ORIGINAL ARTICLE

Clinical Effectiveness of the Obturator Externus Muscle Injection in Chronic Pelvic Pain Patients Shin Hyung Kim, MD; Do Hyeong Kim, MD; Duck Mi Yoon, MD, PhD; Kyung Bong Yoon, MD, PhD Department of Anesthesiology and Pain Medicine, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul, Korea

& Abstract Background: Because of its anatomical location and function, the obturator externus (OE) muscle can be a source of pain; however, this muscle is understudied as a possible target for therapeutic intervention in pain practice. In this retrospective observational study, we evaluated the clinical effectiveness of the OE muscle injection with a local anesthetic in chronic pelvic pain patients with suspected OE muscle problems. Methods: Twenty-three patients with localized tenderness on the inferolateral side of the pubic tubercle accompanied by pain in the groin, anteromedial thigh, or hip were studied. After identifying the OE with contrast dye under fluoroscopic guidance, 5 to 8 mL of 0.3% lidocaine was injected. Pain scores were assessed before and after injection; patient satisfaction was also assessed. Results: Mean pain score decreased by 44.7% (6.6  1.8 to 3.5  0.9, P < 0.001) 2 weeks after OE muscle injection as compared with pain score before injection. In addition, 82% of patients (19 of 23 patients) reported excellent or good satisfaction during 2 weeks after injection. No patients reported complications from OE muscle injection. Conclusions: Fluoroscopy-guided injection of the OE muscle with local anesthetic reduced pain scores and led to a high level of satisfaction at short-term follow-up in patients with

Address correspondence and reprint requests to: Kyung Bong Yoon, MD, PhD, Department of Anesthesiology and Pain Medicine, Anesthesia and Pain Research Institute, Yonsei University College of Medicine, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-752, Korea. E-mail: [email protected]. Submitted: July 02, 2013; Revision accepted: September 15, 2013 DOI. 10.1111/papr.12138

© 2013 World Institute of Pain, 1530-7085/15/$15.00 Pain Practice, Volume 15, Issue 1, 2015 40–46

suspected OE muscle problem. The results of this study suggest that OE muscle injection may be a valuable therapeutic option for a select group of chronic pelvic pain patients who present with localized tenderness in the OE muscle that is accompanied by groin, anteromedial thigh, or hip pain. & Key Words: chronic pain, hip joint, injections, intramuscular, myofascial pain syndromes, obturator externus, obturator nerve, pelvic pain

INTRODUCTION Chronic pelvic pain is broadly defined as a noncyclic and localized pain in the pelvis, including the groin, lumbosacral back, buttock, perineal, and hip area.1 Musculoskeletal dysfunction might contribute to the signs and symptoms of chronic pelvic pain. Piriformis and obturator internus muscles are well known as sources of chronic pelvic pain in patients, and therapeutic intervention provides clinical improvements.2,3 The obturator externus (OE) muscle arises from the external bony margin around the medial side of the obturator foramen and the outer surface of the obturator membrane. The muscle fibers converge and pass posterolaterally and upward, ending in a tendon that runs across the back of the femur neck and the lower part of the hip joint and is inserted into the trochanteric fossa of the femur (Figure 1A).4 Despite several anatomical variations of OE muscle, the anterior branch of the obturator nerve ordinarily reaches the thigh by passing in front of this muscle, beneath the pectineus, and the posterior branch

Obturator Externus Muscle Injection  41

A

B

Figure 1. (A) Anatomical location of obturator externus (OE) muscle. (B) Fluoroscopic anteroposterior view of right OE muscle injection with contrast media.

runs over or pierces the OE muscle.5 The obturator vessels lie between the OE muscle and the obturator membrane.5 On the other hand, the OE muscle functionally participates with the piriformis, superior and inferior gamellus, quadratus femoris, and obturator internus to control the external rotation of the hip.6 In addition, the OE muscle acts on hip adduction together with the adductor brevis, adductor longus, adductor magnus, adductor minimus, pectineus, and gracilis.6 Considering its anatomical location and function, we hypothesized that the OE muscle might be a source of pain and that its treatment could lead to clinical improvements. There are, however, no published reports regarding the OE as a possible target for therapeutic intervention. The aim of this study is to evaluate the clinical effectiveness of the OE muscle injection with a local anesthetic in chronic pelvic pain patients with suspected OE muscle problems.

