Bone Cyst of the Bony Skeleton
A bone cyst is a fluid-filled cavity within bone, and cystic or hollow-looking areas are among the most common incidental findings on skeletal imaging. The great majority are benign and need nothing at all—so much so that a recognised group of them is informally described as don’t-touch lesions, because intervening causes more harm than leaving them alone. What determines whether a finding is reassuring is not its size but its borders: a lesion with a sharp outline and a thin rim of denser bone around it is behaving in a way that destructive processes do not. Age and location narrow the possibilities further, often enough to reach a confident diagnosis without biopsy.
What is it?
Bone consists of a dense outer shell surrounding an internal lattice of fine struts with marrow between them. A cyst is a cavity within this structure containing fluid, and a range of other processes can produce a similar hollow or lucent appearance on imaging without being true cysts.
Rather than listing every possibility, it is more useful to understand how these lesions are assessed, because the same framework applies throughout the skeleton and explains why most require nothing.
The first consideration is the margin, and this carries more weight than any other feature.
A lesion with a sharp, clearly drawn border, particularly one surrounded by a thin rim of denser bone, has grown slowly enough for the surrounding bone to wall it off. Bone reacts to a slow process by building a barrier around it. This appearance is characteristic of benign lesions and is strongly reassuring.
A lesion with an indistinct border that fades into surrounding bone, or one producing a moth-eaten or patchy pattern of destruction, has grown faster than the bone could respond. This appearance indicates an aggressive process, which may be a tumour or infection.
The second consideration is the cortex. An intact outer shell, even if thinned or expanded, indicates a contained process. Destruction of the cortex, particularly with extension into surrounding soft tissue, does not.
The third is the reaction of the bone surface. Slow processes produce a smooth, solid layer of new bone. Aggressive processes produce layered, interrupted, or spiculated patterns.
The fourth, and often the most powerful, is age. The likely diagnoses differ so markedly between a child, a young adult, and someone over 40 that age alone narrows the possibilities more than any other single piece of information. Most benign cystic lesions occur in the first two decades. A lucent bone lesion in someone over 40 raises different considerations, principally secondary deposits from a cancer elsewhere and myeloma.
Taken together these features frequently allow a confident diagnosis without biopsy, which is why many bone lesions are reported as characteristically benign and discharged.
The lesions commonly encountered fall into recognisable groups.
Simple bone cysts, also called unicameral bone cysts, are fluid-filled cavities occurring mainly in children and adolescents, most often in the upper arm bone and the upper thigh bone, and in the foot in the heel bone. They are benign, frequently silent, and often fill in as the skeleton matures. Their main consequence is fracture through the thinned bone.
Aneurysmal bone cysts are blood-filled, divided into compartments, and expand the bone. They occur predominantly under the age of 20, grow more rapidly than simple cysts, and although benign in the sense of not spreading, they are locally destructive and tend to recur, so they require active treatment.
Intraosseous ganglia are benign cavities containing thick fluid, occurring in adults close to a joint, typically well-defined with a sclerotic rim.
Subchondral cysts form in bone beneath a damaged joint surface and are a consequence of joint degeneration rather than a separate condition. They are extremely common with increasing age.
A group of lesions is informally described as don’t-touch lesions, meaning their appearance is so characteristic that biopsy or surgery is unnecessary and potentially harmful. These include fibrous cortical defects and non-ossifying fibromas, which are extremely common in children, appear as well-defined lucencies at the edge of long bones, and resolve spontaneously; bone islands, which are small areas of dense bone; fibrous dysplasia, in which normal bone is replaced by fibrous tissue producing a characteristic hazy appearance; intraosseous lipomas; and healing or healed areas of previous injury. Recognising these prevents unnecessary investigation, and the term exists precisely because they were historically biopsied needlessly.
Enchondromas deserve particular mention because they are common and frequently cause concern. These are benign collections of cartilage within bone, occurring most often in the small bones of the hands and in the long bones of the limbs. In the hands they are the commonest bone tumour and are usually harmless, though they can weaken the bone enough to fracture. In long bones they are frequently found incidentally and have a characteristic appearance with small ring-like or arc-shaped calcifications. The reason they attract attention is that distinguishing a benign enchondroma from a low-grade cartilage cancer can be genuinely difficult, since the two overlap in appearance. Features favouring the latter include pain arising from the lesion itself, growth over time, erosion of the inner surface of the cortex, and larger size. Many are monitored with interval imaging rather than biopsied, since stability is itself diagnostic.
Infection within bone produces a well-defined cavity known as a Brodie abscess, typically in children and young adults, which can closely resemble a benign cyst but usually causes persistent pain including at night.
Giant cell tumours occur in young adults at the ends of long bones, typically around the knee, extending right up to the joint surface. They are locally aggressive and require treatment.
Chondroblastomas occur in the growing ends of bones in adolescents.
Malignant lesions are the reason aggressive features are taken seriously. Primary bone cancers are rare and occur predominantly in adolescents and young adults, with a second smaller peak in later life. Secondary deposits from cancers elsewhere—most commonly breast, prostate, lung, kidney, and thyroid—are considerably more frequent in adults over 40 and are the leading cause of a destructive bone lesion in that age group. Myeloma produces multiple well-defined lucent lesions and is an important consideration in older adults.
Symptoms are absent in most benign lesions.
Where present, a deep ache localised to the bone, frequently activity-related, is typical. Localised tenderness may be found.
A palpable lump occurs where the bone surface is expanded.
Fracture through weakened bone causes sudden pain after minor or no injury.
Certain patterns warrant attention rather than reassurance: pain that is constant rather than activity-related, pain that wakes a person from sleep, pain that is progressively worsening over weeks, and any associated swelling, fever, or unexplained weight loss. Night pain in particular is associated with infection and with tumours and should be reported specifically.
