Intraosseous lipoma is the rarest primary bone tumor, is usually asymptomatic, and is often discovered incidentally during unrelated investigations. Lipomas usually undergo varying degrees of involution, with necrosis, cyst formation, and calcification. Careful radiologicalpathological correlation is required to avoid misinterpretation.
This study describes 12 intraosseous lipomas in 11 patients (6 men, 5 women; mean age, 36 [range, 25-50 years]; mean follow-up, 5 years [range, 2-11 years]). The lesions were located in the calcaneus in 7 patients (bilaterally in 1 patient), the tibia in 2, the femur in 1, and the ilium in 1. All lesions were asymptomatic and discovered incidentally except in 4 patients (5 lesions). Curettage and bone grafting were performed after biopsy in 4 lesions; and in 1 patient, curettage, bone grafting, and reconstructive surgery were performed after pathologic fracture. The remaining 7 patients were followed up conservatively.
Although definite diagnosis of an intraosseous lesion is possible only by histopathologic examination, computed tomography and magnetic resonance imaging findings of intraosseous lipoma are typical, and most patients symptoms are relieved by conservative means. For this reason, we believe that in patients with no signs of an impending pathologic fracture or suspicion of malignancy, clinical and radiological follow-up is sufficient. However, since intraosseous lipoma is an uncommon bone tumor, there is a need to familiarize physicians with the radiological features of this lesion for the correct diagnosis.
Intraosseous lipoma is the rarest primary bone tumor, representing only 0.1% of benign bone tumors, and is usually encountered in the fourth or fifth decade.1-4 The most common sites of involvement are the metaphyses of long bones,2-6 such as the proximal femur5 or fibula,7 as well as the calcaneus,8-10 and cases with metatarsal,11 spinal,12 pelvic,13 and cranial14 involvement have also been reported. Intraosseous lipomas are usually asymptomatic and discovered incidentally during unrelated investigations.
Although many reports of intraosseous lipomas have been published, only a few reports exist in which a series of patients were studied.2-6,15 Controversy exists as to whether to treat patients surgically (biopsy, curettage, and bone grafting)16,17 or follow them conservatively.3,4,15 This study describes 12 intraosseous lipomas in 11 patients.
Materials and Methods
Twelve intraosseous lipomas in 11 patients (6 men, 5 women) were evaluated retrospectively. Mean patient age was 36 years (range, 25-50 years). Mean follow-up was 5 years (range, 2-11 years) (Table).
The lesions were located in the calcaneus in 7 patients (bilaterally in 1 patient), in the tibia in 2, in the femur in 1, and in the ilium in 1. All lesions were asymptomatic and discovered incidentally except in 4 patients (5 lesions). Three patients with calcaneal involvement reported mild blunt pain in the heels on walking, and the patient with proximal femoral involvement reported vague groin pain after heavy activity. All of the other patients lesionsexcept the patient with ilium involvement, whose lesion was detected during a gynecologic investigation (Figure 1), and 1 patient with proximal tibial involvement, whose lesion was detected during a standard investigation for patellar chondromalasia (Figure 2)were discovered with radiographs taken after trauma. All patients had radiographs and routine laboratory investigations, all but 1 had magnetic resonance imaging (MRI), 4 had computed tomography (CT), and 3 had technetium bone scans. One patient with calcaneal involvement was seen in the emergency room with a subtalar dislocation after a fall from a height of 2 m (Figure 3).
Biopsy, curettage, and grafting with allograft bone were performed in the 3 symptomatic patients with calcaneal involvement (only right foot of the bilateral case) (Figure 4), and in the patient with femoral involvement (Figure 5). The patient with bilateral calcaneal lipomas was lost to follow-up, although he had symptoms in his left foot and presented 2 years later with a pathologic fracture of the left calcaneus after he jumped from a height of 2 m (Figure 4). Curettage and bone grafting of the lesion and a subtalar arthrodesis were performed after a biopsy from the pathologic fracture site confirmed intraosseous lipoma. The remaining 6 patients, all asymptomatic, were followed up without surgical intervention.
Milgrams5 classification system is used for staging the lesions: stage 1, the lesion is a solid lipoma composed of viable fat cells; stage 2, part of the lesion is necrotized, forming focal calcification; and stage 3, most of the tumor tissue has died, with variable degrees of cyst formation, calcification, and reactive new bone formation.