METHODS Patients This study is a retrospective observational audit of patients who received OE muscle injection for pain treatment. We reviewed the clinical records and interviewed the patients individually at a follow-up visit or via telephone. The study protocol was approved by the institutional review board, and written informed consent for the procedure was obtained from all patients on an outpatient basis. The study population was defined as chronic pain patients with groin, anteromedial thigh, or hip pain who had localized tenderness on the inferolateral side of the pubic tubercle corresponding to the

obturator foramen on physical examination. Patients with general contraindications for fluoroscopy-guided injection, such as pregnancy, contrast allergy, and coagulopathy, were excluded. Procedure The patients were placed in supine position with a slight external rotation of the hip and were prepared with sterile draping over the inguinal and pubic area. All OE muscle injections were performed by 1 pain physician (K.B.Y.). Fluoroscopy was used to visualize the obturator foramen. After identifying the femoral artery with palpation, to avoid inadvertent puncture or laceration of the femoral vessel, the skin and subcutaneous tissues at the site of needle entry were anesthetized with 1% lidocaine. With an aseptic technique and anteroposterior approach, a 10 cm, 22-gauge block needle was advanced toward the inferolateral side of obturator foramen, according to the tunnel view, until the needle contacted the bone (the ramus of ischium). The needle was then slightly withdrawn from the bone and 1 mL of radio-opaque dye was injected to identify the OE muscle after negative aspiration of blood. If the needle was correctly placed in the OE muscle, the dye spread from the obturator foramen to the greater trochanter in a horizontal or slightly upward direction (Figure 1B). In contrast to the OE muscle, other neighboring muscles such as pectineus, adductor brevis, and adductor longus —which originate in the pubic ramus, and cover the anterior obturator foramen—run downwards and laterally toward the femur shaft. After confirmation of correct needle placement and negative aspiration, 5 to 8 mL of saline-diluted 0.3% lidocaine was injected

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according to the spreading contour of the previously injected contrast dye. Assessment Clinical and demographic data were collected for analysis, including age, gender, location of pain, duration of pain, tender points, walking problems, and previous pain intervention history. Before and 2 weeks after OE muscle injection, patients were asked to rate their pain score using a 10-point numeric rating scale (NRS; 0 = no pain, 10 = worst possible pain). We also assessed the degree of patient satisfaction as excellent, good, fair, or bad 2 weeks after injection. In this period, no analgesic medications were changed for any patients. Complications associated with OE muscle injection, such as hematoma formation, nerve injury, or infection, were investigated. We also observed the presence of sensory change in the groin and medial thigh and monitored difficulty in walking or hip adduction immediately after OE muscle injection. Statistical analysis Continuous data are reported as mean and standard deviation (SD) unless otherwise indicated. Normality of data distribution was assessed using the Shapiro–Wilk test. Categorical data are reported as both the number of patients and percentage. Statistical analyses were performed with SPSS statistical software, version 18.0 (SPSS Inc., Chicago, IL, USA). A paired t-test was used for comparing mean pain scores before and 2 weeks after OE muscle injection. A P value < 0.05 was considered statistically significant.

