What is Unicameral Bone Cyst?

Unicameral bone cysts (UBCs), also known as simple or solitary bone cysts, are common non-cancerous growths that look like tumors in bones. They usually have a specific look on X-rays which can help doctors to identify them easily, reducing the need for other tests or invasive procedures. Most of the time, these bone cysts don’t cause any symptoms and are discovered by accident when a person has imaging tests for another reason.

However, if they do cause symptoms, it’s typically because they have caused a fracture known as a pathologic fracture, leading to pain, swelling, or changes in the shape of the bone. In most cases, treatment isn’t necessary unless the cyst is large, causing symptoms or deformities in shape, or if there’s a risk of a fracture occurring.

What Causes Unicameral Bone Cyst?

Unicameral bone cysts are a type of cyst that contain a clear or slightly bloody fluid and are surrounded by a tough, fibrous layer. Scientists aren’t entirely sure what causes these cysts, but it’s thought that they might be due to a problem with bone growth. This problem could lead to the cyst forming and filling with fluid, causing the bone around the cyst to become thinner and larger.

Another theory is that these cysts form as a result of a blockage in a vein within the bone. Regardless of the cause, it’s important to know about these cysts because they can affect bone health.

Risk Factors and Frequency for Unicameral Bone Cyst

Unicameral bone cysts are typically found in young people, ranging from newborns to 20 years old. They are more common in males, with the ratio being 3 males for every female affected. These cysts account for 3% of all primary bone lesions. However, we don’t know their actual occurrence rate because many times they go unnoticed.

Signs and Symptoms of Unicameral Bone Cyst

When diagnosing bone cysts, a patient’s age plays a significant role. Cysts tend to be more aggressive and likely to return in younger people, with a recurrence rate four times higher than in older individuals. Important information to gather includes the type and onset of pain, as well as any other symptoms the patient is experiencing.

During a physical exam, the doctor will do a full-body check-up with a particular focus on the patient’s bones and muscles. The movement range and the health of the nervous and vascular systems also need to be checked, as serious bone tumors can interfere with these systems.

Unicameral bone cysts, which are one type of bone cyst, usually don’t cause symptoms unless a fracture has occurred. Signs of this might include tenderness, swelling, or bruising at the fracture site. The patient might also have trouble walking or moving the affected limb. These cysts typically don’t lead to infection, so it’s common for laboratory tests to show normal results.

Testing for Unicameral Bone Cyst

Unicameral bone cysts typically occur near the end (metaphysis) of the bone and are often located within the central hollow space of the bone (medullary cavity). As these cysts continue to develop over time, they may move from these initial locations further into the main part of the bone (diaphysis). However, they don’t cross the area of the bone where growth occurs (growth plate).

In children, these cysts tend to occur in the long bones of the body. The most common bones affected are the upper arm bone (humerus), thigh bone (femur), and shinbone (tibia). They can also occur in the smaller lower leg bone (fibula), ribs, outer forearm bone (radius), inner forearm bone (ulna) and finger bones (phalanges). In adults, these cysts can also appear in the heel bone (calcaneus), the large wing-like bone in the pelvis (iliac wing), radius, kneecap (patella), and other parts of the body.

X-ray images usually show these cysts as clear, well-defined areas that are brighter than the surrounding bone. The rim of the cyst might appear denser and the nearby bone may be thinner. However, the outer layer of the bone (cortex) usually stays intact. An unusual bone growth reaction at the affected site is typically not seen unless a fracture is present. If a fracture does occur, a piece of bone might break off and fall into the cyst, which is known as a “fallen fragment.”

A computed tomography scan (CT scan) can also be used to see these cysts. These scans show the same characteristics as x-rays, including thin but intact bone surrounding the cyst and the fallen fragment if a fracture has happened. A CT scan can also show a “rising bubble” sign, which is a small pocket of gas visible in the upper part of the cyst, indicating a pathologic fracture.

Magnetic resonance imaging (MRI) can further confirm the presence of the cyst. The cyst usually appears bright on fluid-sensitive imaging sequences and dark on anatomical imaging sequences. There might be noticeable changes in how the rim of the cyst looks after contrast dye is given. Although less common, the center of the bone cyst may change in appearance as well. The rising bubble sign can also be seen in an MRI, but the fallen fragment is less often visible. It is important to note that while CT and MRI scans do not provide a lot of additional information, they can be useful to rule out other conditions that might look like a unicameral bone cyst.

Treatment Options for Unicameral Bone Cyst

Unicameral bone cysts, which are fluid-filled holes that form inside the bones, do not always require treatment, especially if they are not causing any symptoms. Doctors have different opinions on the best treatment, and sometimes, the treatment fails, resulting in the cyst growing back or the bone breaking.

Some common treatments include injecting corticosteroids (a type of anti-inflammatory medicine) directly into the cyst, scraping out the cyst and filling it with a bone graft (transplanting material into the bone to help it heal). Sometimes, corticosteroid injections may need to be repeated to completely treat the cyst.

The scraping and bone graft method is often used for larger cysts or when the cyst is weakening the bone. Several factors influence the choice of treatment such as the size of the cyst, the strength of the bone, the age of the patient, and the activity level of the patient.

When diagnosing a unicameral bone cyst, doctors consider other conditions that could be causing the symptoms. Some of these conditions include:

Aneurysmal bone cyst
Fibrous dysplasia
Enchondroma
Benign chondroblastoma
Giant cell tumor
Chondromyxoid fibroma

An aneurysmal bone cyst typically appears near the area where the shaft of a bone meets the end, and they usually aren’t located in the center of the bone. This condition also tends to show a lot of expansion in the outer layer or “cortex” of the bone, which isn’t seen in unicameral bone cysts.

Fibrous dysplasia might also be considered because it appears in people of the same age range and is also centrally located within the bone. It can show up in the middle of the bone or near the end. On an X-ray, fibrous dysplasia typically has a “ground glass” appearance, while unicameral bone cysts appear more clear or “lucent”.

An enchondroma, on the other hand, doesn’t have as many hard, thick (“sclerotic”) margins as a unicameral bone cyst. It often contains a different type of tissue known as a chondroid matrix. Enchondromas tend to be found in the end portion of the upper arm bone, which is also a common location for unicameral bone cysts.

What to expect with Unicameral Bone Cyst

Unicameral bone cysts, which are simple, fluid-filled holes in the bone, usually get better on their own in most patients, especially after the growth phase of bones is completed. Less than 10% of patients may see stoppage in the growth of the affected bone. This is mostly observed after a fracture that happened because of disease or a recent surgical operation used to scrape away the cyst. Interestingly, these cysts, whether active or dormant, have a tendency to come back, even after they’ve been surgically removed.

In general, most of these cysts don’t cause any significant health problems, and patients tend to recover well.