Background and objectives Quadriceps sparing adductor canal block has emerged as a viable intervention to manage pain after total knee arthroplasty. Recent studies have defined ultrasound (US) landmarks to localize the proximal and distal adductor canal. US-guided proximal adductor canal injection has not been investigated using these sonographic landmarks. The objectives of this cadaveric study were to evaluate dye injectate spread and quantify the capture rates of nerves supplying articular branches to the knee joint capsule using a proximal adductor canal injection technique.
Methods A US-guided proximal adductor canal injection with 10 mL of dye was performed in seven lightly embalmed specimens. Following injection, specimens were dissected to document dye spread and frequency of nerve staining.
Results Following proximal adductor canal injection, dye spread consistently stained the deep surface of sartorius, vastoadductor membrane, aponeurosis of the vastus medialis obliquus, and adductor canal. The saphenous nerve, posteromedial branch of nerve to vastus medialis, superior medial genicular nerve and genicular branch of obturator nerve were captured in all specimens at the proximal adductor canal. There was minimal to no dye spread to the distal femoral triangle, anterior division of the obturator nerve and anterior branches of nerve to vastus medialis.
Conclusions This anatomical study provides some insights into the mechanism of analgesia to the knee following a proximal adductor canal injection and its motor sparing properties. Further clinical investigation is required to confirm cadaveric findings.
- knee joint innervation
- adductor canal block
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In the USA, over 750 000 knee arthroplasties were performed in 2014.1 Pain after total knee arthroplasty (TKA) can significantly limit early mobilization, rehabilitation and recovery.2 Femoral nerve block (FNB) targets sensory nerves supplying the anterior knee joint. While effective for post-TKA pain, FNB causes quadriceps weakness and increases the risk of fall.3 More recently, quadriceps sparing adductor canal block (ACB) has emerged as a viable alternative to FNB.4 The ACB has been shown to be ‘non-inferior’ to FNB, in terms of pain control and opioid consumption, while its motor sparing property permits rapid functional recovery of the knee joint and postoperative rehabilitation.5
The ACB involves local anesthetic injection into the adductor canal which is a triangular musculoaponeurotic conduit between the distal femoral triangle (apex) and popliteal fossa via the adductor hiatus. Traditionally, the ACB has been performed at the mid-thigh level.6–8 However, a recent ultrasound (US) study has shown this approach may target the distal femoral triangle, rather than the adductor canal, depending on the methodology used to demarcate mid-thigh level.9 This suggests the need to further define sonographic landmarks for targeting the adductor canal.
The adductor canal is bounded anterolaterally by the fascia of the vastus medialis, posteromedially by the fascia of adductor longus/magnus, and the vastoadductor membrane (VAM) forming the roof.10 Previous US studies have defined the proximal opening of the adductor canal as the site where the medial border of the sartorius intersects the medial border of the adductor longus.9 11 The distal femoral triangle is located superior to this landmark and the proximal adductor canal lies inferiorly. Distally, the adductor canal has been demarcated as the site where the femoral artery diverges from the sartorius and courses deep to pass through the adductor hiatus. These sonographic landmarks have been used in more recent cadaveric US-guided injection studies to investigate staining of nerves supplying articular branches to the knee joint capsule using distal femoral triangle and distal adductor canal techniques.12 13 However, US-guided proximal adductor canal injection has not been investigated using these sonographic landmarks. Therefore, the purpose of this cadaveric study was to: (1) evaluate the dye injectate spread using a proximal adductor canal injection technique; (2) determine the relationship of nerves supplying articular branches to the knee joint capsule, relative to the adductor canal, that could be targeted using proximal adductor canal injection technique; and (3) quantify the capture rates of nerves supplying articular branches to the knee joint capsule.
Seven lightly embalmed cadaveric lower limb specimens (3R:4L) with no visible signs of injury, pathology, or previous surgery were used in this study. No further demographic data were available.
