Eur. Radiol. (2001) 11: 1798±1802 DOI 10.1007/s003300000788
L. Xiong Q. Y. Zeng J. R. Jinkins
Received: 8 June 2000 Revised: 9 November 2000 Accepted: 16 November 2000 Published online: 17 March 2001 Springer-Verlag 2001
L. Xiong Department of Radiology, University of Texas Health Science Center, San Antonio, TX 78229, USA Q. Y. Zeng Department of Radiology, General Coal Hospital, Beijing, 100028, China
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J. R. Jinkins ( ) Department of Radiology, School of Medicine, MCP Hahnemann University, 245 North 15th Street, Mailstop 206, Philadelphia, PA 19102-1191, USA E-mail:
[email protected] Phone: +1-2 15-7 62 87 22 Fax: +1-2 15-8 49 14 81
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CT and MRI characteristics of ossification of the ligamenta flava in the thoracic spine
Abstract The purpose of this study was to compare MRI findings with CT findings of mass-forming calcification/ossification of the thoracic ligamenta flava (OTLF). Twentyone Chinese patients presented with clinical evidence of chronic and progressive thoracic spinal cord compression which included: difficulty in walking; weakness; and/or numbness of the extremities, back pain, and lower extremity paresthesias. Axial and sagittal T1-weighted imaging (T1WI) and T2-weighted imaging (T2WI) were performed through the thoracic spine on a 1.0-T Impact unit (Siemens, Erlangen, Germany). Axial CT was obtained with 5-mm contiguous sections through the thoracic region. Decompressive surgery with resection of the OTLF were carried out in all patients. Low signal intensity of the mass-forming OTLF was demonstrated at a single level (n = 1) or at multiple levels (n = 20) on both T1WI and T2WI. The distribution of OTLF was bilateral at all levels identified in 6 cases, unilateral at all levels in 5 patients, and both unilateral and bilateral at different levels in 10 cases. Ossification of the thoracic ligamenta flava involved the upper thoracic spine (T1±4) in 3
cases, midthoracic spine (T5±8) in 3 cases, lower thoracic spine (T9±12) in 10 cases, and more than one thoracic spinal subregion in 5 cases. Computed tomography confirmed the MR findings regarding the location and distribution of OTLF in all cases, as well as the associated evidence of central spinal canal stenosis. In addition, 5 patients revealed associated ossification of the posterior longitudinal ligament. All patients demonstrated gradual, but incomplete, clinical improvement of the radiculomyelopathy following decompressive surgery. Ossification of the posterior longitudinal ligament resulting in thoracic central spinal canal stenosis and clinical radiculomyelopathy is not uncommon in the Asian people. Ossification of the thoracic ligamenta flava can be accurately evaluated equally well by CT and MR with regard to level(s) and side(s) of involvement, as well as to the relative degree of central spinal canal stenosis and the associated compression of the thoracic spinal cord. Keywords MR imaging ´ CT ´ Ossification of the ligamentum flavum
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Fig. 1 a±c Multilevel bilateral ossification of the thoracic ligamenta flava (OTLF). a Sagittal T1WI (TR/TE: 600/12/no. of excitations 2) shows several areas of low signal intensity (asterisks) invaginating into the posterior aspect of the spinal canal at the T3±6 level and replacing the epidural fat at these levels. b Sagittal T2WI (TR/ TE: 5000/112 ms) demonstrates low signal intensity regions in the abnormal areas (asterisks) identified in a. c Axial CT through the T4 vertebral body displays bilateral OTLF (arrows) extending to the midline associated with severe central spinal canal stenosis
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Introduction Ossification of the thoracic ligamenta flava is an uncommon problem in the thoracic spine which in clinical practice can cause myelopathy with or without radiculopathy. This entity was first reported in the late 1920s; however, the full clinical importance and demographics of OTLF was not recognized until the early 1960s [1, 2]. The majority of the reported cases have been described in Japanese patients and have primarily been published in the Japanese literature. The radiologic appearance (conventional radiography, X-ray tomography, and CT) and histologic findings of OTLF have been well documented [3, 4, 5]. This study describes the MR and CT findings in 21 Chinese patients who presented with thoracic radiculomyelopathy caused by OTLF.
