Deep vein thrombosis (DVT) occurs when a thrombus forms in one or more deep veins. It is a common clinical problem and usually affects the lower limbs; moreover, orthopedic surgery is a known risk factor for DVT [1]. DVT can lead to pulmonary thromboembolism or post-thrombotic syndrome, which can result in death. Therefore, orthopedic surgeons usually plan to identify DVT in the lower extremities. Although it is well known that ultrasonography is an excellent modality for postoperative evaluations of orthopedic patients, and venous Doppler ultrasonography is the standard imaging modality for patients with suspected DVT, the procedure can be limited due to issues with the acoustic window and patient discomfort after surgery [2,3]. Moreover, computed tomography angiography (CTA) of the femoral vein is an alternative imaging modality, but involves potentially harmful radiation exposure, the administration of iodinated contrast agents, and a higher degree of invasiveness than ultrasonography. Therefore, it would be helpful to prioritize patients with significant risk factors who would truly benefit from postoperative ultrasonography.

The presence of DVT is associated with multiple factors. A previous study suggested that prolonged surgery (≥2 hours) and a higher body mass index were significantly associated with the development of DVT in patients undergoing major orthopedic surgery [4]. Another study reported that major orthopedic surgery was associated with a greater risk for DVT [5]. However, contradictory studies have also reported that cardiovascular disease (CVD) risk factors, such as age, smoking, adiposity, cholesterol and triglyceride levels, and diabetes mellitus (DM), were associated or not associated with venous thromboembolism [6,7]. The risk factors associated with lower-limb surgery, especially in elderly patients undergoing total knee joint replacement, and controversies regarding the prevalence of DVT, site, and host-related factors, have led to uncertainty regarding which patients require a more rigorous work-up for venous thromboembolism.

Accordingly, the objectives of the present study were to verify which patients need post-orthopedic surgery examinations for DVT by identifying the factors associated with post-orthopedic surgery DVT at our hospital and to determine whether any specific level of vessel is more vulnerable to DVT after lower-extremity surgery in order to help triage patient management in the long run.

Orthopedic surgery is a well-known risk factor for DVT. The release of thromboplastin from dissected soft tissue and reamed bone, as well as venous stasis during surgery and postoperative immobility, contribute to high rates of DVT [8]. The frequency of all major lower-limb orthopedic surgical procedures has increased annually, similar to what has been reported in previous studies [5]. This study investigated the incidence and causes of postoperative DVT, which has multifactorial risk factors. We found that approximately a third of postoperative patients had DVT distal to the knee, and DVT was significantly associated with older age and lower body weight.

Ultrasonography plays a role as an important imaging modality in the accurate diagnosis of postoperative DVT. Most of all, this imaging modality is not affected by metal artifacts, which is especially relevant after musculoskeletal surgery. The susceptibility of magnetic resonance imaging to artifacts, as well as the presence of streak artifacts in CT, can reduce the diagnostic performance of these modalities for DVT. Second, the flexibility of ultrasound examinations in a variety of locations and patients positions is a strength in the postoperative setting. Ultrasonography at various angles may also help overcome the poor sonic window. Third, ultrasonography is a fast, reliable, and cost-effective technique compared to other modalities. Moreover, it can be used to evaluate postoperative pathologic conditions, such as abscesses, muscles or tendon tears, or the extent of hemarthrosis [2].

A previous report showed that knee surgery had a higher incidence of DVT at the localized popliteal location [9]. Unlike the previous study, most DVT sites in our cases were distal to the popliteal vein, including the tibial, peroneal, soleus, (Fig. 2) and gastrocnemius veins; only one DVT was found at the level of the popliteal vein, and no patients had distant thromboembolism. Calf vein thromboses have been reported to be closely related with lack of limb movement. More specifically, the soleal intramuscular veins depend on muscle contraction for venous perfusion, and their valve system does not function sufficiently in immobile patients, so that these veins are the leading points of DVT in the leg. Therefore, postoperative patients, especially after lower-limb surgery, are susceptible to venous stasis due to lack of motion. Our data showed the typical location of DVTs in this context. In contrast, iliac or femoral vein lesions are mainly caused by iliac vein compression, or catheter-related injury, and are symptomatic.

