BONE REMODELING
Bone Anatomy

The skeleton is made up of two types of bones: cortical and trabecular. Cortical bone, also known as compact or lamellar bone, forms the thin outer shell of the long bones and the major portion of the cortex of other bones. Approximately 75 percent of the weight of the skeleton is cortical bone. Trabecular bone is also known as spongy or cancellous bone. It is made of a network of intersecting plates (trabeculae) within the cortex, and serves as the supportive infrastructure of the bone. Trabecular bone is the most metabolically active type of bone, and although it accounts for only 25 percent of the skeleton by weight, it accounts for 75 percent of bone remodeling surface area.¹

The vertebrae and the ends of the long bones of the arms and legs contain a higher percentage of trabecular bone than other areas of the skeleton; vertebrae are composed of more than 65 percent trabecular bone.¹

Normal Bone	Osteoporotic Bone

Osteoporosis is thought to be due to the cumulative loss of bone through an imbalance in bone remodeling. Maximum bone density is attained between ages 20 and 30. Bone is actively remodeled throughout life. During normal remodeling, osteoclasts (bone-resorbing cells) excavate small cavities in the bone, which are subsequently refilled by osteoblasts (bone-forming cells).²

Normally about one million bone remodeling units are active in the skeleton. In osteoporosis, the remodeling balance is shifted, resulting in a net cumulative loss of bone. This occurs as a result of both excessive osteoclast activity and insufficient osteoblast activity. Increased osteoclast activity is pronounced in the immediate postmenopausal phase of bone loss, whereas decreased osteoblast activity is more pronounced during the slow phase of age-related bone loss. In severe cases, excessive osteoclast activity may actually perforate the trabecular structures of bone. This leads to elimination of the subsequent formation phase of the bone.

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