Intracortical remodelling increases in highly-loaded bone after exercise cessation

Abstract

Resorption within cortices of long bones removes excess mass and damaged tissue, and increases during periods of reduced mechanical loading. Returning to high-intensity exercise may place bones at risk of failure due to increased porosity caused by bone resorption. We used microradiographs of bone slices from highly-loaded (metacarpal, tibia, humerus) and minimally-loaded (rib) bones from 12 racehorses, 6 in high-intensity exercise and 6 that had intense exercise followed by at least 35 days of rest, and measured intracortical canal cross-sectional area (Ca.Ar) and number (N.Ca) to infer remodelling activity across sites and exercise groups. Large canals (Ca.Ar > 0.04mm2) were 1.4× to 18.7× greater in number and area in the third metacarpal bone from rested than exercised animals (p= 0.005–0.008), but were similar in number and area in ribs from rested and exercised animals (p= 0.575–0.688). An intermediate relationship was present in tibia and humerus, and when large canals and smaller canals (Ca.Ar > 0.002 mm2) were considered together. The mechanostat may override targeted remodelling during periods of high mechanical load by enhancing bone formation, reducing resorption and suppressing turnover. Both systems may work synergistically in rest periods to remove excess and damaged tissue.