Intraosseous Lipoma. Duk Seop Shin, M.D., Eun Seok Kwak, M.D., Joon Hyuk Choi* MATERIALS AND METHODS

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1 J. of Korean Orthop. Assoc. 2003; 38: Intraosseous Lipoma Duk Seop Shin, M.D., Eun Seok Kwak, M.D., Joon Hyuk Choi* Departments of Orthopedic Surgery and Surgical Pathology*, College of Medicine, Yeungnam University, Daegu, Korea Purpose: We analyzed 10 cases of intraosseous lipoma to elucidate the clinical, radiographic and pathological features, and studied the correlation between of the radiographic and pathological findings. Materials and Methods: Ten cases of 9 patients were the subjects of this study. One patient had intraosseous lipoma in bilateral calcaneus. For the analysis of clinical features, medical records were reviewed. The simple x-ray, MRI and CT images were reviewed. The MRI examinations included T1-weighted, T2-weighted and fat-suppressed (STIR) images in all cases. Histological findings were reviewed and classified with Milgram s stage classification, and the radiographic findings also in 3 stages, and correlation of radiographic and histological stage was studied. Results: The mean age of the patients was 39 years (range, years). Seven patients were men and three were women. The mean size of the lesion was 3.4 cm (range, cm). The most common anatomical location was calcaneus as 7 cases. Only four patients complained pain as symptom and six patients were asymptomatic. We had curettage and bone graft for the treatment. There were no local recurrence or malignant change during the follow up. With Milgram s classification, two cases were stage I, four were stage II and the other four were stage III. In radiographic stage, two cases were stage I, three were stage II and five were stage III. The radiographic stages were very well correlated to the histological stage. Conclusion: Intraosseous lipomas had characteristic radiographic and histological features according to the presenting stages, and their radiographic findings were very well correlated to the histological findings. Key Words: Intraosseous lipoma, Radiographic features, Histological stage Intraosseous lipoma was first described in 1910, and was thought to be one of the rarest primary bone tumors 12). It is composed of mature fat cells with variable small quantities of fibrous and vascular tissue 2,17,21). Intraosseous lipomas may undergo varying degrees of involution, with areas of fat necrosis, cyst formation and dystrophic calcification 21). Diagnosis of a intraosseous lipoma with simple x-ray may not be easy. However, the presence and nature of intra-lesional calcification may give a clue to the diagnosis, particularly in lesions in the calcaneus, where the features are considered to be pathognomonic. The advent of computed tomography (CT) and magnetic resonance imaging (MRI) has enabled the identification of intralesional fat, as well as dystrophic calcification and cyst formation 2,4,8,20,24,26). Although a fair number of reports based on observations in a single patient with this disease have been published, there were only a few reports in which a series of patients were studied 2,9,12,21,22). In Korea, also, there have been only a few case reports Address reprint requests to Duk Seop Shin, M.D. Department of Orthopedic Surgery, Yeungnam University Hospital Daemyung-dong, Namgu, Daegu , Korea Tel: Fax: shinds@med.yu.ac.kr 6,7,14,27). We analyzed our cases of intraosseous lipoma to elucidate the clinical, radiological and pathological features of this disease, and studied the correlation between of the radiographic and pathological findings. MATERIALS AND METHODS Ten cases of 9 patients that we have treated in our hospital from 1993 to 2002 were the subjects of this study. One patient had intraosseous lipoma in bilateral calcaneus. The diagnosis was made by radiographic and histological. For the analysis