BASIC CONSIDERATIONS

Embryologically, all bones of body are mesodermal in origin, developing from the mesenchymal cells by two different mechanisms:

a. Enchondral bone formation, i.e. initial differentiation of mesenchymal cells into cartilage during intrauterine life, which is gradually replaced by bony tissue in utero or postnatally.

b. Membranous bone formation, i.e. direct differentiation of mesenchymal cells into bony tissue, without cartilage formation. Bones of skull vault develop by this mechanism.

Enchondral bone formation, the predominant mode of bone development, begins at the ends of long cartilaginous bones (physes or physeal plate) and extends distally towards the ends (epiphysis) or towards the shaft (metaphysis and diaphysis). Initially, these bones are non-calcified and vulnerable to extrinsic pressure, i.e. intrauterine pressure-a common cause of developmental deformities. Process of ossification begins with appearance of various ossification centers in late gestation or postnatal period, which continues till late childhood or adolescence.

Even after formation, bones are subject to constant turnover with new bone formation by osteoblasts at epiphyseal plate and subperiosteal region, as well as bone resorption by osteoclasts at subperiosteal region.

Structurally, each bone consists of four majorcomponents: a. A mash-work of collagen fibers, derived from osteoblasts, i.e. modified mesenchymal cells.

b. Ground matrix, a cementing substance to bind colla­gen network, composed of various substances, e.g. mucopolysaccharides, mucoproteins and phos­pholipids.

c. Bone cells of three major types—osteocytes, osteoblasts and osteoclasts

d. Inorganic minerals, e.g. Ca++ and PO4^- salts to provide essential strength for above architecture.

Pathologically, rapidly growing bones of a fetus or a child are exposed to various extrinsic and intrinsic adverse factors, leading to various orthopedic disorders, as follows:

• Intrinsic defects in formation of bony framework, e.g. congenital anomalies and skeletal dysplasias.

• Extrinsic factors to affect growth of normally formed bones, e.g. intrauterine pressure or posture, post-natal growth and trauma/disease.

• Defective mineralization or demineralization of bones,

e. g. rickets or osteomalacia

• Defective vascularization or devascularization, e.g. osteochondroses

• Defective resorption or turnover of bones, e.g. post- infective/traumatic healing or tumors.

Etiologically, common orthopedic problems in children may be broadly divided in: (a) developmental bone disorders, (b) inherited skeletal dysplasias, (c) bones and joint infections, (d) skeletal trauma and fractures, (e) osteochondroses or avascular disorders, and (f) bone tumors and masses; which are discussed in this chapter. For disorders of mineralization, e.g. rickets see Chapter 6.5.

23.2