Introduction

Hip surgery is one of the most frequently performed orthopedic surgeries today [1]. Perioperative pain associated with hip surgeries is a significant concern that requires attention as it can lead to complications, morbidity, and reduced overall patient satisfaction [2]. Perioperative pain adversely affects the immediate outcome of surgery and the patient’s long-term prognosis and quality of life. Persistent pain is associated with poor outcomes, with an increased risk of delirium, cognitive impairment, sleep disturbances, and anxiety [3]. In addition, pain interferes with physical rehabilitation. Persistent pain slows recovery, lengthens hospital stays, and increases costs incredibly. It also leads to delayed recruitment with all associated complications, such as thromboembolic symptoms [4].
Among the postoperative complications of hip surgery, the most life-threatening are deep vein thrombosis and pulmonary embolism, which are directly related to a lack of mobility [5]. Therefore, a variety of anesthesia and analgesic procedures are used in THR [6]. These include general anesthesia (GA), patient-controlled anesthesia (PCA), opioid spinal anesthesia, and lumbar epidural anesthesia [7]. Spinal canal anesthesia can also be associated with complications such as spinal hematoma, headache, prolonged hospitalization due to reduced mobility, and delayed mobilization [9]. Pain management usually includes opioids related to various side effects, even when used by the nerve trunk (spinal or epidural) route [10]. Regional nerve blocks like the lumbar plexus block, fascia iliaca compartment block (FICB), femoral nerve block (FNB), obturator nerve block, and sciatic nerve block are also used as a part of multimodal analgesia in hip surgery [12]. FICB and FNB are the current standards for local anesthesia for hip surgery. Both regional blocks provide a femoral nerve block that anesthetizes the femur and causes quadriceps weakness and motor block in the lower extremity, which delays recruitment and discharge [11].
The pericapsular nerve group (PENG) block is a recently reported local anesthetic technique superior to other regional methods, especially in elderly patients, due to its more complete anesthesia of the joint capsule and its motor-sparing effect [13]. The pericapsular nerve group (PENG) block was first described by Girón-Arango et al. [20] in 2018. This block was confirmed by a cadaveric dye study that exhibited pericapsular spread targeting only the sensory branches of the anterior hip capsule with a motor-sparing effect. The PENG block aims at the articular branches of the femoral nerve and obturator nerve [14]. Local anesthetic is administered below the psoas muscle tendon, between the iliopubic eminence and the anterior iliac spine [15]. It causes the motor-sparing effect.

Methods

The literature was reviewed through four electronic databases: PubMed, Cochrane Library, Google Scholar, and Embase. The google scholar search was restricted to the first 200 hundred records. This search was performed in January 2023. We evaluated studies published till the end of January 2023 using the following search terms: the “PENG block” (title), “total hip arthroplasty” (title), “hip surgery” (title), and “hip fracture” (title). The titles, abstracts, and full texts of published studies were screened. We included studies with the following criteria: patients with hip pathologies undergoing surgical procedures for treatment, with the PENG block as intervention and other multimodal analgesic protocols or the placebo as a comparator. In addition, case reports, conference abstracts, and protocols were excluded. T.R. and M.K. holistically assessed article inclusion, with all discordance reviewed for final inclusion by the senior author, M.D. As a result, clinical trials and retrospective studies were included in this review. This entire process is depicted in Fig.1.
Results from the included articles have been summarized as a narrative review to identify the most critical aspects of the known and unknown in this literature.

Results

The initial search yielded 416 articles. Twenty-seven relevant articles were selected based on relevance, recentness, search quality, and citations. Twelve of the studies were related to patients undergoing total hip arthroplasty. Also, 12 studies were related to patients undergoing total hip surgery due to hip fracture. Finally, three studies were related to patients undergoing hip arthroscopy. The results are presented in several tables to facilitate the analysis of the collected material.
Discussion

PENG blocks were initially developed to control pain and analgesia in hip fracture patients [12].

PENG vs. placebo
The ultrasound-guided PENG block improves postoperative pain relief in THR patients without weakening quadriceps muscle strength (Tab. 1). In addition, the PENG block reduces pain scores at rest and movement, reducing analgesic drugs pre- and post surgery [23] and extending the first time to rescue analgesia [27,28]. Research to date suggests that the PENG block should be the primary method of analgesia in Total Hip Arthroplasty due to its potential to impact recovery pathways and contribute to cost-
saving [22].
Pascarella et al. [16] evaluated that the maximum pain score of patients receiving the pericapsular nerve group block was significantly lower than in the control group at all time points. Moreover, the pericapsular nerve group showed a significant reduction in opioid consumption, better hip motion range, and shorter ambulation time. His results suggest that the PENG block improved postoperative functional recovery following total hip arthroplasty.
Also, Chung et al. [28] showed that the PENG block decreased total opioid consumption in the first 24 hours after hip surgery without affecting quadriceps muscle strength.
The PENG block can reduce pain and the need for systemic analgesics in patients with hip fractures in the emergency department and orthopedic ward. In addition, due to the PENG block, mobility in patients with hip fractures is inevitable in situations such as personal needs, undressing for the physical examination, additional imaging needs, and transfer to the operating table [32].
Martin et al. [21] concluded that the PENG block is an effective, safe, regional pain management technique for patients with hip fractures due to metastatic disease. It promotes early mobilization and placement before surgery without pain exacerbation, promoting early mobility and rehabilitation.
The PENG block is a promising modality in the pain management strategy in all hip surgeries [34]. However, hip arthroscopy may be an exception. Patel et al. [26] and Amato et al. [19] demonstrated that a preoperative PENG block did not improve analgesia following arthroscopic hip surgery.

