• LCH is often considered a disease of childhood, but in a larger series,² approximately 40% of the study population was younger than 20 years old; overall, LCH lesions were marginally more common in females than in males.

DISTRIBUTION

• In the combined results of several studies, the involvement of the spine by LCH appears to be a unique feature of the pediatric subpopulation.³

• In this combined series, more than three-quarters of the vertebral LCH lesions were found in the cervical and thoracic segments; multiple level involvement was common.

ANEURSYMAL BONE CYST

EPIDEMIOLOGY

• Aneurysmal bone cysts (ABCs) are characterized by small and large cavernous spaces filled with clotted-blood fluid.

• ABC is a benign and expansile, non-neoplastic lesion that destroys the bony architecture.⁴

• ABC represents approximately 1% of all primary bone tumors.

• The majority of ABCs appear before the age of 20 years, and any bone segment may be affected. Males and females are evenly affected.

• Approximately 10–30% of these tumors arise from the spine. They represent approximately 15% of all primary mobile spine tumors.⁵

DISTRIBUTION

• The cervical, thoracic, and lumbar spines are similarly affected and in most of the reported cases, the posterior elements of the vertebrae were the site of the lesion. Hay et al ⁶ reported a series of 12 sacral lesions out of 92 cases of ABC in the spine.

• The etiology of ABCs is obscure, although the following theory is generally accepted. The aneurysmal bone cysts may be caused by development of a vicious hemodynamic cycle in a bone that is the site of a congenital vascular anomaly. This cycle is triggered by trauma or the development of other pathology, which interferes with venous drainage of the area. As a result, subsequent venous pressure increases, and a dilated and engorged vascular bed develops in the affected bone.⁴

• ABC may arise de novo (primary ABC) or secondarily complicate other benign and malignant bone tumors (secondary ABC) that have undergone hemorrhagic cystic change.

HISTOLOGY

• Pathologically, ABCs contain multiple fluid-filled cavities separated by multiple fibrous septa. The new bone is commonly parallel to fibrous septa (Fig. 2-6). These large spaces filled with blood do not have an endothelial lining.⁷

• The fibrous septa are composed of a moderately dense cellular proliferation of bland fibroblasts, with scattered osteoclastic giant cells and reactive new bone (Fig. 2-7). In approximately one-third of cases, the bone is basophilic and has been termed “blue bone” (Fig. 2-8).

• Mitoses are commonly present and can be numerous, but atypical forms are absent.

• The majority of secondary ABCs develop in association with benign neoplasms, most commonly giant cell tumor of bone, osteoblastoma, chondroblastoma, and fibrous dysplasia.

• ABC-like change may also occur in sarcoma, particularly in telangiectatic osteosarcoma.

Fig. 2-6 The multi-locular cystic architecture of the lesion is evident under low-power examination. Reactive bone formation in septa is oriented along long axes (hematoxylin–eosin, ×40).

Fig. 2-7 A few osteoclastic giant cells are seen in septa of ABC, and the number of nuclei is not so numerous as in giant cell tumor (hematoxylin–eosin stain, ×100).

Fig. 2-8 High-power photograph shows immature blue bone in septa. Stromal cells show round to oval nuclei without atypism (hematoxylin–eosin stain, ×200).

MRI

• Lobulated neural arch mass with cystic spaces of varying sizes and multiple internal septation on T1WI (Fig. 2-10).

• Fluid-fluid levels caused by intracyst hemorrhage (about 30%).

• Low signal rim around mass.

• Peripheral enhancement around the rim.

Fig. 2-10 MR image of ABC. A multiple septated cystic lesion is shown (left, sagittal; middle, coronal; right, axial). Low signal rim means thinned cortex. In the cystic mass, the fluid-fluid level is seen on the sagittal image.

SYNOVIAL CYST

EPIDEMIOLOGY

• Synovial cysts are commonly reported in the spine, particularly within the lumbar spine. Spinal synovial cysts tend to be located principally in the spinal canal, adjacent to the facet joints of the lumbar spine.⁸

• The pathogenesis of synovial cysts remains unclear, but a lot of theories have been proposed to elucidate the causes of spinal synovial cysts, such as degenerative changes in the facet joints, trauma, metaplasia, the presence of developmental rests, excess stress inflicted at the facet joints coupled with the herniation of synovial tissue, and mucinous degeneration in the connective tissues.⁹

• In some reports, trauma has been known to result in cyst enlargement or hemorrhage into the synovial cyst cavity, causing epidural compression of the spinal cord.¹⁰

DISTRIBUTION

Synovial cysts occur in a juxta-articular, posterolateral, or epidural location within the lumbar spinal canal in 0.6–8% of MRI and CT examinations.⁹

HISTOLOGY

Pathologically, synovial cysts exhibit an epithelial-lining cystic lesion (Fig. 2-11, A and B), which can be differentiated from cystic degeneration of the yellow ligament. Facet degeneration accompanies 75% of the synovial cyst cases.

