All other appendicular and axial bones form by enchondral
ossification—that is, by gradual ossification
of a cartilaginous anlage. Instead of converting directly into osteoblasts,
condensations of mesenchymal cells transform into cartilage matrix-secreting
chondroblasts. Long bones are anatomically characterized by a central
shaft (diaphysis) flaring to a broader metaphysis at
either end, and they terminate in a typically bulbous, articular
cartilage-capped, epiphysis (Fig.
210-1). In the central region of each cartilage anlage, capillary
invasion results in the replacement of chondroblasts by osteoblasts,
and the formation of primary centers of ossification.
Ossification of the cartilaginous anlage then continues centrifugally.
The epiphyses at various stages of development, particular to the
individual bone, experience
vascular invasion as well, resulting in the formation of secondary
centers of ossification.1,4 Once the secondary
centers of ossification have appeared, the cartilaginous growth
plate (physis) can be “visualized” radiographically
as the radiolucent disk between the bony metaphysis and epiphysis.
The appearance of the various secondary centers of ossification
typically occurs within a relatively narrow age range, so that their
absence or appearance may be used to estimate the age of the fetus
and infant (eFig. 210.1). Similarly, the
physes will disappear (“close”) at skeletal maturity;
the timing of physeal closure is specific to the individual physes
and is location and gender dependent (eFig. 210.2).
At a more sophisticated level during later stages of growth, careful
analysis of the maturation of the distal radial and ulnar epiphyses,
carpals, metacarpals, and phalanges forms the basis for the calculation
of skeletal (or “bone”) age. These estimations
may used to identify hormonal growth inhibition and allow a calculation
of growth remaining in long bones and, to a lesser extent, the spine.