All laboratory results were within normal limits, and there were no pathologic findings in bone scans. None of the patients in the surgery group had early or late complications.
Histopathology showed Milgram stage 1 intraosseous lipoma in 2 lesions, stage 2 in 2, and stage 3 in 1. The lesions were radiologically staged as stage 1 in 4 lesions, stage 2 in 5, and stage 3 in 3. All histopathology staging results matched with radiological stages. At follow-up, the patient with femoral involvement still reported mild pain after activity, but his reports could not be referred to a specific cause. There were no recurrences.
None of the patients in the conservative group developed symptoms, and none of the lesions showed growth or malignant change. The lesions stages did not change during the follow-up period.
Intraosseous lipoma is the rarest benign tumor of bone, with a reported prevalence of <0.1% among bone tumors.1-6 The first case of intraosseous lipoma was reported in 1880,14 and the total number of reported cases is approximately 300.15 However, in recent years, a considerable number of new cases of intraosseous lipoma were reported, and the real incidence of this tumor seems to be much larger than previously thought.2-4 The reason for this increased incidence is due in part to the heightened awareness of this lesion in recent years and to the widespread use of MRI. In all of our patients but 1 (for whom only CT was obtained), the lesions were confirmed with MRI studies. Although there are many theories about its etiology, it is widely accepted that this lesion is a true primary tumor of bone.1,5,8,9
Men are slightly more commonly affected than women.1-5 Intraosseous lipomas occur throughout the skeleton.5 The most frequent locations include the proximal femur (34%), calcaneus (8%), ilium (8%), tibia (13%), fibula (10%), humerus (5%), and ribs (5%).2,5 Long bone lesions typically occur in the metaphysis, but diaphyseal involvement is not uncommon. Epiphyseal involvement is unusual.18,19 The calcaneus was the involved bone in 7 of our 12 lesions, which we believe is due to the common occurrence of foot and ankle injuries, during which the cystic lesions are incidentally seen.
Clinically, the lesions are often asymptomatic and discovered incidentally.1-5,15,16 When present, mild dull pain is the most common report. Only 4 of our 11 patients (5 lesions) had symptoms, and at follow-up, 1 continued to have symptoms postoperatively, which we thought were unrelated to the intraosseous lesion.
The prevalence of intraosseous lipoma is likely underreported for several reasons.2,4 Besides being asymptomatic, radiographic features can be confused with those of other lesions, and because of its typically benign radiographic appearance, further imaging with CT or MRI is usually not performed. However, the histopathologic features are difficult to interpret if not correlated with radiological studies.2 Finally, the differential diagnosis of the lipoma varies with the stage of the lesion, eg, when ischemic changes are present, it is difficult to distinguish osteonecrosis from intraosseous lipoma.2-5 Lipomas usually undergo varying degrees of involution, with necrosis, cyst formation, and calcification. Careful radiologicalpathological correlation is required to avoid misinterpretation.2-6,16,20 The differential diagnosis may include nonossifying fibroma, aneurysmal bone cyst, simple bone cyst, fibrous dysplasia, giant cell tumor, bone infarct, and chondroid tumors.6 The advent of CT and especially MRI has enabled the identification of intralesional fat, as well as calcification and cyst formation. We, like many other authors,20-23 believe that imaging alone may provide a definitive diagnosis.
Intraosseous lipomas are typically solitary, although bilateral calcaneal lesions have been reported.3-5,10,24-26 Milgram5 also reported multiple site involvement in 2 cases. However, discrete lipomas should be distinguished from multiple lipomatosis.27 One of our patients had bilateral calcaneal involvement.
