A
unicameral bone cyst (UBC) is a common benign fluid-filled
radiolucent lesion of childhood of unknown cause. A UBC probably
represents the third or fourth most common benign bone tumor that
the orthopedic surgeon confronts. These lesions comprise
approximately 3% of all bone tumors. A UBC usually presents as a
unifocal problem, affecting patients who are skeletally immature. It
often leads to thinning of adjacent areas of bone such that fracture
or pain from microfracture may occur. A UBC occurs most frequently
in children aged 5-15 years, with an average age of approximately 9
years. A UBC affects males approximately twice as often as females.
By far, the most common location for the lesion is the proximal
humerus, followed by the proximal femur. The proximal humerus and
femur together account for nearly 90% of all UBCs. However,
virtually any bone may be affected, with the calcaneus being one of
these notable alternate locations.
The specific etiology of a UBC has not been elucidated. Many
theories have been proposed. A commonly quoted theory, proposed by
Cohen in 1960, is that the principal etiological factor is blockage
of the drainage of interstitial fluid in a rapidly growing and
rapidly remodeling area of cancellous bone. Mirra has suggested that
a UBC represents an area of a congenital rest of synovial tissue and
has supported this by demonstrating both synovial type A
(macrophage-like) and type B (fibroblast-like) cells in the lining
of such cysts (Mirra, 1978). Yu et al also have demonstrated how
methylprednisolone influences the cellular physiology of synovial
cells in culture, thus establishing a theoretic basis for steroid
injection treatments for a UBC (Yu, 1991).
Clinical Diagnosis:
Unicameral bone cysts are usually asymptomatic unless fractured,
which is a common occurrence. Most patients with a UBC present to
the orthopedic surgeon after sustaining a pathologic fracture. Such
fractures most commonly involve either the proximal humerus or the
proximal femur. In other instances, patients may present to
emergency department physicians, their primary care physicians, or
orthopedic surgeons for other reasons, and radiographs obtained in
the workup of other complaints may identify asymptomatic UBCs.
In either of these scenarios, a review of the patient's past
medical history, family history, rheumatologic conditions, bone
tumors, endocrine disease, and cancer, is appropriate.
As with all patients who have sustained a fracture, a careful
physical examination of the patient should include efforts to
exclude the possibility of open fracture and neurocirculatory
insult. Physical examination of the patient also should include a
screening examination of the axial skeleton and the uninvolved
extremities. Palpation of major lymph node areas, such as the
axillary and inguinal fields is also appropriate.
Radiologic Overview of the Diagnosis:
Plain film:
Appropriate imaging studies for a UBC should always include
plain radiographs. The plain x-ray appearance of the lesion is
virtually diagnostic. The lesion is typically a 2-3 cm oval
radiolucency with its long axis parallel to the long axis of the
host bone. The lesion is expansile and central (intramedullary) in
location, most commonly involving the proximal metaphysis of the
humerus or femur. Scalloping and erosion of the internal aspect of
the underlying cortex is common. A classic radiographic finding
for a unicameral bone cyst is the fallen-fragment sign, found in
approximately 20% of patients who present with a pathologic
fracture secondary to a UBC. The interior of the bone cyst may
have complete or nearly complete thin bony septations within the
cyst. At the time of pathologic fracture, a portion of one of
these bony segments actually may break free and sink to the
gravity-dependent portion of the lesion. This has not been
described in any other lesion and indicates a fluid-filled cystic
lesion, rather than a lesion filled with matrix.
MRI:
If a UBC is in close proximity to a growth plate and growth
impairment is a concern, magnetic resonance imaging (MRI) may
prove to be quite helpful. MRI should not be a routine part of the
workup of a UBC. Instead it should be reserved for unusual or
atypical situations. One such instance is a cyst where growth
plate damage is a concern. Several authors have documented that
such damage can occur about the proximal humeral growth plate.
Another situation in which preoperative MRI could be of value is
in rare cases in which a more sinister diagnosis is suspected,
such as in persons with pseudocystic osteosarcoma or low-grade
central osteosarcoma. In such instances, MRI is an appropriate
part of preoperative staging of such a tumor. On MR imaging the
cyst fluid is of intermediate signal on T1- and high signal on
T2-weighted images. A UBC can produce a wide variety of
appearances on MRI, including rather heterogeneous fluid signals
and even fluid-fluid levels (a sign much more commonly found in
aneurysmal bone cyst).
Management and Treatment:
An asymptomatic lesion with satisfactory maintenance of cortical
thickness may require only observation. Treatment should be strongly
considered for lesions that have resulted in fracture or marked
weakening of bone. Nonoperative treatment of UBCs usually amounts to
closed fracture care following pathologic fracture through the
lesion. It has been suggested that in as many as approximately 25%
of cases, spontaneous healing of the cyst may occur following such
pathologic fractures. Surgical therapy of a UBC may be divided into
open and percutaneous procedures. Success of such procedures has
been quite varied, and the very definition of success also has
varied from author to author. Open techniques that have been
reported include subtotal resection with and without bone grafting.
Percutaneous steroid injection and simple mechanical disruption of
the cyst wall have also been investigated as a treatment options.
Key Points:
- A unicameral bone cyst is a common benign fluid-filled lesion
of childhood of unknown etiology.
- Most common location is the proximal humerus and proximal
femur.
- Unicameral bone cysts are usually asymptomatic unless
fractured, which is a common occurrence.
- A classic radiographic finding for a UBC is the fallen
fragment sign secondary to a pathologic fracture.
- The plain x-ray appearance of the lesion is virtually
diagnostic. MRI should not be a routine part of the workup of a
UBC and should be reserved for atypical situations.
- Treatment should be strongly considered for lesions that have
resulted in fracture or marked weakening of bone.
References:
- Weissleder R, Wittenberg J, Harisinghani MG. Primer of
Diagnostic Imaging, third edition. Mosby 2003:431.
- Dahnert W. Radiology Review Manual, fifth edition. Lippincott
& Wilkins 2003:161.
- Brant WE, Helms CA. Fundamentals of Diagnostic Radiology,
second edition. Lippincott & Wilkins 1999:974-975.
- Mehlman CT. Unicameral bone cyst. eMedicine 2002.
