Obturator Externus Bursa: Anatomic Origin and MR Imaging Features of Pathologic Involvement 1

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1 Musculoskeletal Imaging Radiology Philip Robinson, MB, ChB 2 Lawrence M. White, MD Anne Agur, MD Jay Wunder, MD Robert S. Bell, MD Index terms: Hip, abnormalities, Hip, anatomy, Hip, arthrography, Hip, diseases Hip, MR, , Published online before print /radiol Radiology 2003; 228: Abbreviation: SE spin echo 1 From the Department of Medical Imaging, Mount Sinai Hospital and the University Health Network, 600 University Ave, Toronto, Ontario, Canada M5G 1X5 (P.R., L.M.W.); and the Departments of Clinical Anatomy (A.A.) and Orthopedic Oncology, Mount Sinai Hospital, University of Toronto, Canada (J.W., R.S.B.). From the 2001 RSNA scientific assembly. Received July 8, 2002; revision requested August 14; revision received October 21; accepted November 22. Address correspondence to L.M.W. ( lwhite@mtsinai.on.ca). Current address: 2 Department of Radiology, St James University Hospital, Leeds, England. Obturator Externus Bursa: Anatomic Origin and MR Imaging Features of Pathologic Involvement 1 PURPOSE: To investigate the anatomy of the obturator externus bursa of the hip and describe the magnetic resonance (MR) imaging features of pathologic involvement. MATERIALS AND METHODS: The authors dissected eight cadaver hemipelvises to assess for the presence of periarticular bursae of the hip and bursal communication with the fascial plane of the obturator externus muscle. In addition, 10 consecutive patients with obturator externus bursa enlargement were prospectively identified with MR imaging. A bursa was considered present when a fluid collection was seen extending along the obturator externus muscle, continuous with the posterior inferior hip joint. The direction, extent, contour, and thickness of the bursa and hip capsule were recorded. Surgical findings were available for eight of the 10 patients, with histopathologic correlation between the bursal lining and hip capsule. RESULTS: At cadaveric dissection, one specimen showed a bursa communicating with and extending from the posteroinferior aspect of the hip joint deep to the obturator externus tendon. In all 10 patients, MR images showed a hip joint effusion with a continuous bursa extending medially and displacing the obturator externus inferiorly. At surgery, a bursa was seen displacing the obturator externus muscle inferiorly and originating from the posteroinferior aspect of the hip joint in all eight patients. Results of pathologic analysis confirmed disease identical to the primary hip abnormality in all eight patients. CONCLUSION: The obturator externus bursa is a potential posteroinferior communication of the hip joint capsule, can be a site of disease spread from the hip joint, and can be accurately identified with MR imaging. RSNA, 2003 Author contributions: Guarantors of integrity of entire study, L.M.W., P.R.; study concepts, L.M.W., P.R.; study design, L.M.W., P.R.; literature research, P.R.; clinical studies, L.M.W., P.R.; experimental studies, P.R., A.A.; data acquisition, P.R.; data analysis/interpretation, L.M.W., P.R.; statistical analysis, L.M.W., P.R.; manuscript preparation, P.R.; manuscript definition of intellectual content, all authors; manuscript editing, L.M.W., P.R.; manuscript revision/review, L.M.W., P.R., A.A.; manuscript final version approval, all authors RSNA, 2003 A bursa is a sac lined with synovial cells that typically forms at an area of tendon friction. Although most bursae are enclosed, it is well recognized that intraarticular communication can occur with periarticular bursae. Although pathologic involvement of the iliopsoas bursa by intraarticular hip disease has been extensively described (1 5), other bursae, such as the obturator externus bursa, have not been fully investigated. The obturator externus bursa has been described in anatomy textbooks as a potential bursa between the tendon of the obturator externus muscle and the posterior hip capsule (6 10). This bursa is thought to be formed by a protrusion of the posterior hip synovium between the posterior femoral capsule (ischiofemoral capsular thickening) and the zona orbicularis (Fig 1) (6). The bursa, therefore, lies between the ischiofemoral capsular ligaments and the tendon of the obturator externus muscle as it spirals, posterior to the femoral neck, toward its insertion into the trochanteric fossa (6 10). To our knowledge, no radiologic or clinical study has been performed to investigate the anatomy of this bursa or its pathologic involvement. The purpose of this study was to investigate the anatomy of the obturator externus bursa of the hip and describe the features of pathologic involvement depicted on magnetic resonance (MR) images. 230

