The principal findings of this study are that the incidence of total VTE was 10.8 and 5.3% after ACL reconstruction and meniscectomy, respectively, without a significant difference. The DVT rates determined using duplex ultrasound within 6 weeks after ACLR and meniscectomy were 3.1 and 1.1%, respectively. ACLR, age and BMI tended to increase the risk of VTE development; however, this tendency failed to reach statistical significance, and no risk factor was detected for major VTE. In the ACLR group, BMI was the only independent risk factor for VTE development following ACL reconstruction.
The reported incidence of VTE after ACLR varies in the existing literature. Oshiba H et al.  found that the incidence of DVT after ACL reconstruction detected by ultrasonography on postoperative day 7 was 6.3% (16/256). Among patients with DVT, the incidence of symptomatic DVT was 12.5% (2/16). Dong et al.  reported that the incidence of DVT among Chinese patients was 12.1% within 7 days of 282 ACL surgeries, and 55.9% (19/34) of the diagnosed DVT were symptomatic. Ye et al.  studied 171 patients with an overall DVT incidence of 14% by venography, but did not report the occurrence of symptomatic DVT. Marieke C et al.  conducted a prospective cohort study and found DVT happened to 13 patients. Among these patients, 69.2% (9/13) were asymptomatic proximal or distal DVT, whereas 30.8% (4/13) were symptomatic. Based on these findings, they suggested that prophylactic measures for DVT should be considered after arthroscopic ACLR to decrease the incidence of DVT, especially when risk factors are present [13,14,15,16], consistent with the guidelines of the French Society of Anaesthesia and Intensive Care .
However, several investigations with large cohorts of patients undergoing ACLR have reported inconsistent results. Jameson et al.  found the rate of symptomatic VTE to be approximately 0.44% (0.30% DVT and 0.18% PE) in 13,941 patients undergoing ACLR. Forlenza EM et al.  reported that the incidence of symptomatic VTE was 1.01 and 1.22% at 30 and 90 days, respectively, in 11,977 patients undergoing ACLR, as recorded in the Humana administrative claims database in Indiana. Gaskill et al.  reported that the incidence of symptomatic VTE was 0.53% among the 16,558 ACLR recorded in the Military Health Care System (MHS) database in the United States. Schmitz et al.  reported that the incidence of symptomatic VTE was 0.4% (0.34% DVT and 0.06% PE) in a cohort consisting of 26,014 primary and revision ACLR procedures obtained from the Swedish Knee Ligament Register in Sweden. These authors recommended against the routine use of thromboprophylaxis, which is in accordance with the American guidelines and recommendations of Swedish surgeons [19, 27].
In our study, all patients underwent duplex ultrasound scans postoperatively, which revealed an incidence of DVT after ACLR of 3.1%. This rate is close to the incidence found by Oshiba H et al.  and Marieke C et al.  but lower than that reported by Dong et al.  and Ye et al. . This may be because 67.4% of the patients underwent reconstruction of the medial and lateral collateral ligament and posterior cruciate ligament at the same time in Dong et al.’s study . In Ye et al.’s study, DVT was detected by venography, which may be more sensitive to the diagnosis of VTE than the ultrasound used in our study. In this present study, the incidence of symptomatic DVT among all DVT was 33.3% (2/6), which is consistent to those reported by some studies (12.5 to 55.9%) [13,14,15,16].
The reported incidence of symptomatic VTE or DVT/VTE (0.4 to 1.2%) in several large cohorts [17, 18, 20, 21] was significantly lower than that in these studies [13,14,15,16] and in our study, which can be mostly explained by the study design and whether screening of VTE was performed for all patients. The data for these large cohorts are generally obtained from local and national databases according to international classification of diseases (ICD) codes. Therefore, the accuracy of the information related to ICD codes should be questioned, as the incidence of asymptomatic VTE cannot be accurately recorded in these health care systems. Moreover, the data from local and national databases are usually reported by patients, and not all patients undergo VTE screening after surgery. In fact, higher rates of VTE are usually reported with the use of prospective diagnostic screening, which was performed in our study and several other studies [13,14,15,16] with a higher reported incidence of VTE.
Compared with ACLR, meniscectomy has been considered to have a lower risk of VTE because of its simplicity and its relatively shorter operative time. In one recently published review , major VTE (symptomatic VTE, DVT and PE) was reported in 4.75% of patients who received ACL reconstruction without prophylaxis and 0.72% of patients who received simple knee arthroscopic surgery (similar to meniscectomy), while the rate of VTE was 8.0% among patients who received ACLR and 1.96% among patients who underwent simple knee arthroscopic surgery, which is almost consistent with the result in our study. However, another investigation with a cohort of 12,595 patients reported conflicting results; specifically, the VTE rate after ligament reconstruction was 0.16%, while that after meniscectomy was 0.56% .
Age as a risk factor for VTE has been previously reported in several series of patients undergoing knee arthroscopic surgeries. Oshiba H et al.  found that patients aged ≥30 years have a higher risk of developing VTE. Ye et al.  reported that female patients and those aged 35 years were at a higher risk for VTE, and they recommended routine thromboprophylaxis in these patients. In several studies with large cohorts, Jameson et al.  found that age over 40 years was associated with an increased VTE risk. Forlenza EM et al.  reported risk factors including age ≥ 45, inpatient surgery, chronic obstructive pulmonary disease (COPD), tobacco use and concurrent posterior cruciate ligament (PCL) reconstruction, meniscal transplant and osteochondral allograft. Gaskill et al.  reported an increased odds of VTE in patients aged 35 years and over with a history of nicotine use, anticoagulant use, concomitant high tibial osteotomy (HTO), or concomitant PCL reconstruction. Schmitz et al.  reported that the only significant risk factor for VTE after surgery was age. In our study, logistic regression revealed that age has a tendency to increase the risk of developing VTE following knee arthroscopic surgery, although the difference failed to reach statistical significance. This may be because we excluded those patients who were older than 60 years in this study.
BMI is directly related to obesity and is usually considered an important part of the preoperative assessment. Increasing BMI has been shown to be associated with increased rates of unplanned hospital admissions and postoperative complications following arthroscopic procedures [30,31,32]. In one recently published study with 141,335 patients following arthroscopic surgery , the authors found that the most common complications were DVT (0.27%), and VTE risk factors included being overweight (25.0 ≤ BMI < 30, OR = 1.474) or diabetic with class I obesity (30.0 ≤ BMI < 35, OR = 1.469), which is partially consistent with the characteristics of the ACLR group in our study. In our study, BMI was associated with an OR of 1.151 for VTE (p = 0.054). A statistically significant difference would likely be obtained with an increase in the sample size in future studies.
Our study has some limitations. First, the retrospective design limited our assessment of the true effect of surgical intervention on the risk of VTE. Second, the number of ACLR patients was relatively small. The power analysis showed that a patient number of 100 per group was sufficient to reach a sufficiently narrow CI, but a study with a larger number of patients would increase its clinical importance. In our study, the tendency of ACLR, age and BMI to be associated with an increased risk of VTE was found, so a much larger patient group may strengthen the statistical significance. Third, the incidence of VTE in this study was mainly identified at 2 weeks after surgery by duplex ultrasound. DVT may develop in patients after they are discharged. All patients underwent follow-up for clinical assessments, but asymptomatic DVT may not have been detected. Therefore, the true overall incidence of DVT after ACL reconstruction is probably slightly higher than that shown in this study. Finally, contraceptive and NSAID use in our study by female patients was not recorded, and this may have an overall effect on the final incidence of DVT [22, 34].