Skeletal development in premature infants: a review of bone physiology beyond nutritional aspects. (Review).

Bone development is usually seen as a process of bone mineral accretion or increase in bone mass, and treatment of bone disorders usually consists of attempts to maximise bone mass accumulation by nutritional means only. However, from a functional perspective, bones should not be as heavy as possible, but rather as stable as necessary. It is therefore important to create conditions that stimulate bones to become more stable.

The prevention and treatment of bone disorders is an important aspect of the care of preterm babies and, accordingly, a considerable amount of literature has been dedicated to this topic. (1-3) However, the focus is almost exclusively on nutritional considerations. Although there can be no doubt that a sufficient and balanced supply of energy, proteins, vitamins, calcium, phosphorus, and other nutrients is an essential prerequisite for normal bone development, the basic facts about bone physiology often receive little consideration. The purpose of this contribution is to highlight some of these topics rather than to give an exhaustive review of bone disease in prematurity.

DEFINITIONS

What is bone density?

In physics, density was defined by Archimedes as the mass of a body divided by its volume (called "physical density" here). In clinical practice and science, "bone density" usually has a different meaning: the degree to which a radiation beam is attenuated by a bone, as judged from a two dimensional projection image (= areal bone density). It is important to remember that the attenuation of a radiation beam not only depends on physical density, but also on bone size--that is, the length of the path that the beam takes across the bone. A small bone therefore has a lower areal bone density than a larger bone, even if the physical density is the same (fig 1). This is true for both the subjective estimation of bone density from standard radiographs and its quantitative assessment by widely available densitometric methods, such as single photon absorptiometry and dual energy x ray absorptiometry. (4) A low areal bone density value can therefore simply reflect the small size of an otherwise normal bone.

What is bone mineralisation?

The physiological process of mineralisation represents the incorporation of mineral (Ca, P. and others) into organic bone matrix (osteoid), after it has been synthesised and deposited by osteoblasts (fig 2). (5 6) 6 However, many reports on bone development in premature babies use statements such as "bone mineralisation has increased" to describe the radiological finding that areal bone density or bone mineral content (the mass of mineral per unit bone length) has increased from one measurement to the other. The meaning of the term "mineralisation" in these two circumstances is by no means identical. An increase in areal bone mineral density or bone mineral content can occur after the incorporation of mineral into pre-existent bone matrix, but can also result from an increase in bone size, thickening of bone cortex or trabeculae, or synthesis of new trabeculae. The latter processes represent formation of new bone tissue, where the …