Bone Remodeling

Strong, healthy bone is continually maintained through bone remodeling. Bone remodeling has two phases: resorption and formation. Bone removal is called resorption. Bone-resorbing cells called osteoclasts remove old bone by releasing acids and enzymes to remove minerals and collagen. Once the osteoclasts have done their job, protein-secreting cells called osteoblasts deposit new tissue.

	During bone resorption, osteoclasts attach to the mineralized bone matrix and excavate small pits on the bone surface, releasing bone collagen and minerals into circulation.
	Cross-linked N-telopeptides (NTx) are released into the bloodstream during osteoclastic activity.
	During bone formation, osteoblasts are recruited to the newly resorbed areas on the bone where they deposit new collagen.
	When resorption and formation are in balance, there is no net change in bone mass. After a resting phase during which the bone is mineralized, the remodeling cycle begins again.

Approximately 20% of bone tissue is replaced annually by this process on a cyclical basis throughout the skeleton. The entire remodeling process occurs over approximately 4 to 8 months, with a range of 3 months to 2 years.

Disease states, such as osteoporosis, occur when resorption is more active than formation, resulting in a net loss of bone. Degradation products of bone resorption are present in urine and blood.