Single-Injection Technique for Medial Parapatellar Arthrotomy

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To the Editor:
We read with interest the article by Bendtsen et al,1 which reviews in detail the anatomy and course of nerve afferents relevant for total knee arthroplasty using a medial parapatellar approach. In this article, special mention is given to the subsartorial plexus, which has contributions from the saphenous, anterior obturator, and posterior branch of the medial femoral cutaneous nerves. This plexus is sandwiched superficial to the vastoadductor membrane (VAM) but deep to the deep fascia of the sartorius muscle. In addition, important contributions to anteromedial knee innervation are given via the medial retinacular nerve (terminal branch of the nerve to vastus medialis) and the infrapatellar branch of the saphenous nerve, which exits the adductor canal by perforating the VAM along with the descending genicular artery branch of the femoral artery. These nerves were also shown in cadaver dissection by Burckett-St Laurant2 to compose the predominance of anteromedial knee innervation. Finally, prepatellar plexus innervation is supplied by a combination of the branches of medial, intermediate (anterior), and lateral femoral cutaneous nerves as well as the infrapatellar branch of the saphenous nerve.
Various techniques have been described to target these separate nerves at a distal site and achieve reliable blockade of nociceptive signals to obtain analgesia while still maintaining motor power.3 Multiple injections often require several needle passes that can be uncomfortable for a patient who is awake. Multiple injections also increase the anesthetic time and is resource intensive. Furthermore, identification of smaller femoral cutaneous nerves can be technically difficult and time consuming.
In our institution, we devised a single “hybrid” injection that blocks most of the nerves previously mentioned using a single needle pass. With the operative leg positioned in hip and knee flexion and external hip rotation, the midpoint between anterior superior iliac spine and the base of the patella is measured and marked. A high-frequency linear transducer is used to scan distally from this point, keeping the subsartorial femoral artery in view. The descending genicular artery is located (Fig. 1) and a point 2 cm proximal to this marked on the patient's skin.
A 50-mm needle is passed in an anterolateral-to-posteromedial direction. Five milliliters of a mixture containing ropivacaine 0.5% and epinephrine 1:200,000 is injected beneath the fascia lata but above the superficial fascia of sartorius to target the branches of the intermediate (anterior) femoral cutaneous nerve (Fig. 2). The needle is withdrawn slightly and advanced between the sartorius and vastus medialis muscles. Five milliliters is gently injected to block the nerve to vastus medialis, which lies in this plane. Care is taken to inject slowly and advance the needle between the muscle layers so as to avoid an inadvertent intraneural injection of the nerve to vastus medialis. The needle is again withdrawn slightly and flattened to perforate the sartorius muscle and 5 mL of mixture is subsequently deposited at the deepest point of the sartorius to account for the variability of the anterior branch of the medial femoral cutaneous nerve. The deep sartorius fascia is breeched, and 5 mL is injected above the VAM to block the subsartorial plexus. Finally, the VAM is perforated by the needle tip, and 15 mL of local anesthetic is injected around the femoral artery. The end point of final local anesthetic placement is confirmed by the presence of the anechoic shadow from the injectate being present between the femoral artery and the VAM. A perineural catheter is then fed into this area through the needle and is subsequently tunneled 8 cm proximally to be out of the surgical field.

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