UPS, we have demonstrated that intra-tumoural injection of a murine-specific STING agonist, DMXAA, results in profound immune mediated tumour clearance. Recently, molecules capable of activating both human and mouse STING pathways have been developed. In pursuit of clinically relevant therapeutic opportunities, the purpose of this study is to evaluate the anti-tumour potential of two agonists of the human and murine STING receptors: ADU-S100 and MSA-2 as monotherapies and in combination with the immune checkpoint inhibitor, anti-PD1 in a murine model of UPS. Immune competent mice were engrafted with murine UPS cells in the hindlimb muscle. Once palpable, mice in the monotherapy group were treated with a single intra-tumoural dose of 1) ADU-S100 or 2) MSA-2 or 3) DMXAA. In additional experimental groups, mice were treated with the different STING agonists and monoclonal anti-PD1. Tumour volume measurements and tumour bioluminescence were measured over time. To quantify dynamic changes in immune populations and in the tumour immune microenvironment, STING treated UPS tumours were evaluated using flow cytometry and mRNA quantification at various timepoints after therapy. DMXAA monotherapy produced complete tumour eradication in 50% of mice, whereas both ADU-S100 or MSA-2 monotherapy only extended survival but did not result in complete tumour clearance. Flow cytometry and transcriptional profiling of tumours at multiple timepoints post-treatment showed similar inflammatory changes and increased TILs numbers across all STING agonists. The addition of anti-PD1 treatment to STING therapy significantly extended survival times with both ADU-S100 and MSA-2, and resulted in 14% complete tumour clearance with ADU-S100. No complete survivors were observed with MSA-2-anti-PD1 combinations therapy. STING activation is a promising immunotherapeutic strategy for UPS. Recently developed human STING agonists are not as effective as DMXAA despite similar immunologic responses to treatment. STING and anti-PD-1 treatment were therapeutically synergistic for both human STING agonists. These results justify further research around STING activation as a therapeutic modality for STS. DMXAA may possess additional off-target therapeutic properties beyond STING activation which warrants further investigation. Elucidating these differences may be critical to further optimize STING therapy for human STS

Aims

There is a lack of biomaterial-based carriers for the local delivery of rifampicin (RIF), one of the cornerstone second defence antibiotics for bone infections. RIF is also known for causing rapid development of antibiotic resistance when given as monotherapy. This in vitro study evaluated a clinically used biphasic calcium sulphate/hydroxyapatite (CaS/HA) biomaterial as a carrier for dual delivery of RIF with vancomycin (VAN) or gentamicin (GEN).

Aims. Staphylococcus aureus is a major cause of septic arthritis, and in vitro studies suggest α haemolysin (Hla) is responsible for chondrocyte death. We used an in vivo murine joint model to compare inoculation with wild type S. aureus 8325-4 with a Hla-deficient strain DU1090 on chondrocyte viability, tissue histology, and joint biomechanics. The aim was to compare the actions of S. aureus Hla alone with those of the animal’s immune response to infection. Methods. Adult male C57Bl/6 mice (n = 75) were randomized into three groups to receive 1.0 to 1.4 × 10. 7. colony-forming units (CFUs)/ml of 8325-4, DU1090, or saline into the right stifle joint. Chondrocyte death was assessed by confocal microscopy. Histological changes to inoculated joints were graded for inflammatory responses along with gait, weight changes, and limb swelling. Results. Chondrocyte death was greater with 8325-4 (96.2% (SD 5.5%); p < 0.001) than DU1090 (28.9% (SD 16.0%); p = 0.009) and both were higher than controls (3.8% (SD 1.2%)). Histology revealed cartilage/bone damage with 8325-4 or DU1090 compared to controls (p = 0.010). Both infected groups lost weight (p = 0.006 for both) and experienced limb swelling (p = 0.043 and p = 0.018, respectively). Joints inoculated with bacteria showed significant alterations in gait cycle with a decreased stance phase, increased swing phase, and a corresponding decrease in swing speed. Conclusion. Murine joints inoculated with Hla-producing 8325-4 experienced significantly more chondrocyte death than those with DU1090, which lack the toxin. This was despite similar immune responses, indicating that Hla was the major cause of chondrocyte death. Hla-deficient DU1090 also elevated chondrocyte death compared to controls, suggesting a smaller additional deleterious role of the immune system on cartilage. Cite this article: Bone Joint Res 2022;11(9):669–678

Aims

To evaluate inducing osteoarthritis (OA) by surgical destabilization of the medial meniscus (DMM) in mice with and without a stereomicroscope.

