Defining the Location of the Adductor Canal Using Ultrasound

Abstract

Background and objectives: The precise location of the adductor canal remains controversial among anesthesiologists. In numerous studies of the analgesic effect of the so-called adductor canal block for total knee arthroplasty, the needle insertion point has been the midpoint of the thigh, determined as the midpoint between the anterior superior iliac spine and base of patella. "Adductor canal block" may be a misnomer for an approach that is actually an injection into the femoral triangle, a "femoral triangle block." This block probably has a different analgesic effect compared with an injection into the adductor canal. We sought to determine the exact location of the adductor canal using ultrasound and relate it to the midpoint of the thigh.

Methods: Twenty-two volunteers were examined using ultrasound. The proximal end of the adductor canal was identified where the medial border of the sartorius muscle intersects the medial border of the adductor longus muscle. The distal end of the adductor canal is the adductor hiatus, which was also visualized ultrasonographically.

Results: The mean distance from the anterior superior iliac spine to the midpoint of the thigh was 22.9 cm (range, 20.3-24.9 cm). The mean distance from the anterior superior iliac spine to the proximal end of the adductor canal was 27.4 cm (range, 24.0-31.4 cm). Consequently, the mean distance from the midpoint of the thigh to the proximal end of the adductor canal was 4.6 cm (range, 2.3-7.0 cm).

Conclusions: In all volunteers, the midpoint of the thigh was proximal to the beginning of the adductor canal, suggesting that an injection performed at this level is in fact a femoral triangle block.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

FIGURE 1

The first row shows three ultrasound images at different levels. The second row shows the identical ultrasound images with color markings. A1 and A2 are at the level of the femoral triangle (FT) corresponding to the red arrows in Figure 2. B1 and B2 depict the beginning of the AC defined as the apex of the FT, where the medial border of the sartorius muscle intersects the medial border of the adductor longus muscle (white arrow in B2). This level corresponds to the blue arrows in Figure 2. C1 and C2 depict the distal end of the AC corresponding to the green arrows in Figure 2. AL (purple), adductor longus muscle; AM (orange), adductor magnus muscle; asterisk (blue), femoral vein; asterisk (red), femoral artery; F, femur; S (green), sartorius muscle; SM (yellow), semimembranosus muscle; VM (blue), vastus medialis muscle.

FIGURE 2

The figure shows the thighs of 4 volunteers. The midpoint of the thigh (red arrow) is defined as half the distance between the ASIS (pink asterisk) and the base of patella (orange stippled line) corresponding to the ultrasound images in Figure 1A. The proximal end of the AC (blue arrow) is defined by the intersection of the medial border of the sartorius muscle (yellow stippled line) and the medial border of the adductor longus muscle (cyan stippled line) corresponding to the ultrasound images in Figure 1B. The distal end of the AC is defined as the adductor hiatus (green arrow) corresponding to the ultrasound images in Figure 1C.

FIGURE 3

The figure shows the ultrasonographic visualization of the adductor canal with the characteristic double contour of the VAM (cyan arrows). AM, adductor magnus muscle; femoral artery (red asterisk); S, sartorius muscle; VM, vastus medialis muscle.