Key Takeaways

• Bone resorption is the breakdown of bone by osteoclasts, releasing stored calcium and minerals.
• It is balanced against bone formation by osteoblasts in a continuous remodelling cycle.
• RANKL drives osteoclast formation and activity; it is opposed by osteoprotegerin.
• Resorption is tracked with markers such as CTX-I, NTX-I, TRAP-5b and cathepsin K.
• Excess resorption causes osteoporosis, osteolysis and increased fracture risk.
• Bone resorption is essential to calcium homeostasis throughout the body.

Contents

• What is Bone Resorption?
• Osteoclasts and the Resorption Process
• Bone Resorption Markers
• Bone Resorption in Disease
• The Role of Calcium
• Frequently Asked Questions

Bone Resorption ELISA Kits

Validated ELISA kits for the collagen breakdown products, osteoclast enzymes and signalling factors used to measure bone turnover.

CTX-I

CTX-I ELISA Kit

Resorption MarkerHuman

C-telopeptide of type I collagen — the leading serum marker of bone resorption.

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NTX-I

NTX-I ELISA Kit

Resorption MarkerHuman

Cross-linked N-telopeptide, released as osteoclasts degrade collagen.

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TRAP-5b

TRAP-5b ELISA Kit

Osteoclast EnzymeHuman

Tartrate-resistant acid phosphatase, a direct marker of osteoclast number.

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Cathepsin K

Cathepsin K ELISA Kit

Osteoclast EnzymeHuman

The protease osteoclasts use to digest the bone collagen matrix.

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RANKL

RANKL (TNFSF11) ELISA Kit

Signalling FactorHuman

The key signal that drives osteoclast formation and activation.

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Bone ALP

Bone Alkaline Phosphatase ELISA Kit

Turnover MarkerHuman

A bone-formation marker used alongside resorption markers to assess turnover.

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What is Bone Resorption?

Bone is not the static, inert material it appears to be. It is a living tissue in a constant state of renewal, continuously broken down and rebuilt in a process called remodelling. Bone resorption is the breakdown half of that cycle: specialised cells called osteoclasts dissolve the mineral and digest the protein matrix of bone, releasing calcium and other minerals back into the bloodstream.

Resorption is balanced against bone deposition, the counter-process carried out by osteoblasts, which lay down new collagen and promote mineralisation to replace the bone that was removed. In a healthy adult the two are tightly coupled, so bone mass stays stable. When resorption outpaces formation — as happens with ageing, hormone loss or disease — the skeleton weakens.

Osteoclasts and the Resorption Process

Osteoclasts are large, multinucleated cells derived from the same myeloid lineage that produces macrophages. Their formation and activity are controlled chiefly by RANKL, a signal produced by osteoblasts and other cells; RANKL is opposed by a decoy receptor, osteoprotegerin, and the balance between them sets the pace of resorption.

To resorb bone, an activated osteoclast clamps tightly onto the bone surface, forming a sealed compartment. It pumps in acid to dissolve the mineral, then secretes the enzyme cathepsin K to digest the exposed collagen matrix. The collagen fragments released — including the C- and N-telopeptides — spill into the circulation, where they can be measured as markers of how much bone is being broken down.

Bone Resorption Markers

Because resorption releases characteristic molecules into blood and urine, it can be tracked without a biopsy. The most widely used resorption markers are the collagen breakdown products CTX-I and NTX-I, together with the osteoclast enzymes TRAP-5b and cathepsin K. These are usually interpreted alongside a formation marker such as bone alkaline phosphatase to give a complete picture of bone turnover.

These markers are invaluable in research and in the clinic, where they are used to monitor osteoporosis, assess fracture risk and track the response to bone-active therapies.

Bone Resorption in Disease

When resorption runs ahead of formation, bone is lost faster than it can be replaced. In osteoporosis this leads to low bone mass, fragile bone architecture and a greatly increased risk of fracture — a process accelerated by the fall in oestrogen after menopause and by glucocorticoid excess.

Localised, excessive resorption also drives osteolysis — the destruction of bone seen around bone tumours and metastases, and in conditions such as rheumatoid arthritis. Subperiosteal resorption, the erosion of bone beneath its outer membrane, is a classic sign of hyperparathyroidism. In each case, resorption markers help quantify how active the disease is.

The Role of Calcium

Beyond the skeleton itself, bone resorption is central to whole-body calcium balance. Bone is the body’s main calcium reservoir, and when blood calcium falls, hormones including parathyroid hormone stimulate osteoclasts to resorb bone and release calcium into the circulation. This makes bone resorption not only a structural process but a key part of the endocrine system that keeps calcium — essential for nerves, muscle and clotting — within its narrow healthy range.