RESULTS Twenty-three consecutive patients who were treated with OE muscle injection were enrolled in this study (11, 8, and 4 of the right, left, and both sides of the OE muscle were injected, respectively). Patient characteristics and pain-related data are shown in Table 1. Five patients previously underwent spine surgery, 1 patient was referred by a spine surgeon because of suspected piriformis syndrome accompanying dysplasia of the hip, 2 patients were referred by an urologist because of chronic perineal pain, 3 patients complained pain developed after a traffic accident, and 2 patients used a wheelchair because of difficulty with walking. Approximately 50% of patients (11 of 23 patients) reported

Table 1. Patients’ Characteristics and Pain Profile Gender (M/F) Age, years Pain duration, months Pain sites, n (%) Groin Anteromedial thigh Perineal Buttock Low lumbar Hip Knee Patients with walking problems, n (%) Previous pain intervention, n (%) Piriformis injection Obturator internus injection Sacroiliac joint injection Epidural block Trigger point injection Other procedures

7/16 48.0  14.5 (23 to 75) 16.1  14.6 (1 month to 5 years) 13 (57) 10 (43) 3 (13) 13 (57) 11 (48) 14 (61) 7 (30) 11 (48) 13 (57) 5 (22) 9 (39) 5 (22) 8 (35) 6 (26)

Values are means  SD (range) or number of patients (%).

various problems with walking (hip pain, discomfort sense, cracking of hip, or limping). Patients enrolled this study previously experienced a variety of interventional procedures with average of 2.8 times. Trigger points injected in the gluteus medius muscle (n = 6), iliacus muscle (n = 2), thigh adductor muscles (n = 4), and pes anserinus (n = 1) were noted as trigger-point injection sites. In addition, some patients previously were treated with psoas compartment block (n = 4), ischial tuberosity bursa injection (n = 2), lumbar sympathetic block (n = 1), lumbar medial branch block (n = 1), caudal block (n = 1), or pudendal nerve block (n = 1). Table 2 shows the change in pain scores before and after OE muscle injection treatment, and the degree of patient satisfaction 2 weeks after injection. Mean pain score decreased by 44.7% (6.6  1.8 to 3.5  0.9, P < 0.001) when compared before and 2 weeks after OE muscle injection. In addition, 82% of patients (19 of 23 patients) reported excellent or good satisfaction during 2 weeks after injection. No patient exhibited complications such as hematoma formation, nerve injury, or infection after OE muscle injection. In addition, no patients reported sensory changes in the groin or medial thigh area, nor was difficulty in walking or hip adduction after OE muscle injection observed.

DISCUSSION This study reports a therapeutic approach for treating OE muscle–generated pain. We demonstrated that fluoroscopy-guided OE muscle injection with local anesthetic led to reduced pain score and to a high level

Obturator Externus Muscle Injection  43

Table 2. Pain Scores before and 2 weeks after Obturator Externus Muscle Injection and the Degree of Patient Satisfaction 2 weeks after Injection Pre-injection pain score, 0 to 10 NRS Post injection pain score, 0 to 10 NRS Patient satisfaction, n (%) Excellent Good Fair Bad

6.6  1.8 (3 to 10) 3.5  0.9 (2 to 5)* 7 12 4 0

(30) (52) (18) (0)

*P < 0.001, Mean pain score decreased by 44.7% when compared before and 2 weeks after obturator externus muscle injection. Values are means  SD (range) or number of patients (%). NRS, numeric rating scale.

of satisfaction at short-term follow-up of chronic pelvic pain patients with suspected OE muscle problems. In the present study, a clue that led us to believe the OE muscle might be involved or responsible for the localized pain was the tenderness noted upon the application of deep pressure toward the obturator foramen. One of the possible causes of tenderness in this localized area is myofascial pain syndrome affecting the OE muscle (Figure 2). Travell and Simons7 briefly mentioned that the OE muscle could become a pain generator similar to the pathophysiology of pirifomis syndrome, although there is no clear evidence. Biomechanically, the OE muscle is more vulnerable to overstretching compared to the piriformis or obturator internus muscles in string models.4 This result implies repetitive microtrauma by overuse injury, bad posture, or unnatural gait may give rise to trigger points on the OE muscle. With regard to a neurogenic mechanism, because the OE muscle—unlike other muscles located in the pelvic area—is innervated by spinal nerves of the L2, L3, and L4, the anatomical convergence of sensory

