The fibrous tissue extended into the adjacent skeletal muscle. A moderate number of osteoclasts were found in the chondroid tissue. Bone trabeculae were thin or irregular and intertrabecular spaces contained various amounts of fibrous tissue. Fibrous osteodystrophy belongs to the group of metabolic bone diseases. It is characterized by extensive osteoclastic resorption of bone and formation of fibrocollagenous tissue.
Osteoclastic activity is enhanced by prolonged and excessive secretion of parathyroid hormone.
In the endosteum, the resorbed bone is replaced initially by highly cellular connective tissue. In later stages, the connective tissue is more fibrous and less cellular. The bone cavity is replaced by fibrocellular tissue that contains irregular bone trabeculae. The original compact bone is reduced in thickness and can be completely replaced by soft tissue. The intense osteoclastic activity in the epiphysis results in destruction of not only the epiphyseal trabeculae, but also of the calcified zone of the articular cartilages.