Fig. 2-11 A, Synovial cyst is seen with relatively thickened surrounding connective tissue. B, Cyst is lined by synovial cells; right side is focally hyperplastic (hematoxylin–eosin stain, A, ×40; B, ×200).

RADIOLOGY

• MRI is the examination of choice for synovial cysts. The typical finding of the synovial cyst is low to intermediate signal intensity on T1-weighted images, and high signal intensity on T2-weighted images (Fig. 2-12).

• However, these cysts also sometimes display low signal intensity on T2-weighted images, which is generally attributed to hemorrhage and blood products within the cyst contents or to calcium deposits ¹¹ (Fig. 2-13).

Fig. 2-12 MRI T2 axial view. High signal mass is seen around the facet joint at the L4–5 level.

Fig. 2-13 Gross feature of the synovial cyst. It has a glistening surface and whitish red color.

FIBROUS DYSPLASIA

EPIDEMIOLOGY

• Fibrous dysplasia is a multisystem disorder that may involve the central nervous system, endocrine organs, skin, and the skeletal system.

• It is characterized by the metaplastic replacement of the medullary component of one bone (monostotic), or less commonly of several bones (polyostotic), with fibrous tissue and irregular osteoid formation.¹²

• In skeletal lesions, it is a common non-malignant fibro-osseous lesion of the bone, accounting for 7% of benign bone tumors and 2.5% of all bone lesions.

DISTRIBUTION

• The usual age of presentation is in the first two decades of life.¹³

• The skeletal involvement can be monostotic (70–80%) or polyostotic (20–30%).

• In the monostotic form, the long tubular bones are often involved. On the other hand, the axial bone is more often involved in the polyostotic form.

• Although fibrous dysplasia accounts for approximately 7% of benign bone lesions, the prevalence of spinal involvement is thought to be very low, particularly in the monostotic form.¹⁴

• Involvement of the spinal column in either monostotic or polyostotic form is very uncommon. Most of the cases of polyostotic fibrous dysplasia of the spine have involvement of the appendicular skeleton.

• Fibrous dysplasia involving the spine has also been seen in individuals with McCune–Albright syndrome, and the prevalence has been reported to range from 7% in the cervical spine to 14% in the lumbar spine.¹⁵

HISTOLOGY

• The lesion is composed of fibrous and osseous components, which are present in varying proportions from lesion to lesion (Fig. 2-14).

• The fibrous component is composed of cytologically bland spindle cells with a low mitotic rate, and the osseous component comprises irregular curvilinear trabeculae of woven bone, described as Chinese character–like (Fig. 2-15).

• Secondary changes, such as foam cells, multinucleated giant cells, aneurysmal bone cyst, or myxoid change may occur.

• Microscopic foci of cartilaginous differentiation are commonly present.

Fig. 2-14 Low-power photograph shows delicate trabeculae of osteoid and woven bone in moderately cellular fibrous matrix. The most common and characteristic pattern of bone consists of slender, curved, and branching trabeculae, frequently described as resembling characters of the Chinese alphabet (hematoxylin–eosin stain, ×40).

Fig. 2-15 High-power view clearly demonstrates the absence of osteoblastic rimming around the bony trabeculae (hematoxylin–eosin stain, ×200).

PYOGENIC OSTEOMYELITIS

• Pyogenic osteomyelitis is a bacterial suppurative infection of vertebrae and the intervertebral disc. It involves thoracic and lumbar vertebrae in most cases.

• Most of these patients are more than 50 years of age and may have other painful back conditions. Erroneous initial diagnoses are commonly made, delaying diagnosis for several weeks or months.¹⁷ It often can be misdiagnosed as metastatic or primary spine tumors.

• The incidence of vertebral osteomyelitis and discitis in one study ¹⁰ was 5.3 cases per 1 million (as compared with a 7.6% and 3.9% prevalence in the general adult population of acute and chronic low back pain, respectively).

• The most common route of infection is bacteremia from an extraspinal primary source. The sources can be lung, gastrointestinal tract, genitourinary tract, and mucous infection. Vascularized subchondral bone adjacent to an endplate is the entrance to the bone marrow. Staphylococcus aureus is the most common pathogen.

