Alexandre Arkader and John P. Dormans
DEFINITION
Unicameral bone cyst (UBC)
Also known as a simple bone cyst, a UBC is a benign, active or latent, solitary cystic lesion that usually involves the metaphysis of long bones (most commonly the proximal humerus and femur, 40% to 80%).
Most (90%) of patients are less than 20 years old.
UBCs represent a cavity filled with yellowish or serosanguineous fluid lined by a thin fibrous membrane.
Aneurysmal bone cyst (ABC)
ABC is a benign, active, and sometimes locally aggressive, solitary, expansile cystic lesion, eccentric in location.
ABCs are most common in the first two decades, and they most often involve the long bones or the spine.
The lesion contains blood-filled cystic spaces that are not lined with vascular endothelium. Between the blood-filled spaces are fibrous septa containing giant cells and immature bone.
PATHOGENESIS
UBC
Cause is unknown; however, theories range from a reactive or developmental process, caused by obstruction to the drainage of interstitial fluid, to a true neoplasm.
Recent isolated cytogenetic analysis has reported on the presence of translocation t(16;20)(p11.2;q13) and TP53 mutations in recurrent UBCs. Further studies are necessary to better understand its pathogenesis.
UBCs are characterized by a fluid-filled cyst lined with a thin fibrous membrane without obvious lining cells. However, owing to the high incidence of associated fractures, several nonspecific changes may be seen, such as hemorrhage, hemosiderin deposits, granulation tissue, new bone formation, and others.
ABC
The neoplastic basis of primary ABCs has been, at least in part, demonstrated by the chromosomal translocation t(16;17)(q22;p13) that places the ubiquitin protease (UBP) USP6 gene under the regulatory influence of the highly active osteoblast cadherin 11 gene (CDH11), which is strongly expressed in bones.
ABCs are characterized by various-size blood-filled cystic spaces without vascular endothelium lining and divided by fibrous septa containing giant cells and immature bone (FIG 1).
NATURAL HISTORY
UBC
Active cysts are generally located near the growth plate and are usually asymptomatic.
Inactive or latent cysts tend to “migrate” away from the growth plate as longitudinal growth occurs.
UBCs may regress spontaneously after skeletal maturity.
ABC
It is usually an active benign lesion with locally aggressive features.
The lesion tends to continue to grow, necessitating intervention.
It can occur as a secondary lesion associated with giant cell tumor, osteoblastoma, chondroblastoma, or fibrous dysplasia.
PATIENT HISTORY AND PHYSICAL FINDINGS
UBC
UBCs are often asymptomatic. The usual presentation is with a pathologic fracture (50% to 65%).
FIG 1 • High-power (200× magnification; A) and low-power (40× magnification; B) photomicrographs of a lesion demonstrate blood-filled spaces surrounded by several focal giant cells, as well as spindle-shaped cells lining the walls of these spaces. Some of the spaces are slit-like, whereas the other spaces are dilated. Hemosiderin is also seen.
FIG 2 • Lateral radiograph of the foot of a 12-year-old boy demonstrates a well-circumscribed, lucent lesion, consistent with unicameral bone cyst.
While about 90% occur before the age of 20 years, calcaneal UBCs tend to occur in a slightly older group. They occur more often in boys (3:1). The most common locations are the proximal humerus and femur, accounting for 50% to 70% of the lesions.
ABC
ABCs often present with mild pain.
Pathologic fracture is not uncommon, following minor trauma.
The spine, femur, and tibia are the most common locations. In the spine, the lesion is more often limited to the posterior elements.
IMAGING AND OTHER DIAGNOSTIC STUDIES
UBC
Plain radiographs typically demonstrate a centrally located, well-defined, radiolucent lesion (FIG 2), usually surrounded by a sclerotic margin and narrow zone of transition.
They may demonstrate cortical thinning and mild expansion (unlike ABCs, UBCs do not exceed the width of the nearest growth plate).
When a pathologic fracture occurs, there is periosteal reaction, and occasionally the typical “fallen fragment” sign is visualized (piece of fractured cortex “floating” inside the cavity).
CT is usually useful to characterize lesions that are difficult to visualize on plain films (ie, spine, pelvis) and to rule out fractures (nondisplaced or minimally displaced).