Materials and methods A retrospective study was undertaken at the General Coal Hospital (Beijing, Peoples Republic of China) from January 1995 to October 1997. A total of 21 Chinese patients were included in this study. There were 11 women and 10 men (mean age 47 years, age range 30±65 years). The patients presented with a radiculomyelopathy of varying severity consisting of low back pain, progressive lower extremity(ies) numbness, paresthesias, and difficulty in walking which was not relieved by conservative treatment. The physical examination revealed decreased lower extremity muscle tone and muscle atrophy, lower extremity dysesthesias or hyperesthesias, and a positive Babinski reflex. Both axial and sagittal T1WI [TR/TE: 600/12 ms, no. of excitations (NEX) = 2] utilizing conventional spin-echo acquisitions and T2WI (TR/TE: 5000/ 112 ms, NEX = 1) utilizing turbo spin-echo MR sequences were performed on a 1.0-T Impact unit (Siemens, Erlangen, Germany). A 5-mm section thickness and a 256 256 acquisition matrix were used. Five-millimeter contiguous axial CT (Siemens, Erlangen, Germany) sections were obtained through the thoracic spine in
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c Fig. 2 a±d Multilevel unilateral OTLF with OPLL and associated intervertebral disc herniations. a Sagittal T1WI (TR/TE: 600/ 12 ms) shows areas of ill-defined low signal intensity anteriorly (asterisks), and in the posterior aspect of the spinal canal at the T4±5 levels (arrows). The thoracic spinal cord appears compressed/atrophic. b Sagittal T2WI (TR/TE: 5000/112 ms) demonstrates low signal intensity regions at the T4±5 levels posteriorly (black arrows), associated with several low signal intensity areas adjacent to the posterior aspects of the T4±6 vertebral bodies (arrowheads). Multilevel intervertebral disc herniations (white arrows) are also noted. c Axial CT through the T4 vertebral body shows the right-sided posterior unilateral OTLF (arrowheads) extending to the midline, and the anteriorly located OPLL (open arrow) causing marked central spinal canal stenosis. d Axial T1WI (TR/TE: 600/12 ms) displays the posterolateral spinal canal hypointensity on the right (asterisk) corresponding to the OTLF and the anterior spinal canal mixed intensity mass representing OPLL (arrow). Note the underlying spinal cord displacement and compression
d order to correlate the MR findings and confirm the presence of ossification. Surgery (laminectomy/facetectomy with resection of the OTLF) was performed in all patients for the purpose of decompressing the thoracic spinal cord. Postoperative MR imaging was not performed because the patients were only followed clinically, and postoperative reevaluation did not form a part of this study.
Results Magnetic resonance imaging demonstrated single-level (n = 1) or multilevel (n = 20) areas of low signal intensity in the region of the thoracic spine ligamenta flava on both T1WI and T2WI. The distribution was bilateral at all levels of enforcement in 6 cases, unilateral at all levels in 6 cases, and mixed unilateral and bilateral in 9 cases (Figs. 1, 2). Ossification of the thoracic ligamenta flava was present in the upper thoracic spine (T1±4) in 3
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cases, in the midthoracic spine (T5±8) in 3 cases, in the lower thoracic spine (T9±12) in 10 cases, and in more than one thoracic spinal subregion in 5 cases. Subtle or obvious increased signal intensity of the spinal cord was demonstrated on T2-weighted images at the level of OTLF in all patients (n = 21). CT confirmed the location and severity of the OTLF unilaterally and/or bilaterally in all cases, as well as the associated evidence of central spinal canal stenosis (Figs. 1, 2). Five patients had associated thoracic ossification of the posterior longitudinal ligament (OPLL) and concomitant posterior thoracic disc herniations (Fig. 2). The clinical radiculomyelopathy only correlated in a general way with the imaging findings. Clinical follow-up postoperatively demonstrated a gradual but incomplete improvement in the radiculomyelopathy in all patients.