of clinical features, the age, gender, anatomical location, presence of symptom, methods and results of treatment, and local recurrence or malignant change in follow up were investigated with review of medical records. The simple x-ray were reviewed and assessed for lesion location, size, calcification, marginal sclerosis, trabeculation and bone expansion. The MRI and CT images were also evaluated for the same features, and especially for the cystic degeneration. The MRI examinations varied in technique, but included T1-weighted, T2-weighted and fat-suppressed (STIR) images in all cases. Histological findings were reviewed and classified with Milgram s stage classification 21) as follows; 1) stage I, lesion composed of mature fat cells with atrophic lamellar bone, 2) stage 526

2 Intraosseous Lipoma 527 II, lesion of fat necrosis and focal calcification, 3) stage III, most of the tumor tissue has died with wide calcification or ossification, cystic degeneration or ischemic bone formation. And, we also classified the radiographic findings in 3 stages as follows; 1) stage I, no calcification in simple x-ray and pure fat component in MRI, 2) stage II, focal calcification in simple x-ray but no cystic degeneration in MRI, 3) stage III, wide calcification in simple x-ray and cystic degeneration in MRI, and investigated the correlation of radiographic and histological stage. RESULTS 1. Clinical Features The mean age of the patients was 39 years (range, years) and the mean follow up period was 18 months (range, months). Seven patients were men and three were women. The mean size of the lesion was 3.4 cm (range, cm). The most common anatomical location was calcaneus as 7 cases, followed proximal tibia as 2 and ilium as one. Only four patients complained pain as symptom, while the three patients had not related with the lesion pain and the other two patients were asymptomatic. We had curettage and autogenous or allogenic bone graft for the treatment of 8 cases. In two cases both autogenous and allogenic bone graft were done due to large size of the tumors. There were no local recurrence or malignant change during the follow up. The clinical features are summarized in Table Radiographic Features The margin of the lesion was well defined in all the cases (100%). Partial or complete marginal sclerosis was seen in 7 of Table 1. The detail features of ten cases Case Age Gen. Location Sx. Calcification Stage 1 34 F Prox.Tibia Pain - I 2 35 M Calcaneus Pain + II 3 35 M Calcaneus - - III 4 36 M Calcaneus Pain + II 5 60 M Ilium Pain + III 6 36 M Calcaneus - + III 7 40 M Calcaneus - + III 8 38 F Calcaneus - - I 9 33 M Calcaneus - + II F Prox.Tibia - - III Gen.: gender; M: male; F: female; Prox.: proximal; Sx.: symptom. A B C Fig. 1. (A) Iintraosseous lipoma in calcaneus shows cystic lesion with calcification in simple x-ray. (B) Stage III intraosseous lipoma shows lesion of iso-intense with subcutaneous fat and cyst on T1- weighted images of MRI. (C) Stage III intraosseous lipoma shows suppression of the fat signal surrounding cyst in the STIR images of MRI.

3 528 Duk Seop Shin Eun Seok Kwak Joon Hyuk Choi 4. Correlation of Radiographic and Histological Features The radiographic features of each case were classified as 3 stages as mentioned in the methods. Two cases were stage I, three cases were stage II, and five cases were stage III. The radiographic stages were very well correlated to the histological stage of Milgram s, but one case. Fig. 2. The mature adipose tissue surrounding atrophic lamellar bone (H&E stain, 100). 