• Mononuclear inflammatory cells are rich in the specimen with necrotic bone fragment (Fig. 2-19).

• On T1WI of MRI, low signal lesion is shown at the disc and adjacent vertebral bodies (Fig. 2-20). The disc space is seen to be narrow. On T2WI, the disc space is of high signal (Fig. 2-21).

• On enhancement with gadolinium, diffuse enhancement is achieved (Fig. 2-22). An epidural mass or paraspinal mass is commonly formed (Fig. 2-23).

Fig. 2-19 Heavy inflammatory infiltrates composed of mononuclear chronic inflammatory cells and occasional neutrophils occupy bone marrow. Bone trabecula present on right shows necrotic feature (hematoxylin–eosin stain, ×100).

Fig. 2-20 Low signal is seen at the T8–9 disc space and adjacent vertebral bodies (T1WI).

Fig. 2-21 High signal is seen at the narrow intervertebral disc of T8–9 (T2WI).

Fig. 2-22 On axial image, diffuse enhancement pattern is seen and paraspinal mass is formed at the right side of the T8 vertebral body.

Fig. 2-23 Epidural mass is seen to compress the spinal cord.

TUBERCULOUS OSTEOMYELITIS

• Tuberculous spondylitis (TS) starts from the initial inoculum in the anterior vertebral body. The mode of transmission is hematogenous spread or through lymphatics from pulmonary origin.

• The spread to the adjacent vertebral body is made beneath the longitudinal ligament, with the intervertebral disc spared. Spondylitis develops in less than 1% of patients with tuberculosis.

• The typical microscopic findings of tuberculosis are the formation of granuloma and caseation necrosis. The granuloma is composed of epithelioid histiocytes and one or two Langerhans giant cells. Lymphocyte infiltration is common (Fig. 2-24).

• The signal change of involved vertebral bodies is low on T1WI and high on T2WI (Figs. 2-25 and 2-26). When gadolinium is administered, diffuse enhancement pattern is observed in the bone marrow, longitudinal ligament, disc, and spinal dura (Fig. 2-27). The disc space is spared until the vertebral body is completely collapsed (Fig. 2-26).

Fig. 2-24 Photomicrograph showing several variable-sized granulomatous lesions, having caseation necrosis, peripheral epithelioid cells, and some Langerhans giant cells (hematoxylin–eosin stain, ×100).

Fig. 2-25 Sagittal MRI of 70-year-old female patient complaining of paraparesis. On T1WI, T6, T7 vertebral body shows low signal change and body collapse.

Fig. 2-26 T2-weighted MR image. T6 and T7 body turned into high signal change, and the T6–7 intervertebral disc is relatively preserved in its height.

Fig. 2-27 On axial image, well-enhancing lesion of the paraspinal and epidural mass is formed.

EXTRADURAL ARACHNOID CYST

EPIDEMIOLOGY

• Spinal extradural arachnoid cysts are thought to be extradural outpouchings of arachnoid that communicate with the intraspinal subarachnoid space through a small defect in the dura.¹⁸

• The pathogenesis of spinal extradural arachnoid cysts is unclear. It is known to be associated with trauma, surgery, arachnoiditis, and neural tube defects.^19,²⁰ In many cases, there is no identifiable cause, and most non-traumatic spinal extradural arachnoid cysts are thought to be congenital.¹⁹ Therefore, they are thought to arise from a congenital dural defect that allows the arachnoid membrane to herniate through the adjacent dura mater.^18,^21,²² The bone changes (thinning of the laminae) observed on MRI and at the time of surgery suggest a long-standing lesion with a congenital origin.

• The mechanism by which the spinal cord is compressed by such cysts is unclear, but several theories have been proposed on the pathogenetic mechanism ²³:

1 A one-way valve causes intermittent increased pressure within the extradural cyst, thereby compressing the spinal cord.

2 A hyperosmolar collection of fluid within the cyst causes free water to enter, resulting in an enlargement of the cyst.

3 The cyst wall secretes fluid and it expands gradually because of the lack of communication with the subarachnoid space. The valve-like mechanism has been speculated as the most common cause for the expansion of the cyst and spinal cord compression.²⁴

HISTOLOGY

The histologic finding of the extradural arachnoid cyst is same as that of the intradural arachnoid cyst.

RADIOLOGY

With all sequences, the signal within the lesion is isointense to CSF.²⁵ Thin-walled septations within the cyst can be visualized (Figs. 2-28 and 2-29). With intravenous contrast enhancement, an exact differentiation between cystic tumors, synovial cysts, and arachnoid cysts can be made.