Mechanical correlations derived from segmental histologic study of the equine superficial digital flexor tendon, from foal to adult.

This study explores changes in the microscopic structure of the superficial digital flexor tendon (SDFT) in horses as they age and engage in physical activity. The study found that tendon fibers become less ‘wavy’ and cell count decreases with age, potentially influencing the tendon’s strength and flexibility. Other observed changes could be associated with specific stress on the tendon during physical activity.

Study Design and Methods

• The study examined SDFT samples taken from 42 horses ranging in age from 0.5 hours to 23 years.
• Each tendon was divided into 7 parts (or segments) and examined under a microscope. The researchers recorded number of cells, wave-like pattern of the fibers (referred to as “fiber undulation”), size and frequency of connective spaces interspaced between the tendon fibers, presence of abnormal cartilage-like cells, and differences in the dorsal (back) and palmar (front) borders of the tendon.

Results and Analysis

• The wave-like pattern and cellularity significantly decreased with age. Less wavy fibers and fewer cells were specifically observed in the mid and upper portion of the tendon (metacarpal segments), mainly in older horses.
• The study found that abnormal cartilage-like cells appeared in specific regions of the tendon (sesamoidean segments) in horses older than 5 years. This is an indication of a mechanism to offset compression stress on the tendon.
• There was a tendency for the dorsal border of the tendon in the metacarpodigital region to change into fibrocartilage as the horse aged. This is contrasted with the front section (palmar border), which typically consisted of dense, non-fascicular connective tissue.
• Activity level appeared to be associated with smaller and fewer connective spaces within the tendon.

Conclusions

• The conclusion from this research is that changes in the wavy structure of tendon fibers, specifically in the mid and upper portions, potentially contribute to a relative weakness within the SDFT. This is reflected in the ‘stress-strain curve’, a measure of the tendon’s response to load and stretching.
• The appearance of cartilage-like cells and conversion of tendon tissue into fibrocartilage on the dorsal segment are considered normal adaptive responses to the compressive forces experienced by the tendon due to activity.