Vol 5 | Issue 3 | September – December 2019 | page: 8-12| Vineet Kurisunkal, Manish Pruthi, Ashish Gulia. 10.13107/jbst.2019.v05i03.017

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Author: Vineet Kurisunkal[1], Manish Pruthi[2], Ashish Gulia[3]
[1]M.S. Ortho, Fellowship in Musculoskeletal Oncology (HBNI)
Senior Clinical Fellow, Royal Orthopaedic Hospital, Birmingham, United Kingdom
[2]M.S. Ortho, DNB Ortho, Fellowship in Musculoskeletal Oncology (HBNI)
Consultant Orthopaedic Oncology, Rajiv Gandhi Cancer Institute, New Delhi
[3]M.S. Ortho, McH Surgical Oncology
Professor, Orthopaedic Oncology, Dept. Of Surgical Oncology
Tata Memorial Hospital, HBNI, Mumbai

Address of Correspondence
Dr. Ashish Gulia
Tata Memorial Centre, Homi Bhabha National Institute, Dr. E Borges Road, Parel, Mumbai – 400 012, India. 

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Abstract

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Introduction:
Unicameral bone cyst (UBC) is a benign cystic lesion of the bone also known as simple bone cyst or solitary bone cyst. It was first recognized by Virchow in 1876. (1) Although exact incidence is not known, UBC’s accounts for 3% of all bone tumors with a male to female ratio of 2:1(2). Proximal humerus and proximal femur are the two most common sites for occurrence and accounts for approximately 80% of cases. Most of the UBC’s are recognized in age between 9 to 13 years, and the lesions usually heal by skeletal maturity. They are unilocular, serous or sero-sanguineous fluid filled bone cavity usually seen in metaphyseal region of long bones. It was originally recognized by Virchow in 1876 and subsequently studied in detail by Jaffe and Lichtenstein in 1942 (1)

Etiopathogenesis:
The etiopathogenesis of UBC is still very unsure, several theories have been postulated namely (3)
1. Virchow’s theory: Hypothesized the occurrence due to abnormalities in the local circulation of the affected area of bone.
2. Jaffe and Lichtenstein theory: Cystic cavity develops secondary to a traumatic insult leading to change in local circulation.
3. Mirra theory: On electron microscopy lesions appeared like intraosseous synovial cysts
4. Cohen’s theory: Hypothesized that the cyst forms as a response to venous occlusion in the intramedullary space.
5. Histopathologically: The lining of cavity contains synovial cells. On further evaluation of the cyst fluid high levels of oxygen free radicals and prostaglandins (prostaglandin E2, interleukin-1 and proteolytic enzymes) were found which have inherent ability to increase bone resorption hence create a cystic intramedullary cavity.
Epidemiology & Clinical features:
UBCs are lesions which arise in first two decades of life (85%) with 50% of lesions being detected in first decade itself and are rare to occur beyond age of 20 years. These Lesions show strong male predominance with male to female ratio of 2:1. (2) About 90% of cases occur in the proximal humerus, proximal femur and proximal tibia out of which most common location is the humerus (55% cases). Other less common sites are the ilium, distal humerus, tibia, calcaneum, talus, radius, ulna, and ribs. (4)
Patients are usually asymptomatic. Most lesions are picked up incidentally on plain radiographs when the patient presents following a history of fall or trauma or sport related injury. Few children present with pain after strenuous work or sports injury. (5)

Radiological Description:
UBC appears as a large, localized lytic lesion in the epimetaphyseal region of long bones. It appears expansile with a narrow zone of transition and a clear matrix. The adjacent matrix appears thinned with a well contained lesion. (6) A loculated appearance is due to the presence of ridges over the inner surface of the cyst rather than bony partitions, but following the healing of a fracture fibro osseous septa may actually form. Occasionally, a fragment of the fractured cyst wall is found in the fluid cavity which gives appearance to the pathognomic ‘fallen leaf sign”. (7) The cystic cavity may appear partially ossified or have septations after fracture or treatment. Mixed lytic sclerotic areas may be present in relapse cases.
Rarely magnetic resonance imaging may be warranted which reveals a single well contained lytic lesion with central fluid collection in primary cases. In the presence of previous healed pathological fractures across the cyst leads to formation of fibroosseous septations giving the appearance of an aneursymal bone cyst showing multiple fluid fluid levels. (8)

Histopathological Description:
Gross examination:
Fusiform expansion of the bone is noted with a classical description of being “egg shell” thin, semi translucent, bluish, and easily penetrable. (5) The cavity is a single lined cavity containing yellow fluid often appearing serosanguinous to hemorrhagic if there is associated history of trauma. Following a healed fracture, the cavity may become divided by fibro osseous septa. A thin layer of grey connective tissue lines the inner surface of the cyst wall, which displays multiple scroll like ridges that account for the pseudoloculated appearance on roentgenograms. (8)

Microscopy:
The cortical wall is composed of loosely trabeculated osseous tissue, and many thin walled vessels. The connective tissue is composed of layers of flattened cells (fibroblasts) lying on vascular collagenous or myxomatous tissue containing multinucleated giant cells, foam cells containing hemosiderin and lipids. (9) Following a fracture, periosteal new bone apposition may be evident. Recent literature reviews have revealed a number of new markers and histone mutations for differentiating benign from malignant tumours ie: Histone 3 mutations like H3F3A which is specific for giant cell tumours and H3F3B which is specific for chondroblastomas, these mutations help in differentiating from other giant cell containing tumours. (10)

