Operative Management of Unicameral Bone Cyst, Aneurysmal Bone Cyst, and Nonossifying Fibroma
Operative Management of Unicameral Bone Cyst, Aneurysmal Bone Cyst, and Nonossifying Fibroma
UNICAMERAL BONE CYST
DEFINITION
Unicameral bone cyst (UBC) or simple bone cyst is a benign, active or latent, solitary fluid-filled cystic lesion that usually involves the metaphysis of long bones.
PATHOGENESIS
The cause is unknown; theories range from a reactive or developmental process to a true neoplasm. The most likely cause is perhaps an obstruction to the drainage of interstitial fluid, leading to the accumulation of fluid under pressure.
Few isolated reports have shown cytogenetic abnormalities including the presence of translocation t(16;20) (p11.2;q13) and t(7;12)(q21;q24.3) and TP53 mutations in recurrent UBCs.
UBCs are characterized by a fluid-filled cyst lined with a thin fibrous membrane without endothelial cell lining. 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.
NATURAL HISTORY
Active cysts are generally located near the growth plate and are usually asymptomatic. Approximately 85% of these cysts are diagnosed at the time of a pathologic fracture.
Inactive or latent cysts tend to “migrate” away from the growth plate as longitudinal growth occurs and populate the midshaft or diaphyseal region.
UBCs may regress spontaneously after skeletal maturity.
PATIENT HISTORY AND PHYSICAL FINDINGS
UBCs are usually asymptomatic; the usual presentation is with a pathologic fracture following a minor trauma.
UBC is more often in boys (3:1), especially during the first two decades. The most common locations are the proximal humerus and the femur, accounting for 50% to 70% of the lesions. Calcaneal UBCs tend to occur in a slightly older group.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain radiographs typically demonstrate a well-defined, centrally located, radiolucent lesion that may be associated with varying degrees of cortical thinning and mild expansion (FIG 1).
When UBCs are associated with bone expansion, it generally does not exceed the width of the nearest growth plate.
When a pathologic fracture occurs, there may be periosteal reaction, and occasionally, the typical “fallen
leaf” sign is visualized (piece of fractured cortex “floating” inside the cavity). Most pathologic fractures are minimally displaced and stable.
Computed tomography (CT) may be useful to characterize lesions that are of difficult visualization on plain films (eg, spine, pelvis) and to rule out fractures (nondisplaced or minimally displaced).
Noninvasive quantitative CT has been applied to evaluation of the risk of pathologic fractures through bone cyst and other benign bone lesions.
Magnetic resonance imaging (MRI) is usually only needed for differential diagnosis of atypical UBCs.
Although the appearance may vary, they present as low to intermediate signals on T1-weighted images and bright and homogeneous signals on T2-weighted images.
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FIG 1 • Lateral radiograph of the foot of a 12-year-old boy demonstrates a well-circumscribed, lucent lesion, consistent with unicameral bone cyst.
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FIG 2 • 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.
DIFFERENTIAL DIAGNOSIS
Aneurysmal bone cyst (ABC) Nonossifying fibroma (NOF)
Fibrous dysplasia (differential with latent/diaphyseal UBC)
Brown tumor of hyperparathyroidism (usually presents with osteopenia and subcortical resorption) Osteomyelitis (commonly shows periosteal reaction)
For calcaneus lesions, the differential also includes chondroblastoma and giant cell tumor.
NONOPERATIVE MANAGEMENT
Lesions with typical radiographic appearance involving non-weight-bearing bones, especially in the absence of significant cortical thinning, 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 15% of UBCs may spontaneously heal, and therefore an attempt for conservative treatment should be made. Exceptions include large lesions and unstable or displaced fractures of lower extremities, especially in older children.
ANEURYSMAL BONE CYST
DEFINITION
ABC is a benign, active, and sometimes locally aggressive, solitary, expansile blood-filled cystic lesion, eccentric in location, most commonly seen in the metaphyseal region of long bones or posterior elements of the spine.
PATHOGENESIS
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. Abnormalities of the short arm of chromosome 17 appear to be recurrent.
The lesion contains blood-filled cystic spaces that are not lined with vascular endothelium, divided by fibrous septa containing giant cells and immature bone (FIG 2).
NATURAL HISTORY
ABC can present as an active or locally aggressive lesion that tends to continue to grow and warrants intervention.
They may also occur in association with other lesions such as giant cell tumor, osteoblastoma, chondroblastoma, or fibrous dysplasia.
PATIENT HISTORY AND PHYSICAL FINDINGS
ABCs often present with pain. Sometimes, swelling or a “mass” can be present.
Pathologic fracture is not as prevalent as in UBC but it may occur following minor trauma.
The most common locations are the metaphysis of long bones, particularly the femur and tibia, and the posterior elements of the spine.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain radiographs show an eccentric, multilobulated, expansile (often expanding beyond the width of the nearest growth plate), radiolucent lesion with a narrow zone of transition.
