Interspace between the Popliteal Artery and the Capsule of the Knee (IPACK) Block for Anterior Cruciate Ligament Reconstruction Surgery: A Two Case Series

This case series describes the use of Interspace between the Popliteal Artery and the Capsule of the Knee (IPACK) block to provide motor-sparing analgesia for two consecutive patients undergoing anterior cruciate ligament reconstruction (ACLR) by the same surgeon. Case 1 demonstrates the use of a proximal IPACK block as a post-operative rescue block for a patient who still experienced severe pain despite having received a femoral nerve block and parenteral opioids. Case 2 describes the use of a modified IPACK block as part of a multimodal approach with opioid and motor sparing effects. In both cases, the IPACK block provided satisfactory pain relief in the immediate postoperative period without motor weakness, making it an effective analgesic method for day surgery. With the IPACK block shown to be an effective nerve block for ACLR, we explore other advantages, limitations and further research required to better define the role of this block.


Introduction
Anterior cruciate ligament reconstruction (ACLR) surgery is associated with moderate to severe postoperative pain which can be attributed to both arthroscopic surgery and the graft donor site [1]. A multimodal analgesia regime with peripheral nerve blocks (PNB) is therefore recommended for this procedure [2].
There are various choices of PNBs available either as a single injection or in combination. Given the multiple innervations of the knee, complete pain relief A. Huang et al. would not be expected from either the femoral nerve block (FNB) or the saphenous nerve block [3] [4] alone.
The recent technique of ultrasound guided local anesthetic infiltration of the interspace between the popliteal artery and the capsule of posterior knee (IPACK) has shown promising results [3] [5] [6]. The technique involves a very selective block of the terminal sensory branches of the posterior aspect of the knee without the involvement of motor branches of the tibial and peroneal nerves leading to reduced pain without motor weakness [7] [8]. This leads to earlier ambulation, rehabilitation and recovery in various knee surgeries [6].
The IPACK block has evolved from being performed in a prone position where the injection occurs at the popliteal crease at the level of the femoral condyles, to being done with the patient supine. In the supine position, the transducer is placed in the medial lower third aspect of the thigh to observe the femoral artery under the sartorius muscle. The transducer is then slid caudally to trace the artery as it dives into the popliteal fossa through the adductor hiatus to become the popliteal artery [3] [7] [8] [9] [10]. We aim to block the popliteal plexus, saphenous nerve and nerve to vastus medialis with a single injection by redirecting the needle after a single puncture. This avoids multiple injections sites.
We describe our experience with the proximal approach of the IPACK block for ACLR.
These 2 patients were selected as they were under the care of the same surgeon and anesthetist. The IPACK block was offered to reduce pain, opioid consumption and facilitate early rehabilitation and discharge.

Case Presentation
Written informed consent was provided by all patients for inclusion in this report.

Case 1
We present a 29 year old ASA physical status 1 male (186 cm, 110 Kg, BMI 32), with allergy to non-steroidal anti-inflammatory drugs, who underwent a left knee ACLR with hamstring autograft, medial meniscus repair and extra-articular tenodesis. He received oral paracetamol 1 g one hour prior to surgery for preemptive analgesia. A femoral nerve block was performed preoperatively under ultrasound guidance with 15 ml of 0.5% Ropivacaine as per our hospital pathway. He received a general anesthetic (GA) with a supraglottic airway for the surgery. GA was maintained with nitrous oxide and desflurane. Intraoperative analgesia comprised of fentanyl 100 mcg, ketamine 50 mg and oxycodone 10 mg. Surgery duration was 80 minutes. The patient had a pain score of 7/10 over the posterior knee 1 hour after the end of surgery whilst in the post anesthesia care unit (PACU). The IPACK block was hence offered. As the knee was bandaged, we used the proximal approach to avoid the surgical dressing. Under ultrasound guidance using the curvilinear probe, we traced the femoral artery caudally beginning under the sartorius muscle to where it dives deep at the adductor hiatus. A 150 mm 21 gauge needle (Stimuplex, B. Braun) was inserted in an anteromedial-posterolateral di-rection to the space between the popliteal artery and the femoral shaft. The needle was advanced 1 -2 cm lateral to the popliteal artery in close proximity to the femoral shaft. After negative aspiration, 25 ml of 0.5% Ropivacaine was injected as the needle was gradually withdrawn. Ultrasound scan confirmed that the local anesthetic agent did not spread to the sciatic nerve ( Figure 1). His pain score decreased to 0/10 within 5 minutes of the block. Plantar flexion and extension of bilateral feet were tested to be equal and full.

Case 2
A 25 year old ASA physical status 1 male (173 cm, 73 kg, BMI 24) who underwent a left knee ACLR with hamstring autograft and lateral meniscus repair. He received oral paracetamol 1 g and etoricoxib 120 mg 1 hour prior to surgery for preemptive analgesia. He was offered a modified IPACK block and adductor canal block which were performed pre-operatively. The patient was placed in a supine position with the left leg externally rotated and flexed ( Figure 2). The proximal IPACK block was performed as described above with 15 ml of 0.5% Ropivacaine given as the needle was withdrawn (Figure 3(a)). The needle is then redirected to deposit 10 mls of Ropivacaine under the Sartorius muscle (Figure 3(b)). General Anaesthesia with a supraglottic airway device was then induced and maintained with nitrous oxide and desflurane. Intraoperative analgesia consisted of fentanyl 100 mcg and ketamine 25 mg. Surgery duration was 50 minutes. Immediate post-operative pain scores were 0/10 at rest and 1/10 on movement. He did not require any opioids in the post-operative period. Patient was able to elevate his operated leg and perform plantar and dorsiflexion in the post anesthesia care unit (Table 1).

Discussion
Multimodal analgesia consisting of two or more modalities is recommended for ACL reconstruction surgeries to reduce opioid consumption and shorten hospital stay [11] [12] [13] [14]. A repertoire of regional anesthesia options includes epidural [15], lumbar plexus block [16] [17], psoas compartment block [18], femoral nerve block (FNB) [19], adductor canal block [20] and sciatic nerve block There has been a move towards finding a more suitable analgesic option that does not impair motor function such as local infiltration of analgesia (LIA) by surgeons, as first described by Kerr [26]. However, the infiltration is performed blind by the surgeon into the posterior capsule and risks blocking the sciatic and common peroneal nerve (Figure 4). There is a wide variation in content, volume and sites performed by the surgeon with thus variable analgesic effect.
IPACK block is an alternative to provide analgesia to the posterior aspect of the knee with less risk of motor blockade. This is accomplished by blocking the articular branches of the obturator, common peroneal (CPN) and tibial nerves within the popliteal fossa [3] [7] [8]. As this technique is performed under ultrasound guidance, the possibility of blocking the sciatic nerve or CPN is reduced.   The second case may support this theory that the adductor canal and the popliteal area is connected. Tran et al found that the dye injected from a medial IPACK approach tracked into the adductor canal and Runge et al found that dye injected in the adductor canal tracks in the popliteal area [7] [8]. If the nerve to the vastus medialis can be blocked with this same injection, it would avoid having to do a second block at the femoral triangle [27] or adductor canal [6] [11].
In their feasibility study by Runge et al., their needle was directed from the vastus medialis toward the femoral artery as it exits the adductor hiatus with the local anaesthetic deposited near the artery [25]. In our second case, we directed our needle more proximally, under the sartorius muscle, similar to a sub-sartorial adductor canal approach. The patient had good analgesia and opioid sparing effect with no significant motor blockade. This would make the IPACK block an ideal preemptive analgesic technique for day surgery as it can be added to a non-opioid multimodal analgesia pathway [2].