In Situ Characterization Reveals an Impaired Fibril Response to Loading Following Unloading during Early Achilles...

Area of Science:

• Biomedical Engineering
• Musculoskeletal Research
• Tissue Mechanics

Background:

• Achilles tendon injuries are common, yet effective rehabilitation guidelines remain elusive.
• Understanding the impact of mechanical loading on tendon healing across multiple scales is critical for improving recovery and preventing re-ruptures.

Purpose of the Study:

• To investigate how mechanical loading (activity vs. immobilization) influences the multiscale mechanical response of healing rat Achilles tendons.
• To elucidate the effects of unloading on collagen fibril structure and nanoscale mechanics during early tendon healing.

Main Methods:

• Combined synchrotron small-angle X-ray scattering (SAXS) mapping with *in situ* tensile stress relaxation.
• Examined early healing (1, 2, and 3 weeks) in rat Achilles tendons under full activity or immobilization conditions.

Main Results:

• Immobilization led to significant alterations in tissue distribution as healing progressed.
• No clear differences were observed at the tissue scale between groups.
• Unloading markedly impaired the nanoscale mechanical response, showing increased spatial variations, reduced fibril recruitment, and decreased elongation capacity after just one week.

Conclusions:

• The nanoscale of healing tendons is highly sensitive to the loading environment, more so than the tissue scale.
• Impaired nanoscale mechanics due to unloading can hinder effective tendon healing and potentially increase re-rupture risk.
• These findings highlight the importance of appropriate loading during rehabilitation for optimal Achilles tendon recovery across all scales.