Osteochondroma is the most common non-cancerous type of bone tumour. The name is made up of three parts that come from Greek words – “osteo-“meaning bone, “chondro-” meaning cartilage, and “-oma” which describes an abnormal growth. As the name suggests, the tumour is made up of both bone and cartilage.
Osteochondromas can range in size from 1 cm to more than 20 cm, with the average size being 3 to 6 cm. The most common location is the edge of a long bone such as those found in the legs and arms and around the knees and elbows. However, this tumour can develop in any bone that is formed by cartilage. Osteochondromas are associated with the growth plate and typically affect younger people with the average age being 21 years old.
Bones are a specialized type of hard tissue that provide our bodies with support and protection. In total there are 206 bones in each of our bodies. Some bones (like the bones in our fingers) are very small while other bones (like the bones in our upper legs) are very big.
Normal bones develop when specialized cells called osteoblasts produce a substance called osteoid. Osteoid is initially soft but becomes hard over time as minerals (such as calcium) are added to it. Osteoid that is still soft is called immature bone while mineralized, hard osteoid is called mature bone. The immature bone starts out with a woven, disorganized appearance under the microscope before undergoing mineralization and remodelling, which makes it hard and strong.
Growth plates, also known as the “physis”, are present at the ends of bones, especially the long bones in our arms and legs. During childhood and puberty, these growth plates are responsible for our growth in height. When we are finished growing our growth plates close and eventually disappear. Osteochondromas are thought to grow from a piece of the growth plate that gets shifted due to trauma and continues to grow, along with the normal growth plate, and form an outgrowth of bone. Osteochondromas also tend to stop growing when the person stops growing.
Cartilage is a specialized elastic type of tissue, which means that it can be bent or compressed (put under pressure) without breaking. Cartilage is found throughout the body, although most cartilage is found between bones where it helps form a cushion that protects the ends of the bones from damage. Some parts of the body, such as the nose and ears, are made almost entirely out of cartilage. Cartilage is made up of cells called chondrocytes, which are surrounded by a specialized type of tissue called a matrix that gives it its rubbery nature.
Most osteochondromas do not cause any symptoms. Some patients notice a firm painless lump under their skin. When symptoms do occur, they vary depending on the size and location of the tumour. Larger tumours can cause muscle soreness, pressure, pain, or decreased movement. If the tumour presses on a nerve or blood vessel, it can lead to numbness/tingling, weakness, or colour change of the affected area. The most serious complication is a broken bone.
Most cases of osteochondromas are single and non-hereditary. Doctors describe these as sporadic because they have no known genetic cause. In contrast, multiple osteochondromas can be a manifestation of a genetic condition called hereditary multiple osteochondromas (or hereditary multiple exostoses).
This diagnosis can be made after a small tissue sample is removed in a procedure called a biopsy or when the entire tumour is removed in a procedure called a resection or curettage. The tissue is then sent to a pathologist for examination under the microscope. In some cases, the diagnosis can be made without a tissue examination because the appearance on imaging is unique (the tumour sticks out from the side of the bone).
Under the microscope, an osteochondroma resembles a disorganized growth plate. The outer layer has dense fibrous tissue with a cartilage cap. Usually, the cartilage cap is less than 1.0 cm. The specialized chondrocytes that make up the cartilage cap typically increase in amount and size in deeper layers. These cells look similar to the chondrocytes in normal cartilage. At the base of the growth plate, the tumour gives rise to newly formed bone by a process called endochondral ossification.
In the rare case that osteochondroma transforms into a type of bone cancer called chondrosarcoma, the amount of cartilage in the osteochondroma will increase and the chondrocytes will look more abnormal. Pathologists use the word atypical to describe abnormal-looking cells.
Your pathologist and radiologist will measure the cartilage cap that forms part of the osteochondroma. When an osteochondroma transforms into chondrosarcoma, the cartilage cap thickness is typically more than 2.0 cm.
No treatment is necessary for single osteochondromas that are not causing any symptoms. These tumours are usually monitored with regular x-rays. If symptoms develop, which includes pain, limitation of movement, or nerve or vessel impingement, then surgical removal is curative. Another indication for surgical removal is when the tumour grows in size quickly as this may indicate progression towards cancer, which is overall a very rare event. Even though surgical removal is curative, re-occurrence can happen when the lesion is not completely removed.