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JBR­BTR, 2007, 90: 388-390.

PAGET'S DISEASE WITH VANISHING BONE PATTERN AND SPINAL FUSION

F Vanhoenacker1,2, D. De Vuyst1, E. Vandervliet1,2, A. Veyt3, J. Vangeneugden3 .M. We present a rare case of Paget's disease involving the spine and pelvis, with unusual imaging features, including severe bony expansion and fatty infiltration, resulting in a "vanishing bone" appearance. Other rare imaging features are the presence of spinal and sacroiliac ankylosis.

Key-word: Osteitis deformans.

Paget's disease (PD) is a frequent finding in the elderly population and may be incidentally detected on imaging studies done for other reasons. Imaging findings have been described extensively in the literature (1-3). The purpose of this case report ­ however ­ is to present a rare case of PD with unusual spinal manifestations, such as "vanishing bone aspect" and localized fusion of the axial skeleton.

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Case report An 81-year-old man was admitted at our department with non-specific abdominal pain. A CT scan of the abdomen revealed no intra-abdominal abnormalities, but showed severe bony expansion and fatty infiltration of the lower lumbar vertebrae (L3-L5) as well as the lower thoracic vertebrae (Fig. 1A). Sagittal reformatted images of the lumbar spine showed bony fusion of the vertebral bodies and posterior elements of L3 to S1 (Fig. 1B). Extensive bone expansion with focal cortical breakthrough and fatty infiltration of the lower vertebral bodies and sacrum resulted in disappearance of the normal trabeculae and cortical bone structure (Fig. 1C). Due to the fatty infiltration of the osseous structures, the dural sac appeared relatively hyperdense compared to the vertebrae (Fig. 1B). Radiographs of the pelvis and hips showed ankylosis of the lower lumbar spine and sacro-iliac joints (Fig. 2A). Thickening of the cortical bone and coarsening of the bony

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Fig. 1. -- A: Coronal reformatted CT image of the abdomen revealing bony expansion and fatty infiltration of the lower lumbar and lower thoracic vertebrae. B: Sagittal reformatted CT image of lumbar spine (soft tissue window) confirming marked fatty infiltration and vertebral fusion. Note the relatively high density of the dural sac, compared to the low attenuation of the osseous spine. C: Axial CT (bone window). Note disappearance of the bony trabeculae and thinning of the cortical bone, resulting in a "ghost" vertebra. The posterior elements are also involved.

trabeculae was seen at the right proximal femur (Fig. 2B). MRI confirmed spinal fusion and ankylosis of the sacro-iliac joints (Fig. 3). Elevated serum alkaline phosphatase was seen on laboratory examination.

Based on the clinical and imaging findings, the diagnosis of Paget's disease was made. Discussion Osteitis deformans or Paget's disease is a noninflammatory condition affecting one or more bones, named after Sir James Paget, an English surgeon who described the clinical course of the disorder in his classic paper published in 1877 (4). The overall prevalence of PD is 3% in the European population. The highest incidence is amongst those of English descent (1). It is rare

From: 1. Dept of Radiology AZ Sint-Maarten, Duffel-Mechelen, 2. Dept of Radiology University Hospital Antwerp, 3. Dept of Neurosurgery AZ Sint-Maarten, DuffelMechelen, Belgium. Address for correspondence: Dr F .M. Vanhoenacker, M.D., Ph.D., Department of Radiology, University Hospital Antwerp, Wilrijkstraat 10, B-2650 Edegem, Belgium. E-mail: [email protected]

PAGET'S DISEASE WITH VANISHING BONE PATTERN AND SPINAL FUSION -- VANHOENACKER et al

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Fig. 2. -- A: Conventional radiography of the pelvis revealing spinal fusion (L3-S1) and indistinct margins of the sacroiliac joint due to ankylosis. B: Conventional radiography of the right hip demonstrating cortical thickening, coarse trabeculae and bony expansion, consistent with Paget's disease.

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Fig. 3. -- A: Sagittal T2 HASTE image of the lower lumbar spine. B: Axial HASTE image of the sacroiliac joints. Marked expansion of the lower lumbar vertebrae with fatty bone marrow replacement within the vertebrae and sacrum. Note continuity of the medullary and cortical bone across the sacroiliac joints, indicating ankylosis of the sacroiliac joints. T1-weighted images also showed fatty marrow replacement (not shown because of degradation of image quality due to patient motion).

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among those of Asian and black African descent. Although the disease can affect any age group, including infants (juvenile Paget's or infantile hyperphoshatasemia), more than 90% of patients are older than 40 years of age (1). Recent data suggest a reduction of the overall prevalence and severity of the disease (5). The etiology of PD is still a matter of debate. Both genetic (6) and non-

genetic factors have been implicated in the pathogenesis of the disease. Potential nongenetic factors include environmental factors such as childhood diet, physical activity, oral bacteria, viruses, zoonotic infections and occupational exposures (5). PD has characteristic radiological and histological features which vary, depending on the stage of the progression of the disorder. The disease passes through an active phase which may be subdivided into osteolytic, mixed and osteoblastic stages and an inactive phase (3). PD is polyostotic in 66% of cases. In other patients, the disease is

