Bone Cyst of the Shoulder

What is it?

The shoulder comprises the head of the upper arm bone sitting against a shallow socket on the shoulder blade, with the collarbone completing the arrangement at the top. The upper end of the arm bone contains a growth plate that is responsible for the majority of that bone’s length, and it is among the most actively growing regions of the skeleton during childhood.

That activity explains the region’s most characteristic finding. Simple bone cysts—fluid-filled cavities, also called unicameral bone cysts—occur predominantly in actively growing bone adjacent to a growth plate, and the upper arm bone is their commonest site in the body, accounting for around half of all cases. They occur mainly between the ages of five and fifteen.

These cysts are benign and are not tumours. They form a single cavity containing clear or yellowish fluid, lined by a thin membrane, and they expand slowly within the bone, thinning the surrounding cortex without destroying it. As the skeleton matures and growth ceases, many fill in with bone spontaneously.

Their clinical significance is almost entirely structural. A cyst occupying a substantial portion of the bone’s width leaves a thin shell that can fracture under relatively minor force, and this is the most common way they are discovered—a child falls, breaks the upper arm, and the cyst is seen on the X-ray taken for the fracture. A particular finding sometimes appears after such a fracture: a small fragment of the cortex falls into the fluid-filled cavity and settles at the bottom, visible on imaging and considered characteristic of a simple bone cyst.

Several other processes produce cystic or lucent appearances in this region.

Aneurysmal bone cysts occur in the upper arm bone in people under twenty. They are blood-filled, divided into compartments, and expand the bone more markedly than simple cysts. They grow faster, are locally destructive, and recur after treatment in a meaningful proportion, so they require active management rather than observation.

Enchondromas, benign collections of cartilage, occur in the upper arm bone in adults and frequently show the characteristic ring-like or arc-shaped calcifications of cartilage lesions.

Intraosseous ganglia occur adjacent to the joint.

A group of findings in the head of the arm bone deserves particular attention, because they appear constantly on shoulder reports and cause disproportionate concern.

Small cystic areas occur at the sites where the rotator cuff tendons attach to the head of the arm bone, particularly on its outer aspect. Some of these are entirely normal—a region at the attachment of one of the tendons commonly shows small lucencies in people with no shoulder problem at all. Others reflect degenerative change at the tendon attachment, and their presence correlates with rotator cuff disease. In either case they are not bone lesions in any meaningful sense, and the clinically relevant finding is the state of the tendons rather than the cysts themselves.

A further finding deserves mention because it is frequently misread. Following a shoulder dislocation, an impression fracture occurs on the back of the head of the arm bone where it strikes the rim of the socket. This produces a defect that can appear cystic on imaging. It indicates previous dislocation rather than any bone disease, and recognising it correctly explains both the appearance and often the person’s history of shoulder instability.

Subchondral cysts form beneath a worn joint surface in shoulder arthritis, as elsewhere, and reflect cartilage loss rather than independent disease.

Malignant lesions in this region are far less common but are the reason aggressive appearances are taken seriously.

Osteosarcoma occurs in the upper arm bone in adolescents and young adults, this being the second commonest site after the knee. It produces persistent pain including at night, often with swelling, and shows aggressive features on imaging.

Ewing sarcoma occurs in a similar age group, more often in the shafts of long bones and the pelvis.

Secondary deposits from cancers elsewhere are considerably more common than primary bone cancers in adults over forty, and the upper arm bone is a recognised site, being one of the regions where red marrow persists into adult life. A destructive lesion here in an older adult raises this possibility before anything else.

Myeloma produces multiple well-defined lucent lesions and is an important consideration in older adults.

The features that distinguish benign from concerning lesions are the same throughout the skeleton.

Reassuring features are a sharply defined border, a surrounding rim of denser bone, an intact cortex even if thinned or expanded, no reaction of the bone surface, and no soft tissue mass. A well-defined lucent lesion in the upper arm bone of a child, adjacent to the growth plate, with a thinned but intact cortex, is overwhelmingly likely to be a simple bone cyst and is frequently diagnosed on that basis alone.

Concerning features are an ill-defined border, a patchy or moth-eaten pattern of destruction, loss of the cortex, layered or spiculated bone surface reaction, extension into soft tissue, and change over a short interval.

Age narrows the possibilities as effectively here as anywhere. A lucent lesion in the upper arm bone of a ten-year-old is almost certainly a simple bone cyst. The same appearance in a seventy-year-old raises entirely different considerations.

Symptoms are absent in most cases.

Where present, a deep ache in the upper arm rather than the shoulder joint itself is typical, often activity-related. The distinction matters, since the great majority of shoulder pain arises from the rotator cuff, the joint, or the neck rather than from bone.

Localised tenderness over the bone may be found.

A palpable fullness occurs where the bone is expanded.

Sudden pain after minor or no injury suggests fracture through weakened bone, and in a child this is frequently the presenting event.

The symptom pattern warranting attention regardless of imaging is pain that is constant rather than activity-related, pain that wakes a person from sleep, and pain progressively worsening over weeks. In an adolescent with upper arm pain of this character, particularly with swelling, imaging is appropriate rather than a period of watchful waiting.

Assessment follows the standard approach.

X-ray is the primary investigation and is frequently sufficient. Simple bone cysts have a characteristic appearance that allows confident diagnosis, and the location and age of the person contribute as much as the appearance itself.

MRI characterises the contents, assesses the surrounding marrow, identifies soft tissue components, and evaluates the rotator cuff, labrum, and joint—which is frequently the reason the shoulder was being imaged. It is essential where a lesion is not clearly benign.

CT demonstrates cortical detail and mineralisation.

Comparison with previous imaging is valuable, and a lesion unchanged over years is benign almost by definition.

Where features are not confidently benign, referral to a specialist bone tumour service before biopsy is important, since an unplanned biopsy can compromise later limb-preserving surgery.