2 MATERIALS AND METHODS Cadaver Study We dissected eight embalmed human cadaver hemipelvises to evaluate for the presence of an obturator externus bursa and locate possible bursal communication with the hip joint involving the fascial plane of the obturator externus muscle. All dissections were performed together by two authors (P.R., A.A.), who were from the division of anatomy of the University of Toronto. The authors followed institutional ethical guidelines. Specimens from cadavers that had previously undergone hip surgery were not included in this study. The average age of the individuals at the time of death was 80 years (range, years). The muscles around the hip joint were sequentially removed to expose the obturator externus muscle and tendon. The tendon of the obturator externus muscle was then carefully reflected from its insertion to document the presence and location of an obturator externus bursa. If an obturator externus bursa was found, dissection was performed to determine if there was a periarticular communication between the bursa and hip joint. Further dissection was performed to assess the hip joint space for degeneration. Arthrography was performed in two additional cadaver hemipelvises by one author (P.R.) after the administration of contrast material (diatrizoate meglumine 60%, Hypaque; Nycomed Amersham, Princeton, NJ). A 20-gauge needle was inserted into the hip joint by using an anterior approach and fluoroscopic guidance. After the intraarticular position was confirmed by using the iodinated contrast material (diatrizoate meglumine 60%), the joint was slowly distended with diatrizoate meglumine 60% and normal saline until resistance was met or extravasation occurred (22- and 19-mL volumes were injected in the two specimens). Anteroposterior and lateral radiographs were obtained during and after completion of injection. Clinical Study Between 1997 and 2000, 10 consecutive patients with obturator externus bursa enlargement associated with an intraarticular hip abnormality were prospectively identified with MR imaging. All 10 patients were referred for MR imaging by experienced orthopedic surgeons (J.W., R.S.B.) for investigation of hip pain and dysfunction. The patients had consented to be included in the musculoskeletal database. Institutional ethical board approval was obtained for the prospective and retrospective analyses of the clinical details. There were five men and five women, with a median age of 45.5 years (range, years). All MR images were obtained with a 1.5-T unit (Signa; GE Medical Systems, Milwaukee, Wis) and a quadrature extremity coil (Medical Advances, Milwaukee, Wis). Transverse and coronal T1- weighted conventional spin-echo (SE) MR images (repetition time msec/echo time msec, 583/8) and transverse, coronal, and sagittal T2-weighted fat-suppressed fast SE images (4,166/104, echo train length, eight) were obtained through the pelvis. All MR images that were prospectively identified as depicting an obturator externus bursa were retrospectively assessed by two experienced musculoskeletal radiologists (L.M.W., P.R.); decisions were reached by consensus. An obturator externus bursa was diagnosed if a collection with the same signal intensity as that of fluid (low signal intensity at T1-weighted imaging, high signal intensity at T2- weighted imaging) was seen to extend along the superior margin of the obturator externus muscle, which was contiguous laterally to the hip joint. In all cases, the posterior and inferior aspects of the hip joint were assessed for continuity of fluid extension from the hip capsule into the bursal collection along the tendon of the obturator externus muscle. The direction and extent of the bursa and any displacement of the obturator externus muscle were noted. The contours of the bursal margin and hip capsule were classified as normal or thickened (consistent with capsular disease or synovitis). Surgical correlation was available for eight of the 10 patients (mean time from MR imaging to surgery, 41 days; range, 2 58 days). Surgical findings