MRI is useful for differential diagnosis and atypical UBCs.
Although variable, they present as low to intermediate signals on T1-weighted images and bright and homogeneous signals on T2-weighted images.
ABC
Plain radiographs show an eccentrically located, multilobulated, expansile (expands beyond the width of the nearest growth plate), radiolucent lesion with a narrow zone of transition.
A rim of sclerotic bone is often seen.
Cortical thinning, disruption, and periosteal reaction are common.
CT is helpful, especially because these lesions are commonly located in the spine (FIG 3A,B).
It shows the typical ridges in the interior of the cyst.
Soft tissue extension can be appreciated, but there is no true soft tissue mass.
MRI is useful to confirm the lobulated nature of the lesion and the cystic cavities filled with fluid (fluid–fluid levels on T2-weighted signal; characteristic but not pathognomonic; Fig 3C,D).
There is variable signal intensity in both T1- and T2-weighted images due to the nature of the cyst contents (fresh blood, mixed with degraded blood products).
DIFFERENTIAL DIAGNOSIS
UBC
ABC
Nonossifying fibroma
Fibrous dysplasia
Brown tumor of hyperparathyroidism (usually presents with osteopenia and subcortical resorption)
Bone abscess (commonly shows periosteal reaction).
For calcaneus lesions, the differential also includes chondroblastoma and giant cell tumor.
Latent UBC (diaphyseal.
Fibrous dysplasia
ABC
UBC
Giant cell tumor
FIG 3 • Aneurysmal bone cyst of L4. A. Axial CT scan of the lower lumbar spine shows a destructive, lytic, expansile lesion, involving the body and posterior elements of L4 with disruption of the spinal canal. B. Three-dimensional-reconstruction CT image demonstrates the asymmetric collapse of the vertebra. C. Sagittal T1 MRI shows the collapsed L4 with posterior disruption by the tumoral mass. D. Axial T2-weighted MRI shows the characteristic fluid-fluid level and disruption of the medullary canal.
Osteoblastoma
Telangiectatic osteogenic sarcoma
NONOPERATIVE MANAGEMENT
UBC
Lesions with typical radiographic appearance and lesions in non-weight-bearing bones can be followed with serial radiographs.
Small lesions (ie, those that involve less than one third to half of the bone width) in weight-bearing bones that have low risk for fracture can also be observed.
Following pathologic fracture, up to 14% of UBCs may spontaneously heal, and therefore an attempt for conservative treatment should be made.
ABC
There is little place for conservative treatment of ABCs. At least an incisional biopsy for diagnosis confirmation is recommended.
For small, asymptomatic lesions located in non-weightbearing bones, observation can be indicated.
SURGICAL MANAGEMENT
The first indication for surgical treatment of benign bone cysts is to confirm the diagnosis (ie, incisional biopsy).
Large lesions that involve more than one third to half of the bone width and lesions of weight-bearing bones are usually treated with surgery.
Pathologic fractures are not an absolute indication for surgical treatment, but when they are associated with ABCs, or are located in the lower extremities, surgery is usually indicated.
ABCs in nonessential bones (eg, rib, fibula) can be treated with wide resection to avoid recurrence.
Preoperative Planning
It is very important to pay attention to the details, formulate the differential diagnosis, and rule out a malignant process. It is also very important to rule out physeal growth arrest before any intervention.
For ABCs, an open biopsy is often needed since needle biopsy may get only blood and no tissue.
An image intensifier is useful for accurate localization of the lesion, during percutaneous procedures, and for intraoperative confirmation that the entire lesion is being addressed.
For open surgery, headlamps for illumination and loupes for magnification are recommended.
Positioning
Positioning depends on the lesion's location. For all extremity lesions, the entire affected extremity should be entirely free draped. It is important to confirm that the extremity can be properly imaged by the image intensifier.
Approach
UBCs are less aggressive lesions and therefore are prone to a percutaneous technique.
ABCs can be quite aggressive and an open and more aggressive approach is usually necessary.
For lesions in the proximal femur, a more aggressive approach is often necessary; particularly if a pathologic fracture is present, open reduction and internal fixation is indicated (FIG 4).
