Bone Cyst of the Knee

What is it?

The knee is formed by the lower end of the thigh bone, the upper end of the shin bone, the smaller bone beside it, and the kneecap. The ends of these long bones are among the most metabolically active regions of the adult skeleton, with the growing zones nearby being the fastest-growing parts of the body during adolescence.

This activity explains why the knee is the most common site in the skeleton for bone lesions of all kinds. Rapidly growing and remodelling bone is where developmental abnormalities arise, and it is also where the primary bone cancers of young people occur.

A bone cyst is a fluid-filled cavity within bone, and several distinct processes produce cystic or lucent appearances here.

Subchondral cysts are the most common in adults. They form in the bone immediately beneath a damaged joint surface, where either joint fluid is forced through the worn cartilage into the bone or small areas of bone fail under excessive load. They are a feature of osteoarthritis rather than an independent condition, and their presence indicates that the cartilage above them has worn. In the knee they occur beneath the worn compartment, most often on the inner side.

Intraosseous ganglia are benign cavities containing thick fluid, occurring near the joint in adults, typically well-defined with a thin rim of denser bone, and found in knees that are otherwise normal.

Non-ossifying fibromas and fibrous cortical defects are extremely common in children and adolescents and are concentrated around the knee. They appear as well-defined lucent areas at the edge of the bone near the growing zone, often with a scalloped border and a sclerotic rim. They are developmental rather than neoplastic, cause no symptoms in the great majority, and resolve spontaneously as the skeleton matures, filling in with normal bone. They are the archetypal don’t-touch lesion, and recognising them prevents a great deal of unnecessary investigation. Their only clinical significance arises where a large one thins the bone enough to risk fracture.

Simple bone cysts occur around the knee though less frequently than in the upper arm bone, and aneurysmal bone cysts occur here in people under 20, expanding the bone and requiring active treatment.

Cysts associated with cartilage injury occur beneath areas where the joint surface has been damaged, including osteochondritis dissecans—a condition of adolescents and young adults in which a fragment of cartilage and underlying bone separates from the end of the thigh bone, typically on the inner side of the inner condyle.

Several other lesions characteristically occur at the knee and deserve mention because they appear on reports from this region.

Giant cell tumours occur in young adults, typically between 20 and 40, at the very end of the bone extending up to the joint surface. They appear as expansile lucent lesions without a sclerotic rim. They are classified as benign because they rarely spread, but they are locally destructive, recur after treatment in a meaningful proportion, and occasionally behave more aggressively. They require treatment rather than observation.

Enchondromas, benign collections of cartilage, occur in the thigh and shin bones and frequently show characteristic ring-like calcifications.

Osteochondromas are bony outgrowths from the surface near the growing zone, extremely common and usually harmless, though they can cause symptoms through pressure on adjacent structures.

A normal variant worth knowing is the cortical desmoid, an irregular area on the back of the inner side of the lower thigh bone where a tendon attaches. It can look alarming on imaging, with an irregular appearance suggesting erosion, and is entirely benign and a recognised source of unnecessary concern.

Malignant lesions are the reason aggressive features matter here.

Osteosarcoma is the principal primary bone cancer of adolescence and young adulthood, and the knee is its commonest site, arising in the lower thigh bone or upper shin bone. It is rare, but it occurs precisely in the age group and location where benign lesions are also most common, which is why the distinction is taken seriously. It typically produces persistent pain including at night, often with swelling, and shows aggressive features on imaging: an ill-defined destructive lesion, interrupted or spiculated reaction of the bone surface, and frequently a soft tissue mass.

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

Secondary deposits from cancers elsewhere are considerably more common than primary bone cancers in adults over 40, and the knee is an occasional site.

The features distinguishing benign from concerning lesions are the same throughout the skeleton, and understanding them is the most useful thing a person can take from a report.

Reassuring features are a sharply defined border, a thin rim of denser bone surrounding the lesion, an intact outer cortex, no reaction of the bone surface, and no soft tissue mass. A lesion with these features, in a characteristic location and age group, can frequently be diagnosed with confidence on imaging alone.

Concerning features are an ill-defined or indistinct border, a patchy or moth-eaten pattern of bone 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 substantially. In a child or adolescent, a well-defined lucency near the growing zone of the knee is overwhelmingly likely to be a non-ossifying fibroma. In a young adult, a lucent lesion extending to the joint surface raises the possibility of a giant cell tumour. In an older adult, cysts beneath a worn joint surface are the usual explanation, while a destructive lesion raises the question of secondary cancer.

Symptoms are absent in most benign lesions.

Where present, a deep ache localised to the bone rather than to the joint line is typical, often activity-related. Localised bone tenderness may be found, distinguishable from the joint line tenderness of meniscal problems or the diffuse ache of arthritis.

Swelling or a palpable lump occurs with expansile lesions.

Fracture through weakened bone causes sudden pain after minor or no injury.

The symptom pattern that warrants attention regardless of imaging is pain that is constant rather than activity-related, pain that wakes a person at night, and pain that is progressively worsening. In a young person with knee pain of this character, particularly with swelling, imaging is appropriate rather than a period of watchful waiting, since this is the presentation of osteosarcoma and delay in that diagnosis has consequences.

Assessment follows the standard sequence.

X-ray is the primary investigation and remains the most informative for characterising a bone lesion. In many cases a confident diagnosis follows from the X-ray alone, particularly for the characteristic benign lesions of childhood.

MRI assesses the contents of the lesion, its extent within the marrow, any soft tissue component, and the surrounding cartilage, ligaments, and menisci. It is essential where a lesion is not clearly benign, and it is also the investigation that identifies cartilage injury and other causes of knee symptoms.

CT demonstrates mineralisation within a lesion and cortical detail.

Comparison with previous imaging is frequently decisive. Many incidental lesions are visible in retrospect on earlier studies, and stability over years establishes benignity more convincingly than any single appearance.

Where features are not confidently benign, referral to a specialist bone tumour service before biopsy is important. An unplanned biopsy taken through the wrong approach can contaminate tissue planes and, if the lesion proves to be a primary bone cancer, can convert a limb-preserving operation into a more extensive one. National guidance in many countries reflects this, and the sequence matters.