Milgram5 divided intraosseous lipomas into 3 stages based on their respective histology, reflecting the degree of involution of the lesion. Stage 1 lesions consist of viable fat cells; there is no cellular atypia, mitoses, or capsular tissue. Stage 2 lesions are composed of fat cells with areas of necrosis and calcification. Stage 3 lesions have necrosis, calcification, and cyst and reactive new bone formation. The staging system proposed by Milgram5 suggests that lipomas may progress over time from stage 1 to stage 2 or 3. However, there is no tendency of stage 2 and 3 lesions to occur in older patients. Unfortunately, there is little literature on the natural history of intraosseous lipomas. One case showed no change other than increased host bone sclerosis over 16 years.28
The radiological appearance of intraosseous lipoma depends on the histological stage of the lesion. Milgram stage 1 lesions are radiolucent, well-circumscribed lesions due to resorption of preexisting bone.5,6 However, the radiographic features of intraosseous lipoma containing only fat is nonspecific and resembles the features of unicameral bone cyst, fibrous dysplasia, and plasmacytoma. Intraosseous lipoma containing only fat is easily differentiated from other lesions on MRI and CT, because both modalities are able to document the adipose tissue. Computed tomography demonstrates the low attenuation of fat (<60 to 100 HU),2,4,9,20,29 and on MRI, the fat in the lesion shows high-signal intensity on both T1- and T2-weighted images, and fat suppression on short-tau inversion recovery or other fat suppression sequences are similar to subcutaneous fat.2,18-21,23 Milgram5,6 reports that stage 1 lesions show expansion or remodeling in half of all cases. However, bone expansion was absent in our series, and in other reports.3
Milgram stage 2 intraosseous lipomas show variable amounts of central or peripheral ossification or calcification.5,6,23 Enlargement of the lipoma in an enclosed space leads to increased pressure, which compromises blood flow, causing ischemia and necrosis. The necrotic fat then calcifies. The ossification and calcification in an intraosseous lipoma produce a distinctive radiographic appearance (Figure 6). Central or ringlike calcification in a lucent lesion involving the body of the calcaneus is pathognomonic of an intraosseous lipoma and allows it to be distinguished from unicameral bone cyst.2 Partially mineralized lesions may be mistaken for a chondroid lesion or osteonecrosis on radiographs. Computed tomography and MRI are useful in differentiating intraosseous lipoma with calcification from other lesions. Fat is seen in portions of the lesion and distinguishes the intraosseous lipoma from tumors of chondroid, osteoid, or fibrous origin. On MRI, calcification, which is common in stage 2 and 3 lesions, is seen as areas of low-signal intensity on both T1- and T2-weighted images.2,20,21,23 After intravenous administration of gadolinium, no contrast material enhancement is seen on MRI in the low-intensity sites, which proves the presence of calcification.
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Figure 6: Patient 9. A cystic lesion with central calcification and peripheral sclerosis in the calcaneus is seen on a lateral radiograph of the foot (A). A lesion with hyperintense signal intensity, which is similar to subcutaneous fat with a central area showing low-signal intensity on T1-weighted sagittal and T2-weighted axial images, is seen on MRI (B). The low-intensity site proves the presence of calcification. The intraosseous lipoma was radiologically staged as Milgram stage 2.
Milgram stage 3 intraosseous lipomas show thick peripheral ossification at the lesion margin with variable amounts of central ossificationcalcification on radiographs.5,6 With progressive ischemia and involution, the fat undergoes cystic degeneration, and central cystic areas are sometimes surrounded by a rim of ossification. This central ring of ossification may then be surrounded by fat, which in turn is surrounded by a rim of ossification demarcating the periphery of the lesion. The resulting bulls-eye appearance is distinctive for intraosseous lipoma.2,4,5 Cystic degeneration may be the primary feature shown on CT or MRI in stage 3 intraosseous lipomas. Despite the heterogeneous appearance of an involuted intraosseous lipoma on CT and MRI, identifying fat in the lesion leads to definitive diagnosis of intraosseous lipoma.2,20,23
The need for surgical treatment is controversial. Curettage with bone grafting is the treatment of choice when surgical intervention is needed. Most lipomas, however, can be managed conservatively. Some surgeons feel that in asymptomatic cases with no signs of an impending pathologic fracture or suspicion of malignancy, nonoperative treatment with clinical and radiological follow-up is a wise approach.5 Malignant transformation is rare.5,30 While some surgeons think that biopsy is unnecessary because radiological features are characteristic,20,21,31 others believe that the lesion must be diagnosed histologically.16,17 However, reports stating that biopsy is required usually predate the common and efficient use of MRI, when an accurate radiological diagnosis was almost impossible.
