Radial magnetic resonance imaging and pathological findings of acetabular labrum in dysplastic hips

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1 Pathophysiology 7 (2) Radial magnetic resonance imaging and pathological findings of acetabular labrum in dysplastic hips Toshikazu Kubo a, *, Motoyuki Horii a, Junko Yamaguchi a, Ryu Terauchi a, Yaoping Wu b, Yukiharu Hasegawa c, Yasuyuki Enoki d, Yasunari Tsuchihashi d, Yasusuke Hirasawa a a Department of Orthopaedic Surgery, Kyoto Prefectural Uni ersity of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto , Japan b Department of Orthopaedic Surgery, Xi-Jing Hospital, The Fourth Military Medical Uni ersity, Xi-an, People s Republic of China c Department of Orthopaedic Surgery, Nagoya Uni ersity School of Medicine, Nagoya, Japan d Hospital Department of Pathology, Kyoto Prefectural Uni ersity of Medicine, Kyoto, Japan Received January 2; accepted 7 April 2 Abstract Radial MRI findings and pathological changes were comparatively examined in the acetabular labrum of hips of patients, who underwent total hip arthroplasty for osteoarthritis due to acetabular dysplasia. Diffuse high signal pattern on the radial MR images corresponded to histological degeneration of the labrum. High signal pattern which was equivalent to the synovial fluid, showed an intralabral tear. In the obscure areas of MR images, severe impairment of the labrum such as rupture, detachment, and displacement were found. 2 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Radial MR imaging; Dysplastic hip; Labrum; Pathology. Introduction Acetabular labrum, which is located on the acetabular rim, is thought to play an important role for the hip stability [], and to relate colosely to the hip disorders [2]. However, its function and pathology are not fully understood. Non invasive monitoring of the labrum in both normal and abnormal conditions is necessary to clarify them in detail. The magnetic resonance imaging with radial sections allows to observe the labrum covering wide range of its circumference (Fig. ) [3]. Our previous studies using the technique showed that (i) shapes of the labral sections varies according to the monitoring portion even in healthy subjects (Fig. ) [4], (ii) the labral size is large in dysplastic hips, and the size corresponds to the severity of dysplasia [5], (iii) abnormally high signal intensity appears within the labrum * Corresponding author. Tel.: ; fax: address: tkubo@koto.kpu-m.ac.jp (T. Kubo). even before the radiographic manifestation of osteoarthritis [5], and (iv) various abnormalities appears not only within the labrum but also on the regions adjacent to the labrum during the early and advanced stages of osteoarthritis [6]. In order to know what happens on the labrum when certain changes are depicted on radial MR images, we compared the MRI fingings with macroscopic and histological findings of the labrum resected at the total hip arthroplasty (THA). 2. Subjects and methods 2.. Patients The current study examined hips of patients who received THA for secondary osteoarthritis caused by acetabular dysplasia. There were one male (one joint) and ten females (ten joints), and their ages ranged from 45 to 75 years old (mean: 62 years). The stages of OA [7] were advanced in four hips and terminal in seven hips //$ - see front matter 2 Elsevier Science Ireland Ltd. All rights reserved. PII: S ()48-

2 T. Kubo et al. / Pathophysiology 7 (2) Fig.. Positions of the 2 radial sections and the normal labral images corresponding to the antero-superior 3, mid-superior and postero-superior 3 planes. The intact labrum was depicted at a homogenous low signal intensity. The shapes of labrum are varied according to the planes. Radial MRI was performed less than one month before THA using MAGNEX 5X (.5 telsa, Shimadzu Corp., Kyoto, Japan) and a local surface coil. T2*-weighted images were obtained with small tip angle gradient echo (STAGE) method (TR: 5 ms, TE: 2 ms, flip angle: 3 ) [8,9]. At imaging, a scout view of the entire circumference of the acetabular margin was obtained by making adjustment for the anterior open angle and the acetabular inclination angle. On this imaging, 2 radial sections with 5 interval (slice thickness: 4 mm), which include 24 labral planes of acetabular labrum, were determined centered on the mid-point of the acetabulum (Fig. ). The current study examined 7 planes in the range from antero-superior 45 to postero-superior 45 through the mid-superior plane. The labra were resected ranging from antero-superior to postero-superior portion as a mass at surgery. The resected labra were devided into three specimens corresponding to the antero-superior plane, mid-superior plane, and postero-superior plane, and they were histologically examined. Each were cut into 4 mm thick blocks, fixed in 5% formalin, embedded into paraffin, sectioned into 5 mm slices, stained with HE. The degree of labral degeneration was examined and compared with MRI findings on each corresponding plane. 3. Results 3.. Preoperati6e MRI findings of the labrum (Table and Fig. 2) Two planes from one hip showed almost normal low signal, but the other planes had some abnormalities. Table Abnormalities found by preoperative radial MRIa Antero-superior 45 Low signal Diffuse high signal Central high signal Obscure a 2 9 Figures represent number of patients. Mid-superior 3 5 Postero-superior

