Juxtacortical Lesions

Juxtacortical lesions of bone are a group of entities with differing histogenesis and pathogenesis that arise in association with the periosteum. Periosteal stromal cells participate in fracture repair, and as such, they are able to differentiate into fibroblastic, osteoblastic, and chondroblastic lineages. Therefore, lesions of the periosteum present a variety of tissue components, particularly in those entities considered to be reactive, proliferative lesions. In many cases, the differentiation of reactive, benign, and malignant entities relies on clinical presentation, location, and radiographic findings as much as on histologic appearance.

Florid Reactive Periostitis

Florid reactive periostitis (FRP) presents as a periosteal-based fibroblastic proliferation with intermixed fibro-osseous elements that can histologically mimic osteosarcoma. It has histologic similarities to nodular fasciitis, subungual exostosis, and bizarre parosteal osteochondromatous proliferation (BPOP).

Historical Overview

Reactive fibro-osseous lesions of the digits were described in 1933 by Mallory in a review of “metaplastic and neoplastic bone and cartilage containing tumors of soft parts.” Recognizing the clinical and histologic similarity to nodular fasciitis, the term “parosteal fasciitis” was commonly used until the 1980s. Clinicopathologic and radiologic characterization of FRP was described in 12 cases by Spujt and Dorfman in 1981.

Incidence and Demographics

FRP is uncommon. In a survey of 203 benign and reactive lesions of the hands and feet, Ostrowski and Spujt found 16 cases of FRP, slightly greater than the number of cases of BPOP. Most patients are between 20 and 40 years of age (range, first to fifth decade). The condition occurs equally in both sexes. Although a history of trauma is usually not elicited, the favored etiology is traumatic.

Localization and Clinical Manifestations

More than 75% of FRP cases occur in the bones of the hand, followed by the bones of the foot; the most common site is the proximal or middle phalanx. It has occasionally been described in the long bones of the upper and lower extremities. Presenting symptoms are usually pain and swelling and, when erythema is present, an infectious etiology is frequently favored clinically. When FRP occurs in the foot, discomfort is typically related to wearing shoes, and the clinical differential of Morton neuroma may arise.

Radiologic Features and Gross Pathology

In an early stage lesion, radiographs show a periosteal-based soft tissue swelling, often circumferential, that with maturation becomes partially ossified. There is adjacent subperiosteal new bone formation, sometimes laminated, and rarely scalloping of the underlying cortex ( Fig. 8-1 ). More mature lesions may assume an osteochondromatous appearance but lack continuity with the marrow space as seen in conventional osteochondroma.

Histopathology

Depending on the age of the lesion, there are varying degrees of fibroblastic and chondro-osseous differentiation. Early lesions are fibroblastic with variable cellularity, often with myxoid regions similar to nodular fasciitis. As in nodular fasciitis, mitotic figures may be frequent, albeit normal. Early bone formation appears as wisps of osteoid that coalesce into trabeculae of woven bone with osteoblast rimming ( Fig. 8-2 ). One third of cases have immature chondroid elements, similar to those seen in fracture callus. Maturing lesions develop a zonal appearance, with more organized arcades of woven and lamellar bone at the periphery and immature woven bone centrally. Although hypercellularity and reactive changes (open chromatin, nucleoli) may be present in the cartilaginous component and elicit concern for chondrosarcoma, osteoblastic areas lack pleomorphism, and the appearance is more reminiscent of fracture repair or BPOP.

Differential Diagnosis

Malignant entities in the differential diagnosis include high-grade juxtacortical osteosarcoma, parosteal osteosarcoma, and surface involvement by a medullary osteosarcoma. Radiologic correlation is helpful; lack of cortical destruction and medullary involvement argues against a primary medullary osteosarcoma as does the low incidence of primary medullary osteosarcoma in the hands and feet. Parosteal osteosarcomas are broad based, often nearly circumferential, and heavily ossified. In contrast, FRP is periosteal based, has less ossification, and lacks a lobulated appearance. Histologically, parosteal osteosarcoma has well-developed, long bony trabeculae with an intervening fibroblastic stroma with low cellularity and lacks the nodular fasciitis–like areas and small trabeculae of woven bone as seen in FRP. High-grade juxtacortical osteosarcoma displays obvious pleomorphism, a high nucleus-to-cytoplasm ratio, and atypical mitotic figures. Osteoid formation in high-grade tumors is variable and fishnet-like or lacelike, instead of the trabecular pattern seen with osteoblast rimming as in FRP, fracture, or other reactive lesions.

Ancillary Diagnostic Studies and Genetics

Much of the literature on FRP consists of case reports in radiologic or clinical journals, with little published immunohistochemical or molecular information. One study has found MDM2 and CDK4 immunostaining to be useful in differentiating low-grade osteosarcoma, including parosteal osteosarcoma, from a number of benign and reactive conditions, including FRP. FRP has not been shown to have a chromosomal abnormality as found in BPOP [t(1;17)] and subungual exostosis [t(X;6)(q13-14;q22)], although they share histologic similarities.

Bizarre Parosteal Osteochondromatous Proliferation

Bizarre parosteal osteochondromatous proliferation (BPOP), or Nora lesion, is a benign, self-limited chondro-osseous proliferative lesion, believed to be associated with trauma that has histologic features similar to fracture repair.