We believe that pain alone is not an indication for surgical intervention or any other invasive treatment, including biopsy. When we reevaluate our cases now, we likely would not have performed surgery on the cases with femoral and calcaneal involvement. The cause of pain is unclear, but it may be mechanical due to expansile remodeling of bone, or it may be related to the ischemic changes that frequently accompany these lesions.3 It is also possible that the pain is referable to nearby joint disease and that the intraosseous lesion is incidentally discovered.2,3 It was reported that symptoms may recur after surgical treatment or resolve spontaneously on conservative treatment, thus suggesting that many intraosseous lipomas are incidental findings and that patients may have another, unidentified cause of symptoms.3 Microtrabecular fracture in areas of weakened bone following episodes of minor trauma may be 1 cause of pain.3 Areas of diffuse increased signal were observed on MRI within the lipoma in some series, which may represent a stress response.3,20 Whether these areas lead to pathologic fractures is not known, and our patient with pathologic calcaneus fracture had not noticed an increase in his symptoms since he was diagnosed with intraosseous lipoma. We concluded that he fractured his weakened calcaneus because of the height from which he jumped. As far as we know, this is the second case that has a pathologic fracture after being diagnosed as having an intraosseous lipoma.32
We believe that in this age of imaging, clinical and radiological follow-up is sufficient in patients with no signs of an impending pathologic fracture or suspicion of malignancy. Since intraosseous lipomas are usually asymptomatic and discovered incidentally, with a probable higher incidence than previously reported, it is easy to assume that many people with this lesion are never diagnosed or treated. Our case with subtalar dislocation is a good example for this scenario; if the patient had not injured his ankle, his incidentally discovered intraosseous lipoma would likely never have been diagnosed. However, since intraosseous lipoma is an uncommon tumor of bone, there is a need to familiarize physicians with the radiological features of this lesion for the correct diagnosis.
- Unni K. Lipoma and liposarcoma. In: Unni K, ed. Dahlins Bone Tumors: General Aspects and Data on 11087 Cases. Philadelphia, PA: Lippincott-Raven; 1996:349-353.
- Murphey MD, Carroll JF, Flemming DJ, Pope TL, Gannon FH, Kransdorf MJ. From the archives of the AFIP: benign musculoskeletal lipomatous lesions. Radiographics. 2004; 24(5):1433-1466.
- Campbell RS, Grainger AJ, Mangham DC, Beggs I, Teh J, Davies AM. Intraosseous lipoma: report of 35 new cases and a review of the literature. Skeletal Radiol. 2003; 32(4):209-222.
- Radl R, Leithner A, Machacek F, et al. Intraosseous lipoma: retrospective analysis of 29 patients. Int Orthop. 2004; 28(6):374-378.
- Milgram JW. Intraosseous lipomas. A clinicopathologic study of 66 cases. Clin Orthop Relat Res. 1988; (231):277-302.
- Milgram JW. Intraosseous lipomas: radiologic and pathologic manifestations. Radiology. 1988; 167(1):155-160.
- Ongürü O, Pabuçcu Y, Celasun B. Intraosseous lipoma of the fibula. Clin Imaging. 2002; 26(1):55-57.
- Hart JA. Intraosseous lipoma. J Bone Joint Surg Br. 1973; 55(3):624-632.
- Reig-Boix V, Guinot-Tormo J, Risent-Martinez F, Aparisi-Rodriguez F, Ferrer-Jimenez R. Computed tomography of intraosseous lipoma of os calcis. Clin Orthop Relat Res. 1987; (221):286-291.
- Yildiz HY, Altinok D, Saglik Y. Bilateral calcaneal intraosseous lipoma: a case report. Foot Ankle Int. 2002; 23(1):60-63.
- Uguz B, Aydin AT. Intraosseous lipoma of the fifth metatarsal: a case report. J Foot Ankle Surg. 2004; 43(2):119-122.
- Chang H, Park JB. Intraosseous lipoma of lamina of the first thoracic vertebra: a case report. Spine (Phila Pa 1976). 2003; 28(13):E250-251.
- Buckley SL, Burkus JK. Intraosseous lipoma of the ilium. A case report. Clin Orthop Relat Res. 1988; (228):297-301.
