Edematous granulation tissue consisting of vessels and fibrous tissue along with lymphoplasmacytic infiltrate in marrow is consistent with a diagnosis of subacute osteomyelitis.
Bone fragments against a background of dense and loose fibrovascular tissue. As chronic ostemyelitis progresses, the fibrovascular tissue becomes more fibrotic.
Irregular bone fragments embedded within the fibrovascular background. Note the line of osteoblasts building new bone.
Another image of bone remodeling and lymphoplasmacytic inflammation against a fibrovascular background.
It is worth mentioning that the diagnosis of acute or chronic osteomyelitis can only be made after integrating clinical, radiographic and histologic data. The terms acute, subacute and chronic osteomyelitis refer to the duration of the disease and not the composition of the inflammatory infiltrate.1 Neutrophils, lymphocytes and plasma cells in varying proportions can be seen in both acute and chronic osteomyelitis.
However, certain patterns of inflammation can be seen in each stage of the disease. In acute osteomyelitis, a vigorous neutrophilic infiltrate is seen in the marrow. In subacute osteomyelitis, the marrow is replaced by an edematous granulation tissue containing a mixture of inflammatory cells. In chronic osteomyelitis, marrow fibrosis with a mixed inflammatory infiltrate is present; appositional deposition of new bone on existing trabeculae can also be seen. Note that appositional growth is similar to adding layers to the outside, like rings on a tree.
Occasionally, the dominant cell population is the plasma cell, leading to the term plasma cell osteomyelitis, and if there are numerous foamy macrophages, xanthogranulomatous ostemyelitis may be used.2 One may find necrotic bone "sequestrum" surrounded by granulation and new bone "involucrum". The formation of sequestrum is part of the reason chronic ostemyelitis can be refractory to antibiotics; this necrotic bone protects bacterium from antibiotic penetration.
If treated inadequately with antibiotics, it may continue for many years with recurrent episodes of local infection, which may be accompanied by sinus formation. Rare complications include amyloidosis with formation of squamous cell carcinoma in the sinus tract.
Antibiotics of adequate coverage and sufficent duration. Surgical removal of the infected sequestrum may be necessary to resolve the infection.
Sinus tract formation connecting bone to the overlying cutaneous surface may occur as a result of chronic infection. These tracts are lined by squamous epithelium and may progress into squamous cell carcinoma, although this long-term complication is quite rare. Approximately 1% of patients develop squamous cell carcinoma, and it may be as long as 40 years after the original infection.1
1 Rosai, J. Rosai and Ackerman's Surgical Pathology. 9th Ed. Philadelphia, PA: Elsevier; 2004:2140-2142.
2 Sternberg SS, ed. Diagnostic Surgical Pathology. 4th Ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004: 261-2.