Aims. To develop an early implant instability murine model and explore the use of intermittent parathyroid hormone (iPTH) treatment for initially unstable implants. Methods. 3D-printed titanium implants were inserted into an oversized drill-hole in the tibiae of C57Bl/6 mice (n = 54). After implantation, the mice were randomly divided into three treatment groups (phosphate buffered saline (PBS)-control, iPTH, and delayed iPTH). Radiological analysis, micro-CT (µCT), and biomechanical pull-out testing were performed to assess implant loosening, bone formation, and osseointegration. Peri-implant tissue formation and cellular composition were evaluated by histology. Results. iPTH reduced radiological signs of loosening and led to an increase in peri-implant bone formation over the course of four weeks (timepoints: one week, two weeks, and four weeks). Observational histological analysis shows that iPTH prohibits the progression of fibrosis. Delaying iPTH treatment until after onset of peri-implant fibrosis still resulted in enhanced osseointegration and implant stability. Despite initial instability, iPTH increased the mean pull-out strength of the implant from 8.41 N (SD 8.15) in the PBS-control group to 21.49 N (SD 10.45) and 23.68 N (SD 8.99) in the immediate and delayed iPTH groups, respectively. Immediate and delayed iPTH increased mean peri-implant bone volume fraction (BV/TV) to 0.46 (SD 0.07) and 0.34 (SD 0.10), respectively, compared to PBS-control mean BV/TV of 0.23 (SD 0.03) (PBS-control vs immediate iPTH, p < 0.001; PBS-control vs delayed iPTH, p = 0.048; immediate iPTH vs delayed iPTH, p = 0.111). Conclusion. iPTH treatment mediated successful osseointegration and increased bone mechanical strength, despite initial implant instability. Clinically, this suggests that initially unstable implants may be osseointegrated with iPTH treatment. Cite this article: Bone Joint Res 2022;11(5):260–269

Aims. Osteoarthritis (OA) is the most prevalent systemic musculoskeletal disorder, characterized by articular cartilage degeneration and subchondral bone (SCB) sclerosis. Here, we sought to examine the contribution of accelerated growth to OA development using a murine model of excessive longitudinal growth. Suppressor of cytokine signalling 2 (SOCS2) is a negative regulator of growth hormone (GH) signalling, thus mice deficient in SOCS2 (Socs2. -/-. ) display accelerated bone growth. Methods. We examined vulnerability of Socs2. -/-. mice to OA following surgical induction of disease (destabilization of the medial meniscus (DMM)), and with ageing, by histology and micro-CT. Results. We observed a significant increase in mean number (wild-type (WT) DMM: 532 (SD 56); WT sham: 495 (SD 45); knockout (KO) DMM: 169 (SD 49); KO sham: 187 (SD 56); p < 0.001) and density (WT DMM: 2.2 (SD 0.9); WT sham: 1.2 (SD 0.5); KO DMM: 13.0 (SD 0.5); KO sham: 14.4 (SD 0.7)) of growth plate bridges in Socs2. -/-. in comparison with WT. Histological examination of WT and Socs2. -/-. knees revealed articular cartilage damage with DMM in comparison to sham. Articular cartilage lesion severity scores (mean and maximum) were similar in WT and Socs2. -/-. mice with either DMM, or with ageing. Micro-CT analysis revealed significant decreases in SCB thickness, epiphyseal trabecular number, and thickness in the medial compartment of Socs2. -/-. , in comparison with WT (p < 0.001). DMM had no effect on the SCB thickness in comparison with sham in either genotype. Conclusion. Together, these data suggest that enhanced GH signalling through SOCS2 deletion accelerates growth plate fusion, however this has no effect on OA vulnerability in this model. Cite this article: Bone Joint Res 2022;11(3):162–170

Aim. Staphylococcus aureus is the leading pathogen in fracture-related infection (FRI). Virulence factors vary between different strains, which may have a decisive influence on the course of infection. Previous in vitro experiments, in vivo testing in wax moth larvae, and genomic analysis of S. aureus isolates from FRI identified a low- and high-virulent strain. These findings correlated with the acute course of FRI induced by the high-virulent pathogen, whereas the low-virulent strain caused a chronic FRI in its human host. However, the role of bacterial virulence in FRI is not completely understood. Therefore, the present study aimed to compare the identified high- and low-virulent S. aureus isolates in a murine FRI model. Method. Skeletally mature C57Bl/6N mice received a femoral osteotomy stabilized by titanium locking plates. FRI was established by inoculation of either high-virulent S. aureus EDCC 5458 or low-virulent S. aureus EDCC 5464 in the fracture gap. Mice were euthanized 4 and 14 days after surgery, respectively. Severity and progression of infection were assessed in terms of clinical presentation, quantitative bacteriology, semiquantitative histopathologic evaluation, and serum cytokine profile. Results. Quantitative bacteriological results 4 days after surgery revealed a higher bacterial load in soft tissue samples in high-virulent infected animals (p =0.026). Mice infected with the high-virulent strain also displayed higher rates of organ dissemination (24/36 organs in high-virulent, versus 5/36 organs in low-virulent infected animals; p <0.0001). In the histopathological assessment, bacterial agglomerations at the fracture ends were present to a greater extent in the high-virulent cohort and barely detectable in low-virulent infected mice. In both cohorts, no bone healing was observed after 4 days. On day 14, bone healing at the fracture site was visible in low-virulent infected animals, whereas callus formation was observed in only one animal from the high-virulent infected cohort. Furthermore, osteonecrosis and osteolysis were increased in high-virulent infected animals. Regarding serum cytokines, innate immune markers were elevated in both groups at day 4. By day 14, a more pronounced proinflammatory response indicated by increased serum cytokine levels of IFN-γ, IL-1β, and IL-6 was observed in high-virulent infected animals. Conclusions. The present study demonstrated distinct bacteriological and histopathological differences between two different virulent S. aureus strains previously shown to have different courses in human patients. While host physiology is often considered to have a major impact on the course of FRI, this study highlights the critical influence of the invading pathogen and its virulence characteristics