The anterior hip joint is innervated by the femoral, obturator, and accessory obturator nerves and is therefore a focal point for hip analgesia (Fig. 3) [3]. PENG block, introduced in 2018, targets the sensory branches, including the femoral nerve articular branch and accessory obturator nerve [1]. Studies have highlighted the application of PENG block for fracture- or hip surgery-related acute pain [1, 4, 5]. In OA-related chronic hip joint pain, a single PENG block with a long-acting local anesthetic and steroid was initially as effective as conventional intra-articular steroid injections over the short term; however, its effect diminished over the ensuing weeks [2]. Presently, no evidence demonstrates the longer-term efficacy of utilizing short-acting local anesthetics repeat.

Branches of the lumbar plexus that innervate anterior hip capsule. The obturator nerve exhibits a comparatively greater distance from the intended injection site (X). The femoral and accessory obturator nerves pass between the anterior inferior iliac spine (AIIS) and the iliopubic eminence (IPE). aFN, articular branch of the femoral nerve; AIIS, anterior inferior iliac spine, AON, accessory obturator nerve; ON, obturator nerve; IPE, iliopubic eminence

In our case, repeated PENG blocks maintained an analgesic effect. Unlike a single-shot PENG block, which shows reduced analgesic effectiveness after 4 weeks [2], repeated treatments at 2–4 week intervals provided longer-lasting relief. While such repeat blocks may increase medical costs and complications, they are a convenient and safe alternative to intra-articular injections and avoid the risk of septic arthritis. Therefore, repeated PENG blocks could serve as a viable therapy option.

PENG block emerges as a potentially valuable diagnostic and therapeutic tool for patients like ours, in whom severe hip pain persists despite relatively minor joint deformity. Plain X-ray findings do not reliably correlate with pain symptoms in OA [6]. Here, post-fall imaging showed no significant changes, suggesting that the pain likely stems from inflammation resulting from minor soft tissue damage. Inflammatory conditions cause the sensitization at the spinal dorsal horn cell. Speculatively, blocking nerve inputs with a short-acting local anesthetic can reduce hypersensitivity and offer prolonged pain relief for 6 months. Additional studies are required to determine the suitability of PENG block for chronic hip pain.

We initially perceived that using a PENG block would be less likely to cause muscle weakness [1] because it blocks the articular branches (Fig. 3). However, quadriceps muscle weakness arises in 25% of the patients after PENG block [4]. Therefore, we suggested that a high injection pressure or large drug volumes may precipitate muscle weakness [7]. The injection site in the PENG block aligns with the synovial capsule. Synovial capsule rupture because of injection can result in drug dispersion across the fascial surface and between muscles reaching the primary trunk of the femoral or obturator nerve, inducing anesthesia [7]. In the original report [1], 20 mL was employed for the PENG block; even with 10 mL, dispersion into the anterior hip joint capsule was observed [8]. Although 15 mL may be considered excessive, it is possible that a high-pressure or inappropriate injection site may have existed. We chose a short-acting local anesthetic without a steroid and observed that the muscle weakness disappeared after 1 h, only minimally extending the outpatient visit duration. For safety in an outpatient context, appropriate local anesthetic dosage and injection sites must be meticulously considered.

In managing OA-related chronic hip pain, PENG block presents as a promising option for conservative therapy. Ultrasound-guided PENG block offers simplicity within an outpatient clinical setting. However, the potential risk of inducing muscle weakness, emphasizing the need for meticulous deliberation over optimal dosage and administration sites, must be analyzed.