Bone-loading response varies with strain magnitude and cycle number

Area of Science:

• Biomechanics
• Bone Physiology
• Mechanobiology

Background:

• Mechanical loading is crucial for bone health, influencing bone size, shape, and strength.
• Key variables in bone stimulation include strain magnitude, strain rate, and frequency.
• Previous research indicated that a low number of loading cycles (36) with high strain (2,000 microepsilon) maximized bone formation.

Purpose of the Study:

• To investigate if the number of loading cycles directly impacts bone formation.
• To determine if the cycle number required for bone formation activation varies with load magnitude at a low frequency.

Main Methods:

• Adult rat tibiae were subjected to four-point bending at specific loads (-800 or -1,000 microepsilon) and cycle numbers (0, 40, 120, or 400) at 2 Hz for 3 weeks.
• Histomorphometry was used to analyze differences in periosteal and endocortical bone formation.
• Loading parameters included constant frequency (2 Hz) and varying strain magnitudes.

Main Results:

• Loading at 40 cycles did not stimulate bone formation.
• At -800 microepsilon, 400 cycles increased periosteal bone formation by 2.8-fold, with no endocortical changes.
• At -1,000 microepsilon, 120 or 400 cycles significantly increased periosteal formation (8-10 fold), formation surface (2-3 fold), and endocortical formation surface (1-fold).

Conclusions:

• The number of loading cycles needed to activate bone formation is dependent on the applied strain magnitude.
• Increasing the number of loading cycles generally enhances the bone's response to mechanical stimuli.
• Bone formation activation requires a strain-dependent number of cycles, with higher strains being more effective at lower cycle counts.