Cortical thinning, disruption, and periosteal reaction are common.
CT is helpful, for better characterization and treatment planning of axial lesions (FIG 3A,B).
CT demonstrates 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 levels on T2-weighted signal are 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).
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FIG 3 • ABC 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.
DIFFERENTIAL DIAGNOSIS
UBC NOF
Giant cell tumor Osteoblastoma
Telangiectatic osteogenic sarcoma
NONOPERATIVE MANAGEMENT
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-weight-bearing bones, observation can be indicated. For lesions that are of difficult access/approach, serial embolization or sclerotherapy is another option.
NONOSSIFYING FIBROMA
DEFINITION
NOF is a benign latent, cystic, eccentric, and cortical-based lesion that is most often found incidentally in the metaphyseal region of long bones. NOFs are the most common benign fibrous lesions, and it is estimated that up to 20% of all children have an NOF or the smaller counterpart, fibrous cortical defect. These lesions can be multicentric or synchronous.
PATHOGENESIS
NOF is part of a heterogeneous group of benign connective tissue tumors that are considered as reactive by many authors rather than a true neoplasm. Typically, they do not contain any bone and are characterized by a spindle cell proliferation with storiform arrangement of cells. Theories surround the notion that NOF is a developmental defect of the growth plate.
There have only been three case reports demonstrating cytogenetic abnormalities in NOF, including a complex translocation involving chromosomes 3, 4, 11, and 14.
NATURAL HISTORY
The majority of NOF and fibrous cortical defects ossify (heal) spontaneously over time. At times, they can be associated with stress fractures, especially in very active children, or multicentric tumors.
PATIENT HISTORY AND PHYSICAL FINDINGS
The most common presentation is an incidental finding. At times, patients will report pain or discomfort, especially with physical activities; these cases are usually a reflection of a stress reaction or pathologic fracture. Most lesions are located around the knee and shoulder.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Plain radiographs are usually diagnostic and demonstrate a well-defined, cortical-based/eccentric, lytic lesion with bubbly appearance, surrounded by a sharp sclerotic margin (FIG 4A,B).
CT and MRI are usually not necessary for the diagnosis; however, they may be helpful for adequate
evaluation of the lesion's size and extent as well as the presence of associated stress fracture (FIG 4C,D).
DIFFERENTIAL DIAGNOSIS
UBC ABC
Osteofibrous dysplasia Chondromyxoid fibroma
NONOPERATIVE MANAGEMENT
Because NOF heals or ossifies over time, most lesions are amenable to conservative treatment. Large lesions in weight-bearing areas and/or associated with chronic pain or pathologic fracture may warrant surgical treatment.
Unlike UBC, pathologic fracture associated to NOF will heal but the lesion will persist.
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FIG 4 • NOF of the distal tibia. AP (A) and lateral (B) radiographs of a well-defined, cortical based, lytic lesion in the distal tibia metaphysis, surrounded by sharp sclerotic border, consistent with a NOF. MRI (C,D) shows low intensity on T1 and better defines the extent of the lesion and cortical involvement. There is no soft tissue extension or periosteal reaction.
UNICAMERAL AND ANEURYSMAL BONE CYSTS AND NONOSSIFYING FIBROMAS
SURGICAL MANAGEMENT
Indications
Diagnosis: The first indication for surgical treatment of these benign bone lesions is to confirm the diagnosis when the clinical and/or radiographic appearances are not classic.
Symptoms: Although UBC and NOF are usually painless, ABC can be associated with significant pain that warrants treatment.
Risk of fracture: Large lesions usually involving more than one-third to half of the bone width and lesions of weight-bearing bones are at a higher risk of pathologic fracture and may warrant surgery.
Pathologic fracture: Although not an absolute indication for surgical treatment (especially in the upper extremities), fractures of weight bearing bones, especially around the hip, often need acute management. In general, it is preferred to wait for pathologic fractures to heal prior to treatment of the lesion.
Preoperative Planning
It is imperative to formulate the differential diagnosis and rule out a malignant process. If biopsy is planned, open biopsy yields best results, especially for ABC (filled with blood).
Because most of these lesions are juxtaphyseal, it is important to rule out physeal involvement prior to any intervention.
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.
Because most of these lesions are resected through a minimally invasive or limited approach, headlamps for illumination and loupes for magnification are recommended.
Lesions in nonessential bones (eg, rib, fibula), in particular for locally aggressive ABC, can be treated with wide resection to avoid recurrence.
Positioning
Positioning depends on the lesion's location. For all extremity lesions, the entire affected extremity should be draped free. It is important to confirm that the extremity can be properly imaged by the image intensifier, especially for proximal lesions, such as around the shoulder and hip.
Approach
UBCs and NOFs are often latent lesions and therefore amenable to a percutaneous technique.
ABCs are often locally aggressive, and a formal open approach with extensive curettage and adjuvants is recommended.
