Lesions Characterized by Osteoid Deposition and Nonaggressive Radiology

Definition and Synonyms

Osteoma and bone island or enostosis are benign slow-growing non-neoplastic lesions composed of dense mature compact or cancellous bone. The basic difference between the two is their location—osteoma being a surface (parosteal, periosteal) lesion, whereas bone island an intramedullary lesion.

Brief Historical Overview

A literature search yielded two case reports of surgical management for orbital osteoma by Norris and Fridenberg, dated 1884 and 1902, respectively. Norris described a bony “growth” or “enormous thickening” of the right orbital floor in a 32-year-old woman without actually using the term “osteoma.” Fridenberg described a mass of the right orbit in a 21-year-old man with a history of trauma to the region. He referred to this osseous protrusion as being an osteoma and suggested a traumatic etiology, citing 17 cases reported by Schuchardt, Moser, and Miodowsky, with 12 of the 17 patients having a history of trauma.

In 1951, Geschickter and Copeland reported the first case series of parosteal osteoma in 16 patients. The authors attributed local recurrence and even metastatic potential to parosteal osteoma, a conclusion most likely resulting from the inclusion of several cases of surface osteosarcoma in the study group.

In 1905, bone island was first described by Stieda, who at the time called these intramedullary lesions “compact bone nuclei.” Seven years later, the term “island” was coined by Fishers, who also emphasized the diagnostic importance of these lesions.

Incidence and Demographics

Osteoma is common in the craniofacial bones and very rare in the rest of the skeleton, with an incidence in the general population ranging between 0.014% and 0.43%, according to different authors. However, incidences as high as 3% have been reported for paranasal sinus osteomas. In patients with Gardner syndrome, the incidence of craniofacial osteoma is reported to be 21% to 24%. The lesion has a predilection for males, with a male-to-female ratio of 1.3 to 2 : 1, and it is more common in adults, with a maximum incidence in the fourth to fifth decades of life for parosteal osteoma and slightly earlier (third to fourth decades) for craniofacial osteoma. Osteomas are rare in adolescence except for patients with Gardner syndrome, although many patients are asymptomatic for long periods of time before diagnosis, and therefore the age of onset is often difficult to establish.

Bone islands are common lesions equally seen in all age groups and both sexes, and they are usually discovered incidentally on radiographs obtained for unrelated reasons.

Localization and Clinical Manifestations

Osteomas have a predilection for craniofacial bones, with approximately 75% of cases arising in the frontal and ethmoidal sinuses. The skull, both inner and outer tables, and the mandible are also involved. The rest of the skeleton is rarely affected, with the metaphysis and diaphysis of long bones, vertebrae, and pelvis being the most common locations.

Bone islands are usually solitary lesions located in the medullary metaepiphysis of the long bones, especially the femur, as well as the pelvis, ribs, and vertebral bodies. Involvement of the skull is exceedingly rare.

Most osteomas are asymptomatic, with some patients seeking medical care solely for cosmetic reasons. In the craniofacial location, the lesions become symptomatic when they interfere with the normal sinus drainage, with subsequent sinusitis, headaches, nasal discharge, and sometimes anosmia. Osteomas of the orbit can cause exophthalmos, diplopia, and even vision loss. In the long bones, a history of long-standing dull pain is sometimes the only complaint.

Multiple craniofacial osteomas are associated with Gardner syndrome, an autosomal dominant disorder also characterized by intestinal adenomas, epidermal inclusion cysts, fibromas, aggressive fibromatoses, and sometimes dental malformations. The soft tissue tumors and craniofacial osteomas usually precede the onset of intestinal symptoms by an average of 17 years, which is why their presence should prompt genetic testing and a screening colonoscopy in these patients.

Radiologic Features and Gross Pathology

Craniofacial osteomas are smooth, radiodense lesions protruding within paranasal sinuses and orbit, or from the outer or inner table of the calvarium. They have a very typical appearance and thus the risk of radiographic misdiagnosis is extremely low. Computed tomography (CT), particularly three-dimensional CT scan, is recommended for treatment planning purposes ( Figs. 5-1 and 5-2 ).

Conventional radiographs show parosteal osteomas as broad-based, lobulated, smooth radiodense lesions located on the surface of the long bones without continuation with the medullary cavity of the underlying bone. The degree of radiodensity is similar to that of cortical bone. An intralesional radiolucent area (nidus) is not seen on conventional radiography or CT, and there is no overlying periosteal reaction. Magnetic resonance imaging (MRI) can be used to exclude the presence of a soft tissue mass.

Bone islands are small, intramedullary, radiodense lesions mimicking metastatic disease. They are usually round to ovoid, although they can have an irregular contour studded with radiating bone spicules protruding from the lesion into surrounding spongiosa. The size can be quite variable, and they range in diameter from 1 to 40 mm ( Figs. 5-3 and 5-4 ). Of help in differentiating bone islands from osteoblastic metastases is bone scintigraphy, which is uniformly negative in bone islands, whereas it shows avid radionuclide uptake in metastatic foci.

