Bone accrual in children: adding substance to surfaces.

The mass of growing bones increases through changes in outer dimensions and through the net addition of tissue on inner bone surfaces. In this overview I examine bone accrual as it occurs on trabecular (inner) and periosteal (outer) surfaces. In the axial skeleton, the amount of trabecular bone increases during development, because trabeculae grow thicker as a result of bone remodeling with a positive balance. Remodeling is a process in which osteoblasts and osteoclasts are tightly linked ("coupled") in time and space. In contrast to trabecular thickness, trabecular number and material density change little throughout development. Bone accrual on periosteal surfaces leads to an increase in bone size, which is a crucial determinant of bone strength throughout life. Periosteal osteoblasts deposit new bone on an extended surface area and over an extended period of time without being interrupted by osteoclasts. This type of bone metabolic activity is called modeling, which is much more efficient than remodeling for increasing bone mass. In the past, research has focused on bone remodeling on trabecular surfaces. However, the key to an improved understanding of bone mass and strength development in children will lie with studies on bone modeling on periosteal surfaces.