Historical Overview

Nora and colleagues described 35 cases of a distinctive osteochondromatous proliferation of the extremities in 1983. Most cases involved bones of the hand, although involvement of the small bones of the foot was also seen. More recently, the acronym BPOP has been favored over the eponymous designation.

Incidence and Demographics

The largest single institution experience, an update of the original series, was published in 1993, reporting 65 cases. Patients ranged in age from 8 to 73 years (mean, 34 years), and the study included 35 lesions in the hand, 10 in the feet, and 17 in the long bones. In both series, there was no predilection by gender and the local recurrence rate was 55%. In a survey of 203 benign and reactive lesions of the hands and feet, Ostrowski and Spujt found nine cases of BPOP.

Localization and Clinical Manifestations

BPOP usually presents as a painless palpable mass adjacent to the shaft of one of the phalanges or metacarpals. About 30% of patients give a history of trauma.

Radiologic Features and Gross Pathology

Radiographically, BPOP is a well-marginated surface lesion with variable periostitis, edema, and tumefaction, depending on the duration of the lesion ( Fig. 8-3 ). In contrast to a conventional osteochondroma, ossification and calcification are present in a disorderly pattern. Also, the lesion is not attached to the cortex by a pedicle and does not communicate with the medullary cavity. Grossly, the lesion is usually less than 2 cm and is composed of intermixed bone and cartilage, without the orderly maturation seen in osteochondroma.

Histopathology

Microscopically, BPOP has variable amounts of three components: cartilage, bone, and spindle cells. Thus, it resembles a fracture callus ( Fig. 8-4 ). The usually predominant cartilage component is hypercellular with large immature chondrocytes, raising concerns for malignancy. The osteoid component includes trabeculae of woven bone intermixed with the fibroblastic element and areas of ossification of the lesional cartilage matrix. Osteoblast rimming is present, and the reactive bone frequently has a distinctive bluish tint, similar to that seen in osteoid osteomas. The fibroblastic component, although mitotically active and reactive in appearance, lacks pleomorphism. Occasionally, a pseudocartilage cap without a perichondrial layer is present, causing confusion with an osteochondroma, further emphasizing the need for radiologic correlation.

Differential Diagnosis

The differential diagnosis of BPOP includes conventional osteochondroma; juxtacortical osteosarcoma (parosteal or periosteal); and if cartilage predominates, surface chondroma and chondrosarcoma. As described above, BPOP lacks the orderly maturation from cartilage cap to mature bone and marrow space seen in an osteochondroma. Both parosteal and periosteal osteosarcoma are extremely rare in the hands and feet, with fewer than 10 cases in the literature. However, 25% of cases of BPOP occur in the long bones, where the lesions are frequently larger and the differential diagnosis of juxtacortical osteosarcoma is of real concern. Parosteal osteosarcoma has a broad, sessile attachment to the cortex, and microscopically has long, well-formed trabeculae of mixed woven and lamellar bone separated by a relatively bland fibroblastic proliferation. In contrast, the trabeculae in BPOP are short, often have a distinctive blue tint, and are intermixed with fibroblastic and chondroid elements. Malignant chondroid lesions, whether on the surface or in soft tissue, are extremely rare in the hands and feet. In the long bones, they would lack the intermixture of cartilaginous, bony, and fibroblastic elements seen in BPOP.

Ancillary Diagnostic Studies and Genetics

A recent cytogenetic and fluorescence in situ hybridization (FISH) study found evidence of a t(1;17)(q32;q21) translocation in all of five cases of BPOP examined, suggesting that BPOP is a benign locally recurrent neoplasm as opposed to a reactive proliferative process. The genes involved in the translocation have not as yet been identified.

Treatment and Prognosis

Conservative excision is followed by recurrence in 50% of cases. Thus, one or more recurrences should not necessarily suggest the lesion is actually a missed juxtacortical osteosarcoma.

Subungual Exostosis

Subungual exostosis is a benign, self-limited proliferative lesion that occurs in the distal phalanx. It is believed to be secondary to trauma, and is histologically similar to BPOP.

Historical Overview

Subungual exostosis was first described by Dupuytren in 1817 and was followed by his series of five cases in 1847. It shares common clinical and histologic features with BPOP.

Incidence and Demographics

Its peak incidence is in the second and third decades. The lesion occurs more frequently in men than in women.

Localization and Clinical Manifestations

The lesion occurs only in the distal phalanx, usually below the nail bed. The great toe is involved in 80% of cases. Symptoms of swelling and pain are common, the latter sometimes quite intense due to its association with the nail bed. There is usually a history of trauma.

Radiologic Features and Gross Pathology

Radiographically, subungual exostosis evolves from a soft tissue periosteal density to a calcifying mass with trabeculated bone extending distally from beneath the nail bed ( Fig. 8-5 ). In contrast to an osteochondroma, it is not continuous with the marrow space of the underlying bone.

Histopathology

Histologically, early lesions may have a reactive fibroblastic appearance similar to a fasciitis or periostitis, but as the lesion matures it displays a mixture of immature cartilage and bone similar to BPOP ( Fig. 8-6 ).

Differential Diagnosis

The histologic differential diagnosis is similar to the discussion above for BPOP. However, the distinctive location and radiologic appearance should signal the high likelihood of subungual exostosis.

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