Grossly, osteomas are dome-shaped, lobulated, ivory, hard lesions whose texture depends on the combination of trabecular and compact bone ( Fig. 5-5 ).

Histopathology

Histologically, both osteoma and bone island are composed of dense compact bone with intraosseous Haversian systems. Parosteal osteomas can be trabecular. The lesional bone shows multiple parallel cement lines, indicating a slow growth rate ( Figs. 5-6 and 5-7 ).

Histologic Variants and Grading

Two histologic types of osteoma have been described: compact or “ivory” and trabecular or spongy.

The compact variant is composed of dense lamellar bone with well-formed Haversian systems and few narrow spaces. The spongy osteoma is composed of irregular bony trabeculae lined by osteoblasts enclosing fatty or hematopoietic marrow.

Differential Diagnosis

Craniofacial osteomas have a very characteristic radiographic appearance and do not pose major problems in differential diagnosis.

The differential diagnosis of parosteal osteoma includes parosteal osteosarcoma, osteoid osteoma, sessile osteochondroma, and juxtacortical myositis ossificans. Some of these entities can be differentiated radiographically whereas, for others, biopsy and histologic examination are warranted.

The most important entity entering the differential diagnosis of solitary osteoma is parosteal osteosarcoma. Although radiographic examination may not always be conclusive, a biopsy of the lesion is usually diagnostic. The pattern of bone deposition can be similar between the two entities, but the intervening stroma is different, with a spindle cell, somewhat pleomorphic cellular proliferation characterizing parosteal osteosarcoma, as opposed to a fibrovascular or adipose stroma seen in parosteal osteoma.

Osteoid osteoma is usually excluded radiographically by the presence of a nidus and clinically by characteristic nocturnal pain.

Sessile osteochondroma shows the typical cartilage cap and blending of the lesional spongiosa with the spongiosa of the bone of origin.

Osteoblastic metastatic disease is the most important entity that enters the differential diagnosis of enostosis. An isotope bone scan is usually sufficient to settle the diagnosis; metastatic disease shows avid isotope uptake compared to enostosis, a scintigraphically cold lesion.

Genetics

Gardner syndrome is a variant of familial adenomatous polyposis (FAP) with several extracolonic manifestations, including craniofacial osteoma, epidermal inclusion cyst, aggressive fibromatosis, fibroma, and lipoma. The condition is caused by truncating mutations at codons 1403 and 1578 of the adenomatous polyposis coli gene. Although the same gene is mutated in classical FAP, attenuated FAP, and congenital hypertrophy of retinal pigment epithelium, other codons are affected in those conditions. The incidence of Gardner syndrome is estimated to be about 1 in 7500 births. Patients with Gardner syndrome present in adolescence with multiple craniofacial osteomas and soft tissue tumors, the onset of which precedes the onset of intestinal adenomatosis by about a decade. Genetic testing and screening colonoscopy is therefore of paramount importance in these patients; if the condition is left untreated, they all develop intestinal adenocarcinoma by 40 years of age.

Treatment and Prognosis

Small asymptomatic osteomas and bone islands do not require treatment. Simple observation is sufficient. Large and rapidly growing craniofacial osteomas, as well as symptomatic ones, regardless of size, require resection followed by some form of local reconstruction. Endoscopic resection is advocated for small and medium-size craniofacial lesions, and a combination of external and endoscopic approach can be used for large frontal sinus osteomas.

When not associated with Gardner syndrome, the prognosis of osteoma and bone island is excellent.

Definition and Synonyms

Osteoid osteoma is a benign bone-forming tumor characterized by its small size, limited growth, and disproportionate pain.

Brief Historical Overview

The term “osteoid osteoma” was coined by Jaffe in 1935 when he published his first series of five cases. All cases were located within the medullary cavity of various bones and clinically diagnosed as “localized osteomyelitis” or “bone abscess,” although none showed histologic evidence of inflammation. Instead, the common denominator of all five lesions was a “nidus-like focus of osteoid and new bone” whose removal “promptly and permanently abolished the patient's complaints.” A few years later, Jaffe realized that cases being diagnosed as “sclerosing nonsuppurative osteomyelitis” and “osteomyelitis with cortical bone abscess” showed the same lack of inflammation and same pattern of bone formation as his previous intramedullary cases, but the lesions were located within the cortex of long bones. Through the study of resection specimens from patients clinically diagnosed with osteomyelitis by 1940, the concept of osteoid osteoma was fully launched. The term was formulated to, in Jaffe's own words, “characterize the lesion's pathological pattern.” Jaffe's masterful description of the clinical presentation, radiographic aspect, and histology of osteoid osteoma is to this day exquisitely accurate.

