Bone cysts are benign, fluid-filled cavities that develop inside bones. They are not cancerous, but they can weaken the surrounding bone enough to cause pain, swelling, or fractures. Most bone cysts occur in children and teenagers, particularly during growth spurts, and the two main types behave quite differently from each other.
The Two Main Types
The vast majority of bone cysts fall into one of two categories: simple (unicameral) bone cysts and aneurysmal bone cysts. Understanding which type you’re dealing with matters because their causes, behavior, and treatment outlook are distinct.
Simple bone cysts are solitary cavities filled with clear, yellowish fluid. They most commonly form in the upper arm bone (proximal humerus) and upper thigh bone (proximal femur) in children. In adults, they tend to appear in the pelvis or heel bone. These cysts are considered reactive lesions rather than true tumors, meaning they develop in response to a disruption in normal bone physiology rather than from abnormal cell growth. They are most active during childhood growth spurts and often heal on their own once a person reaches skeletal maturity.
Aneurysmal bone cysts are rarer and more aggressive. Instead of clear fluid, they’re filled with blood and divided into multiple chambers by internal walls called septations. They most commonly develop near the ends of long bones, particularly around the knee (distal femur, proximal tibia), the upper arm, and the spine. Unlike simple cysts, aneurysmal bone cysts can grow rapidly, sometimes fast enough that their expansion on imaging raises initial concern about a malignant tumor. About 75 to 90% of cases occur in people under 20, with an average age of around 17.
What Causes Bone Cysts
Simple bone cysts appear to form when veins inside the spongy interior of a bone become blocked. This venous obstruction leads to a buildup of pressure and fluid, which gradually dissolves the surrounding bone tissue and thins the outer shell (cortex). The cyst wall itself has no epithelial lining; it’s made up of fibrous connective tissue rather than a true membrane. This mechanism explains why decompression, simply creating an outlet for the trapped fluid, is one of the treatment strategies.
Aneurysmal bone cysts have a genetic component. Roughly 70% of primary cases involve chromosomal rearrangements that create abnormal fusions with a gene called USP6. These genetic changes drive the cyst’s locally aggressive behavior and help distinguish true aneurysmal bone cysts from cyst-like changes that can appear alongside other bone tumors. Because they involve actual genetic alterations in cells, aneurysmal bone cysts are classified as indeterminate tumors rather than simple reactive lesions.
Symptoms and How They’re Discovered
Many bone cysts, especially simple ones in flat bones like the pelvis, cause no symptoms at all and are found incidentally when imaging is done for another reason. When symptoms do appear, the most common is mild to moderate pain in the affected area.
For simple bone cysts, the first sign is often a fracture. About two-thirds of simple bone cysts are diagnosed only after the weakened bone breaks from a relatively minor injury, like a fall during sports. Aneurysmal bone cysts are more likely to cause noticeable pain and swelling before a fracture occurs, though pathological fractures still happen in about 8% of cases (rising to 21% when the spine is involved). Aneurysmal cysts in the spine can also press on nearby nerves, causing radiating pain or neurological symptoms like numbness or weakness.
A visible lump or deformity is possible with aneurysmal bone cysts that have grown large enough to expand the bone’s outer surface.
How Bone Cysts Are Diagnosed
X-rays are typically the first step. Simple bone cysts appear as well-defined, oval areas of thinned bone, usually centered in the shaft near the growth plate. One classic finding on X-ray is the “fallen fragment sign,” where a piece of broken bone sinks to the bottom of the fluid-filled cavity, confirming the cyst is hollow.
MRI provides more detail and is particularly useful for aneurysmal bone cysts. On MRI, aneurysmal cysts show characteristic fluid-fluid levels inside their multiple chambers, where blood components have separated like oil and water. An intact rim of low-intensity signal surrounding the entire lesion, along with internal septations, allows a specific diagnosis in many cases. This imaging pattern helps distinguish aneurysmal bone cysts from malignant tumors, though biopsy is sometimes still needed to confirm.
Treatment Options
Treatment depends on the type of cyst, its size, its location, and whether it’s causing symptoms or fracture risk.
Observation and Activity Changes
Small, asymptomatic simple bone cysts can often be monitored with periodic X-rays. Since these cysts tend to heal spontaneously once a child finishes growing, watchful waiting is a reasonable approach when fracture risk is low. Limiting high-impact sports and activities during this period helps reduce the chance of a break through the weakened bone. The trade-off is that consolidation through observation alone takes longer than active treatment.
Injections
Steroid injections into the cyst cavity are a long-established, minimally invasive option for simple bone cysts. The procedure involves draining the cyst fluid and injecting a steroid to promote healing. In a randomized controlled trial, steroid injection proved more effective than bone marrow injection, though the overall healing rate was about 42%. Nearly half of patients needed a second injection, and 44% of those who had a repeat injection ultimately still failed to heal. Because of these numbers, injections are often tried first as a less invasive step, with surgery held in reserve.
Surgery
Open curettage, where the cyst lining is scraped out and the cavity is packed with bone graft material, remains the most reliable treatment. It has a higher healing rate than injections: studies show a failure rate of about 23% for curettage with bone grafting, compared to roughly 61% for conservative (non-surgical) management. Another surgical approach involves inserting a cannulated screw or pin into the cyst to provide continuous decompression, addressing the underlying fluid pressure that caused the cyst in the first place.
For aneurysmal bone cysts, surgery is more commonly needed because of their aggressive growth pattern. Recurrence rates after surgical treatment range from 10 to 59%, with higher recurrence in children younger than 10. An initial surgery achieves an overall success rate of about 82%, but some patients require additional procedures if the cyst returns.
Risks for Growing Children
The biggest concern with bone cysts in children is damage to the growth plate, the layer of cartilage near the ends of bones where lengthening occurs. Cysts that sit near or cross the growth plate can disrupt normal bone development, even after the cyst itself has healed. In children younger than 12 to 14, this can lead to limb length differences and angular deformities that worsen over time.
Aneurysmal bone cysts in contact with the growth plate also have higher recurrence rates, compounding the risk. One documented case involved a child whose aneurysmal bone cyst crossed the growth plate of the thigh bone, eventually resulting in a 5-centimeter leg length difference and a 21-degree angular deformity by age 14, requiring complex reconstruction. These cases are uncommon but illustrate why cysts near the growth plate in young children are monitored closely and treated more proactively.
Long-Term Outlook
The prognosis for bone cysts is generally favorable. Simple bone cysts in children often resolve completely once skeletal growth is finished, though treatment may be needed in the meantime to prevent fractures or speed healing. Even when treatment is required, most patients recover full function of the affected limb.
Aneurysmal bone cysts require more vigilance. Their recurrence rates are meaningful, particularly in younger children, and follow-up imaging over several years is standard practice after treatment. Still, these are benign lesions. With appropriate management, the large majority of patients do well, though some need more than one round of treatment to achieve a lasting result.