were obtained from the surgical notes and discussions with the surgeons (J.W., R.S.B.). The surgeons prospectively recorded the presence or absence of periarticular bursal collections at surgery, and all detected bursae were resected and sent for pathologic analysis for determination of the presence of disease involvement and synovitis. Pathologic analysis consisted of routine gross pathologic examination and histopathologic analysis of the resected specimens with hematoxylin-eosin stain. The hip capsule was analyzed in a similar manner for a comparison between the pathologic findings of the hip joint and those of the obturator externus bursa. Figure 1. Illustration of a dissected hip joint (posterior view) shows hip synovium (curved arrows) protruding from the margin of the ischiofemoral capsular ligament (straight arrow). (Reprinted, with permission, from reference 11.) Figure 2. Photograph of a dissected cadaver hip. Posterior inferior view with the tendon ( ) of the obturator externus muscle elevated shows the communication of the bursa (arrow) with the hip joint. RESULTS Cadaver Study At dissection, one hemipelvis with a small (9 6-mm) posterior bursa was seen communicating with and extending from the posterior and inferior aspects of the hip joint. Communication with the hip joint occurred between the area of ischiofemoral capsular thickening and the zona orbicularis (Fig 2). The bursa was deep to the tendon of the obturator externus muscle, proximal to its insertion into the trochanteric fossa (Fig 2). No bursae were identified in the remain- Volume 228 Number 1 Anatomic Origin and MR Imaging of Obturator Externus Bursa 231

3 Figure 3. T2-weighted fat-suppressed fast SE MR images (4,166/104) obtained in a patient with synovial osteochondromatosis of the left hip. (a) Coronal MR image shows a hip effusion with fluid and capsular synovitis (arrows) that extends medially and inferiorly and displaces the obturator externus muscle ( ) inferiorly. (b) Transverse MR image shows the bursa ( ) extending along the plane of the obturator externus muscle and separate from the inferior hip capsule (arrows). ing seven cadaver specimens. No evidence of severe hip joint degeneration was found in any of the cadavers. Arthrographic injection did not produce distention of an obturator externus bursa, and anterior extravasation occurred in both cases. Clinical Study In each of the 10 patients, MR images showed the presence of a hip joint effusion with a fluid collection (ie, bursa) extending from the posterior and inferior aspects of the hip joint, separate from the inferior recess of the hip capsule (Figs 3, 4). In all 10 patients, the bursa displaced the myotendinous and muscular components of the obturator externus muscle inferiorly while extending medially toward the obturator foramen (Figs 3 5). In all 10 patients, this bursa could be followed on contiguous MR images to communicate with the hip joint through the posterior capsule deep to the obturator externus tendon (Fig 5). The contour of the bursal lining was similar to that of the hip capsule in all patients. Irregular thickening was seen in eight patients and was due to synovial osteochondromatosis in one patient, pigmented villonodular synovitis in two patients, osteoarthritis in three patients, intraarticular myxoma in one patient, and tuberculous synovitis in one patient. The bursal lining had a smooth contour and normal thickness in two patients one with seronegative arthritis and one with avascular necrosis. A bursa extending along the superior margin of the obturator externus muscle was confirmed in all eight patients who underwent surgery. The surgical procedures performed in these eight patients were synovectomy (n 4), prosthetic hip replacement (Charnley type) (n 3), and radical resection with prosthetic hip replacement (Kotz type) (n 1). At surgery, the bursa was found to be displacing the obturator externus muscle inferiorly and originating from the posterior aspect of the hip joint. Each bursa extended medially, where it was anatomically delineated as a bursa. This finding correlated with the extent of the bursa identified at preoperative MR imaging (Fig 3). Gross pathologic and histopathologic analyses of the surgically resected bursae (n 8) and the hip joint capsules that were sampled at biopsy (n 3) and/or resected (n 5) helped confirm the presence of bursal disease identical to the primary hip abnormality