What are the other Names for this Condition? (Also known as/Synonyms)

• Simple Benign Bone Cyst
• Solitary Bone Cyst
• Unicameral Bone Cyst (UBC)

What is Simple Bone Cyst? (Definitions/Background Information)

• Simple Bone Cysts are common benign tumors that form fluid-filled cysts on bones. These tumors are also known as Unicameral Bone Cysts, and are typically observed in children and teenagers
• Normally, only a single bone tumor is observed that may be located near the growth plate. Simple Bone Cysts may cause mild pain in the affected bone. Tumors that are large may cause the bones to weaken, making it vulnerable for easy fractures
• Complications may include deformation of the affected limb, if the tumor affects the epiphyseal plate (bone growth area), and recurrence of tumor following treatment. Nevertheless, a Simple Bone Cyst is not considered a malignant or premalignant lesion
• Symptomatic tumors or tumors that affect the bone strength may be treated through surgical measures. Small-sized tumors that do not present any significant symptoms may be observed, by the healthcare provider. In some cases, Simple Bone Cysts may resolve as the child grows and develops
• The prognosis of Simple Bone Cysts is typically excellent in a majority of cases. Following treatment, in many cases, the condition heals within a few months

There are 2 types of Simple Bone Cyst, depending on whether the cyst involves the growth plate, or is present away from the growth plate. The 2 types include:

• Active Simple Bone Cyst: Here, the cyst is present within the growth plate of the bone. As the bone grows in the child, it can weaken and deform the bone
• Latent Simple Bone Cyst: Here, the cyst is away from the growth plate. In such cases, it does not cause any deformity of the bone. However, large-sized cysts can weaken bones, resulting in easy fractures. These cysts are also called Inactive Simple Bone Cysts

Who gets Simple Bone Cyst? (Age and Sex Distribution)

• Simple Bone Cyst is usually diagnosed in children and adolescents, between ages 5-15 years. Approximately, 80% of the cases are detected in individuals below the age of 20 years
• In some cases, the tumor may form in adults too
• Both males and females are affected. Boys are affected more than girls in a 2:1 or 3:1 ratio
• There is no known racial or ethnic preference
• Studies indicate that Simple Bone Cysts constitute 3% of all bone tumors. The incidence may be higher, since many tumors are known to be undiagnosed or underdiagnosed

What are the Risk Factors for Simple Bone Cyst? (Predisposing Factors)

• No clear risk factors have been established for the development of a Simple Bone Cyst

It is important to note that having a risk factor does not mean that one will get the condition. A risk factor increases one’s chances of getting a condition compared to an individual without the risk factors. Some risk factors are more important than others.

Also, not having a risk factor does not mean that an individual will not get the condition. It is always important to discuss the effect of risk factors with your healthcare provider.

What are the Causes of Simple Bone Cyst? (Etiology)

The exact cause and mechanism of formation of Simple Bone Cyst may be due to genetic abnormalities.

• Abnormalities in chromosomes 4, 6, 8, 12, 16, and 21 have been detected in the tumor. These genetic abnormalities are still being researched
• An isolated case involving chromosomal translocation t(16;20)(p11.2;q13) has been reported. Also, one case of recurrent tumor exhibited mutation in the TP53 gene

There are numerous theories proposed for formation of the tumor that include:

• Abnormality in the development of the growth plate
• Abnormal pockets of synovium from the joint cavity in the vicinity of the cyst
• Abnormalities in blood circulation resulting from venous obstruction leading to tumor formation

Trauma, as a cause of tumor development, has not been definitely proven.

What are the Signs and Symptoms of Simple Bone Cyst?

The signs and symptoms of Simple Bone Cyst include:

• These fluid-filled cystic tumors are usually solitary in nature. Very infrequently, in adults, solidified bone cyst tumors have been reported
• Some tumors are small, while others are large; small-sized cysts usually are asymptomatic
• Large-sized tumors can also be without any significant signs and symptoms, but may occasionally result in bone pain and pathologic fractures
• Pathologic fractures occur due to weakening of the bone from minor trauma. The fracture may occur in the area of the tumor
• In some individuals, there may be swelling of the affected region or restricted joint movement, if the tumor is present near a joint
• Commonly, tumors are observed near the zone of bone growth, near the growth plate (known as the metaphyseal region)
• If tumors form in the epiphyseal plate, or in the metaphyseal region near the epiphyseal plate, in a growing child, it can deform the bone. The epiphyseal plate is a part of the bone which is responsible for bone growth
• The size and shape of the bone cyst can change over time, if tumors involve the epiphyseal plate in a growing child
• The most prominent locations of Simple Bone Cyst include:
  • Upper arm bone (humerus) and thigh bone (femur): Proximal humerus and femur constitute 75-90% of all cases in the body
  • In older adults, involvement of the hip bone (ilium) and heel bone (calcaneus), are occasionally noted

How is Simple Bone Cyst Diagnosed?