Discussion Spinal stenosis of either a congenital or acquired nature is not uncommonly encountered in clinical practice, and tends to be most common in the lumbar and cervical regions of the spine. Acquired spinal stenosis caused by OTLF, however, is more frequently seen in the thoracic spine and in Asian populations. Only a very few cases of OTLF have been reported in Caucasians [6, 7]. The ligamentum flavum (LF) is a paired structure that approximates in the midline. The LF attaches inferiorly to the posterosuperior surface of the lamina of the vertebral body below and superiorly to the anteroinferior surface of the lamina above. The normal thickness of the LF is approximately 2.0 mm in the thoracic spine. Histologically, the LF is made up of connective tissue with some collagen fibers interspersed among a majority of elastic fibers (i.e., elastin containing). The LF contains deep and superficial components which are firmly adherent to each other [8,9]. With regard to function, the LF: (a) provides a static, elastic force to aid the return to a neutral position after spinal flexion/extension; and (b) maintains a generally smooth surface forming a part of the dorsal aspect of the central spinal canal. The normal appearance of the LF on CT and MR has been extensively studied and compared with anatomic correlations [10, 11, 12,13]. The LF is intermediate MR signal intensity between spinal cord and cortical bone on both T1WI and T2WI and is slightly hyperintense as compared to other ligaments. The overall prevalence of OTLF is estimated to be 6.2 % for males and 4.8 % for females in the Oriental population [5]. OTLF is a well recognized cause of acquired central spinal canal stenosis in this specific population resulting in myelopathy, radiculopathy or a combination of both. Patients typically present clinically with a long-term history of gait disturbance, lower extremity muscle weakness with or without atrophy,
numbness of the lower extremities, lower extremity cutaneous sensory disturbances, bladder incontinence, intercostal neuralgias and mid-back pain. The clinical presentation is nonspecific, and the diagnosis is often made late in the course of the disease process. Radiologic examinations play an important role in the diagnosis and evaluation of OTLF [14, 15, 16, 17, 18, 19]. On lateral conventional radiography and/or conventional X-ray tomography, OTLF appears as beaklike or nodular bony densities projecting into the posterior aspect of the central spinal canal [5]. On conventional water-soluble contrast myelography, a complete block or partial obstruction of flow may be demonstrated at the most severely affected levels. Both CT and MR can accurately demonstrate the shape, location, distribution, and level(s) of the OTLF as well as the relative degree of associated central spinal canal stenosis. Good correlation between CT and MR imaging findings was found in the present study regarding the level, side, and general degree of severity of the central spinal canal stenosis caused by the OTLF. On T2WI, the compression of the underlying spinal cord was identified as well as cord hyperintensity, representing edema and/or myelomalacia [16, 19]. These imaging findings also correlated reasonably well with the gross clinical findings of myelopathy. The lower one-third of the thoracic spine was the most common overall location for OTLF both in the published literature as well as in our study. The association of thoracic OTLF with both thoracic OPLL and thoracic intervertebral disc herniation as seen in the current study has also been reported in the literature [14, 16]. The development and pathogenesis of OTLF is poorly understood. It may occur secondary to developmental±hereditary predisposing factors, mechanical causes, metabolic disorders, trauma, and/or chronic degenerative changes. Although it is not the major cause in the Asian population, the relationship between OTLF and X-linked hypophosphatemia [20] and calcium pyrophosphate dihydrate crystal deposition disease (i.e., pseudogout) has been documented [21, 22, 23]. Histologically, in OTLF the normally predominating elastin content of the LF is replaced by collagen [10]. Numerous irregular fragments of bone, cartilage, fibrous tissue, and otherwise normal ligamentous tissue are frequently identified in surgical specimens. Ossification starts at the edges of the adjacent laminae near the ligamentous insertions, and then extends into the central areas of the affected LF (Figs. 1c, 2c). The differential diagnosis of OTLF is relatively narrow due to the specific anatomic location and the characteristic imaging appearance. However, calcified LF hematomas [24], calcified thoracic meningiomas, and epidural calcifying hemangiomas can on occasion mimic monofocal OTLF when such lesions are located in the dorsal aspect of the spinal canal. With regard to treat-
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ment, a standard decompressive bilateral laminectomy has been widely used; however, a unilateral laminectomy with bilateral resection of the LF has been recommended by Poletti [7]. These surgical procedures may have to be extended to other or all levels of involvement, depending upon the clinical syndrome and the severity of OTLF as demonstrated on CT and/or MR imaging. In conclusion, whereas CT is an excellent tool for evaluating the local extent of OTLF and the axial configuration of the central spinal canal, MR imaging provides useful overall information concerning the level(s) of the segmental involvement, the presence of associated disease (e.g., OPLL, disc herniation), and the effects
on the underlying spinal cord. Although OTLF resulting in thoracic central spinal canal stenosis and clinical radiculomyelopathy is not common in the West, it may be encountered in immigrants from the East and may be rarely seen in non-Asian patients. In part because the clinical course of OTLF is nonspecific, it may go undiagnosed until late in the course of the disease process, and for this reason it should be considered in the differential diagnosis in any patient presenting with thoracic radiculomyelopathy. Acknowledgements The authors thank T. Clifton for assistance with manuscript transcription.
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