Proximal adductor canal injection technique
US-guided dye injection (10 mL containing 0.5 mL of methylene blue diluted in 9.5 mL of water) into the proximal adductor canal was performed using a 10–12 MHz linear probe (SonoSite Edge, Bothell). The specimens were placed supine. The US probe was positioned over the thigh to visualize the sartorius muscle, the vastus medialis muscle, adductor longus muscle and femoral shaft (figure 1A). To facilitate localization of the femoral artery, a stainless stain wire was inserted into the vessel. A 22 G, 80 mm echogenic needle (PAJUNK Germany) was inserted in-plane in the lateral to medial direction. The needle was advanced to reach the proximal adductor canal at the level of the intersection of the medial borders of the sartorius and adductor longus (figure 1B). The endpoint of the needle was anterior-lateral to the femoral artery, identified by the stainless steel wire (figure 1C). This was followed by a 10 mL dye solution injection over 10 s producing a clear sonographic endpoint of fluid expansion.
Dissection and data analysis
Following US-guided injection, each specimen was carefully dissected using a 3.5× magnification lens. The skin, subcutaneous tissue and deep fascia were removed from the thigh. Next, the sartorius was carefully reflected laterally to visualize the VAM. While maintaining the integrity of the VAM, the gracilis, adductor magnus, adductor longus, vastus medialis and rectus femoris were exposed. Next, the VAM was carefully reflected to reveal the underlying neurovascular bundle proximally and the aponeurosis of vastus medialis obliquus (VMOA) distally. The full extent of the dye spread was defined and documented. Next the nerve to vastus medialis (NVM), superior medial genicular nerve (SMGN), saphenous nerve (SN), anterior and posterior division of obturator nerve (ON) and genicular branch of obturator nerve (GBON) were traced to their termination. The course and relationship of each nerve was carefully documented in relation to the adductor canal. This was followed by an assessment of nerve staining in each specimen. Photographs were taken throughout the dissection process. The capture rate of each nerve was quantified as a percentage and analyzed using descriptive statistics.
In all specimens, the dye was found to stain the deep surface of sartorius, VAM, VMOA, adductor canal and posteromedial branch of NVM lying between the fascia and muscle belly of the vastus medialis (figure 2).
Minimal dye spread to the following regions was observed in all specimens: (1) distal femoral triangle (figure 3); (2) posteromedial aspect of vastus medialis muscle (figures 4B and 5), with the exception of one specimen where anterior dye spread was significant (figure 6); and (3) through the adductor hiatus into the superomedial part of the popliteal fossa (figure 7A), with the exception of one specimen where popliteal fossa dye spread was extensive (figure 7B).
The VAM could be visualized in its entirety, deep to sartorius, with the SN piercing it distally close to the adductor magnus tendon (figure 2C). Following removal of VAM, the proximal SN, posteromedial branch of NVM, SMGN and VMOA could be visualized (figure 5C). The SN, proximally, was enveloped with the femoral artery and vein in a thin fascial covering as it coursed through the proximal adductor canal. In the distal adductor canal, the SN first traveled deep to the VMOA then pierced the aponeurosis and VAM, with the saphenous branch of the descending genicular artery, to continue distally along the posteromedial aspect of the knee joint deep to sartorius (figure 5C). The NVM, originating from the femoral nerve, was found to give rise to numerous branches deep to the fascia of vastus medialis muscle (figure 3). The posteromedial branch divides distally at the VMOA into the SMGN and numerous branches to the vastus medialis obliquus (figure 5). At the proximal adductor canal, the posteromedial branch of NVM, including the SMGN, was found to be separated from the contents of the adductor canal by the thin fascia of the vastus medialis which blends with the VAM anteriorly (figure 4B).