The diagnostic tests for Simple Bone Cyst may involve:

• Complete physical examination, and evaluation of the individual’s medical history
• X-ray, CT or MRI scan of the affected region, to aid in obtaining a clear image of the tumor
• Bone scans, to rule out the tumor in other parts of the body
• Fine needle aspiration (FNA) of the cyst, for analysis of fluid within the cyst
• Tissue biopsy: A core biopsy is performed and sent to a laboratory for a pathological examination. A pathologist examines the biopsy under a microscope. After putting together clinical findings, special studies on tissues (if needed) and with microscope findings, the pathologist arrives at a definitive diagnosis. Examination of the biopsy under a microscope by a pathologist is considered to be gold standard in arriving at a conclusive diagnosis

A differential diagnosis may be undertaken to rule-out the following tumors that may present similar features:

• Giant cell tumor of bone
• Traumatic bone cyst
• Aneurysmal bone cyst

Note: Majority of the tumors are discovered incidentally during radiological studies undertaken for some other medical conditions (such as a bone fracture). 

Many clinical conditions may have similar signs and symptoms. Your healthcare provider may perform additional tests to rule out other clinical conditions to arrive at a definitive diagnosis.

What are the possible Complications of Simple Bone Cyst?

Complications due to Simple Bone Cyst could include: 

• Pathological fracture of a bone, if the tumor is large
• Deformed limb; uneven length of the limb, if the growth plate is affected in children
• Tumors are known to recur following treatment: Some studies report a 10-20% recurrence rate, while others report up to 50% recurrence

How is Simple Bone Cyst Treated?

The treatment of Simple Bone Cyst depends upon a variety of factors such as:

• Age of the individual; whether the patient is a child or an adult
• Size of the tumor
• Whether the condition is symptomatic or asymptomatic
• Location of the tumor in the affected bone; the bone location
• Strength of the affected bone
• Whether the tumor is in the epiphyseal plate (growing part of the bone), or is located away from the epiphyseal plate

Small tumors presenting no symptoms may be observed.

• In such cases, a ‘wait and watch’ approach may be adopted by the healthcare provider
• Follow-ups are performed through periodic X-rays of the affected bone
• It may result in the cyst disappearing as the individual becomes older

In individuals where there is significant signs and symptoms, a presence of pathologic fractures, when individuals are at an increased risk for fractures, or large tumors causing bone stability issues, surgical measures may be adopted.

The surgical treatment measures for Simple Bone Cyst may include the following:

• Aspiration of the fluid
• Injection of steroids into the cavity: Steroids help in prevention of re-accumulation of the cyst fluid; it also assists in filling-up of the cyst with normal surrounding bone (solidified T). Multiple steroid injections, over a period of several months to years, may be necessary
• Curettage of the cyst with bone grafts:
  • In this method, the lining of the cyst is scraped to prevent the re-accumulation of the fluid
  • After curettage, the cavity of the cyst is filled with bone grafts
  • The bone grafts can either be an allograft (bone from a donor), or an autograft (pieces of bone from different body parts, from the same individual)
  • Occasionally, the cyst cavity can be filled with a cement-like foreign material
• In some rare cases, the bone needs to be stabilized further, to increase the bone strength. In such cases, bone stabilization procedures, such as internal fixation and intramedullary rodding, may be adopted
• Occasionally, some Simple Bone Cyst tumors are described as resolving spontaneously, following a fracture (i.e., after the fracture heals)
• Follow-up care with regular screening may be recommended by the healthcare provider, since tumor recurrence is not uncommon

How can Simple Bone Cyst be Prevented?

• Current medical research has not established a method of preventing the formation of a Simple Bone Cyst
• Regular medical screening at periodic intervals with blood tests, scans, and physical examinations, are recommended, due to the possibility of recurrence. Often several years of active vigilance is necessary

What is the Prognosis of Simple Bone Cyst? (Outcomes/Resolutions)

• The prognosis of a Simple Bone Cyst is usually excellent with accurate diagnosis and appropriate treatment. The tumors are benign and no metastasis, or spread of the tumor to other body locations, is observed
• In many cases, in children, small cysts get filled-in with normal bone, as the child becomes an adult, resulting in the disappearance of the cyst
• Following treatment, individuals may take several months (between 3-6 months) to fully recover the bone strength
• However, as many as 1 in 2-4 individuals may experience tumor recurrence despite surgery/treatment. The recurrence rate is higher in a child, when compared to an adult

Additional and Relevant Useful Information for Simple Bone Cyst:

• Aneurysmal bone cyst (ABC) is a tumor-like lesion of the bone, primarily found in children and adolescents. This benign tumor grows rapidly and destroys any surrounding bone tissue

Please visit the following DoveMed website link for additional information:

http://www.dovemed.com/diseases-conditions/aneurysmal-bone-cyst-abc/