Incidence and Demographics

Osteoid osteoma is a common benign neoplasm representing approximately 10% to 15% of benign bone tumors. It is most frequently seen in children and young adults and has a peak incidence in the second decade of life. Only rarely does osteoid osteoma occur in children younger than 5 years of age or adults older than 35. The tumor displays a male predilection with a male-to-female ratio of about 2 to 3 : 1, and all races are affected equally.

Localization and Clinical Manifestations

The tumor can involve any bone but has a distinct predilection for the long bones of the lower extremity, with the femoral neck being the single most frequently affected site. The long bones of the upper extremity, particularly around the elbow, can also be involved. The flat bones are uncommon sites for osteoid osteoma, and the craniofacial bones are almost never involved. The location within a long bone is either diaphyseal or metaphyseal, usually intracortical, although subperiosteal, endosteal, or intramedullary osteoid osteomas have been described. They can also be intra-articular or extra-articular, with the most common intra-articular location affecting the hip joint. When located in the spine, the vertebral bodies are spared, the lesion having a clear predilection for the posterior arch.

Clinically, 80% of patients present with pain of several months' duration. Intermittent at first, the pain becomes relentless and is more severe at night. Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) are very effective in alleviating the pain for several hours at a time. The pain is believed to be elicited by increased secretion of prostaglandins, particularly prostaglandin E ₂ , by the neoplastic osteoblasts as a response to increased vascular pressure within the nidus. When the lesion is located near a joint, patients may complain of joint effusion and arthritis-like symptoms, whereas its presence near a growth plate can stimulate abnormal bone growth, resulting in significant limb asymmetry. Muscle atrophy is present in about 20% of patients. Tumors located in the vertebral column may cause muscle spasm with subsequent scoliosis. Soft tissue edema, local tenderness, and increased temperature are seen in superficially located lesions.

Radiologic Features and Gross Pathology

Conventional radiographs of an intracortical osteoid osteoma typically show the nidus as a small (less than 1.5 cm) radiolucent area surrounded by a zone of dense fusiform osteosclerosis and periosteal reaction ( Fig. 5-8 ). When prominent, the periosteal reaction and osteosclerosis can obscure a small nidus on conventional radiographs ( Fig. 5-9 ). The nidus shows different degrees of mineralization and subsequent variable central radiodensity. For lesions located near a joint, the absence of periosteum results in significantly less osteosclerosis, making their identification difficult ( Fig. 5-10 ). Similarly diminished surrounding osteosclerosis is seen in lesions located within the medullary cavity.

Bone-windowed CT done in 1- to 2-mm slices and adequately reconstructed with a bone algorithm is the best radiographic method to diagnose osteoid osteoma. It should be used in all patients with specific symptoms, even when conventional radiographs are nondiagnostic. Contrast enhancement may help differentiate an osteoid osteoma nidus from a Brodie abscess ( Fig. 5-11A and B ).

Technetium-99 bone scintigraphy is always positive, showing a small area of intense isotope uptake in a less positive and diffuse uptake halo known as the “double density sign” ( Fig. 5-12A and B ). Although MRI can be used to diagnose osteoid osteoma, it is not recommended by most authors because of its capacity to highlight the rather intense surrounding soft tissue edema, thus creating a great deal of diagnostic confusion. However, in a 2003 study comparing the capacity of gadolinium-enhanced MRI and thin-section CT to highlight osteoid osteoma, Liu and colleagues found the two methods similarly accurate, with the latter being slightly inferior.

Grossly, the nidus of osteoid osteoma appears as a small, gritty, red lesion sharply demarcated from a surrounding hypervascular zone within an adjacent sclerotic cortex ( Fig. 5-13 ).

Histopathology

Histologically, osteoid osteoma is composed of interconnecting seams of osteoid and woven bone rimmed by active osteoblasts, in a loose fibrovascular background with occasional intermixed osteoclasts. The nidus displays gradual bone maturation from the periphery toward the center. A peripheral zone of dense osteosclerosis sharply demarcated from the nidus can be seen in resection specimens ( Figs. 5-14 and 5-15 ). However, for intramedullary osteoid osteomas, the degree of reactive bone formation is much less prominent.

Differential Diagnosis

Osteoid osteoma should be differentiated from osteomyelitis, Brodie abscess, bone island, osteoblastoma, and intracortical osteosarcoma. The absence of pain, together with a negative radionuclide bone scan, can differentiate intramedullary osteoid osteoma from a bone island. However, the same does not apply to osteomyelitis, a bone abscess, and intracortical osteosarcoma, for these conditions show avid radionuclide uptake as well. Furthermore, a small intracortical sequestrum can be mistaken for the nidus of an osteoid osteoma on conventional radiographs and CT scans. Biopsy of the lesion is in most cases diagnostic, provided thorough clinical and radiologic correlation is performed.

Osteoblastoma shows identical histologic features with osteoid osteoma, being different only in its unrestricted growth potential and subsequent increased local aggressiveness as well as its pre­dilection for the axial skeleton. Correlation with radiographic appearance is paramount in differentiating osteoid osteoma from osteoblastoma.