Due to the risk involved with pathologic fractures and mal-union around the hip, lesions in this region warrant a more aggressive approach, and often, open reduction and internal fixation is indicated (FIG 5).
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FIG 5 • 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, and 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: Because 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
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Percutaneous Intramedullary Decompression, Curettage, and Grafting with Medical-Grade Calcium Sulfate Pellets
Under fluoroscopic guidance, a Jamshidi trocar needle (Cardinal Health, Dublin, OH) is percutaneously inserted into the cyst cavity/lesion.
The cyst is aspirated to confirm the presence of straw-colored fluid, which is typical of previously untreated or unfractured UBCs; this step is not done for NOFs.
Three to 10 mL of 50% diluted Renografin dye (E.R. Squibb & Sons, Princeton, NJ), or similar, is injected to perform a cystogram and confirm the single fluid-filled cavity (this step is not done for NOFs) (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 (cortical window) is enlarged manually (TECH FIG 1B).
Under fluoroscopic guidance, percutaneous removal of the cyst pseudo-lining or tumor contents is done, and curettage is performed using a pituitary rongeur and various-sized angled curettes (TECH FIG 1C).
For UBCs, it's recommended to perform intramedullary decompression and this can be achieved using an angled curette or flexible intramedullary nail, in one direction (toward the diaphysis) or in both directions (when the growth plate is far enough away, avoiding penetration of the physis) (TECH FIG 1D,E).
For NOFs, curettage does not need to extend beyond the surrounding sclerotic bone into the medullary cavity, as this bone is structurally supportive.
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.
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TECH FIG 1 • UBC. A. Anteroposterior (AP) radiograph showing a pathologic fracture through a well-defined, lytic, central lesion in the proximal humerus metaphysis of a 6-year-old boy. B. At 4 weeks, the fracture has healed but the cyst persisted. C. Fluoroscopic image showing the placement of the Jamshidi needle. D. A 0.5-cm incision is made following the cystogram that showed some uneven filling, typical of postfractured cysts. The arthroscopy trocar is used to open a cortical window. E,F. The cyst is curetted using curettes and a pituitary rongeur. Material is then sent for pathology analysis. G. Intramedullary decompression of the cyst is done using flexible intramedullary nails or angled curettes. H. The cyst is filled with medical-grade calcium sulfate pellets. I. Three-month follow-up radiograph shows complete healing.
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Four-Step Approach with Open Curettage and Bone Grafting
A more extensive, open approach is preferred for ABCs and for recurrent UBCs that have been previously treated with percutaneous techniques.
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 ABC.
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 (extension of margins) (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.
Adjuvants are usually not needed for NOF; although recurrence is uncommon, incomplete excision will lead to persistence of the tumor.
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The cavity is now tightly packed with the bone graft of choice. For lesions that are not involving the
subchondral bone, a combination of cancellous or corticocancellous allograft and demineralized bone matrix paste is usually preferred (TECH FIG 2C-F). For lesions that are juxta-articular and involving the subchondral bone, especially in the weight-bearing bones, a synthetic graft material, such as calcium sulfate/phosphate combo, is recommended for extra and acute structural support.
The wound is closed in a layered fashion.
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TECH FIG 2 • A. Anteroposterior (AP) radiograph of the proximal humerus shows an expansile, lytic, loculated ABC. 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.
PEARLS AND PITFALLS
Diagnosis
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UBC and NOF diagnosis can be made solely based on radiographic findings.
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The surgeon should always biopsy ABCs to rule out malignancies and other associated conditions.
Growth
arrest
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Growth arrest can occur without or prior to the treatment. The family should be
informed if there is a suspicion of growth plate damage before the treatment. MRI helps confirm physeal involvement.
Pathologic ▪ UBCs may spontaneously resolve after fractures, so adopt a conservative
fracture treatment (if possible).
Natural
history
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UBCs tend to grow away from the growth plate and will not grow after skeletal
maturity. ABCs continue to grow beyond skeletal maturity. NOF heals over time and are most amenable to observation.
Recurrent
lesions
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Bone cysts may “upgrade.” UBCs may recur with an ABC component and ABCs
may recur with a more aggressive behavior than the initial lesion. NOF may develop a secondary ABC.
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Most recurrences following treatment occur in the first 24 months.
POSTOPERATIVE CARE
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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, that can take up to 3 months.
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%; the intermediate- to long-term results (>2 years) show 80% complete or partial response following one surgical intervention.
Nonetheless, the success rate increases to 94% after a repeat surgery, reaching a 100% healing rate in patients who undergo more than two repeat surgeries.
These results compare favorably with outcomes after other 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 (younger 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
Allergic reaction to the graft (<5%) Fracture (intraoperative or postoperative)
Intraoperative bleeding: For aggressive-looking ABCs and lesions in difficult locations such as the pelvis and spine, preoperative embolization is helpful.
SUGGESTED READINGS
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