Differential Diagnosis for UBC:
Varied benign and inflammatory bone lesion often lead to a misdiagnosis of unicameral bone cyst (5, 6) e.g.: Aneurysmal Bone Cyst (ABC), Fibrous Dysplasia with cystic changes, Enchondroma, Eosinphilic granuloma
ABC’s are eccentrically located lesions occurring near the metadiaphysis. Radiologically ABC’s show a significant expansion of the cortex, unlike the mild concentric expansion was seen in UBCs and show multiple fluid fluid levels which is pathognomic for ABC’s
Fibrous dysplasias are centrally located in the metadiaphyseal area occurring within the same age distribution. Radiographically lesion is usually “ground-glass” density.
An enchondroma has less sclerotic margins than the unicameral bone cyst and often contains a chondroid matrix and are usually located in the short tubular bones of the hands and feet.
Eosinphilic granuloma frequently involves axial skeleton than appendicular skeleton. Spinal Colum involvement is mostly seen in children. These are most destructive lytic lesion with associated soft tissue component and vertebral fracture or vertebra plana.

Treatment of UBC:
UBC’s are treated based on their stage at diagnosis. UBC’s could be latent or active, if a UBC has been diagnosed during its latent stage its most likely to be an incidental pick up which can be observed closely and should be followed up clinico – radiologically till healing. If during the observation phase there is an increase in the size or symptoms, it implies that lesion should be in the active stage of the disease and this should be intervened and treated. (6)
Treatment approach depends on the stage of UBC at presentation. Goal of treatment in UBC is to regain cortical thickness, bone strength and achieve healing of cyst. (3,11) If a lesion presents with a pathological fracture through the upper limb and is minimally displaced should be splinted and closely observed, there are studies which show cases spontaneously heal without any surgical intervention, the remaining cases have to be surgically stabilized. (4)
In lower limb defects, the treatment approach aims at aggressive surgical intervention so as to prevent any untoward event like occurrence of a pathological fracture. In a meta-analysis of 3217 cysts, healing after conservative treatment was 65% (27-100%). (3) In another study by Traub et al, healing after a conservative treatment was 70%. (12) However, in the study by Neer et al, healing of cysts after a conservative treatment was very low (4%) and was statistically low from healing after surgical procedures (77%) (p<0.001). (13) Also, subsequent fractures, growth disturbances and deformity were more common in the conservative treatment group.
In the meta-analysis of 62 studies (3217 cysts) mentioned earlier, the pool estimate of bone marrow injections was similar to steroid injections, 77.9% (65.9-89.9%). (3) The healing rates were improved when demineralized bone matrix was added. Healing rates after curettage were 90% while treatment with Intramedullary nails had almost 100% healing rates for UBC’s of long bones.
Based on the published literature, various treatment options have been suggested for the treatment of UBC’s. These can be broadly divided into 2 categories – (4, 8, 11, and 14)
a) Percutaneous interventions:
i. Percutaneous aspiration of the cyst fluid and steroid injection – When a corticosteroid is injected into a unicameral bone cyst, the cyst will heal in most cases by new bone formation.

Method: This is done by two needle method. Needles are introduced into the cavity to allow free escape of cystic fluid. Removal of fluid should be thorough so that the injected suspension of prednisolone acetate crystals can cover the whole cyst wall cavity. Up to 200 mg can be injected depending up on the size of the cavity. Generally, repeat injections are required. The injection does not appear to impair the growth of the epiphyseal plate, and no biochemical disturbances have been noted. Commercially available bone substitute material can be injected in to the cavity to provide osteogenic potential and rapid healing.

ii. Percutaneous aspiration of the cyst fluid and autogenous bone-marrow injection.
iii. Percutaneous aspiration of the cyst fluid and demineralized bone matrix injection
iv. Percutaneous aspiration of the cyst fluid and demineralized bone matrix combined with bone-
marrow injection.
v. Percutaneous drilling and Kirschner wires, Cannulated screws, or flexible intramedullary nails insertion to provide continuous decompression of the cyst.

b) Surgical procedures:
Principles of surgical intervention in a case of UBC’s:
i. The fracture should be allowed to heal with splinting if required maintaining limb length
ii. Adequate sized bone window should be made for curettage
iii. The reduce recurrence cyst wall cavity should be completely excised
iv. Avoid damaging the growth plate in order to avoid angular deformities
v. Cavity may be filled with bone grafts or bone graft substitutes
vi. Periosteum should be preserved for inducing osteogenesis
vii. Recurrent lesions may require subperiosteal resection

Described surgical techniques are:
i. Limited Curettage and bone grafting: This is the classical method of curettage where a small bony window is made and curettage is attempted through this cavity. This method is associated with a high rate of local recurrence due to inadequate disease clearance
ii. Extended intralesional curettage and bone grafting: Extended curettage is done by making an adequately sized bony window and post curettage all intervening bony septae and cyst membrane are broken down using a high-speed burr. The residual cavity is reconstructed using auto or allografts.
iii. Partial Resection and Bone Grafts: Subtotal excision of the cavity is done leaving only a small longitudinal portion of host bone to maintain length and stability. A strut of auto/allograft or in combination with morsellized bone graft can be used to fill the defect. The preserved periosteum is closed over the grafts.

Complications:
• Immediate Complications:
i. Pathological Fractures
ii. Angular Deformities
iii. Limb Length Discrepancy
• They are also associated with increased chance of local recurrence which ranges from 10 to as high as 80 percent

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References

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2) Kaelin, Andre & D. MacEwen, G. (1989). Unicameral bone cysts. International Orthopaedics. 13. 275-282. 10.1007/BF00268511.
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How to Cite this article: Shetty N, Hegde P, Singh H, Gulia A. Aneurysmal Bone Cyst – Review. Journal of Bone and Soft Tissue Tumors Sep-Dec;5(3): 8-12.

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