PENG block vs. LIA
In combination with LIA, the PENG block can be considered a valuable part of the multimodal analgesic management of postoperative pain after hip replacement surgery for optimal opioid-sparing strategies and rapid recovery [38,42].
Motor-sparing regional anesthesia modalities, such as local infiltration analgesia (LIA) and the PENG block, have become the mainstay of multimodal approaches used during hip surgery. As seen in table 2, the PENG block provides similar analgesia to LIA.

The PENG block vs the fascia iliaca block
For primary total hip arthroplasty, the pericapsular nerve group block results in the better preservation of the motor function than the fascia iliaca block [34]. Also, as seen in table 3, the PENG block outperformed the fascia iliaca block in providing adequate analgesia before positioning patients undergoing hip surgery under spinal anesthesia[37], which is especially important in patients undergoing hip surgery due to fracture [35].
Aliste et al. [32] showed that the pericapsular nerve group block resulted in a lower incidence of quadriceps motor block at 3 hours (45% vs. 90%; p<0.001) and 6 hours (25% vs. 85%; p<0.001) compared with the fascia iliaca block. Furthermore, the pericapsular nerve group block also provided better preservation of hip adduction at 3 hours (p=0.023) as well as a decreased sensory block of the anterior, lateral, and medial thighs at all measurement intervals (all p≤0.014). No clinically significant intergroup differences were found regarding postoperative pain scores, cumulative opioid consumption at 24 and 48 hours, ability to perform physiotherapy, opioid-related side effects, and length of hospital stay.
Also, Natrajan et al. [39] concluded that the PENG block provides better postoperative analgesia and reduces rescue analgesics requirement in 24 hours compared to the fascia iliaca block in patients undergoing hip surgery. In addition, Zheng et al. [25] received that the PENG block provided lower VAS scores, more extended time of the first analgesic consumption, and lower total dose of morphine consumption compared with FICB.

The PENG block vs. the femoral nerve block
Compared to a femoral nerve block, an ultrasound-guided PENG block provides better postoperative pain relief and less pronounced quadriceps weakness [40], as noted in table 4.
Lin et al. [29] randomized 60 patients and showed that patients receiving the PENG block for intraoperative and postoperative analgesia experience less postoperative pain (p=0.04) with better-preserved quadriceps strength (p=0,004). Also, Allard et al. [30] observed that the PENG block in hip fractures improved the mobility of the operated limb (p=0,001). However, it did not change the total morphine consumption (p=0,458). In addition, Lin et al. [31] evaluated that the PENG block lasted longer than the FNB, resulting in a faster recovery and shorter time to discharge readiness.

The PENG block vs. epidural analgesia
The PENG block is equivalent to epidural analgesia regarding pain scores and opioid consumption (Tab. 5).

The motor-sparing effect of the PENG block
The PENG block has become a prevalent, ultrasound-guided, regional technique as an announced motor-sparing hip block. However, quadriceps weakness after the PENG block was observed, is specified in table 6. The precise mechanism of femoral nerve involvement after the PENG block is fought to result from local anesthetics spread via a plane between the pectineus and psoas major or intramuscularly [43]. To avoid quadriceps weakness after the PENG block, a laterally placed needle tip, away from the undersurface of the iliopsoas tendon, and a reduction in injection volume should be considered [44]. Çiftçi et al. [15], in their cadaveric study, showed that a high volume PENG block might result in motor weakness. For example, 30mL of dye resulted in a more extended spread around the femoral nerve trance from the inguinal to the knee, around the femoral cutaneous nerve and obturator nerve, compared to 20ml of dye. Therefore, clinicians should be aware of motor weakness after performing a high-volume PENG block. In addition, volumes up to 20mL do not appear to cause quadriceps weakness and can be successfully used in hip surgery.

Conclusion

Our review shows that the PENG block can improve pain control and reduce opioid use while retaining mobility and quadriceps strength, which is especially important during the postoperative period and rehabilitation.
However, more evidence is needed to confirm the safety and efficacy of PENG block technology. Therefore, more well-designed studies with larger sample sizes are required.
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