The periosteum—a surface for all seasons

Abstract

Fetal limb buds removed in utero and grown ex vivo develop the shape of the proximal femur [1]. Thus, the structure needed to accommodate the stresses to be borne is assembled by instructions from the ancestorial genetic ‘blue print’. Although this seems obvious, it is considerably less obvious that the reverse also applies; the stresses imposed on bone determine its material composition, structural design and so its strength [2]. Bone has the ability to adapt—to modify its material composition and structural design to accommodate prevailing loads. This remarkable feat is achieved by the cellular machinery of bone modeling and remodeling [3, 4]. Bone modeling, or construction, is the deposition of new bone on a quiescent surface resulting in changes in bone size and shape. Bone remodeling, or reconstruction, is the formation of bone within a previously excavated site on a bone surface. Bone is perceived as solid and unchanging because emphasis has been on bone as a mineralized ‘hard’ tissue. This view is ossified. Bone is a fibrous collagen scaffolding reinforced in, around and within its fibres by crystal platelets and needles of calcium hydroxyapatite mineral. This composite material moves; it can be indented and recoils elastically. It can deform plastically sustaining some damage to return incompletely to its original length under those circumstances. The material is woven as alternating higher and lower density lamellae as osteons in cortical bone or hemiosteons in trabecular bone [5]. Osteons have a central longitudinal Haversian canal from which Volkmann canals branch off finding their way to the periosteum or endosteum (Fig. 1). Osteons also contain a fluid-filled lacunar-canalicular system within which osteocytes communicate with each other and bone lining cells by dendritic cytoplasmic extensions. Thus, bone is not an impenetrable solid; it is a metropolis of highways and freeways, turnpikes and underground railways; a house of many rooms and corridors, with fluid-filled canals and canaliculi, wired with nerve fibres, fed with a vasculature no less complex than that of the hepatobiliary, pulmonary-alveolar or renal glomerulotubular systems.