The present study showed three main findings. First, PAs run approximately 7–16 mm away from the posterior cortex in the osteotomy planes for both OWHTO and hybrid CWHTO without significant differences. Second, PA locates approximately at 42–54% and 3–15% of the osteotomy line in OWHTO and hybrid CWHTO, respectively. Third, the minimum screw drilling angle from the tibial tuberosity toward the PA, with the medial tibial cortex as reference, was 35.2°, suggesting that the safe angle in DTO was less than 35° to the MC line.

Previously, several studies examined the distance between PA and the posterior cortex in the osteotomy planes for HTO. Bisicchia et al. examined the distance between the PA and the posterior cortex in cadavers using contrast-enhanced CT images and reported that the mean distance was 9.6 ± 2.1 mm in the osteotomy planes for OWHTO [18]. Kang et al. examined the distance between the PA and the posterior cortex in the virtual osteotomy planes of OWHTO using contrast-enhanced CT in patients with peripheral artery diseases and reported a value of 15 mm with a 99% confidence interval of 9–21 mm [19]. Choi et al. reported that the mean distance measured from MR images was 13.0 ± 2.0 mm in osteotomy planes of OWHTO [20]. In this study, the mean minimal distance between the posterior cortex and PA in the osteotomy planes for OWHTO was 10.8 ± 2.5 mm (Table 5). Although similar mean values of dPC-PA were reported from previous studies, there were differences in the mean distance between posterior cortex and PA among the previous studies. The difference could be partly due to methodological difference among the studies. In the study using cadavers, the distance may have changed during dissection. In addition, it would have been difficult to consistently determine the closest point from PA. While similar measurement methods to Kan’s report were used in this study, approximately 5 mm difference in average dPC-PA was observed. One reason could be the difference in osteotomy plane. In our study, the osteotomy plane was created to include the point 4 cm distal to the medial joint line and the point 1.5 cm distal to the lateral joint level. In addition, the sagittal plane was adjusted by the tibial joint surface. In Kang’s study, the osteotomy plane was reconstructed at 3.5 cm below the joint line and perpendicular to the tibial axis. Therefore, the difference in osteotomy plane may have caused the measurement difference. In the report by Choi, dPC-PA was measured in the experimental models using MR images combined with contrast-enhanced CT from different subjects. Therefore, the values may be different from the actual distances. Together with our data, these reports suggest that PAs run approximately 10 mm away from the posterior cortex in the osteotomy planes for OWHTO. Further, the distance was also measured in the planes for hybrid CWHTO in the present study. The mean distance was 10.3 ± 2.2 mm in hybrid CWHTO, and there was no significant difference between the planes for OWHTO and hybrid CWHTO. Therefore, PAs run approximately 10 mm away from the posterior cortex at proximal tibia levels, and the risk of PA injury was similar regardless of the osteotomy technique. Since it is very difficult to avoid the PA just by controlling the bone saw during osteotomy and the distance can be even shorter in some patients, careful attention and adequate protection are essential to prevent PA injuries during HTO.

Table 5 Previous reports and this study on the distance between the posterior cortex and the popliteal artery in the osteotomy plane

Kang et al. reported that the working width of the saw oscillates reached up to 35 mm [19]. Notably, the minimal distance in our study was approximately 7 mm. Taking the working length of the bone saw into consideration, the dangerous point based on the working length of the bone saw was also examined. The mean distance to the dangerous point from the most medial point in OWHTO was 25.9 ± 2.9 mm, which corresponds to 47.6 ± 3.7% of the osteotomy line, when the center of the saw was positioned 6.0 ± 2.0 mm posteriorly. The results suggest that, if osteotomy starts from the position approximately 5–6 mm posterior to the most medial point and progresses in parallel to the ground, the tip of the saw will come into contact with PA. Therefore, surgeons should exercise great caution when cutting the bone close to this point. Meanwhile, in hybrid CWHTO, PA was located within close proximity to the starting point of osteotomy. Therefore, surgeons should cut the posterior cortex while retracting PA posteriorly.

