Bone Remodelling

Bone remodelling is the coupled, site-specific renewal of bone by a temporary structure called a basic multicellular unit. Osteoclasts excavate a resorption cavity over roughly two to three weeks; osteoblasts then refill it with osteoid that subsequently mineralises, taking several months. Because formation lags resorption by an order of magnitude in time, anything raising the number of active units transiently lowers measured bone mass by opening more unfilled cavities. Control runs through RANK ligand, its decoy receptor osteoprotegerin, and sclerostin from osteocytes, which restrains formation and is suppressed by mechanical loading.

Turnover is followed biochemically rather than structurally, using C-telopeptide of type I collagen for resorption and the N-terminal propeptide of type I procollagen for formation. Denosumab, an anti-RANKL antibody approved in 2010, is a pure antiresorptive and suppresses both markers; teriparatide raises formation first, creating an anabolic window before resorption catches up; romosozumab briefly raises formation while lowering resorption.

The coupling explains an important hazard. Stopping denosumab releases a wave of remodelling that had been suppressed, and multiple vertebral fractures within months of discontinuation are a documented rebound effect, which is why transition to another agent is standard.

The common misreading is treating turnover markers as strength. They move within weeks and are highly responsive, which makes them attractive endpoints for products with no fracture data, but a marker reports the rate of a process, not whether the resulting bone bears more load. A supplement or secretagogue promoted on a rise in a formation marker is claiming one half of a cycle whose net effect depends on what resorption did at the same time.