FIG 4 • Classification and treatment algorithm for proximal femur pathologic fractures through a bone cyst. Type IA: A small cyst is present in the middle of the femoral neck, the lateral buttress is intact, cannulated screws are used avoiding the physis. Type 1B: A larger cyst is present, there is compromise of the lateral buttress, and a pediatric dynamic hip screw (DHS) is used. Types IIA and IIB: There is not enough bone between the growth plate and the lesion; therefore, the patient can be kept in traction or a cast until initial healing occurs, or parallel Steinmann pins across the physis can be used. Type IIIA: Since the growth plate is closed, cannulated screws purchasing the femoral head are used. Type IIIB: Because of the loss of the lateral buttress, a pediatric DHS is recommended. A spica cast is generally recommended after the surgical treatment. Internal fixation should be preceded by a four-step approach. (Adapted from Dormans J, Flynn J. Pathologic fractures associated with tumors and unique conditions of the musculoskeletal system. In Beaty JH, Kasser JR, eds. Rockwood and Wilkins' Fractures in Children, ed 5. Philadelphia: Lippincott Williams & Wilkins, 2001:151.)
TECHNIQUES
UNICAMERAL BONE CYST
Percutaneous Intramedullary Decompression, Curettage, and Grafting With Medical-Grade Calcium Sulfate Pellets
Under fluoroscopic guidance, a Jamshidi trocar needle (CardinalHealth, Dublin, OH) is percutaneously inserted into the cyst cavity.
The cyst is aspirated to confirm the presence of straw-colored fluid, which is typical of previously untreated or unfractured UBCs.
Three to 10 mL of Renografin dye (E.R. Squibb, Princeton, NJ) is injected to perform a cystogram and confirm the single fluid-filled cavity (TECH FIG 1A).
A 0.5-cm longitudinal incision is then made over the site of the aspiration and a 6-mm arthroscopy trocar is advanced into the cyst cavity through the same cortical hole, and the cortical entry is enlarged manually (TECH FIG 1B).
Under fluoroscopic guidance, percutaneous removal of the cyst lining is done and curettage is performed using a pituitary rongeur and various-sized angled curettes (TECH FIG 1C).
An angled curette or flexible intramedullary nail is used to perform the intramedullary decompression in one direction (toward the diaphysis) or in both directions (when the growth plate is far enough, avoid physeal injury) (TECH FIG 1D,E).
Medical-grade calcium sulfate pellets (Osteoset, Wright Medical Technology, Arlington, TN) are inserted through the same cortical hole and deployed to completely fill the cavity (TECH FIG 1F,G).
Angled curettes can be used to advance pellets into the medullary canal.
Tight packing of the cyst is preferred.
The wound is closed in a layered fashion.
TECH FIG 1 • Unicameral bone cyst. A. Fluoroscopic image showing a proximal femur unicameral bone cyst filling with dye. B. A 0.5-cm incision is made where the Jamshidi needle for the cystogram was placed. C. The cyst is curetted using curettes and a pituitary rongeur. Material is then sent for pathology analysis. Intramedullary decompression of the cyst is done using flexible intramedullary nails (D) or angled curettes (E). F. The cyst is filled with medicalgrade calcium sulfate pellets. G. Four-month follow-up radiograph shows complete healing.
ANEURYSMAL BONE CYST
A four-step open surgical approach is used.
Under fluoroscopic guidance, a Jamshidi trocar needle is percutaneously inserted into the cyst cavity.
The cyst is aspirated to confirm the presence of blood-filled cavities or soft tissue septations typical of ABCs.
A longitudinal incision of roughly the same size of the cyst is made, the neurovascular structures are protected, and the periosteum is opened and retracted.
Perforation of the thinnest part of the cyst wall is performed with a curette, burr, or drill.
The fibrous lining of the lesion is completely curetted. Septations are opened and removed to access all components of the cyst.
The use of headlamps for illumination and loupes for magnification is recommended to ensure a thorough excision. Image intensifier may be helpful to ensure that all cavities were opened.
A high-speed burr is used to improve the curettage and help with complete excision of any macroscopic tumor (TECH FIG 2A,B).
The cavity is then cauterized with electrocautery and in selected cases (eg, lesions distant from the growth plate and main neurovascular structures) phenol 5% solution is applied to the cyst wall with a cotton-tipped applicator to extend the margins.