- Arslan G, Karaali K, Cubuk M, Senol U, Lüleci E. Intraosseous lipoma of the frontal bone. A case report. Acta Radiol. 2000; 41(4):320-321.
- Kapukaya A, Subasi M, Dabak N, Ozkul E. Osseous lipoma: eleven new cases and review of the literature. Acta Orthop Belg. 2006; 72(5):603-614.
- Chow LT, Lee KC. Intraosseous lipoma. A clinicopathologic study of nine cases. Am J Surg Pathol. 1992; 16(4):401-410.
- Gonzalez JV, Stuck RM, Streit N. Intraosseous lipoma of the calcaneus: a clinicopathologic study of three cases. J Foot Ankle Surg. 1997; 36(4):306-310.
- Yamamoto T, Akisue T, Marui T, Nagira K, Yoshiya S, Kurosaka M. Intraosseous lipoma of the humeral head: MR appearance. Clin Imaging. 2001; 25(6):428-431.
- Latham PD, Athanasou NA. Intraosseous lipoma within the femoral head. A case report. Clin Orthop Relat Res. 1991; (265):228-232.
- Propeck T, Bullard MA, Lin J, Doi K, Martel W. Radiologic-pathologic correlation of intraosseous lipomas. AJR Am J Roentgenol. 2000; 175(3):673-678.
- Kamekura S, Nakamura K, Oda H, Inokuchi K, Iijima T, Ishida T. Involuted intraosseous lipoma of the sacrum showing high signal intensity on T1-weighted magnetic resonance imaging (MRI). J Orthop Sci. 2001; 6(2):183-186.
- Levin MF, Vellet AD, Munk PL, McLean CA. Intraosseous lipoma of the distal femur: MRI appearance. Skeletal Radiol. 1996; 25(1):82-84.
- Blacksin MF, Ende N, Benevenia J. Magnetic resonance imaging of intraosseous lipomas: a radiologic-pathologic correlation. Skeletal Radiol. 1995; 24(1):37-41.
- Rosenblatt EM, Mollin J, Abdelwahab IF. Bilateral calcaneal intraosseous lipomas: a case report. Mt Sinai J Med. 1990; 57(3):174-176.
- Tejero A, Arenas AJ, Sola R. Bilateral intraosseous lipoma of the calcaneus. A case report. Acta Orthop Belg. 1999; 65(4):525-527.
- Futani H, Fukunaga S, Nishio S, Yagi M, Yoshiya S. Successful treatment of bilateral calcaneal intraosseous lipomas using endoscopically assisted tumor resection. Anticancer Res. 2007; 27(6C):4311-4314.
- Szendroi M, Karlinger K, Gonda A. Intraosseous lipomatosis. A case report. J Bone Joint Surg Br. 1991; 73(1):109-112.
- Richardson AA, Erdmann BB, Beier-Hanratty S, et al. Magnetic resonance imagery of a calcaneal lipoma. J Am Podiatr Med Assoc. 1995; 85(9):493-496.
- Ramos A, Castello J, Sartoris DJ, Greenway GD, Resnick D, Haghighi P. Osseous lipoma: CT appearance. Radiology. 1985; 157(3):615-619.
- Milgram JW. Malignant transformation in bone lipomas. Skeletal Radiol. 1990; 19(5):347-352.
- Goto T, Kojima T, Iijima T, et al. Intraosseous lipoma: a clinical study of 12 patients. J Orthop Sci. 2002; 7(2):274-280.
- Weinfeld GD, Yu GV, Good JJ. Intraosseous lipoma of the calcaneus: a review and report of four cases. J Foot Ankle Surg. 2002; 41(6):398-411.
Drs Bagatur and Gur are from the Department of Orthopedic Surgery and Traumatology, Medical Park Bahcelievler Hospital, and Drs Yalcinkaya, Dogan, Mumcuoglu, and Albayrak are from the Department of Orthopedic Surgery and Traumatology, Istanbul Education and Research Hospital, Istanbul, Turkey.
Drs Bagatur, Yalcinkaya, Dogan, Gur, Mumcuoglu, and Albayrak have no relevant financial relationships to disclose.
Correspondence should be addressed to: A. Erdem Bagatur, MD, Funda 07-01, No. 37, Bahcesehir 34850, Istanbul, Turkey (firstname.lastname@example.org).