Distally, the roof of the adductor canal had two distinct layers (figure 5). The superficial layer was the VAM which was continuous with the fascia of the vastus medialis, adductor longus and adductor magnus muscles. The deep layer was the VMOA, an aponeurosis that attached to the adductor magnus tendon posteriorly and anteriorly provided origin for fiber bundles of the vastus medialis obliquus. The branches to vastus medialis obliquus coursed deep to the VMOA to enter the muscle, whereas the SMGN coursed superficial to the aponeurosis (figure 5C).
The anterior division ON coursed anterior to the adductor brevis and terminated in the subcutaneous fascia of the distal medial thigh without entering the adductor canal (figure 6A,B). In contrast, the posterior division ON descended posterior to the adductor brevis and continued distally along the anterior belly of the adductor magnus. The GBON, the most distal branch of the posterior division ON, emerged from the inferior border of the adductor longus to enter the distal adductor canal (figure 6C). The GBON continued through the adductor hiatus to enter the popliteal fossa prior to terminating in the posterior knee joint capsule.
The SN, posteromedial branch of NVM, SMGN, and GBON were captured in all specimens at the proximal adductor canal. More specifically, the SN was stained in all specimens prior to piercing the VMOA and VAM (figure 5). Anterior branches of NVM were not stained as they entered the vastus medialis muscle belly proximally, in all but one specimen where there was significant intramuscular spreading of dye anteriorly (figure 6). The posteromedial branch of NVM, terminating as SMGN and branches to vastus medialis obliquus, was stained as it coursed along the posteromedial border of vastus medialis separated from the contents of the adductor canal by the thin fascia of the vastus medialis (figure 4B). This thin fascial layer was permeable, allowing dye to spread into the vastus medialis muscle (figures 4B and 5). The GBON, a terminal branch of the posterior division ON, was also stained in all specimens when it entered the distal portion of the adductor canal (figure 6C).
Our current cadaveric study assessed the extent of dye spread following a 10 mL methylene blue injection in the proximal adductor canal. This study was novel as it reported staining of the posteromedial branch of NVM and SMGN, in addition to SN and GBON, using a US-guided proximal adductor canal injection technique. Previous studies have reported staining of SN and GBON using ACB but not the posteromedial branch of NVM and SMGN.12–14 To provide insights regarding possible injection targets, two summary illustrations (transverse sections), one of the anatomy and contents of the proximal adductor canal and the second of the distal adductor canal have been included (figure 8).
The roof of the adductor canal was found to have thin proximal and thick distal portions in the current study consistent with previous literature.15 Moreover, the thick distal portion was composed of two distinct layers (figures 5 and 8). The superficial layer was the VAM which extended from the proximal opening of the adductor canal to the adductor magnus tendon. The deep layer was the VMOA, an aponeurosis which has been reported to originate from the adductor magnus tendon.16 17 This finding would account for the US description of the VAM, at the distal adductor canal, by Wong et al as having a ‘double contour’.9
In the current study, the origin of SMGN is consistent with more recent cadaveric dissection literature.18 19 The SMGN was found to be a distal branch of the posteromedial branch of NVM which originated from the femoral nerve. Therefore, SMGN can be considered a terminal branch of the femoral nerve.18 The posteromedial branches of NVM and SMGN were found to course along the posteromedial border of the vastus medialis. Although the posteromedial branches of NVM and SMGN were located deep to the VAM it was separated from the structures within the proximal adductor canal by the fascia of the vastus medialis (figure 8). Previous literature has discussed the implication of the location of the NVM in relation to the adductor canal and whether an ACB could capture the sensory branches of NVM.9 11–14 20 21 Most recently, in a cadaveric study, Johnston et al concluded that the NVM will be captured by the distal femoral triangle injection and spared with the distal adductor canal injection.13 Similarly, in the current study, the proximal adductor canal injection was found to consistently stain the posteromedial branch of NVM. Thus, both the distal femoral triangle (20 mL) and the proximal adductor canal (10 mL) injections could capture the NVM. However, the proximal adductor canal injection (10 mL) was found to spare the anterior branches of NVM which would likely preserve greater vastus medialis activation, contributing to the quadriceps motor sparing characteristic of the ACB.4 5
Moreover, in the current study using a proximal adductor canal injection, dye injectate was shown to penetrate through the thin fascia of the vastus medialis to stain the posteromedial branches of NVM and SMGN in addition to the SN and GBON. Spreading of injectate through a fascial layer has been previously reported with supporting histological evidence.22 Previous cadaveric studies using a distal adductor canal injection have not reported staining of the posteromedial branch of NVM and/or SMGN.12 13 This suggests the proximal adductor canal injection may have greater analgesic effect by capturing more nerves that supply articular branches to the knee joint capsule as compared with the distal adductor canal injection. This is supported by the findings of a recent randomized controlled trial, where Abdallah et al reported the proximal adductor canal injection was superior in providing postoperative pain relief at 0 and 6 hours’ time points, in addition to having decreased 24 hours’ morphine consumption.23 Conceivably, these analgesic advantages may be a result of the proximal ACB/distal femoral triangle block capturing the posteromedial branch of NVM and SMGN, as reported in the current study.
