Bone Cyst

What is Bone Cyst?

Bone cysts often don’t show any symptoms and are usually discovered by accident during x-ray scans. However, sometimes they can cause pain because of repeated internal bleeding or fractures that happen without a clear cause. Types of bone cysts include simple bone cysts and aneurysmal bone cysts.

A simple bone cyst is a single, fluid-filled, non-cancerous cyst that can be found in any bone in the arms or legs. They are most commonly found near the shoulder and hip joints. They can also form in adults in the hip bone and heel bone. These cysts are usually more reactive during growth periods and can heal on their own once the growth phase of that bone is over. Two-thirds of simple bone cysts are accompanied by a fracture. These cysts don’t show symptoms in flat bones unless they are noticed on a scan.

On the other hand, an aneurysmal bone cyst is a rare kind of bone tumor that is non-cancerous but can cause local bone damage. This cyst is filled with blood and commonly occurs at the ends of long bones in children and young adults. They can occur anywhere, but are most commonly found near the knee, shoulder, and spinal cord. Most people experience light to moderate pain with this cyst. Its quick growth can sometimes give the impression of a malignant or cancerous growth. In the spine, these cysts might cause radiating pain or nerve-related symptoms. In rare cases, aneurysmal bone cysts can develop in the soft tissues of the body, not just the bones.

What Causes Bone Cyst?

UBC, or Unicameral Bone Cyst, is a type of bone lesion that arises in response to the slow flow, or stasis, of blood in a particular type of bone (cancellous bone). This leads to a build-up of pressure, inflammation, and causes the bone to break down or get reabsorbed.

ABC, or Aneurysmal Bone Cyst, is a type of bone tumor that grows locally but is aggressive. It was previously believed to be caused by bleeding inside the bone due to slow blood flow and bone breakdown by cells called osteoclasts. However, the latest understanding is that ABCs are not primarily caused by these factors. About 70% of primary ABC lesions, or the original tumors, are linked with changes in chromosomes, leading to gene fusions between a gene called USP6 and various other genes.

A secondary ABC is linked to many different benign (non-cancerous) and malignant (cancerous) bone lesions, such as chondroblastoma, osteosarcoma, simple bone cysts, giant cell tumors, and telangiectatic osteosarcoma. These secondary ABC lesions, or tumors that develop as a result of the primary tumor, do not have the chromosomal changes observed in primary ABCs.

Risk Factors and Frequency for Bone Cyst

UBC and ABC are two types of lesions that are commonly found. UBC is more common and it usually appears in people’s second decade of life. It affects men twice as much as women. On the other hand, ABC is a rarer lesion, with only 1.4 cases per 10,000 people each year, accounting for 9.1% of all bone tumors. It is more common in females and 80% of these cases also occur in one’s second decade of life. They’re most often found in patients under 20 and are quite rare in people aged 30 and over.

• UBC lesions are quite common and they typically appear in people’s second decade of life, more in males than in females.
• ABC lesions are more rare, with 1.4 cases per 10,000 people annually, making up 9.1% of all bone tumors.
• ABC lesions are more frequently found in females.
• About 80% of ABC lesions are found in people in their second decade of life.
• Both UBC and ABC are most common in people under 20 years old.
• It’s quite rare to find both UBC and ABC lesions in people aged 30 and over.

Signs and Symptoms of Bone Cyst

Unicameral Bone Cysts (UBC) often do not exhibit any symptoms. These cysts, usually found in the femur or humerus bones, are often discovered by chance. Infrequently, these cysts can lead to pain if a spontaneous fracture occurs through the cyst.

Aneurysmal Bone Cysts (ABC) mostly affect the long bones and spine. These type of cysts can cause symptoms such as mild to moderate pain that can persist for weeks or even months. This is typically the symptom that brings patients to the clinic. If a cyst occurs in the spine, it can present itself as back pain, a curved spine (scoliosis), or twisting of the neck (torticollis). Sometimes, swelling in soft tissue may occur. It is also possible for a patient to have a bone fracture due to these cysts.

During a physical examination, a doctor may notice tenderness, swelling in the local area, scoliosis or torticollis (when the spine is involved), and negative effects on the nervous system (neurological deficits) if there is a cyst in the spine.

Testing for Bone Cyst

UBC (Unicameral Bone Cyst):

In simple terms, UBC looks like a ‘bubble’ or ‘cyst’ in a long bone, typically near the joints (this area is known as the metaphysis). This bubble-like lesion is well-defined, hollow, and might appear slightly expanded or larger. This cyst doesn’t cause the outer layer of the bone (the cortex) to crack, but it may have several ridges inside that give it a multi-chambered or ‘split’ appearance. However, there is no reaction from the surrounding layer of the bone (the periosteum).

If a fracture occurs, a small bone fragment might fall into the bottom of the cyst, creating a ‘fallen fragment sign’. This is a typical sign of a fractured UBC. The cyst often starts near the joints in children, then gradually moves into the central part of the bone (called the diaphysis) over time. The cyst is considered ‘active’ when it’s within 1 cm of the growth plate and ‘stable’ when it’s nearer to the diaphysis.

