Bone Cyst of the Ankle

What is it?

Bone is not solid throughout. Beneath a dense outer shell lies a lattice of fine struts with marrow between them, and this internal architecture varies from place to place according to the forces each region carries. A cyst is a cavity within this structure containing fluid rather than bone.

Several distinct things are described as bone cysts in the ankle and foot, and they differ considerably in what they mean.

Subchondral cysts are the most common in adults. They form in the bone immediately beneath a joint surface where cartilage has worn away. Two mechanisms are described: joint fluid being forced under pressure through the damaged surface into the bone beneath, and small areas of bone dying under excessive load and being replaced by fluid-filled cavities. Either way, they are a consequence of joint degeneration rather than a separate condition. In the ankle they occur in the lower shin bone and the talus, and their presence on a report generally indicates that the joint above them has suffered damage—which is the finding of significance rather than the cyst itself.

Cysts associated with osteochondral lesions of the talus are a specific version of this. Where an area of cartilage and underlying bone on the dome of the talus has been damaged, usually by injury, a cyst commonly forms beneath it. These are relevant to treatment decisions because their size and depth influence which procedures are appropriate.

Intraosseous ganglia are benign cavities containing thick gelatinous fluid, occurring in adults close to a joint, most often in the lower shin bone or the talus. They resemble the ganglion cysts familiar on the outside of the wrist, and are thought to arise similarly. Unlike subchondral cysts they occur in joints that are otherwise normal. They are typically well-defined with a thin rim of denser bone around them, and most cause no symptoms.

Simple bone cysts, also called unicameral bone cysts, are fluid-filled cavities occurring mainly in children and adolescents. In the foot, the heel bone is a characteristic site, where they appear in a particular location in the front part of the bone. They are benign, frequently cause no symptoms, and often stop growing and fill in as the skeleton matures. Their significance is structural: a large cyst thins the surrounding bone and can lead to fracture through the weakened area, which is sometimes how they are discovered.

Aneurysmal bone cysts are different in character. They are blood-filled cavities divided into compartments, they expand the bone, and they can grow rapidly. They occur predominantly in people under 20 and, while benign in the sense of not spreading, they are locally destructive, tend to recur after treatment, and require active management. They may arise on their own or alongside another bone lesion.

One further entity deserves specific mention because it causes more unnecessary concern than any genuine cyst. The heel bone contains a region in its front-central portion where the internal struts are naturally sparse, because forces pass around rather than through it. On X-ray this appears as a well-defined lucent area that looks exactly like a cyst, and it is present in essentially everyone. It is known as a calcaneal pseudocyst, and it is entirely normal anatomy. A report describing a lucency in this location in a person with no symptoms is usually describing this.

Other lesions can appear cystic and must be considered.

Intraosseous lipoma, a benign collection of fat within bone, occurs characteristically in the same part of the heel bone and has a recognisable appearance, often with a small central calcification.

Infection within bone can produce a well-defined cavity, known as a Brodie abscess, typically in children and young adults, which may closely resemble a benign cyst and usually causes persistent pain including at night.

Benign tumours including giant cell tumour and chondroblastoma occur near joints and can appear lucent, the latter characteristically in the growing end of a bone in adolescents.

Malignant lesions in this region are rare but are the reason that lesions with aggressive features are investigated rather than assumed benign.

The features that distinguish a benign lesion from a concerning one are well established and are what a radiologist assesses.

Reassuring features include a sharply defined border, a thin rim of denser bone around the lesion indicating the bone has had time to wall it off, an intact outer cortex, no reaction of the bone surface, and no associated mass in the surrounding soft tissue. A lesion with these features, particularly in a characteristic location, can frequently be diagnosed confidently without any further investigation.

Concerning features include an ill-defined or indistinct border, destruction of the outer cortex, a layered or spiculated reaction of the bone surface, extension into the surrounding soft tissues, and rapid change on sequential imaging.

This distinction is the single most useful thing to understand about any bone lesion report, because it explains why one person is told their finding needs nothing and another is referred for biopsy.

Symptoms are absent in the majority.

Where they occur, a deep ache in the ankle or heel, worse on weight bearing and after activity, is typical. Localised tenderness may be present.

Swelling is uncommon except with expansile lesions.

Fracture through a weakened area produces sudden pain after minor or no injury, and in children this is a common first presentation of a simple bone cyst.

Certain symptom patterns raise concern and warrant attention: pain that is constant rather than related to activity, pain that wakes a person at night, and pain that is progressively worsening over weeks. Night pain in particular is a feature associated with infection and with tumours, and is worth reporting specifically.

Assessment is usually straightforward.

X-ray is the primary investigation and frequently sufficient. It demonstrates the location, size, margin, and the state of the surrounding bone, and in characteristic lesions in characteristic places allows a confident diagnosis.

MRI characterises the contents, confirming fluid and distinguishing it from solid tissue, assesses the surrounding marrow for oedema, evaluates the adjacent joint and cartilage, and examines the soft tissues. A particular finding—fluid-fluid levels, where the contents separate into layers within compartments—suggests an aneurysmal bone cyst.

CT demonstrates the cortical shell and any fracture better than other methods and is useful before surgery.

Comparison with any previous imaging is valuable. A lesion unchanged over years is benign almost by definition, and many cysts found incidentally are visible in retrospect on earlier films.

Biopsy is performed where the appearance is not confidently benign, where there are aggressive features, or where a person has a known cancer and a bone lesion could represent spread.