The cavity is now tightly packed with bone graft. We prefer to use a combination of allograft cubes and demineralized bone matrix paste (TECH FIG 2C–F).
The wound is closed in a layered fashion.
TECH FIG 2 • A. AP radiograph of the proximal humerus shows an expansile, lytic, loculated aneurysmal bone cyst. B. The cyst is thoroughly curetted and a high-speed burr is used to remove any residual cyst lining. C. The cyst is completely filled with packed allograft chips and demineralized bone matrix. D. Radiographic aspect 2 weeks after the procedure; the cyst was entirely removed and the cavity was entirely grafted. E,F. At 4-month follow-up, AP radiographs of the right proximal humerus in internal and external rotation demonstrate that the lesion is completely healed, with good incorporation of the bone graft.
POSTOPERATIVE CARE
In most cases, the extremity is protected from weight bearing for about 4 to 6 weeks. Before the patient is allowed to return to physical activity, radiographic evidence of bone healing is necessary.
OUTCOMES
The minimally invasive technique for UBC has shown promising results on a short-term evaluation, with reported success rate (eg, complete or partial healing or opacification) of about 95%. Recently the intermediate to long-term results in a larger cohort were reviewed, and the observed rate of complete or partial response achieved with one surgical intervention was 80%. Nonetheless, the success rate increased to 94% after a repeat surgery, reaching a 100% healing rate in patients who underwent more than two repeat surgeries. These results compare favorably with other outcomes after surgical treatment of UBCs.
The recurrence rate for any surgically treated ABC varies from 10% to 59% according to the reported results. In a large cohort including 45 children with primary ABC treated by the described four-step approach technique and at least 2 years of follow-up, the recurrence rate was only 18%. Although the recurrence rate was slightly higher among younger children (less than 10 years old), this difference did not show statistical significance.
COMPLICATIONS
Persistence or recurrence: varies from 10% to 20% for all techniques described
Infection
Fracture
Intraoperative bleeding: for aggressive-looking ABCs and lesions in difficult locations such as the pelvis and spine, arteriography, and sometimes embolization, is helpful.
REFERENCES
· Arkader A, Mik G, Manteghi A, et al. Intermediate to long term results of a minimally invasive technique for the treatment of unicameral bone cysts. CORR. June 2009. In press.
· Boriani S, De Iure F, Campanacci L, et al. Aneurysmal bone cyst of the mobile spine: report on 41 cases. Spine 2001;26:27.
· Campanacci M, De Sessa L, Trentani C. Scaglietti's method for conservative treatment of simple bone cysts with local injections of methylprednisolone acetate. Ital J Orthop Traumatol 1977;3:27.
· Cohen J. Etiology of simple bone cyst. J Bone Joint Surg Am 1970;52A:1493.
· Dormans JP, Hanna BG, Johnston DR, et al. Surgical treatment and recurrence rate of aneurysmal bone cysts in children. Clin Orthop Relat Res 2004;421:205–211.
· Dormans JP, Sankar WN, Moroz L, et al. Percutaneous intramedullary decompression, curettage, and grafting with medicalgrade calcium sulfate pellets for unicameral bone cysts in children: a new minimally invasive technique. J Pediatr Orthop 2005;25:804.
· Garg S, Mehta S, Dormans JP. Modern surgical treatment of primary aneurysmal bone cyst of the spine in children and adolescents. J Pediatr Orthop 2005;25:387–392.
· Mankin HJ, Hornicek FJ, Ortiz-Cruz E, et al. Aneurysmal bone cyst: a review of 150 patients. J Clin Oncol 2005;23:6756.
· Oliveira AM, Perez-Atayde AR, Inwards CY, et al. USP6 and CDH11 oncogenes identify the neoplastic cell in primary aneurysmal bone cysts and are absent in so-called secondary aneurysmal bone cysts. Am J Pathol 2004;165:1773.
· Ramirez AR, Stanton RP. Aneurysmal bone cyst in 29 children. J Pediatr Orthop 2002;22:533.
· Sullivan RJ, Meyer JS, Dormans JP, et al. Diagnosing aneurysmal and unicameral bone cysts with magnetic resonance imaging. Clin Orthop Relat Res 1999;366:186–190.