10 cases (70%). Calcification was noted in 6 of 10 cases (60%), typically central in location, and varied in size (Fig. 1A). Trabeculation was seen in 3 of 10 cases (30%) and bony expansion in 5 of 10 cases (50%). In MRI examinations, all lesions were composed entirely or partly of fat, which was iso-intense with subcutaneous fat on both T1-weighted images (high signal, Fig. 1B) and T2-weighted images (intermediate to high signal). Suppression of the fat signal was observed in all cases with STIR images (Fig. 1C). Areas of low signal, which corresponded with calcification or ossification on simple x-ray were also found. Marginal sclerosis was manifested by a rim of low signal on all pulse sequences. Cysts were seen in 5 of 10 cases (50%) in MRI, and they appeared as well-demarcated areas of intermediate signal on T1-weighted images and high signal on T2- weighted and STIR images (Fig. 1B, C). In one case, CT was done and demonstrated area of low attenuation equivalent to fat density. The detail imaging appearances are summarized in Table Histological Features Characteristic histological findings included areas of mature adipose tissue surrounding atrophic bone (Fig. 2). No cellular atypia or mitoses were encountered. The atrophic lamellar bones were noted in 9 of 10 cases (90%). In 5 of 10 cases (50%), foci of fat necrosis were observed in association with large aggregates of foamy histiocytes. Calcifications were seen in 7 of 10 cases (70%) and ischemic bone formations were seen in 4 of 10 cases (40%). Cystic degenerations were seen in 4 of 10 cases (40%). In 3 of 10 (30%) cases, fibroses were noted. According to the Milgram s classification, two cases were stage I, four cases were stage II, and the other four cases were stage III. DISCUSSION The sex predilection in intraosseous lipoma is controversial. Some authors 15,21) have reported that there was no sex predilection, while others have reported bias to males 5,12). In our study, seven patients out of 10 were male. The reported age distribution is fairly even, from youth to elderly, with high incidence in fourth, fifth, and sixth decades of life 12,15,21), and findings in our patients were in good agreement with this distribution. The calcaneus and long tubular bones are the common anatomical locations and multifocal locations are rare 10,11,21,28). In our study case 2 and 3 were bilateral calcaneal lesion of one patient. Multiple intraosseous lipomas should be distinguished from multiple lipomatosis 10,11,28), in which the fat deposition may be due to associated endocrine abnormalities such as type IV hyperlipoproteinemia 11), and other conditions such macrodystrophia lipomatosa. Some authors reported up to 70% of patients with intraosseous lipomas presented with pain 5,20), while other authors reported most of the patients were asymptomatic 12,21). In our study only four patients had lesion-related pain. Microtrabecular fractures in areas of weakened bone following episodes of minor trauma might be one cause of pain 5). Milgram proposed three stages based on the histological appearances of intraosseous lipomas 21). Stage I lesions contain viable mature lipocytes interspersed with fine bony trabeculae 2,17,21). The fat is identical to subcutaneous fat 4). There is no cellular atypia, mitoses 17,23) or capsular tissue 3,17,18). In our study two cases were in stage I. Stage II lesions develop areas of infarction due to expansion of fat cells within rigid trabeculae. The adipose tissue is partly necrotic with loss of nuclei, and foamy macrophages may be present. Some portions of necrotic fat become calcified, possibly as a result of hemorrhage and trauma 1). Four cases were stage II in our study. Extension of infarction to involve the whole lesion leads to a stage III, with necrotic fat, calcified fat, cyst formation and reactive peripheral or central new bone formation, although these features may be seen to a lesser extent in the stage II lesions. In this respect intraosseous lipomas resemble bone infarcts, although the bony trabeculae are resolved in stage III, which is considered to be a distin-

