In vitro fatigue behavior of the equine third metacarpus: remodeling and microcrack damage analysis.

The research aims to understand the effects of mechanical loading on microcrack damage and remodeling in a particular bone in thoroughbred racehorses. The study also analyzes the regional variations in the remodeling of the bone and its correlation with mechanical properties, as well as how remodeling and microcrack damage are influenced by age.

Methodology and Experiment

• The researchers conducted their study on specimens from the third metacarpal bone (also known as the equine cannon bone) of Thoroughbred racehorses.
• The bones were subjected to monotonic or fatigue failure, reflecting the stress they would typically undergo during horse racing events.
• They created beams from various areas of the bone and loaded them to fracture either monotonically, or cyclically by an initially set load.
• They then used basic fuchsin to stain the specimens and prepared cross sections from loaded and load-free regions.
• Using histomorphometry, they determined the current and past remodeling, porosity, and microcrack density and length.

Observations and Findings

• The research showed that strained and unstained microcracks could be observed. Unstained microcracks were often associated with regions of woven bone and appeared to be damaged Sharpey’s fibers.
• The length of the unstained cracks increased after failure, particularly near the surfaces of the beam. However, the density of these cracks did not increase.
• Stained microcracks were wider and longer than unstained ones and were primarily located near the fracture surface, and on the side of the beam that faced compression.
• Stained cracks increased in number after failure when the beams had higher elastic modulus, shorter fatigue life or higher deformation at failure.

Impact of Age and Regional Variation on Bone Remodeling

• The research highlighted that the extent of prior bone remodeling increased with the age of the racehorse, especially in the medial region of the bone.
• The rate of ongoing remodeling tended to decrease with age, though not in the dorsal region, which was found to have the best resistance to fatigue.
• However, it was noted that the impact of remodeling on the bone structure did not greatly influence microdamage. This indicates that the changes due to aging and regional variations, though visible, do not significantly affect the microdamage in the bone.

Staining Observations and Conclusions

• The staining of damage in fractured equine bone with basic fuchsin was found to be independent of both age and regional factors.
• This staining was confined to areas near the fracture surfaces. The microdamage observed appeared to consist only of subtle disruptions of Sharpey’s fibers, suggesting the absence of any major structural anomalies.