in all patients. The primary hip abnormality was synovial osteochondromatosis in one patient (Fig 3), pigmented villonodular synovitis in two patients, synovitis in osteoarthritis in three patients (Fig 4), intraarticular myxoma in one patient, and tuberculous synovitis in one patient. The diagnosis of hip disease in the two patients who underwent nonsurgical treatment was confirmed by means of serologic-clinical (seronegative arthritis) and radiologic (avascular necrosis) examinations (Fig 5). DISCUSSION To our knowledge, this is the first study to describe pathologic involvement of the obturator externus bursa, its appearance at MR imaging, and its origin from a potential communication of the posterior and inferior aspects of the hip joint capsule. Pathologic involvement of the iliopsoas bursa has been extensively reported (1 5). In 15% of cases, the iliopsoas bursa communicates with the hip joint; however, the frequency increases when an intraarticular abnormality is present (5). This is thought to be due to a potential capsular defect between the iliofemoral and pubofemoral capsular ligaments of the anterior hip joint (5). The development of an obturator externus bursa seems to involve a similar process; we found it to originate from the margin of the ischiofemoral capsular ligaments. In a previous article, Guerra et al (12) described a potential weakness of the inferior hip capsule adjacent to the plane of the obturator externus muscle. In their study, they injected six thawed cadaver hips with methylmethacrylate and two with iodinated contrast material. Although only one hip (injected with methylmethacrylate) showed inferior extravasation, the authors believed that the area of hip joint weakness involved 232 Radiology July 2003 Robinson et al

4 Figure 4. (a) Conventional coronal T1-weighted SE (583/8) and (b) T2-weighted fat-suppressed fast SE (4,166/104) MR images obtained in a patient with osteoarthritis of the right hip show a small obturator externus bursa with fluid and synovitis. The bursa extends medially (solid arrows) and indents the obturator externus muscle (open arrow). (c) Transverse T2-weighted fat-suppressed fast SE MR image (4,166/104) shows the bursa (large arrow) extending along the plane of the obturator externus muscle and separate from the inferior hip capsule (small arrows). the iliofemoral and pubofemoral ligaments. The extent of extravasation reported in that article did not entirely follow the pattern that we identified, because no medial extravasation was described. In addition, we believe that the origin of the bursa is more posterior at the margin of the ischiofemoral ligament. Moss et al (13) injected two cadaver hips with increasing volumes of saline (maximum, 20 ml). Subsequent MR imaging of these specimens did not depict a bursa in the plane of the obturator externus muscle. Lequesne et al (14) reviewed maximal arthrographic distention of 13 hips with synovial osteochondromatosis. The maximum volume injected was 20 ml (limited by patient pain), and the authors did not describe the formation of a bursa adjacent to the obturator externus muscle. In a review of 158 hip arthrograms, Steinbach et al (1) found that 9% of examinations revealed periarticular bursae and abscesses. Again, a bursa deep to the obturator externus muscle was not described. In that article, it was hypothesized that periarticular bursae became pathologically involved only when intraarticular pressure was chronically increased and synovitis of the hip joint occurred (1). The results obtained in our study patients concur with this hypothesis and may help explain why this bursa is difficult to reproduce experimentally when the hip joint is acutely distended (1,13,14). While reviewing the literature, we found an interesting case report that described synovial osteochondromatosis of the hip with periarticular extension (15). The surgical finding in that case was that of a bursal sac in continuity with the hip joint (15). The radiograph presented in that case seemed to show disease extending medially and inferiorly from the hip joint. No other images were presented, and no mention of the relationship between the bursal sac and the obturator externus muscle was made in the surgical report. The abnormality was presumed to be an iliopsoas bursa. Limitations of our study include the fact that the cadavers studied were chemically preserved and not frozen. However, as already mentioned, obturator externus extension could not be reproduced in previous injection studies of fresh cadavers and live patients (1,13,14). Our patients were referred with hip pain to specialist orthopedic oncologic and joint replacement surgeons. Therefore, the true prevalence of bursal involvement cannot be accurately estimated from our Volume 228 Number 1 Anatomic Origin and MR Imaging of Obturator Externus Bursa 233

