Peri-operative SARS-CoV-2 infection increases postoperative mortality. The aim of this study was to determine the optimal duration of planned delay before surgery in patients who have had SARS-CoV-2 infection. This international, multicentre, prospective cohort study included patients undergoing elective or emergency surgery during October 2020. Surgical patients with pre-operative SARS-CoV-2 infection were compared with those without previous SARS-CoV-2 infection. The primary outcome measure was 30-day postoperative mortality. Logistic regression models were used to calculate adjusted 30-day mortality rates stratified by time from diagnosis of SARS-CoV-2 infection to surgery. Among 140,231 patients (116 countries), 3127 patients (2.2%) had a pre-operative SARS-CoV-2 diagnosis. Adjusted 30-day mortality in patients without SARS-CoV-2 infection was 1.5% (95%CI 1.4-1.5). In patients with a pre-operative SARS-CoV-2 diagnosis, mortality was increased in patients having surgery within 0-2 weeks, 3-4 weeks and 5-6 weeks of the diagnosis (odds ratio (95%CI) 4.1 (3.3-4.8), 3.9 (2.6-5.1) and 3.6 (2.0-5.2), respectively). Surgery performed ≥ 7 weeks after SARS-CoV-2 diagnosis was associated with a similar mortality risk to baseline (odds ratio (95%CI) 1.5 (0.9-2.1)). After a ≥ 7 week delay in undertaking surgery following SARS-CoV-2 infection, patients with ongoing symptoms had a higher mortality than patients whose symptoms had resolved or who had been asymptomatic (6.0% (95%CI 3.2-8.7) vs. 2.4% (95%CI 1.4-3.4) vs. 1.3% (95%CI 0.6-2.0), respectively). Where possible, surgery should be delayed for at least 7 weeks following SARS-CoV-2 infection. Patients with ongoing symptoms ≥ 7 weeks from diagnosis may benefit from further delay.

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This study provides for the first time a strong characterisation of oMSC and establishes the basis for a model of cartilage engineering in a large animal.

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SARS-CoV-2 has been associated with an increased rate of venous thromboembolism in critically ill patients. Since surgical patients are already at higher risk of venous thromboembolism than general populations, this study aimed to determine if patients with peri-operative or prior SARS-CoV-2 were at further increased risk of venous thromboembolism. We conducted a planned sub-study and analysis from an international, multicentre, prospective cohort study of elective and emergency patients undergoing surgery during October 2020. Patients from all surgical specialties were included. The primary outcome measure was venous thromboembolism (pulmonary embolism or deep vein thrombosis) within 30 days of surgery. SARS-CoV-2 diagnosis was defined as peri-operative (7 days before to 30 days after surgery); recent (1-6 weeks before surgery); previous (≥7 weeks before surgery); or none. Information on prophylaxis regimens or pre-operative anti-coagulation for baseline comorbidities was not available. Postoperative venous thromboembolism rate was 0.5% (666/123,591) in patients without SARS-CoV-2; 2.2% (50/2317) in patients with peri-operative SARS-CoV-2; 1.6% (15/953) in patients with recent SARS-CoV-2; and 1.0% (11/1148) in patients with previous SARS-CoV-2. After adjustment for confounding factors, patients with peri-operative (adjusted odds ratio 1.5 (95%CI 1.1-2.0)) and recent SARS-CoV-2 (1.9 (95%CI 1.2-3.3)) remained at higher risk of venous thromboembolism, with a borderline finding in previous SARS-CoV-2 (1.7 (95%CI 0.9-3.0)). Overall, venous thromboembolism was independently associated with 30-day mortality ). In patients with SARS-CoV-2, mortality without venous thromboembolism was 7.4% (319/4342) and with venous thromboembolism was 40.8% (31/76). Patients undergoing surgery with peri-operative or recent SARS-CoV-2 appear to be at increased risk of postoperative venous thromboembolism compared with patients with no history of SARS-CoV-2 infection. Optimal venous thromboembolism prophylaxis and treatment are unknown in this cohort of patients, and these data should be interpreted accordingly.

Osteochondritis dissecans is a rare cause of painful knees in children. Only 10 cases of lateral tibial condyle involvement have been reported in the literature. Congenital agenesis of both cruciate ligaments has been described even less, and its prevalence is unknown. The authors report an atypical association of osteochondritis dissecans of the tibia with congenital absence of both cruciate ligaments. A 12-year-old male soccer player presented with a painful right knee. Magnetic resonance imaging revealed the diagnosis. The child was treated conservatively. At 18-month follow-up, radiographs showed osseointegration of the osteochondritis dissecans, and the patient had resumed normal athletic activity without pain. To the authors' knowledge, this is the only report describing such an association. The authors discuss the possible etiology of osteochondritis dissecans associated with agenesis of the cruciate ligaments and highlight the possibility of this association when osteochondritis dissecans of the tibia is diagnosed in a child with a painful knee. In this patient, the strain due to anteroposterior instability may have been the cause of osteochondritis. Conservative treatment should be considered in this setting.

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24Background 25Circulating mesenchymal stem cells contribute to bone repair. Their incorporation in fracture 26 callus is correlated to their bioavailability. In addition, Granulocyte-colony stimulating factor 27 induces the release of vascular and mesenchymal progenitors. We hypothesized that this 28 glycoprotein stimulates fracture healing, and analyzed the effects of its administration at low 29 doses on bone healing. 30Methods 31 27 adult male Sprague-Dawley rats underwent mid-femur osteotomy stabilized by centromedullar 32 pinning. In a post (pre) operative group, rats were subcutaneously injected with 5µg/kg per day of 33Granulocyte-colony stimulating factor for 5 days after (before) surgery. In a control group, rats 34 were injected with saline solution for 5 days immediately after surgery. A radiographic 35 consolidation score was calculated. At day 35, femurs were studied histologically and underwent 36 biomechanical tests. 37