Assessment follows a predictable sequence.
X-ray is the primary investigation and, unusually in modern imaging, remains the most informative for characterising a bone lesion. The pattern of destruction and the bone’s response are better demonstrated on plain films than on cross-sectional imaging, and a radiologist experienced in bone lesions can frequently reach a confident diagnosis from X-ray alone.
MRI characterises the contents, distinguishes fluid from solid tissue, assesses the extent within the marrow, and demonstrates any soft tissue component. It is more sensitive than X-ray for detecting lesions but less specific for characterising them, which is why both are frequently used together.
CT demonstrates mineralisation within a lesion, cortical integrity, and fine bone detail, and is particularly useful for cartilage lesions and for surgical planning.
Comparison with previous imaging is frequently decisive. A lesion unchanged over several years is benign almost by definition, and many incidental findings are visible in retrospect on earlier studies, which immediately resolves the question.
Where a person has a known cancer, or where features are not confidently benign, further assessment follows—which may include bone scan or PET-CT to assess for other lesions, blood tests, and ultimately biopsy.
One procedural point is important. Where a bone lesion may be malignant, referral to a specialist bone tumour service before biopsy is recommended in national guidance in many countries. The reason is that an unplanned biopsy performed through the wrong approach can contaminate tissue planes, and if the lesion proves to be a primary bone cancer, this can turn a limb-preserving operation into a more extensive one. The sequence—specialist referral first, biopsy planned by the team who would operate—materially affects outcomes.
Important to Know
Most bone lesions found incidentally are benign and require nothing further. Care involves orthopaedic services, with specialist bone tumour centres for lesions that are not confidently benign.
The most useful thing to understand about a report describing a bone lesion is that the descriptive terms carry specific meaning. A lesion described as well-defined with a sclerotic margin, with intact overlying cortex and no periosteal reaction or soft tissue component, is being described as benign in all the ways that matter. Terms such as ill-defined, permeative, moth-eaten, cortical destruction, aggressive periosteal reaction, and soft tissue mass carry the opposite implication.
The word lesion itself causes considerable unnecessary alarm. In radiological usage it simply means an abnormality of any kind, without any implication about cause. A benign cyst, a healed fracture, and a cancer are all lesions.
For confidently benign findings, no treatment, no biopsy, and no follow-up imaging are needed. Being discharged after a single X-ray is an appropriate outcome rather than a dismissal, and the don’t-touch category exists specifically because intervening on these lesions historically caused harm without benefit.
For lesions that are probably benign but not certainly so, interval imaging is a reasonable approach. Stability over six to twelve months provides strong reassurance, and this strategy avoids biopsy in most cases.
Treatment is considered where a lesion causes pain, has caused a fracture, or has thinned the bone enough to risk one. Curettage with filling using bone graft, bone substitute, or cement is the usual approach, with recurrence rates varying by lesion type.
Simple bone cysts in children are frequently observed, particularly where small and asymptomatic, since many resolve with skeletal maturity. Larger or symptomatic ones may be injected with steroid or bone marrow, decompressed, or curetted and grafted. A fracture through one is usually allowed to heal first, and the cyst sometimes heals with it.
Aneurysmal bone cysts require active treatment given their tendency to enlarge and recur, with curettage and grafting standard, supplemented in some centres by embolisation, injection techniques, or targeted medication.
Enchondromas in long bones are frequently monitored rather than treated, with interval imaging to confirm stability. Those causing pain, growing, or showing features suggesting a low-grade cartilage tumour are referred for specialist assessment. In the hands they are treated where they cause symptoms or fracture.
Giant cell tumours require treatment given their local aggressiveness, usually curettage with adjuvant measures and sometimes targeted medication.
Where a lesion proves to be a secondary deposit from a cancer elsewhere, management is directed by the oncology team, with local treatment considered where a bone is at risk of fracture.
Several situations warrant a lower threshold for investigation.
A new bone lesion in anyone with a known cancer is assessed carefully, since secondary deposits are far more common than coincidental benign lesions in this group.
Multiple lucent lesions in an older adult raise the possibility of myeloma and warrant blood tests.
A lesion with any aggressive feature is referred regardless of how well the person feels.
Night pain, progressive pain, and pain unrelated to activity warrant assessment even where imaging appears reassuring.
A word about incidental findings is appropriate given how often these arise. Imaging performed for one reason frequently identifies unrelated abnormalities, and bone lesions are among the most common. The majority are longstanding, benign, and of no consequence. The appropriate response is characterisation rather than either dismissal or alarm, and in most cases this is achieved quickly with an X-ray and, where available, comparison with previous imaging.
Care is managed by orthopaedics, with specialist bone tumour services where required. Imaging features—margin, rim, cortical integrity, periosteal reaction, and soft tissue involvement—together with age and location, carry more weight than size alone.
Patient education plays an important role. Understanding that most bone lesions are benign and incidental, that a well-defined lesion with a sclerotic rim is reassuring, that some lesions are deliberately left alone because intervention causes harm, that stability over time establishes benignity, and that night pain or progressive pain warrants attention regardless of imaging all contribute to appropriate care.
Red flag symptoms include pain that is constant rather than related to activity; pain that wakes you at night; pain that is progressively worsening over weeks; sudden pain or inability to use a limb after minor or no injury, suggesting fracture through weakened bone; a visible or palpable lump over a bone, particularly if enlarging; swelling, redness, warmth, or fever, suggesting infection; unexplained weight loss, night sweats, or fatigue; a new bone lesion in anyone with a history of cancer; and any lesion reported as having ill-defined margins, cortical destruction, aggressive periosteal reaction, or an associated soft tissue mass. These warrant prompt medical evaluation.