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initially or totally monostotic (3). A predilection exists for the axial skeleton. Particularly characteristic is involvement of the pelvis (3075%), sacrum (30-60%), spine (3075%), especially the lumbar segment, and skull (25-65%). In addition, the proximal portions of the long bones are commonly affected, particularly the femur (25-35%). Abnormalities of the axial skeleton or proximal part of the femur are present in approximately 75 to 80% of cases (3). However, any part of the skeleton may be involved. Detailed discussion of the imaging features of PD by anatomic site is described in excellent review articles and textbooks (1-3) and is beyond the scope of this short report. Well known pathognomonic features of PD of the spine include coarsening of the vertical trabeculae, enlargement of the body as compared to its normal neighbours above and below, extension of the sclerotic process into the posterior elements and a mild degree of flattening or rounding of the normally concave anterior curve (2, 7). Bone enlargement may be responsible for spinal stenosis. Increased sclerosis along all four margins of the vertebral body, especially the superior and inferior margins, may result in a "picture-frame" vertebra. A more generalized increase in density, often extending to the posterior elements, may give rise to an "ivory" vertebra. Vertebrae with lytic or mixed components are weakened and generally begin to develop biconcave deformities ("fish" vertebrae), which in later stages can culminate in fracture collapse and neurologic impairment (3, 7). Our case is unusual because of the combination of two rare spinal manifestations of PD. First, fusion of the vertebral bodies and the sacroiliac joints are rare complications of PD. The mechanism of fusion is not completely clear. Intervertebral disc degeneration can be caused by mechanical stress induced by the asymmetry at the end plate attachment of annulus fibrosus between a pagetic and a normal vertebra, and also by direct invasion of the disc

space by the pagetic process. Aggressive pagetic invasion at the disco-vertebral junction leads to pagetic tissue replacement of the cartilage end-plate and subsequently of the intervertebral disc. This process can lead to vertebral ankylosis. Pagetic vertebral fusion can also occur by direct extension of PD along large pre-existing degenerative bridging osteophytes (7, 8). A similar cartilaginous joint destruction may lead to pagetic ankylosis of the sacroiliac joints, mimicking sacroiliitis (3, 9). The second unusual feature in our case is the extensive osteolysis and expansion of multiple vertebrae resulting in thinning of the cortical bone and almost complete disappearance of the bone trabeculae. This results in a "vanishing bone" or "ghost" pattern of the lower lumbar spine and the sacrum. Marked fat replacement of the lower lumbar spine causes a relative hyperdense CT appearance of the dural sac compared to the osseous spine, which is a peculiar imaging finding. The preservation of fatty marrow attenuation on CT-scan and fatty marrow signal on MR images respectively is a key finding to the correct diagnosis of PD in our patient. Indeed, other osteolytic processes causing extensive bone disappearance, such as vanishing bone disease (Gorham disease) will display a low signal intensity on T1weighted images. The same applies to other diseases causing widespread osteolysis, such as myeloma or osteolytic metastases. Marrow fat appears essentially preserved in all phases of PD, except in the presence of sarcoma or acute fracture (10). Preservation of fat has previously been described as a very confident sign to exclude sarcomatous change in PD. This underscores the usefulness of MR imaging as a clinical problem-solving modality, obviating the need for biopsy (10). Furthermore, pathognomonic radiological imaging features, such as vertebral expansion, involvement of the posterior elements of the spine, thickened cortical bone and coarse trabeculae of the right hip contribute to the diagnosis of PD

and exclude other disorders in the differential diagnosis. Conclusion In conclusion, radiologists should be aware of potential spinal and sacroiliac ankylosis and a vanishing bone pattern, complicating longstanding PD. The clue to the correct diagnosis is the presence of the combination of other pathognomonic radiographic and CT features of PD and the preservation of fatty marrow on MR imaging. Correct interpretation of these imaging features avoids unnecessary biopsy of the lesion. References

1. Mirra J.M., Brien E.W., Tehranzadeh J.: Paget's disease of bone: review with emphasis on radiographic features, part I. Skeletal Radiol, 1995, 24: 163-171. 2. Mirra J.M., Brien E.W., Tehranzadeh J.: Paget's disease of bone: review with emphasis on radiographic features, part II. Skeletal Radiol, 1995, 24: 173-184. 3. Resnick D.: Paget's Disease. In: Diagnosis of Bone and Joint Disorders, 4th ed. Edited by Resnick D. Printed by W.B Saunders Company, Philadelphia 2002, p 1947-2000. 4. Paget J.: On a form of chronic inflammation of bones (osteitis deformans). Trans Med Chir Soc Lond, 1877, 37. 5. Cundy T.: Is Paget's disease of bone disappearing? Skeletal Radiol, 2006, 35: 350-351. 6. Beyens G., Van Hul E., Van Driessche K., et al.: Evaluation of the role of the SQSTM1 gene in sporadic Belgian patients with Paget's disease. Calcif Tissue Int, 2004, 75: 144152. 7. Dell'Atti C., Cassar-Pullicino V.N., Lalam R.K., Tins B.J., Tyrell P .N.: The spine in Paget's disease. Skeletal Radiol, 2007, 36: 609-626. 8. Lander P Hadjipavlou A.: Intradiscal ., invasion of Paget's disease of the spine. Spine, 1991, 16: 46-51. 9. Bezza A., Lechevalier D., Monreal M., el Maghraoui A., Magnin J., Eulry F .: Sacro-iliac involvement in the course of Paget disease. Report of 6 cases. Presse Med., 1999, 28: 1157-1159. 10. Kaufmann G.A., Sundaram M., McDonald D.J.: Magnetic resonance imaging in symptomatic Paget's disease. Skeletal Radiol, 1991, 20: 413418.

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