An unusual localization of intraosseous schwannoma: the hamate bone

Abstract

Intraosseous schwannoma of the hamate bone presented in this case is a very rare benign tumor, and its diagnosis com[1]bined with clinical, imaging and needle biopsy is important to guide further therapy. The diagnosis of schwannoma of the hamate was proved histologically following its surgical treatment by curettage. 

Introduction

Schwannomas (neurilemmomas) are the most common tumors derived from schwann cells of nerve fibers. It is a rela[1]tively common tumor, approximately 5% of all benign soft tissue tumors. Intraosseous schwannoma, however, is very rare, accounting for less than 1% of all benign bone tumors.1-8 They are generally aympto[1]matic. Radiologically, schwannomas pres[1]ent as well-circumscribed lytic, expansile intramedullary lesions with sclerotic bor[1]ders. There is no predilection of sex, age and location. There is no case of intraosseous schwannoma around the wrist has been described in literature. We present the first case report of an intraosseous schwannoma of hamate bone of the wrist.

Case Report

A 34-year-old woman presented with 2- year history of the right wrist pain. The pain generally occured in the morning and increased with daily activations. There was mildly significant swelling on the wrist. Physical examination showed a 2×2-cm firm, round and hard mass on the dorsal aspect of the right wrist. The wrist had almost full range of motion. The overlying skin was intact, and there was no evidence of wamth, erythema, or induration. Plain radiographs revealed a lytic lesion with well-defined sclerotic borders in the hamate bone. It was multilocular, with no inner calcifications; the dorsal cortical part of the bone was disrupted without periosteal reaction (Figure 1A). Computerized tomog[1]raphy (CT) scans showed the lesion was expansile with cortical break-through in its its dorsal aspect. It has sclerotic, well[1]defined borders with narrow transition zone (Figure 1B). Magnetic resonance imaging (MRI) revealed again cortical disruption of the dorsal cortex and extraosseous soft tis[1]sue extension of the lesion to the dorsal side of the hand (Figure 2A). The lesion was hyperintense on PD-weighted sequences, and demonstrated solid and homogenous enhancement on contrast-enhanced T1- weighted sequences. Despite its soft tissue extension, it has still well-defined borders within bone and soft tissues. Immunohistochemistry studies of the tissue obtained by a needle biopsy showed S-100 protein positive spindle cells, addressing a benign neurogenic tumor. Then, we per[1]formed totally resection of the lesion by curettage via dorsal longitudinal incision. After curettage, bone defect was recon[1]structed by autograft harvested from the iliac crest. The gross specimen showed a solid lobular tumor with clear boundary, partly located within the bone. Histolo - gically, the tumor was composed of multi[1]ple nodules of spindled Schwann cells with[1]in the bone parenchyma (Figure 3A and B). Antoni A pattern was predominant. Nuclear palisading and Verocay bodies were present (Figure 3C). Immunohi stochemically, tumoral cells were strongly positive for S[1]100 protein (Figure 3D). The final diagnosis was plexiform schwannoma of the bone. The patient did not receive any adjuvant therapy after the surgery. She had been fol[1]lowed up for seven months with no sign of recurrence and pain (Figure 2B).

Discussion

Schwannomas are the most common tumors derived from schwann cells of nerve fibers. Intraosseous schwannoma, however, is very rare, accounting for less than one per-cent of all benign bone tumors.1-8 In addition to the sacrum, iliac wing and mandible, intraosseous schwannomas also occur in the long bones, vertebra, fibula and frontal bone,1-5,8 however, there is no case in literature intraosseous schwannoma of hamate bone. Schwannomas may involve bone tissue via three mechanisms: i) they may be intramedullary, producing rarefac[1]tion of the bone; ii) they may be located within the nutrient canal, with the formation of a dumbbell-shaped tumor; or iii) they may be extraosseous, eroding into the bone.1,6,7 On the base of imaging, intraoper[1]ative findings of dumbbell-shaped tumor inside and outside the hamate bone and gross examination, this case demonstrates an example of intraosseous schwannoma. Intraosseous schwannomas have char[1]acteristic radiographic features such as a well-defined lytic lesion, sclerotic margins, narrow transition zone, lobulated contours, cortical expansions and absence of central calcification. However, all these radi[1]ographic findings are non-spesific and does not help clinicians in differential diagnosis. On the basis of CT and direct X- ray, It is difficult to differentiate the intraosseous schwannoma from other lytic benign bone lesions of such simple bone cyst, aneurys[1]mal bone cyst, and giant cell tumor of bone, fibrous dysplasia, enchondroma and non[1]ossifying fibroma. On MRI, shwannomas are solid lesions contrary to simple bone cyts and aneurysmal bone cyts. T2 hyperin[1]tensity of the schwannomas can be bright as much as that of enchondroma. However, enhcondromas may contain calcifications with typical contrast enhancing views on MRI. Giant cell tumors, fibrous dysplasia and non-ossiying fibromas are heteroge[1]nous tumors having both T2 hyper and hypointense areas. On CT, giant cell tumors have lytic borders due to its local agresive-

ness and fibrous dysplasias have ground[1]glass apperance with wide transition zone. CT and MRI scans in our case showed that the lesion was expansile with cortical break-through and extra osseous extension but well defined borders within both bone and soft tissues. The sclerotic, well-defined borders have narrow transition zone on CT and plain radiography. These findings may be the characteristic for intraosseous schwannomas.1-4,7,8 The final diagnosis of intraosseous schwannoma was made by his[1]tologic examination of tumor. The histolog[1]ic features of this lesion are similar to those of soft tissue schwannoma.

Conclusions

Making diagnosis of an intraosseous schwannoma pre-operatively with clinical and radiological findings, and needle biopsy was beneficial to the patient to avoid unnecessary adjuvant therapy. Surgical excision is the pre[1]ferred treatment option. If significant bony defect occurs, bone grafting should be consid[1]ered. These tumors are generally well encapsu[1]lated without invasion of the surrounding struc[1]tures, enabling complete resection possible. Recurrence is rare after complete resection.