3 T. Kubo et al. / Pathophysiology 7 (2) Fig. 2. Abnormalities on MR images. (a) Low signal, (b) diffuse high signal, (c) central high signal, (d) obscure. Eleven planes from five hips showed a diffuse high signal pattern, i.e. diffusely increased signal at moderate severity throughout the labral section. In a plane from a hip showed a central high signal pattern, which is as high a signal area as the synovial fluid in the labrum without extension to the surface. The labral margin was indistinguishable in 59 planes from ten hips, and this finding is classified as a obscure pattern, which was appeared more frequently in the anterosuperior and mid-superior planes than in the posterosuperior ones Macrospic findings The acetabular labrum in all patients had a large width (5 2 mm) on the weight bearing portion with irregular surface. Continuity of the labrum was interrupted in ten of the hips. A longitudinal tear (Fig. 3a) was found in 6 hips, and a flap formation, which means a more sever injury than the longitudinal tear, was found in four hips (Fig. 3b). Most of these changes occurred in the range between the mid-superior and antero-superior portions, and they were evaluated as obscure on MRI. There was a intralabral tear on a postero-superior portion of a hip (Fig. 4). The central high signal pattern was appeared at the corresponding postero-superior 45 plane on MRI. No labral tear was found on postero-superior portion in other ten hips. Fig. 3. Macroscopic findings of the labrum. (a) Longitudinal tear, (b) flap formation. Arrows show the mid-superior portion. Antero-superior portion is on their left. L, displaced labrum; S, scar at the tear.

4 T. Kubo et al. / Pathophysiology 7 (2) Fig. 4. Intralabral tear at the antero-superior portion Histological findings of the labrum On the portions which showed diffuse high signal or obscure pattern on MR images, the dense and homogenous matrix became flocculous with fibrous separation and mucoid deposition (Alcian blue stain) (Fig. 5a). Irregularly distributed fibroblastic cells with decreased cellularity was observed (Fig. 5a). On the other hand, the area with low signal pattern, which meant an almost normal MR finding, had only a little abnormalities mentioned above (Fig. 5b). 4. Discussion The authors have proposed radial MRI which allows the labrum to be viewed in radial cross-section along its entire circumference [3 6]. On the hips with acetabular dysplasia, insufficiency of the stability produces the excessive stress on the acetabular labrum, resulting in the labral degeneration and rapture with frequency [2]. Radial MRI depicts abnormal high signal within the acetabular labrum on dysplastic hips even before the radiological manifestation of osteoarthritis [5]. Labral rupture and the stripping of the labral and/or capsular insertion are indicated on the image at the advanced stage [6]. In this study, we investigated what really occurs macroscopically and histologically on the labra with abnormal findings on MRI. Macroscopic abnor- Fig. 5. Histological findings of the labrum. (a) Tissues corresponding to the area of diffuse high signal pattern. Mucinous degenerations with fibrous separation were observed. (b) Tissues corresponding to the area of low signal pattern. There was only a little degeneration. malities consisted of large labrum, longitudinal tear and flap formation. These changes were more frequently found on the mid-superior and antero-superior portions, and their MR images were usually evaluated as obscure. This pattern indicates the presence of severe impairment, and at the same time, it suggests the limitation of radial MRI to depict the tear and/or flap formation with severe labral degeneration. Considering that the intralabral tear was depicted as a high signal area, the signal in the labrum would represent a tear when its intensity is as high as synovial fluid. On the other hand, a diffuse high signal pattern would mean a degeneration, while a low signal pattern would indicate an almost normal condition, according to the histological consideration. References [] M.A.M. Harty, Anatomy, in: M.E. Steinberg (Ed.), The Hip and Its Disorders, W.B. Saunders, Philadelphia, PA, 99, pp [2] I.V. Ponseti, Growth and development of the acetabulum in the normal child. Anatomical, histological, and roentgenographic studies, J. Bone Joint Surg. (Am.) 6 (978)

5 T. Kubo et al. / Pathophysiology 7 (2) [3] T. Kubo, M. Horii, Y. Hirasawa, Magnetic resonance imaging of rheumatic diseases, in: O. Abe, K. Inokuchi, K. Takasaki (Eds.), The XXX World Congress of the International College of Surgeons, Monduzzi Editore, Bologna, 996, pp [4] T. Kubo, M. Horii, Y. Harada, Y. Noguchi, Y. Yutani, H. Ohashi, Y. Hachiya, H. Miyaoka, S. Naruse, Y. Hirasawa, Radial-sequence magnetic resonance imaging in evaluation of acetabular labrum, J. Orthop. Sci. 4 (999) [5] T. Kubo, M. Horii, J. Yamaguchi, K. Ueshima, Y. Hirasawa, Acetabular labrum in hip dysplasia evaluated by radial magnetic resonance imaging, J. Rheumatol. 27 (2) [6] M. Horii, T. Kubo, Y. Hirasawa, Radial MR imaging of the hip joint with moderate osteoarthritis, J. Bone Joint Surg. (Br.) 82 (2) [7] K. Takaoka, T. Sakamaki, S. Yanagimoto, T. Matsumoto, N. Sugano, S. Saito, S. Kobayashi, The Asian hip, in: J.J. Callaghan, A.G. Rosenberg, H.E. Rubash (Eds.), The Adult Hip, Lippincotto Raven, Philadelphia, PA, 998, pp [8] A.D. Elster, Gradient-echo MR imaging: techniques and acronyms, Radiology 86 (993) 8. [9] M. Horii, T. Kubo, S. Naruse, Y. Hirasawa, Relationship between pulse sequences and signal intensity of joint fluid in the gradient-echo MR imaging, Magn. Reson. Imaging 5 (997)