Clinically, optimization of the area of coverage of peripheral nerve blocks may provide better postoperative knee analgesia. The articular branches and their area of supply of the knee joint capsule have been previously documented in two anatomical studies.18 24 The extensive nerve supply to the knee joint suggests a single block likely will not be adequate to provide complete analgesia for postoperative knee pain.18 19 24 Recent clinical studies have investigated the effectiveness of supplementary blocks in managing postoperative knee pain.25 26 Thobhani et al supplemented FNB and ACB with the infiltration of the interspace between the popliteal artery and capsule of knee (IPACK) block and reported reduced opioid consumption compared with FNB or ACB alone.25 Similarly, Sankineani et al reported significantly better visual analog scale scores in patients who received ACB plus IPACK block as compared with ACB alone.26 The anatomical explanation for these observations may be a result of greater area of capsular coverage.
In the previous literature, Runge et al reported extensive dye injectate spread into the popliteal fossa using a distal ACB.12 Similar results were reported using the IPACK block,27 suggesting supplementation of a distal ACB with an IPACK block may be redundant. However, the proximal adductor canal injection, used in the current study, captured posteromedial branch of NVM and SMGN with minimal spread into the popliteal fossa, with the exception of one specimen. This suggests that using a proximal ACB in combination with an IPACK block may provide greater analgesic effect as a result of larger area of capsular coverage. Other combination blocks involving the femoral nerve or distal femoral triangle blocks, with their potential quadriceps motor blockade, would be less favorable. Further anatomical and clinical investigations are required to confirm this concept.
The current anatomical study has three limitations: (1) sampling bias may be a confounding factor due to the small sample size; (2) dye injectate spread in cadaveric specimens may not replicate in vivo conditions due to differences in fascial dynamics; and (3) nerve staining pattern cannot definitively confirm analgesic effect and will require further clinical evaluation.
In conclusion, this anatomical study suggests the proximal adductor canal injection has a consistent dye spread pattern capturing the posteromedial branch of NVM, SMGN, SN and GBON. This may explain why a proximal adductor canal injection confers greater analgesic effect than a distal injection, while also preserving quadriceps muscle function. Further clinical investigation is required to confirm cadaveric findings.
The authors thank Ian Bell and Benjamin Kozlowski for their valuable technical assistance. We also thank the individuals who donate their bodies and tissue for the advancement of education and research.
Contributors JT, VWSC, PWHP, and AMRA contributed to the experimental design, data acquisition, analysis of data, drafting and revising the manuscript critically for important intellectual content.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests VWSC has received an honorarium from Philips Healthcare. PWHP received equipment support from SonoSite Fujifilm Canada. AMRA is an Anatomy Faculty with the Allergan Academy of Excellence.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement All data relevant to the study are included in the article or uploaded as supplementary information.