Ossification

replacement of the primary spongiosa by secondary bone, formation of bone marrow or conversion of the spongiosa into primary cortical bone by filling of spaces between the trabeculae.

In

intramembranous ossification

, transformation of mesenchymal cells into osteoblasts is accompanied by formation of a meshwork of collagen fibers and amorphous ground substance (osteoid matrix) that undergoes calcification. Some of the osteoblasts become entrapped within the substance of the matrix lacunae, after which they are termed osteocytes. The osteocytes, which maintain some contact with each other by sending out elongated processes through canaliculi, are involved primarily in maintaining the integrity of the surrounding matrix and are not involved directly in bone formation. This process results in formation of cancellous bone, bone marrow, and in areas of compact bone, cylindrical masses containing a central vascular channel (haversian system).

In

endochondral (intracartilaginous) ossification

, cartilaginous tissue derived from mesenchyme is replaced with bone within sites called ossification centres. Through a process identical to intramembranous ossification, the osteoblasts produce a subperiosteal collar of bone enclosing the central portions of the cartilaginous tissue. Vascular channels perforate the shell of bone and disrupt the lacunae, creating spaces that fill with embryonic bone marrow. Subsequently osteoblasts lay down osteoid tissue in the cartilage matrix and then become trapped within the developing bone as osteocytes. As tubular bones become ossified the periosteal collar spreads toward the ends of the bone. Some of the initially formed trabeculae become resorbed and create a marrow space, which develops into the bone cortex. Ultimately a system with longitudinally arranged compact bone surrounding vascular channels (haversian system) is formed. As bone formation proceeds an advancing frontier moves toward the end of the bone; this region ultimately becomes located between the epiphysis and diaphysis of a tubular bone and forms the growth plate or physis. The growth plate is the predominant site of longitudinal growth of the bone. At the ends of the tubular bones, secondary centres of ossification develop in the epiphyses. Eventually the growth plates disappear and the epiphyseal and diaphyseal ossification centres fuse. In some cases transient aberrations of ossification are marked by growth recovery lines.

Abnormalities of ossification

occur in a number of disorders, including rickets, endocrine disorders (acromegaly), neoplasms, Severs phenomenon and ischiopubic osteochondrosis. Ossification may also occur at abnormal sites (ectopic or heterotopic ossification).

Heterotopic bone formation

may occur in central nervous system and spinal cord disorders, probably as a consequence of immobilization periarticular soft tissues may undergo ossification, particularly in the pelvis. Other causes of soft tissue ossification are neurologic diseases, metastases, melorheostosis, Pellegrini Stieda syndrome, Eagle syndrome and surgery.

Ossification of ligaments

is common. The anterior longitudinal ligament of the spine may become ossified in diffuse idiopathic skeletal hyperostosis DISH . The ligamentum flavum and supraspinous ligament may be involved to various extents. A specific disorder, ossification of the posterior longitudinal ligament (OPPL) of the spine, is characterized by presence of a linear band of ossification along the posterior margin of vertebral bodies and intervertebral discs, especially in the cervical spine. Conventional tomography, CT, and MR imaging are helpful in diagnosis of this condition.Also, see bone maturation.

HC

DR