CT scans can show cysts with false separators (pseudo-septations), which can be helpful for assessing the risk of cyst fracture and the involvement of neighbouring structures. MRI results usually show bright or ‘hyperintense’ cysts, with enhancement of the cyst wall and separators. Bone Scintigraphy and Positron Emission Tomography (PET scan) unfortunately, don’t offer clear results. Cystography (imaging of the cyst) can be used to study the flow of blood within the cyst.

ABC (Aneurysmal Bone Cyst):

An initial suspicion of an ABC may be made through a clinical examination and imaging. The imaging characteristics may reveal an expanding, hollow, abnormal bone growth near the joint. The cyst may be well-lined or mildly invasive, similar to a malignant tumor. A smooth reaction usually covers this cyst from the surrounding layer of the bone.

A bone scan might show peripheral “tracer” uptake (when a radioactive substance, or tracer, collects in certain areas of the body) and a central area of decreased uptake, known as the “doughnut sign”. CT scans and MRIs can further pinpoint the cyst characteristics, involvement of soft tissue, and potential tumor aggressiveness. A CT scan can be particularly useful in outlining cysts in areas like the spine and pelvis.

When differentiating ABC from UBC using an MRI, the presence of a ‘double-density’ fluid and internal partitions or ‘septa’ within the lesion can indicate an ABC. Imaging can further aid in planning surgical management.

In all cases, a biopsy (a procedure to remove a small segment of tissue for examination under a microscope) should be performed to confirm the diagnosis. Do note that the sample for biopsy must be of high quality and very representative of the lesion. Fine needle aspiration cytology (a procedure where a small sample of cells is taken using a fine needle) is sometimes done by radiologists but often doesn’t provide clear-cut results. The need for biopsy arises when there is a high clinical and radiological suspicion for ABC. Patients with unmanaged bleeding disorders are usually advised not to undergo this procedure.

Treatment Options for Bone Cyst

If a person has mild symptoms from a bone lesion in their upper limbs, doctors often opt to monitor the condition rather than resorting to immediate treatment. Regular x-rays are taken to keep an eye on the lesion. However, if the lesions are large, at risk of causing a fracture, or found in the lower limbs, doctors tend to opt for treatment options like curettage or aspiration, which are methods to remove or drain the abnormal tissue, and injections, which apply medication directly to the lesion site.

If a person who has these lesions experiences a fracture in one of their upper limbs, the focus is usually on treating the fracture first, typically by immobilization for 4 to 6 weeks. However, in cases of unstable fractures or fractures in weight-bearing areas like the lower extremities, treatment will involve both fracture fixation and treatment of the bone cyst.

Non-aggressive techniques are generally preferred for treating other types of bone lesions. For example, corticosteroids, a type of anti-inflammatory medication, can be injected directly into the cyst, after draining its contents. This technique is effective more than 90% of the time and may work by decreasing internal pressure within the cyst.

Another benign bone lesion is the aneurysmal bone cyst (ABC). The aim of ABC treatment is disease elimination, preventing it from coming back, and reducing pain or functional impairment. Traditional treatment modes were surgical curettage and bone grafting. However, to avoid extensive surgery, disability, and rehabilitation costs due to the benign nature of the cyst, less aggressive options have become popular. Treatments like medical therapy with denosumab, sclerotherapy with polidocanol (a type of injection that causes the cyst to shrink), radionuclide ablation (use of radiation to treat the cyst), and selective embolization of the feeding vessel to the cyst (blocking the blood supply to the cyst) have emerged as alternatives.

In some cases where surgical removal of the cyst is not possible due to its location, arterial embolization can be used as an alternative. Calcitonin and steroids can also be injected directly into the cyst, guided by CT scans, yielding positive results. However, if the lesion continues to recur or if any suspicious findings occur on imaging, more aggressive treatment may be required.

Similarly, a simple treatment like an injection with an alcoholic solution of zein (corn protein), could also be applicable. This option has properties that help stop the growth of the cyst and start the formation of new bone along the inner layer of the cyst. However, it may necessitate repeated injections.

What else can Bone Cyst be?

UBC (unicameral bone cysts) and ABC (aneurysmal bone cysts) can often be mistaken for one another, as they affect the same age group, appear in similar locations, and are both fluid-filled. This can make diagnosis a challenge. One way to distinguish the two is observing that ABCs tends to be more off-center, aggressive, and show more bone branching. Still, if a UCB has been subject to trauma, it can look much like an ABC, and in these instances, a biopsy might be required. The presence of a cement-like substance indicates a UBC, whereas bluish areas of fibrous cartilage are typical of ABC.

Additionally, diseases like Telangiectatic Osteosarcoma can display features similar to ABC on radiographic images. Therefore, a biopsy is critical to exclude osteosarcoma, which is identified by unusual cells, cell division, and irregular bone matrix. Molecular biology techniques can be used to identify gene rearrangement, a hallmark of primary ABC.