5 Figure 5. Sequential sagittal T2-weighted fat-suppressed fast SE MR images (4,166/104) obtained in a patient with avascular necrosis of the femoral head show a hip effusion with obturator externus bursal extension. (a) A posterior communication (arrows) with the hip joint is seen. (b d) Sequential images extending medially show the bursa (arrow) at different positions along the obturator externus muscle. series, and no patients with acute hip abnormalities were evaluated. The determination of the prevalence of bursal involvement, however, was not an aim of this study: We wished to describe pathologic involvement of the bursa and increase awareness among radiologists and other clinicians. It could also be argued that the obturator externus bursa is a recess of the hip capsule rather than a true bursa. In classic anatomy texts, this capsular extension is described as a bursa (6 10); however, we imaged only those patients with an underlying hip abnormality and an associated effusion. The terminology used to describe periarticular fluid collections connecting to an underlying joint is somewhat confusing, given the associated controversy regarding their exact origin (16). Although it has been hypothesized that these collections are cysts or localized capsular ruptures, many are thought to represent distended bursae (16). Herein, we use the term bursa because we believe our study findings were due to a chronic increase in the intraarticular pressure of the hip that causes decompression into the adjacent obturator externus bursa. In conclusion, we found that the obturator externus bursa is a potential posteroinferior communication of the hip joint capsule, can be a site of disease spread from the hip joint, and can be accurately identified with MR imaging. Recognition of the bursa as being contiguous with the hip joint and familiarity with the pattern of bursal distention are important in the radiologic evaluation of the extracapsular extent of hip abnormality to determine disease extent and possible surgical planning. References 1. Steinbach LS, Schneider R, Goldman AB, Kazam E, Ranawat CS, Ghelman B. Bursae and abscess cavities communicating with the hip. Radiology 1985; 156: Pritchard RS, Shah HR, Nelson CL, Fitz- Randolph RL. MR and CT appearance of iliopsoas bursal distension secondary to diseased hips. J Comput Assist Tomogr 1990; 145: Steiner E, Steinbach LS, Schnarkowski P, Tirman PF, Genant HK. Ganglia and cysts around joints. Radiol Clin North Am 1996; 34: Eisenberg KS, Johnston JO. Synovial chondromatosis of the hip joint presenting as an intrapelvic mass. J Bone Joint Surg 1972; 54: Sim FH, Dahlin DC, Ivins JC. Extra-articular synovial chondromatosis. J Bone Joint Surg 1977; 59: Anson BJ. Musculature of the inferior member. In: Morris human anatomy. 12th ed. New York, NY: McGraw-Hill, 1977; Hamilton WJ. The lower limb. In: Textbook of human anatomy. 9th ed. New York, NY: MacMillan, 1994; Romanes GJ, ed. In: Cunningham s textbook of anatomy. 10th ed. New York, NY: Oxford University Press, 1964; Woodburne RT. The lower limb. In: Essentials of human anatomy. 6th ed. New York, NY: Oxford University Press, 1978; Gray H, Williams P, Warwick R, Dyson M, Bannister L, eds. Anatomy of the human body. 37th ed. Edinburgh, Scotland: Churchill Livingston, 1989; Agur AMR. In: Grant s atlas of anatomy. 9th ed. Baltimore, MD: Williams & Wilkins, 1991; Guerra JG Jr, Armbuster TG, Resnick D, et al. The adult hip: an anatomic study. II. The soft-tissue landmarks. Radiology 1978; 128: Moss SG, Schweitzer ME, Jacobson JA, et al. Hip joint fluid: detection and distribution at MR imaging and US with cadaveric correlation. Radiology 1988; 208: Lequesne M, Becker J, Bard M, Witvoet J, Postel M. Capsular constriction of the hip: arthrographic and clinical considerations. Skeletal Radiol 1981; 6: Ginai AZ. Case report 607. Skeletal Radiol 1990; 19: Resnick D, ed. In: Diagnosis of bone and joint disorders. 4th ed. W.B. Saunders, Radiology July 2003 Robinson et al