Bone Cysts & Benign Bone Growths

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What is a Bone Cyst?

Bone cysts are most commonly found in kids, although ovearll they are fairly uncommon in anyone. A cyst is a fluid filled cavity, and sometimes they can form within bone (in contrast to a ganglion cyst, which forms in a joint, see talk). 

The two most common cysts in kids are an aneurysmal bone cysts (ABCs) and a simple bone cysts (unicameral bone cysts).

What is a Simple Cyst?

A simple bone cyst is a solitary cavity (hole) in the bone, that fills with water-like fluid.  It almost always occurs before someone is 20 years old, and they are twice as common in boys.  They are benign (meaning no risk of cancer).  They are usually solitary (only one is found in the body) and occur in the upper arm in 50% of cases, around the knee in 20% of cases, and in other bones the rest of the time. The cause is unknown but is thought to be a result of abnormal flow of blood (like a block to venous return) through the bone.  

Cysts occur in the metaphysis (a region of soft bone near the growth plate) and if they are very near the growth plate (within 0.5 cm) they have increased risk for getting bigger and are called "active" (while those with low risk for getting big are called "latent").  Cysts stop growing once you stop growing, so as long as they are small, they don’t make the bone too weak, there is not need for treatment.  However, If the cyst is big it can make the bone weak putting it at higher risk of breaking.  Its not uncommon for the bone to break around a big cyst, or cause microfractures (which cant be seen on x-ray but are very painful).  In 10% of kids they fracture and cause growth arrest.  

This may cause scarring and small subdivisions to form within the cyst (making it appear as multiple small cavities, or something more complicated...but its still simple!).  The cysts can be mistaken for fibrous dysplasia, abcs, or eosinophilic granulomas.  ABCs are more commonly identified when double-density fluid levels, septations. 

When treated surgerically (like when the cause pain or they caused a fracture) there are a few approaches, with the goal to prevent recurrence (which happens about 8% of the time).  The cyst can be drained of its fluid and then steroids (methylprednisone) are injected.  The results are variable, which success between 40-80% after 2-5 injections.   Another option is to perform surgery, where the walls of the cyst are scrapped out and then the hole is filled with bone graft (calcium phosphate paste, or porous hydroxyapitate). 

What is an Aneursymal Bone Cyst (ABC)?

Aneursysmal bone cysts are also solitary lensions in the metaphyseal (soft bone near the growth plate), however they are caused by an abnormal blood vessel within the bone (an AV fistula: meaning an artery and vein come together without capillary bed in between) or because of abnormal blood flow due to a near by tumor (some of which are benign, like a nonossifying fibroma, some can be malignant like a giant cell tumor, or telangectatic osteosarcoma).  These cysts can occur in adults, but 70% occur in people under 20 years old, they can occur in any bone (unlike simple cysts which usually occur in the arm or leg), and are most commonly found in the femur, pelvis, spine (especially the posterior elements), humerus.  The cyst is usually identified in the same way as a simple cyst, by causing pain secondary to microfractures of the bone which has been structurally weakened by the enlarging bone.  On x-ray you often see a “blowout” lesion, which means that the cyst has expanded beyond the normal shape of the bone its within, the weakened thin bone that contains the cyst has actually bowed outward.  This will form during the growth phase of the cyst, when it rapidly expands with destruction to surrounding bone, sometimes it forms Codmans triangles (periosteal ossification) and can appear similar to a malignant lesion, then the cyst slows or stops its growth, and will exibit a bubbly appearance from the multiple locualtions inside it.  Because these cysts can grow destructively, treatment with surgery is recommended, but a CT scan is ordered first to confirm the diagnosis and look for signs of other tumors.  Treatment consists of 4 phases: aspiration, curratage, cauterization, and grafting. 

What is a Nonossifying Fibroma?

Nonossifying fibroma, also known as a fibrous cortical defects, is the most common benign lesions of childhood, and is often found incidentally in the metaphysis of long bones (esp. femur and tibia). They present as a sharply delineated lucency with a sclerotic border on x-ray, and don’t require further work up unless they are painful, then a CAT scan is ordered to look for fractures or differentiate it from other things like a cyst or granuloma. 

They don’t require treatment, and usually get smaller with time.

What is Fibrous Dysplasia?

Fibrous dysplasia is a benign intramedullary (within the center of the bone) growth of soft tissue (probably a failure of woven bone to mature).  Its identified in late childhood and adolescence, and occurs in just one bone, or in some people it occurs in multiple bones, often in one leg (which may cause a limb length discrepancy) and often in the face, and they are often found in conjunction with café au lait spots. 

On x-ray they can appear bubbly b/c surrounded by sclerotic bone, but often as they grow they replace cortex, and can even become eccentric (grow outside the normal shape of bone) yet remain contained by periosteal bone.  The result of replacing cortical bone is weaking the bone and causing angular deformity by repeated microfractures over time.  Sometimes confused with simple cysts, or encondromas. 

The initial treatment is observation if nonpainful, but doctors can give bisphosphonates which help reduce pain and decrease remodeling and replacement of bone.  If significant bone deformity occurs or weakening occurs and results in a fracture, then an intramedullary nail can be placed to strengthen the bone.

What is an Osteochondroma?

An osteochondroma is a "bump" caused by the abnormal growth of cartilage around a joint.  Its most commonly found around the knee joint, its also seen in the arm bone (near the shoulder) but it can occur around any joint in the body.  Its typically seen as a solitary lesion in growing adolescents, and its believed to occur when cartilage contributing to bone growth within a growth plate escapes its normal position and starts growing at the side of the bone.  The bump is normal bone with a small cartilage cap (like a branch forming of a tree trunk).  

An osteochondroma usually occurs as a solitary lesion, however, a mutation in the EXT gene can cause a condition called Multiple Hereditary Exostosis, which causes many osteochondromas to form throughout the body.  The vast majority of solitary lesions are benign (no risk for causing cancer) however very rarely a malignant transformation can occur, whereby a benign bump turns into chondrosarcoma cancer.  In Multiple Hereditary Exostosis, this malignant transformation is far more common, in about 5-10% of cases (especially osteocondromas in the arms).   

Most osteochondromas are noticed as a painless bump around the knee (or shoulder).  Sometimes they can press on a tendon or nerve and cause pain, but generally they are asymptomatic.  X-rays are ordered by the treating doctor to evaluate the bump.  Most of these look pedunculated (like a mushroom), however if they are flat and wide doctors are more concerned for an aggressive growth.  Also similar to a mushroom, these bumps have a "cap", made of cartilage.  Usually the cap is thin, however, if it starts to grow larger (>2 mm) doctors are also more concerned for aggressive growth.

Treatment can be surgical or nonsurgical.  Most cases dont need surgery unless the bump is irritating to nearby tendons or if its blocking normal motion of the arm or leg.  Occasionally they need to be removed if a doctor is concerned that the benign bump is starting to look malignant (an aggressive cancer).  Lastly some people dont like they way it looks and will ask for it to be removed for aesthetic reasons.  There is a low risk that the bump grows back (about 2%), and therefore most people are either happy to leave it alone, or happy to remove it and never see the bump again.