Solitary Exostosis and Multiple Hereditary Exostosis


What is an exostosis?

Exostosis (or osteochondroma) is a very frequent benign lesion in children, characterized by a hard protuberance starting from the bone plane, usually near one end of a bone segment.
More precisely, the exostosis is a hamartoma, that is an overgrowth of the cartilage that normally contributes to the growth of the bone; in this case the cartilage grows in an aberrant seat and direction.
This cartilage behaves similarly to the rest of the cartilage, that is, starting from the the apex of the protuberance (cartilaginous cap) becomes progressively bone, thus increasing the exostosis. 

Solitary exostosis of the distal femur (lateral and anteroposterior view)

Solitary exostoses are more frequently located on the long bones (distal part of the femur, proximal humerus, proximal tibia), at the level of the metaphysis, that is, just above the cartilage of growth of that bone segment. They can have very variable shape and dimensions; the base of implantation can be wide (sessile exostosis) or narrow (pedunculated exostosis).

Exostoses tends to grow in a variable way as the child grows and stop their growth at the skeletal maturity.

Can solitary exostoses turn into a malignant lesion?

The risk of transformation of an exostosis into a malignant lesion (peripheral chondrosarcoma) is extremely low (below 1%) and does not however occur in children.

What are the symptoms of exostosis?

Usually, the only symptom of exostosis is the perception of a mass, mostly discovered by chance. This mass appears hard, starting from the plane of the bone and does not cause pain.

In some cases, the growing exostosis causes compression on some surrounding structure (vessels, nerves, etc.) causing disturbances.

Multiple Hereditary Exostosis

Multiple Hereditary Exostosis (MHE) is a rare disease (incidence of 1-2 cases per 100,000 people), characterized by the formation of exostoses in different parts of the body.

These exostoses are located mostly at the level of the metaphyses of the long bones, but frequently also at the level of scapulae, ribs and other districts

Multiple Hereditary Exostosis, 11-year-old patient

Multiple Hereditary Exostosis is characterized by an autosomal dominant genetic transmission. 

The morphology of exostoses tends to be different according to the anatomical localization: for example, at the level of the lower (distal) femur the exostoses tend to assume a pedunculated appearance in the proximal direction, at the proximal tibia tend to assume pedunculated appearance in the distal direction, at the level of the fibula and humerus a sessile aspect, at the level of the ulna tend to localize in the distal part of the segment (at the wrist) and cause shortening of the bone.

The clinical picture of Multiple Hereditary Exostoses is characterized by:

• multiple swellings (exostoses) from the bone plane;
• functional limitation (case-to-case variable);
• aesthetic disturbances (from case to case);
• short stature;
• asymmetry of limb length;
• angular limb deformities;
• possible malignant transformation.

Typical involvement of forearm bones with deformation and restriction of movement 

The phenotype of clinical presentation of Multiple Hereditary Exostoses can be very variable, from severe cases (with numerous exostoses and significant clinical features) to very mild cases.

The follow-up of multiple exostosis disease:

Exostoses tend to grow in a continuous and simultaneous way throughout the growth period. Therefore, it is necessary not to exaggerate with radiographic examinations (to limit radiation exposure) and to avoid operate on all lesions.

Follow-up examinations are recommended every 6/8 months during growth. It can be helpful to perform periodic clinical photos to monitor the evolution.

Radiographic examinations may be useful in the event of significant clinical changes (noticeable increase in volume, disturbances, etc.).

MRI and CT scans can be performed in case of suspicion of transformation or to study a possible compression of an exostosis on the nearby vascular-nerve structures.

Risk of malignant transformation 

The incidence of malignant transformation varies considerably among different authors (0.5-25%) and occurs more easily in adulthood, at the level of girdles, trunk and around the knee. Usually it manifests with an increase in the size of a mass and with pain.

Surgical treatment of Multiple Hereditary Exostoses

Surgical indications must follow very precise principles.

The main indications to surgical removal of an exostosis are:

• functional limitation
• pain (rare)
• compression of an exostosis on nearby vessels and nerves
• suspicion of malignant transformation of an exostosis. 

Six-year-old, proximal femur exostosis with nerve compression and pain. Excision was performed 


The growth of exostoses can cause limb deformation by variable mechanisms (compression, subtraction of growth, etc)

Deformities can be corrected with various techniques:

• osteotomies
• asymmetrical epiphysiodesis
• lenghtening with axis correction

Please refer to the respective pages for explanations on surgical technique

Asymmetrical epiphysiodesis for knee valgus in boy with Multiple Hereditary Exostoses


Patients with Multiple Hereditary Exostoses may have differences in length of the upper and lower limbs or length difference between forearm bones (radius vs ulna).

There are several surgical possibilities to consider:

• Symmetrical epiphysiodesis to slow the growth of the longer limb 

• Lenghtening of shortened bone segments

Please refer to the respective pages for explanations on surgical technique.

Lenghtening of the femur with external fixator in boy with Multiple Hereditary Exostoses

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