information in the spinal cord could easily generate a trigger point in the OE muscle and affect its sensitivity in patients with a primary pathology in the lumbar spine.8,9 Travell and Simons suggested that the tenderness arising from trigger points in the OE muscle is best located by applying deep pressure in between the pectineus and adductor brevis in the groin, thereby exerting pressure on the muscle against the external surface of the obturator membrane.7 Based on this method, we examined OE muscle tenderness by applying deep pressure toward medial and cephalad directions on the inferolateral side of the palpable pubic tubercle, corresponding to the obturator foramen area. In our clinical experience, however, a precise palpation of the trigger point in OE muscle is very difficult because the muscle is located deep beneath several superficial muscles in the groin, and muscle depth in each patient varies widely. For similar reasons, the typical referred-pain pattern and local twitch-response elicited by snapping or injection could not be checked clearly in our study population. Therefore, pathological conditions of OE muscle (presenting as localized tenderness) may not be fully explained by myofascial pain syndrome alone. Another probable cause for localized tenderness of OE muscle is the irritation or entrapment of the obturator nerve (Figure 2). The obturator nerve enters the thigh through the obturator foramen and divides into an anterior and a posterior branch.10 These branches are either separated by some fibers of OE muscle, or run over the OE muscle.10 The OE muscle is known to be an entrapment site of the obturator nerve in proximal medial thigh region.11 Considering the close anatomical relationship between the OE muscle and the

Pathologic conditions

Pain presentations

Myofascial trigger point Spasm

Obturator externus

Obturator nerve

Inflammation Nerve irritation

Groin pain Thigh pain

Nerve compression

Hip movement - external rotation - adduction

Hip destabilization

Sacroiliac joint Piriformis

Hip pain

Low lumbar pain Buttock pain

Obturator internus

Figure 2. The obturator externus muscle as a possible pain generator in chronic pelvic pain: a proposed explanation.

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obturator nerve, irritation or compression of obturator nerve can result from the inflamed, spastic, or stretched OE muscle. Because of this, thigh pain or knee pain, not only groin pain, may present as associated symptoms (Figure 3). Injection into the pathologically changed OE muscle may have beneficial effects in terms of decreasing spasm of the muscle, modulating neurotransmitters, and, consequently, breaking the cycle of pain. In this context, we used relatively large volumes of lidocaine for spreading local anesthetic to a larger area of OE muscle belly, not to a targeted focal point. In piriformis syndrome, 5 to 10 mL dilute local anesthetic is injected into the belly of the piriformis muscle as part of perisciatic injection.12,13 We chose a somewhat smaller volume of lidocaine for OE muscle injection than that used in the perisciatic injection technique, because the size of the OE muscle is a little smaller than the piriformis muscle.4 After OE muscle injection, we looked for the presence of unintended obturator nerve block due to leakage of local anesthetic onto the obturator nerve. However, no patients had obturator nerve block–related symptoms after OE muscle injection. Due to inconsistency and complexity of the obturator nerve and its branches in inguinal and thigh region,10,14 we cannot completely exclude the possibility of partial obturator nerve block and its effect on pain relief on groin and medial thigh. Nonetheless, clinical improvements in this study cannot be fully explained by

the obturator nerve block effect following OE muscle injection. Thus, pain relief after OE muscle injection likely results as a form of stabilization of the muscle and the affected nerve. In fact, most patients receiving OE muscle injection showed reduced tenderness at the OE muscle area, and typical jump sign disappeared 2 weeks after injection. Furthermore, an overall high level of patient satisfaction including functional improvements in our study cannot be explained by a transient anesthetic effect after obturator nerve block. An interesting observation in our study was a noticeable relief of hip pain and an increase in walking comfort subsequent to OE muscle injection in patients with hip problems as compared with that observed in their previous modes of injection treatment. The OE muscle aids in both external rotation and adduction of hip movement.6 Considering its action, chronic muscle strain or injury triggered by repetitive activities and cumulative overwork can lead to OE muscle dysfunction. In addition, isolated OE problems such as pathological involvement of OE bursa by intra-articular hip disease,15 abscess,16 and impingement17 were previously reported as a primary cause of hip pain. On the other hand, anatomical structures supporting the hip joint and pelvis are in close anatomic proximity, and they work as a functional unit. In this functional unit, the OE muscle participates as a stabilizer of the hip as it inserts into the greater trochanter along with the piriformis and the

Figure 3. Patterns of pain referred from the obturator externus muscle. X = tender point.

