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496soleal veins11). Therefore, for distal thrombosis with a tendency towards enlargement, it is necessary to check for thrombus at regular intervals.Regarding imaging diagnosis, noninvasive venous US is the first choice for thrombosis detection in the lower extremities because of high diagnostic accuracy17-19). Contrast-enhanced CT examination involves administration of a contrast agent and radiation exposure. We sometimes encountered rare cases in which clinical VTE was highly suspected without positive US findings. In such cases, contrast-enhanced CT or magnetic reso-nance imaging should be performed18, 20-22). US can continuously visualize the femoral, popli-teal, and lower leg veins in B mode. A compression US technique was used, in which the vein was compressed with a probe, allowing the determina-tion of the presence or absence of a thrombus. Color, power, and pulse Doppler methods are commonly used to visualize blood vessels and check for obstructions18, 23, 24). The mean sensitivity for proxi-mal-type DVT in the femoral and popliteal veins, compared with venography, was reported to be 97% (range, 89-100%) for symptomatic patients and 62% (range, 38-100%) for asymptomatic patients25). Furthermore, the mean sensitivity of US for distal-type DVT was 73% (range, 0-100%) for symptomatic patients and 53% (range, 0-92%) Figure 4　Time points when thrombus became positive for the first time in the DVT-positive groupIn the DVT-positive group, the frequencies for first observation of DVT were 20.0% on POD 0, 46.7% on POD 1, 13.3% on POD 3, 6.7% on POD 7, and 0% on POD 14 and POD 21. POD, post-operative day; DVT, deep venous thrombosisfor asymptomatic patients25). However, the speci-ficity compared with venography was 96% (95% CI, 95.2-96.8%)18). The result of 27% of DVT posi-tive cases in our study may be lower than the actual result because the sensitivity of US is low in the distal-type. However, since the specificity is high, there is little possibility that false positives are included.In the present study, the patients in the DVT-pos-itive group were significantly older than those in the DVT-negative group. The risk of DVT was reported to gradually increase every 10 years with age, and the risk of DVT in patients aged >65 years was 2.1 times higher26, 27). The results relative to the mean age in the present study correspond to those of previous studies. In addition, the DVT-positive ratio was 9.4% (6/64) in men and 31.3% (89/284) in women. There was a significant difference between the groups (p = 0.0002). The factors that cause DVT more frequently in women than in men may be that the average age of women is older than that of men and that older people are more likely to delay rehabilitation progress28, 29). In addi-tion, it is possible that lower preoperative activities of daily living due to advanced age may affect post-operative walking ability28, 29), and lower limb pumping ability due to differences in muscle mass may contribute to venous stasis30).