4 Intraosseous Lipoma 529 guishing feature 21). In our study, four cases were in stage III. The appearances of intraosseous lipomas on simple x-ray and MRI correspond to the pathological staging system. Milgram 21) states that stage I lesions are purely radiolucent with resorption of pre-existing bone and expansion or remodelling in half of all cases. However, bone expansion was noted only four cases in our series. In stage II lesions, localized areas of calcification may be seen and are typically central, but may be peripheral. At stage III reactive ossification around the calcified fat in the outer rim of the lesion is prominent. Peripheral or central calcification fills much of the lesion, but expansion is present in the minority of cases 21). The intrinsic lucency of the lesion may be obscured by internal ossification, which should be distinguished from chondroid calcification 18). The fat component of the intraosseous lipoma is easily recognized on MRI by high signal intensity on both T1-weighted and T2-weighted images, and fat suppression on STIR or other fat suppression sequences. Cysts are commonly seen on MRI and have well-demarcated borders. Signal intensity is intermediate on T1-weighted sequences and very high on T2-weighted and fat-suppressed images 24). In our study, the radiographic stages of all cases but one corresponded very well to histological stages. Therefore, many authors now believe that imaging study alone may provide a definitive diagnosis 24). Intraosseous lipomas are considered to represent true benign tumors of fat, although this view has been challenged 3,8,19,26). It has been postulated that lipomatous masses may develop from infarcts 3), although Milgram distinguishes infarcts by the presence of trabeculae, zonal calcification at the periphery and the lack of stellate myxomatous cells 22). Other hypotheses suggest that lipomas may be the end stage of infection and other inflammatory processes 3). Mechanical factors and even increased venous pressure have also been implicated as etiological agents. Furthermore a heterogeneous group of lesions termed polymorphic fibro-osseous lesions, or liposclerosing myxofibrous tumours (LSMFT) have been described 25). These lesions show a variety of histological patterns, similar to conditions such as fibrous dysplasia, non-ossifying fibroma, intraosseous lipoma, simple cysts and bone infarcts. The presence of fine bony trabeculae within intraosseous lipomas may make them indistinguishable histologically from osteoporosis, especially in Ward s triangle of calcaneus 13). It is a recognized area of normal porosity, which has been referred to as a pseudo-tumor, and may be very pronounced, even in the young. This area of porosity occurs because of two lines of biomechanical stress that are manifest by prominent trabeculae, leaving a triangular area in which there are relatively fewer trabeculae 16). Therefore correlation with radiology is essential to determine whether a lipoma is actually present. Most intraosseous lipoma can be managed conservatively. Goto et al. 12). suggested surgical indication of intraosseous lipoma as follows; 1) painful tumor, 2) occurrence of pathological fracture, 3) necessity for histological diagnosis, and 4) need to decrease the risk of malignant transformation. While most intraosseous lipomas can be managed conservatively, we treated 10 cases with surgery because we could not accumulate enough experience about this rare tumor presented once or twice in a year in our clinic. With this long experience and recent knowledge reported, we could observe the other patients without symptom after radiographic diagnosis. Surgical treatment is usually consisted of curettage and packing with bone chips. In our study, even though the numbers of cases were not enough, and follow up periods were short, there were no local recurrence and malignant change after surgery. CONCLUSION Intraosseous lipomas were located in calcaneus and asymptomatic in most cases clinically, and each case had characteristic radiographic and histological features according to the presenting stages. Their radiographic findings were very well correlated to the histological finding. REFERENCES 1. Appenzeller J and Weitzner S: Intraosseous lipoma of os calcis. Case report and review of literature of intraosseous lipoma of extremities. Clin Orthop, 101: , Barcelo M, Pathria MN and Abdul-Karim FW: Intraosseous lipoma. A clinicopathologic study of four cases. Arch Pathol Lab Med, 116: , Barker GR and Sloan P: Intraosseous lipomas: clinical features of a mandibular case with possible etiology. 