“Hamstrings block” for ACLR with hamstring tendon graft
The optimal regional anesthetic technique for minimising pain after ACLR involves continuous femoral block combined with obturator and sciatic blocks. However, this technique is associated with significant motor block, which interferes with the ability to weight bear, and for ambulatory management, necessitates substantial patient education to minimise the risk of falls. It may also hinder aggressive physiotherapy if this to be initiated during the first few days after surgery. This has led to increased interest in motor block minimising techniques such as the adductor canal block, which can reduce quadriceps weakness. Compared to a proximal femoral block, the adductor canal block is more likely to block only the saphenous nerve, therefore sparing the nerves supplying the quadriceps muscle.
A second largely uninvestigated strategy for minimising motor block after ACLR involves replacing the obturator (innervating gracilis) and sciatic (innervating semitendinosus) blocks with blocks restricted to the gracilis and semitendinosus muscles – the muscles used for tendon graft harvesting. Through sciatic block avoidance, foot drop is minimised, thus eliminating the most inconvenient side effect of the combined femoral/obturator/sciatic block approach.
An ultrasound guided “hamstrings block” is proposed as an alternative to the aforementioned combined femoral, oburator and sciatic blocks. It involves:
2. LA placement on the deep surface of the semitendinosus and gracilis muscles (the most common muscles used for hamstring tendon graft harvesting).
In most patients, a high frequency linear probe will provide optimal imaging although sonograms using a curvilinear probe are illustrated here as they better demonstrate the anatomy. Unless specifically stated, the following description assumes the lateral position with the ipsilateral leg down, the medial thigh facing upwards, the upper hip flexed (thereby exposing the medial thigh), and the operator facing the patients front i.e. standing on the left side of the bed/table for the left leg; right side of bed/table for right leg.
Patient position: lateral or supine.
First, the sartorius muscle and subsartorial (adductor) canal are identified (Fig. 1). This canal contains the femoral vessels, the posterior branch of the obturator nerve, and terminal branches of the femoral nerve – the saphenous (+/- infrapatellar) nerves. The nerves themselves cannot often be sonographically visualised.
Fig. 1. Section of the thigh depicting the hamstrings block (medial side uppermost).
LA placement is deep to the sartorius fascia on both sides of the femoral artery just proximal adductor hiatus (the point where the femoral artery dives deeply) (Fig. 2 and 3). The adductor magnus muscle tendon may be sometimes imaged within the adductor canal. Consider also blocking any prominent subcutaneous nerves imaged superficial to the sartorius muscle as rarely, these may represent the infrapatellar nerve emerging proximally to the adductor hiatus.
Fig. 2. Needle direction for subsartorial (adductor canal) block with the patient in the lateral position.
For this position (ipsilateral medial thigh facing upwards), also consider a posteromedial to anterolateral needle approach while standing behind the patient (rather than in front).
Fig. 3. Sonogram and diagram of sartorius (anterior) and gracilis (posterior) muscles (probe placed on the medial thigh; beam directed medial to lateral).
Fig. 4. Sonogram of the gracilis and semitendinosus muscle relations in the posteromedial thigh (probe placed on the posterior thigh; beam directed anteriorly).
Arrow depicts the sciatic nerve. Pressure on the probe has distorted the anatomy slightly.
Patient position: lateral or prone.
Next, identify the gracilis and the semitendinosus muscles:
Gracilis. The gracilis muscle is a relatively small muscle with a lenticular (lens) shape lying posterior to the sartorius and superficial to the deep hamstring muscles (adductor magnus anteriorly, semimembranosus posterolaterally) (Fig. 3). The gracilis may be followed distally where it becomes a discrete and superficial tendon passing posteromedial to the knee.
Semitendinosus. Identifying semitendinosus begins with identifying the semimembranosus immediately lateral to the posterior part of the gracilis (Fig. 4). If the semimembranosus and semitendinosus appear together, the semitendinosus belly predominates in the upper thigh and the semimembranosus belly in the lower thigh. At the adductor canal level (the block level), the semimembranosus usually predominates. The semitendinosus muscle is located posterolateral to the semimembranosus. With the probe placed on the posterior aspect of the thigh (beam facing directly anterior), the two muscles may not be distinctly distinguishable as the fascial plane between them passes parallel to the ultrasound beam. Moving the muscles through probe manipulation makes the intermuscular fascial plane more distinct. The semitendinosus muscle may also be identified by scanning distally and observing the muscle form its long tendon, or by scanning the tendons posteromedially to the knee joint (gracilis is anterior and semitendinosus posterior) and following the semitendinosus proximally.
For LA placement, first position the ultrasound probe at the adductor canal just proximal to the adductor hiatus (where the femoral artery dives deeply), and identify the muscles. If a subsartorial block has already been placed with the patient in the lateral position using an anterior to posterior needle direction, the needle is simply passed further posteriorly to a position deep to the gracilis muscle. Approx. 6 ml LA is injected into the plane deep to the gracilis muscle (Fig. 5).
Fig. 5. Needle placement for gracilis block.
LA placement for semitendinosus block may be achieved by withdrawing the needle back to a subcutaneous position and redirecting deep to the femoral artery towards semitendinosus, or by moving the percutaneous puncture point posteromedially. Approx. 10 mL LA is placed in the fascial plane either side of the semitendinosus with or without scanning to avoid excessive injectate reaching the sciatic nerve (Fig. 6, 7 and 8). Inadvertant sciatic block is unlikely to occur as significant sciatic blockade requires the needle to have penetrated the sciatic nerve sheath.
Fig. 6. LA placement at the lateral border of semitendinosus.
Fig. 7. LA placement at the medial border of semitendinosus.
Fig. 8. Sonogram of appropriate LA distribution (arrows).
Initial experience with the hamstrings block has been encouraging. It has often been combined with LA infiltration around the knee by the surgeon, but it is unclear if this adds to the analgesia. Please contact Dr Peter Hebbard to report your experience with the block or if you would like to collaborate with a formal study.
Without a conventional inguinal level femoral block, the hamstring block will not provide analgesia as complete as that achieved when using a combined femoral, obturator and sciatic block. This is because the posterior division of the femoral nerve sends branches to the 3 main parts of the quadriceps muscle (particularly vastus lateralis and intermedius) at varying levels proximal to the adductor canal. Each branch gives rise to articular filaments, which together with the saphenous and infrapatellar terminations of the femoral nerve, provide sensory innervation to the knee joint. The clinical relevance of these proximally arising femoral sensory nerves, in terms of mediating pain after knee surgery (compared to the saphenous and infrapatellar nerves), is at present not known.