BACKGROUND:Knee arthroscopy (KA) is a routine orthopedic procedure recommended to repair cruciate ligaments and meniscus injuries and in eligible patients, to assist the diagnosis of persistent knee pain. KA is associated with a small risk of thromboembolic events. This systematic review aims to assess if pharmacological or non-pharmacological interventions may reduce this risk. This review is the second update of the review first published in 2007. OBJECTIVES:To assess the efficacy and safety of interventions, whether mechanical, pharmacological, or in combination, for thromboprophylaxis in adult patients undergoing KA. SEARCH METHODS:For this update, the Cochrane Vascular Information Specialist searched the Cochrane Vascular Specialised Register, the CENTRAL, MEDLINE, Embase and CINAHL databases, and the World Health Organization International Clinical Trials Registry Platform and ClinicalTrials.gov trials registries, on 14 August 2019. SELECTION CRITERIA:We included randomized controlled trials (RCTs) and controlled clinical trials (CCTs), whether blinded or not, of all types of interventions used to prevent deep vein thrombosis (DVT) in males and females aged 18 years and older undergoing KA. There were no restrictions on language or publication status. DATA COLLECTION AND ANALYSIS:Two authors independently selected studies for inclusion, assessed trial quality with the Cochrane 'Risk of bias' tool, and extracted data. A third author addressed discrepancies. We contacted study authors for additional information when required. We used GRADE to assess the certainty of the evidence. MAIN RESULTS:This update adds four new studies, bringing the total of included studies to eight and involving 3818 adult participants with no history of thromboembolic disease undergoing KA. Studies compared daily subcutaneous (sc) low-molecular-weight heparin (LMWH) versus control (five studies); oral rivaroxaban 10 mg versus placebo (one study); daily sc LMWH versus graduated compression stockings (GCS) (one study); and aspirin versus control (one study). The incidence of pulmonary embolism (PE) in all trials combined was low, with seven cases in 3818 participants.There were no deaths in any of the intervention or control groups. LMWH versus control When compared with control, LMWH probably results in little to no difference in the incidence of PE in patients undergoing KA (risk ratio (RR) 1.81, 95% confidence interval (CI) 0.49 to 6.65; 1820 participants; 3 studies; moderate-certainty evidence). LMWH showed no reduction of the incidence of symptomatic DVT (RR 0.61, 95% CI 0.18 to 2.03; 1848 participants; 4 studies; moderate-certainty evidence). LMWH may reduce the risk of asymptomatic DVT but the evidence is very uncertain (RR 0.14, 95% CI 0.03 to 0.61; 369 participants; 2 studies; very low-certainty evidence). There was no evidence of an increased risk of all adverse events combined (RR 1.85, 95% CI 0.95 to 3.59; 1978 participants; 5 studies; moderate-certainty evidence). No evidence of a clear effect on major bleeding (RR 0.98, 95% CI 0.06 to 15.72; 1451 participants; 1 study; moderate-certainty evidence), or minor bleeding was observed (RR 1.79, 95% CI 0.84 to 3.84; 1978 participants; 5 studies; moderate-certainty evidence). Rivaroxaban versus placebo One study with 234 participants compared oral rivaroxaban 10 mg versus placebo. No evidence of a clear impact on the risk of PE (no events in either group), symptomatic DVT (RR 0.16, 95% CI 0.02 to 1.29; moderate-certainty evidence); or asymptomatic DVT (RR 0.95, 95% CI 0.06 to 15.01; very low-certainty evidence) was detected. Only bleeding adverse events were reported. No major bleeds occurred in either group and there was no evidence of differences in minor bleeding between the groups (RR 0.63, 95% CI 0.18 to 2.19; moderate-certainty evidence). Aspirin versus control One study compared aspirin with control. No PE, DVT or asymptomatic events were detected in either group. Adverse events including pain and swelling were reported but it was not clear what groups these were in. No bleeds were reported. LMWH versus GCS One study with 1317 participants compared the use of LMWH versus GCS. There was no clear difference in the risk of PE (RR 1.00, 95% CI 0.14 to 7.05; low-certainty evidence). LMWH use did reduce the risk of DVT compared to people using GCS (RR 0.17, 95% CI 0.04 to 0.75; low-certainty evidence). No clear difference in effects was seen between the groups for asymptomatic DVT (RR 0.47, 95% CI 0.21 to 1.09; very low-certainty evidence); major bleeding (RR 3.01, 95% CI 0.61 to 14.88; moderate-certainty evidence) or minor bleeding (RR 1.16, 95% CI 0.64 to 2.08; moderate-certainty evidence). Levels of thromboembolic events were higher in the GCS group than in any other group. We downgraded the certainty of the evidence for imprecision resulting from overall small event numbers; risk of bias due to concerns about lack of blinding, and indirectness as we were uncertain about the direct clinical relevance of asymptomatic DVT detection. AUTHORS' CONCLUSIONS:There is a small risk that healthy adult patients undergoing KA will develop venous thromboembolism (PE or DVT). There is moderate- to low-certainty evidence of no benefit from the use of LMWH, aspirin or rivaroxaban in reducing this small risk of PE or symptomatic DVT. There is very low-certainty evidence that LMWH use may reduce the risk of asymptomatic DVT when compared to no treatment but it is uncertain how this directly relates to incidence of DVT or PE in healthy patients. No evidence of differences in adverse events (including major and minor bleeding) was seen, but data relating to this were limited due to low numbers of events in the studies reporting within the comparisons.