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5 530 Duk Seop Shin Eun Seok Kwak Joon Hyuk Choi and Delcambre B: Intraosseous lipoma: role and limitation of modern imaging techniques. Rev Rhum Engl Ed, 62: , DeLee JC: Intra-osseous lipoma of the proximal part of the femur. Case report. J Bone Joint Surg, 61-A: , Dohler R, Poser HL, Harms D and Wiedemann HR: Systemic lipomatosis of bone. A case report. J Bone Joint Surg, 64-B: 84-87, Freiberg RA, Air GW, Glueck CJ, Ishikawa T and Abrams NR: Multiple intraosseous lipomas with type-iv hyperlipoproteinemia. A case report. J Bone Joint Surg, 56-A: , Goto T, Kojima T, Ijima T, et al: Intraosseous lipoma: a clinical study of 12 patients. J Orthop Sci, 7: , Gunterberg B and Kindblom LG: Intraosseous lipoma. A report of two cases. Acta Orthop Scand, 49: 95-97, Ha KI, Han SH and Kang JK: A case of multiple intraosseous lipomas. J of Korean Orthop Assoc, 16: , Hart JA: Intraosseous lipoma. J Bone Joint Surg, 55-B: , Jensen NC, Madsen LP and Linde F: Topographical distribution of trabecular bone strength in the human os calcanei. J Biomech, 24: 49-55, Latham PD and Athanasou NA: Intraosseous lipoma within the femoral head. A case report. Clin Orthop, 265: , Lauf E, Mullen BR, Ragsdale BD and Kanat IO: Intraosseous lipoma of distal fibula. Biomechanical considerations for successful treatment. J Am Podiatr Med Assoc, 74: , Lagier R: Case report 128: lipoma of the calcaneus with bone infarct. Skeletal Radiol, 5: , Levin MF, Vellet AD, Munk PL and McLean CA: Intraosseous lipoma of the distal femur: MRI appearance. Skeletal Radiol, 25: 82-84, Milgram JW: Intraosseous lipomas. A clinicopathologic study of 66 cases. Clin Orthop, 231: , Milgram JW: Intraosseous lipomas with reactive ossification in the proximal femur. Report of eight cases. Skeletal Radiol, 7: 1-13, Milgram JW: Malignant transformation in bone lipomas. Skeletal Radiol, 19: , Propeck T, Bullard MA, Lin J, Doi K and Martel W: Radiologicpathologic correlation of intraosseous lipomas. Am J Roentgenol, 175: , Ragsdale BD: Polymorphic fibro-osseous lesions of bone: an almost sitespecific diagnostic problem of the proximal femur. Hum Pathol, 24: , Richardson AA, Erdmann BB, Beier-Hanratty S, et al: Magnetic resonance imagery of a calcaneal lipoma. J Am Podiatr Med Assoc, 85: , Song JM, Kim JB, Park JS and Rah SK: Intraosseous lipoma in long tubular bone: 3 case report. J of Korean Orthop Assoc, 32: , Szendroi M, Karlinger K and Gonda A: Intraosseous lipomatosis. A case report. J Bone Joint Surg, 73-B: , 골내지방종 신덕섭 곽은석 최준혁 * 영남대학교의과대학정형외과학교실, 병리학교실 * 목적 : 골내지방종 10 예를분석하여, 임상적, 방사선학적및병리학적특성을규명하고, 방사선학적특성과병리학적특성이잘부합하는지를조사하고자한다. 대상및방법 : 양측종골에골내지방종을동시에가진환자를포함한 9 명의환자, 10 예를대상으로하였다. 임상적및방사선학적특성을알아보기위하여의무기록과단순방사선사진, CT, MRI 를검토하였다. MRI 는모든예에서 T1, T2, 강조영상과 fat suppressed image (STIR) 를시행하였다. 조직학적특성을알기위하여병리조직슬라이드를검토하였고, 결과에따라 Milgram 의 stage 로분류하였다. 방사선학적특성도 stage 를나누어분류하고조직학적분류와일치하는지를조사하였다. 결과 : 환자들의평균연령은 39 세 (34-60 세 ) 였고, 남자가 7 명, 여자가 3 명이었다. 종양의평균크기는 3.4 cm ( ) 였다. 해부학적위치로는종골이 7 예로가장많았다. 네명의환자만이통증을호소하였고, 나머지는종양과관계된증상은없었다. 종양은소파술및골이식수술로치료하였으며, 추시상재발이나악성변화는없었다. Milgram 의분류상 stage I 이 2 예, stage II 는 4 예, stage III 은 4 예였다. 방사선학적 stage 는각각 2 예, 3 예, 5 예였으며, 조직학적 stage 와잘부합하였다. 결론 : 골내지방종은각 stage 에따라특징적인방사선학적및조직학적특성을보이며, 방사선학적 stage 는조직학적 stage 와잘부합하였다. 색인단어 : 골내지방종, 방사선학적특성, 조직학적시기