This volunteer anatomical study compared the point of needle entry of the two most common approaches described for the ACB and found that the mid-thigh approach is proximal to the superior border of adductor canal in both male and female volunteers, and hence outside the true adductor canal. The distal ACB entry point (dACB) was within the true adductor canal. The midthigh approach is proximal to the distal AC approach by approximately 11 cm. To our knowledge, this is the first study evaluating the actual distance between these commonly described approaches to ACB.
Understanding the exact anatomy of the AC is important as multiple needle entry points have been described for AC blocks. Several cadaveric studies and one human volunteer study have previously evaluated the anatomy of adductor canal. Relevant measurements from previous cadaver studies are summarized in Table 2. Anagnostopoulou et al. concluded that the “superior foramen of the AC” (corresponding to the superior border of AC) was at a mean distance of 6.5 cm from the midpoint of thigh.15 In an earlier study, Horn et al. found the median distance from the proximal patella to the distal end of AC was 10.25 cm (7 to 11.5 cm).16 In a recent cadaver study, Elazab et al. described a continuous layer of subsartorial fascia called the fascia vasto-adductoria in the midthigh.17 They further subdivided this fascial layer into the a proximal fascia vasto-adductoria (a thin, quadrangular layer of fascia stretching between the vastus medialis and adductor longus muscles) and the distal fascia vasto-adductoria, also known as vasto-adductor membrane (a thick, pentagonal layer arising from the adductor magnus spreading anterolaterally to the vastus medialis muscle). The length of the vasto-adductor membrane in their study was comparable to that from an earlier cadaveric study by Tubbs et al.18
In a recent, small (N = 22) ultrasound study in predominantly male volunteers, Wong et al. showed that the midthigh approach was proximal to superior border of AC, but reported a mean AC length of 11.5 cm.19 In contrast, the median length of adductor canal in our study was 8 cm. Wong et al. stated that the AC is roofed by the vasto-adductor membrane in its entire length, and hence the proximal border of the vasto-adductor membrane marks the beginning of the AC. Using the same principle, we can assume that the distal border of the vasto-adductor membrane marks the distal end of the AC and the vasto-adductor membrane length corresponds to AC length. Previous cadaveric studies have reported a mean vasto-adductor membrane length of 7.6 cm and 7.9 cm, consistent with an AC length of 8 cm in our study.17,18 The differences in measurements between our study and the Wong et al. study may be related to the method used to localize the inferior border of AC. We used the method described by Manickam et al. to identify the adductor hiatus.11 By comparison of results it is clear that the inferior border of AC identified by Wong et al. was distal to that used in our study (Table 3). The distance from base of patella to the inferior border of adductor canal in our study (9 cm) also correlates better with previous cadaveric studies. Finally, it is important recognize that ultrasonography relies on generated images of deep structures, and the identified landmarks and measurements may not fully correspond to cadaveric dissections, producing minor differences. This is a limitation of ultrasound studies on anatomical measurements, including our study. Nevertheless, the midpoint of thigh has consistently been reported as proximal to the proximal border of AC and the results from our larger study with both male and female subjects closely correlate with the previous cadaveric studies.
Based on this knowledge of AC anatomy, we reviewed recently published literature to identify various needle entry points described for adductor canal blocks. We searched the PubMed (January 1, 2017–January 31, 2019) using the key word phrase “Adductor canal block.” A total of 115 articles were retrieved; the abstracts and methods of all retrieved articles were reviewed by two authors (YR, AM) and 46 prospective clinical trials were identified and reviewed in detail (Figure 3). Case reports, retrospective studies, cadaveric studies, non-English articles, review articles and meta-analyses were excluded (N = 69) (Included articles are listed in Supplemental Digital Content). The points of needle entry were grouped into 3 different regions (Figures 4A and 4B): i) proximal blocks,20,21 including block entry points that target the SFA after it has just beneath the medial border of the sartorius muscle, or the proximal-thigh approach by Meier et al. involving the needle insertion at a location where the SFA is underneath the medial third of the sartorius muscle; ii) mid-thigh blocks,3,10,20,22 including blocks performed at the midthigh level (between ASIS and base of patella), and the distal approach described Meier et al. where the SFA lies underneath the middle third of sartorius; and iii) distal blocks,11,22,23 including blocks performed at the dACB and other true AC blocks . Figure 4A shows the anatomical locations and Figure 4B shows the corresponding ultrasound image of the levels of these blocks. In 34 out of 40 studies (the needle entry site was not specified in 6 studies), the needle entry point was at or above the midpoint of the thigh, and hence anatomically proximal to the true adductor canal.
Clinical relevance of the blocks at the different locations
Irrespective of the site of injection, and whether it is within the true anatomical adductor canal, it is important to understand if there are clinically significant differences between these block injection sites. In a cadaveric study, Runge et al. injected 10 mL of dye into distal AC in 10 legs and into distal femoral triangle in 3 legs.24 There was consistent spread of the dye to the popliteal plexus in the distal AC group but no spread of dye into popliteal fossa after the femoral triangle group. Andersen et al and Goffin et al have also demonstrated spread of the injectate to the popliteal fossa with distal AC injection in previous cadaveric studies (Table 4).25,26 This spread may improve the quality and duration of analgesia of distal AC injections. In a study using 4 fresh frozen cadavers which acted as their own control, Johnston et al. injected 20 mL of dye into one thigh in the distal FT, into the other thigh in the distal AC.27 When the injectate was introduced into the distal FT, both the saphenous nerve and the nerve to vastus medialis were stained. In contrast, dye administered into the distal AC stained only the saphenous nerve. They concluded that the injection into the distal AC may be suboptimal for knee analgesia as it may spare the nerve to the vastus medialis, while an injection in the distal FT may provide greater analgesia to the knee, but may result in undesirable motor blockade from spread to the vastus medialis nerve.27 Clinical studies comparing different locations used for ACB in terms of efficacy, undesirable effects, complications and outcomes are limited (Table 5). Three studies compared midthigh injections with more proximal site injections and found that both approaches provided comparable analgesia with minor differences.20,21,28 A fourth study compared midthigh injection with a distal site injection and showed improved analgesia the day after knee arthroplasty with catheter insertion at the midthigh compared to a more distal insertion point.22 Another recent study compared catheter insertions at mid FT and proximal AC ( 2 cm distal to the ACsup) and found no differences in immediate postoperative functional mobility, analgesia, and opioid consumption .29 In summary research groups that advocate more distal injections believe that analgesia will be better because of blockade of branches of sciatic and obturator nerves while groups that prefer proximal injections believe that consistent blockade of the nerve to vastus medialis is important to block success.