The major threat of a soleal thrombus is that it can propagate silently to the thigh, and form a freely floating distant thromboembolism to the lung. Therefore, it is important to remain vigilant for calf lesions, especially in patients with a known risk for DVT [10-12].

The risk factors known to be related with DVT were analyzed in our study, and interestingly, instead of the usual suspects (e.g., prolonged surgery time or cardiovascular risk factors), we found that lower body weight was associated with a higher incidence of postoperative DVT. In one study, the relative muscle volume of the vastus lateralis at the level of the thigh was negatively associated with the prevalence of symptomatic and radiologically detected DVT [13]. Lower body weight can be accompanied by loss of muscle volume, which is known as sarcopenia. Sarcopenia is defined as the loss of skeletal muscle mass and strength with aging, and is a recognized component of geriatric syndrome [14]. It has been associated with a multitude of poor outcomes including low survival rate, high infection rate, prolonged hospital stay, treatment toxicity, and physical disability [15]. Although obesity is a known risk factor for DVT, it is uncertain whether only a higher body weight is an independent risk factor for DVT, or whether the effect is secondary to DM and dyslipidemia [16]. We suggest future research on sarcopenia, especially in the leg, which may be related to the occurrence of DVT.

In addition, low muscle mass has also negative correlations with an increased prevalence of coronary arterial disorder(s) and metabolic syndrome [17]. Future studies investigating the associations of DVT with the Framingham risk score and leg muscle mass are warranted.

The management of below-knee DVT remains inconsistent because current guidelines are based on weak evidence. The American College of Chest Physicians recommends anticoagulation if symptoms are severe; otherwise, it is recommended to perform surveillance ultrasonography 2 weeks after surgery to monitor for clot propagation, at which point anticoagulation should be initiated if there is any evidence of proximal extension [18]. Patient symptoms and a D-dimer test in the early postoperative period, however, may lead to clinical confusion and mask the thrombosis. Therefore, it is the responsibility of radiologists to clearly report sonographic findings in a way that most appropriately facilitates patient management, and recommend an alternative study such as CTA in cases of severe postoperative changes in overlying soft tissue shadowing the deep venous structure. When postoperative ultrasonography for DVT showed the presence of a thrombosis, the clinicians at our hospital treated the respective patients with antithrombotic agents, even for isolated calf vein lesions, closely monitored them at the outpatient clinic, and followed up with ultrasonography. Most DVT sites in our patients were distal to the popliteal vein, with multiple sites, including the tibial vein in five patients, the peroneal vein in 13 patients, the soleal vein in 28 patients, and the gastrocnemius veins in 25 patients. None of them had adverse events related with anticoagulation therapy or distant thromboembolism. All showed resolution of the thrombosis on follow-up. We believe that early sonographic recognition and treatment may have prevented thrombotic extension or recurrence, although this possibility needs future study.

Several limitations of this study should be acknowledged. First, data analysis was performed retrospectively at a single tertiary referral center, and some asymptomatic patients may have missed the postoperative evaluation. Second, DVT ultrasonography is considered to be an operator-dependent imaging procedure. The variable expertise of the operators may have affected the reported incidence of DVT. To decrease inter-operator variability, the study was performed using a standardized protocol, and the expert musculoskeletal radiologists confirmed the examinations. Additionally, with more expert examinations, the incidence of distal DVT might even increase, since it is more difficult to diagnose a thrombosis in a calf vein than at the femoropopliteal level in postoperative patients [19]. Third, DVT prophylaxis regimens and prevention programs may have affected the outcomes of our study.

In conclusion, isolated distal DVTs were found to be common in patients after lower-limb surgery. We showed that advanced age and lower body weight were associated with a higher incidence of postoperative DVT. The typical location was below the popliteal vein, which may be related to venous stasis. Therefore, physicians should carefully examine the calf level, especially in elderly and fragile patients. In the aging society, where more and more elderly patients with DVT risk factors undergo major surgery, we hope that the typical ultrasonography findings shown in our study will help radiologists screen at-risk patients.