Other conditions with a similar appearance include:

• Giant Cell Tumor
• Eosinophilic granuloma
• Osteoblastoma
• Malignant tumors
• Nonossifying fibroma and solid ABC

There are also other types of cystic bone lesions, such as intraosseous ganglion cysts and epidermoid cysts. Intraosseous ganglion cysts, which usually occur at the ends of long bones (like the lower leg bone, knee, and shoulder), look like an outlined, single or multiple chambered bone defect with a thin rim of hardened tissue on X-rays. These are usually treated by removal and scraping of the affected area. Epidermoid cysts, on the other hand, are filled with a protein material and are lined with a particular type of skin-like cell.

What to expect with Bone Cyst

UBC, or unicameral bone cyst, starts growing in the part of the bone most distant from the center, called the metaphysis. It usually grows away from the growth plate or the part of the bone that allows growth in children. Interestingly, these cysts can disappear on their own, which is why we don’t see them often in adults.

Active cysts are close to the growth plate, while inactive cysts have grown further away or are no longer in contact with the growth plate. Certain areas are high-risk because a cyst there can seriously impact the patient’s quality of life. For example, a cyst in the neck of the thigh bone (femur), if it breaks, may lead to tissue death and a difference in leg length.

Cysts that do not cause symptoms and have a low risk of causing fractures are usually left as they are. Many children experience pain because the cysts can cause fractures. Differences in growth may also occur because of the pressure the cyst exerts on the growth plates, complications of treating the cyst near the growth plate, or a fracture through a cyst in the neck of the femur. These can all cause differences in limb length and deformities. Therefore, choosing the right treatment method is essential in managing these cysts.

Deep cleaning (curettage) and bone grafting, which replace damaged or lost bone with healthy bone, plus internal fixation, where metal hardware stabilizes the bone, are generally reserved for larger, pain-causing cysts in high-risk areas, such as the femur. Other cysts are treated with injections performed through the skin. Factors that may indicate a worse outcome after percutaneous treatment include large size, multiple locations, active lesions, and age younger than 10.

As for ABC (aneurysmal bone cyst), there is about a 10 to 20% chance that it can come back after being removed via deep cleaning (curettage). Factors known to increase the risk of recurrence include being younger than 15 years, having the cyst located centrally in the bone, and incomplete removal of the cystic cavity. Complete surgical removal is usually reserved for bones that the body can spare, like the clavicle (collarbone) and fibula (smaller bone in the lower leg), where removing part of the bone doesn’t cause significant long-term problems.

Possible Complications When Diagnosed with Bone Cyst

Certain complications can occur along with these lesions:

• Pathological fracture: This is the main complication of these lesions. Signs of a possible fracture include pain, earlier age of onset, presence of the lesion in the upper arm, changes in the size of a cavity over time, closeness to the growth plate, numerous partitions, and early recurrence.
• Pain
• Rare transformation into a malignant tumor: This is uncommon, but there have been reports of cancers appearing years after treatment.
• Arthritis
• Pressure symptoms
• Growth disorders: In children, there might be inconsistencies in limb length or deviations in axis if the cysts breach the growth plate or involve the end of a long bone.
• Recurrence: No treatment can guarantee a complete cure except for total surgical removal. The recurrence rate after treatment is 10 to 30% for UBC, which is why minimally invasive treatment is the initial option.

A more aggressive surgical approach is generally recommended for lesions in the neck of the femur due to the serious complications that can occur if these fracture, such as loss of blood supply to the bone, residual deformity, etc. There are general risks involved in managing these cysts: unpredicted leftover structural deformities and functional disabilities may end up occurring, as well as potential harm to important nearby structures like blood vessels.

Preventing Bone Cyst

It’s crucial to inform both patients and their main caregivers about the nature of their tumor. This is especially necessary if the patient is a child. Everyone involved needs to understand the course of the tumor, what changes to anticipate in the patient’s appearance or abilities, potential complications, and chances of the tumor returning after treatment. Patients and their families should also receive guidance about different treatment options as well as the potential risks and benefits of these treatments. Providing proper information, counselling, and family support plays an essential part in delivering appropriate healthcare for these patients.

In about 75% of patients, a complication known as a ‘pathological fracture’ is present when they first discover they have the condition known as Unicameral Bone Cyst (UBC). A pathological fracture is when a bone breaks due to a disease, rather than an injury. In children, UBC is often the reason for the pathological fracture.

For diagnosing and managing this, doctors often use something called “Mirels criteria”. This assessment helps predict how likely it is that a long bone – like those in arms or legs – will fracture due to a lytic or sclerotic lesion (damaged or hardened areas often associated with cancer). The Mirels criteria consider four factors: the area affected, the specific location on the bone, the state of the bone matrix (interior structure of the bone), and whether the patient is experiencing pain. Each of the factors has a scoring system that sums up to a maximum of 12 points, with a minimum of 4 points. A score above 9 suggests a high fracture risk, and doctors may choose to undertake preventive action to support and strengthen the bone.

Such information helps the doctors plan the best healthcare strategy for each patient with a bone cyst, taking into account their individual risk of fracture.