This study used CT images taken in the knee joint extension position. Several reports have examined the effect of knee flexion angle on the distance between the PA and the posterior cortical bone of the tibia. Kim et al. reported that, between 0° and 90° flexion, the PA was farthest away from the posterior tibia at 90° flexion with a mean of 11 mm at 2.0 cm below the joint line [23]. Meanwhile, Choi et al. reported that the distance increased by only 1.3–1.7 mm at 90° flexion compared with 0° flexion in the virtual osteotomy planes [20]. Shetty et al. reported that, in 85% of the cases, the PA receded from the posterior tibia in knee flexion, whereas in the remaining cases, it approached the posterior tibia [24]. Thus, there is no clear consensus. In any case, the PA may not move to a position where a bone chisel or bone saw cannot reach it by knee flexion alone. Although the artery is movable, its mobility is not likely to help prevent damage to the arterial wall once the bone saw hits PA. Meanwhile, when chisels contact with PA, there may be a chance that PA moves aside posteriorly without being damaged. However, as this is not guaranteed, correct placement of the retractor just below the posterior cortical bone of the tibia is a sure way to prevent damage to PA.

Recently, DTO has attracted the attention of surgeons considering previous reports on the deterioration of patellofemoral joints after OWHTO [25,26,27]. In DTO, screw fixation for the tibial tuberosity is generally required. Meanwhile, case reports on PA injuries by tibial tuberosity fixation during tibial tuberosity transfer have been reported [28, 29]. Hernigou et al. previously examined the safe zone and danger zone for screw drilling during tibial tuberosity transfer and reported that the direction to the medial one-third of the tibia was the safest zone and the lateral upper side of tibia was the most dangerous zone [30]. Yang et al. also examined the safe zone for surgeries on proximal tibia using MR images. They reported that neurovascular structures were observed lateral to the posterior middle line of the tibia and suggested that the safe zone was the medial half of the tibia if penetration of the posterior cortex was needed during surgeries [31]. Furthermore, they measured the angle between the line passing through the most anterior part of the tibial tuberosity to the PA and the AP axis, and the angle was < 10°. However, those references including the AP axis could be affected by the rotation of the lower leg, and the estimation of the safe direction may not be fully reliable. In the present study, the safe angle for screw drilling was determined in relation to the medial cortex line since the medial cortex line can be easily identified during surgery and the effects of rotation can be avoided. Considering that the minimum angle in our study was 35.2°, it was suggested that an angle of less than 35° against MC line would be safe for screw drilling in DTO.

This study has several limitations. First, since the PAs were identified using contrast-enhanced CT, the vascular wall was not considered. Therefore, the distance from the posterior cortex to the PA, when including the vascular wall, will be shorter than our measured dPC-PA. Second, although the minimal distance between the posterior cortex and PA in our patients was 7 mm, the distance can be shorter than 7 mm in other patients. Third, during DTO, the distal part shifts laterally after opening the gap. Therefore, the vascular course changes, and consequently the distance and safe angle may change. However, the lateral shift of the distal part will shift the neurovascular structure laterally, and the safe angle is not likely to be less than 35°. Fourth, the subjects of this study were 20 patients with cardiovascular disease who underwent contrast-enhanced CT for cardiovascular diseases, not patients with knee OA with altered alignment who are eligible for HTO surgeries. However, Lee et al. reported that, in their MRI study, the location of PA distal to the joint level was not significantly different between the arthritic and non-arthritic groups [32]. Fifth, this study evaluated only CT images taken in the knee joint extension position. Since the distance between PA and the posterior cortical bone can change depending on the knee flexion angle, the values in our study may not be utilized if a surgeon uses a different knee flexion angle during surgery.

Despite the limitations, we believe that the present study provides useful information for preventing PA injury during HTO and DTO.

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