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obturator internus muscle.4 Thus, OE muscle may affect or be affected by hip destabilization accompanying low lumbar and buttock pain caused by lumbosacral spine, sacroiliac joint, piriformis, or obturator internus problems (Figure 2). Collectively, it seems possible that the OE muscle can be a primary or secondary cause of hip pain and closely associated with chronic pelvic pain caused by musculoskeletal dysfunction (Figure 3). We used fluoroscopy and contrast to help us identify the OE muscle. A recent study regarding ultrasoundguided obturator nerve block may help to identify the OE muscle.18 Ultrasonography is useful for detecting local twitch responses for myofascial trigger points, especially in deeply located muscles.19 Furthermore, real-time ultrasound imaging may help in the determination of a proper volume of injectate. We used only a local anesthetic for treating OE muscle. The evaluation of other drugs use such as steroids or botulinum toxin for OE muscle injection may prove useful in the future. Our study has several limitations that suggest directions for future research. The number of patients was small and there was no control group. Also, we did not conduct long-term follow-up. We cannot suggest specific diagnostic methods that identify OE muscle problems other than typical tenderness and related clinical manifestations. In the future, similar to that in piriformis syndrome, electromyography20 and imaging modalities such as computed tomography and magnetic resonance imaging21 may help identify pathology of the OE muscle and problems associated with it. Despite these considerations, our study will hopefully provide impetus for investigating the pathophysiology, diagnosis, and treatment of OE muscle–related problems. We demonstrated clinical improvements in patients with chronic pelvic pain after OE muscle injection. Fluoroscopy-guided injection of the OE muscle with local anesthetic reduced patients’ pain scores and led to a high level of satisfaction at short-term follow-up in patients with suspected OE muscle-generated pain. The results of this study suggest OE muscle injection is a valuable therapeutic option for a select group of chronic pelvic pain patients who present with localized tenderness in the OE muscle that is accompanied by groin, anteromedial thigh, or hip pain.

ACKNOWLEDGEMENTS We are grateful to Dong-Su Jang, Medical Illustrator, for his help with the figures in our article. There was no external funding in the preparation of this article.

CONFLICT OF INTEREST None.

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15. Robinson P, White LM, Agur A, Wunder J, Bell RS. Obturator externus bursa: anatomic origin and MR imaging features of pathologic involvement. Radiology. 2003;228: 230–234. 16. Fowler T, Strote J. Isolated obturator externus muscle abscess presenting as hip pain. J Emerg Med. 2006;30:137–139. 17. Muller M, Dewey M, Springer I, Perka C, Tohtz S. Relationship between cup position and obturator externus muscle in total hip arthroplasty. J Orthop Surg Res. 2010;5:44. 18. Taha AM. Brief reports: ultrasound-guided obturator nerve block: a proximal interfascial technique. Anesth Analg. 2012;114:236–239.

19. Rha DW, Shin JC, Kim YK, Jung JH, Kim YU, Lee SC. Detecting local twitch responses of myofascial trigger points in the lower-back muscles using ultrasonography. Arch Phys Med Rehabil. 2011;92:1576–1580. 20. Fishman LM, Zybert PA. Electrophysiologic evidence of piriformis syndrome. Arch Phys Med Rehabil. 1992;73:359–364. 21. Jankiewicz JJ, Hennrikus WL, Houkom JA. The appearance of the piriformis muscle syndrome in computed tomography and magnetic resonance imaging. A case report and review of the literature. Clin Orthop Relat Res. 1991; 262:205–209.

Clinical effectiveness of the obturator externus muscle injection in chronic pelvic pain patients.

Because of its anatomical location and function, the obturator externus (OE) muscle can be a source of pain; however, this muscle is understudied as a...
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