PERIOSTEAL REACTION

PERIOSTEAL REACTION

By Dr. KM Liau

The periosteum is a membrane several cell layers thick that covers almost all of every bone. The only parts not covered by this membrane are the parts covered by cartilage.

PERIOSTEAL REACTION

Besides covering the bone and sharing some of its blood supply with the bone, it also produces bone when it is stimulated appropriately.

What does it take to make this happen?

Practically anything that breaks, tears, stretches, inflames, or even touches the periosteum.

So, when some anonymous process stimulates this reactive bone formation, eventually we see evidence of it on some imaging study.

SPEED OF PERIOSTEAL REACTION

The main determinant of how the new bone formation looks is how fast the abnormal process grows, and has little to do with any intrinsic properties of the periosteum.

Evidence of the speed at which these processes are growing is the main thing we look for when assessing periosteal reaction.

SLOW PERIOSTEAL REACTION

With slow-growing processes, the periosteum has plenty of time to respond to the process.

It can produce new bone just as fast as the lesion is growing.

Therefore, we can see solid, uninterrupted periosteal new bone along the margin of the affected bone.

RAPID PERIOSTEAL REACTION

With rapidly growing processes, the periosteum cannot produce new bone as fast as the lesion is growing.

Therefore, rather than a solid pattern of new bone formation, we see an interrupted pattern.

If the lesion grows unevenly in 'stops' and 'starts', then the periosteum may have time to lay down a thin shell of calcified new bone before the lesion takes off again on its next growth spurt.

This result in a pattern of one or more concentric shells of new bone over the lesion.

This pattern is sometimes called lamellated or "onion-skin" periosteal reaction.

RAPID PERIOSTEAL REATION

If the lesion grows rapidly but steadily, the periosteum will not have enough time to lay down even a thin shell of bone.

In such cases, the tiny fibers that connect the periosteum to the bone (Sharpey's fibers) become stretched out perpendicular to the bone.

When these fibers ossify, they produce a pattern sometimes called "sunburst" or "hair-on-end" periosteal reaction, depending of how much of the bone is involved by the process.

CODMAN'S TRIANGLE

Another pattern seen in rapidly growing processes is called the Codman's triangle.

This is a bit of a misnomer, since there really is not a complete triangle.

When a process is growing too fast for the periosteum to respond with even thin shells of new bone, sometimes only the edges of the raised periosteum will ossify.

When this little bit of ossification is seen tangentially on a radiograph, it forms a small angle with the surface of the bone, but not a complete triangle.

So, when a process is growing too fast for even the Sharpey's fibers to ossify, one may only see a soft tissue mass arising from the bone, perhaps with small Codman's triangles at its margins.

COMPLEX PERIOSTEAL REACTION

If we see a solid pattern of periosteal reaction, we can be fairly confident that we are dealing with a benign process.

As with many rules in medicine, there are some caveats associated with the use of this rule. The main caveat with this rule is that benign processes and malignant processes may coexist.

The usual way that this may manifest is when there is a fracture or infection in the same area as a tumour.

In this case, you may see a fairly complex pattern of periosteal reaction that demonstrates some elements that look benign and some that look very aggressive.

CAUSES OF SOLID PERIOSTEAL REACTION

1. Infection - Brodie's abscess

2. Benign neoplasms - Osteoid osteoma

3. Healed stress fracture

4. Eosinophilic granuloma

5. Hypertrophic pulmonary osteoarthropathy

OSTEOID OSTEOMA

Note the central lucent nidus within the thickened periosteum.

BRODIE'S ABSCESS

HEALED STRESS FRACTURE

CAUSES OF AGGRESSIVE PERIOSTEAL REACTION

1. Acute Osteomyelitis

2. Malignant neoplasms

A) Osteosarcoma

B) Ewing Sarcoma

C) Chondrosarcoma

D) Lymphoma

E) Leukemia

F) Metastasis

OSTEOSARCOMA

Note the sunburst appearance and Codman's triangle

CHONDROSARCOMA

Note the bony calcification which signifies cartilage formation within the diaphysis.

Cortical breach with very little periosteal reaction signifying a very aggressive type of tumour.

EWING SARCOMA

LEUKEMIA

References

Ragsdale BD, Madewell JE, Sweet DE. Radiologic and pathologic analysis of solitary bone lesions. Part II: Periosteal reactions. Radiol Clin N Am 1981; 19: 749-783.