MECHANICAL CORRELATIONS DERIVED FROM SEGMENTAL HISTOLOGIC-STUDY OF THE EQUINE SUPERFICIAL DIGITAL FLEXOR TENDON, FROM FOAL TO ADULT

Objective-To assess histologic Variations of the equine superficial di gital flexor tendon (SDFT) according to site and to horse age and acti vity, and to correlate these data with reported segmental mechanical r esults. Sample Population-Superficial digital flexor tendons isolated from 42 horses 0.5 hour to 23 years old. Procedure-7 segments of each SDFT were delimited and submitted for conventional histologic examinat ion. Each segment was examined and graded for fiber undulation, cellul arity, number and size of interfascicular connective spaces (ICS), pre sence or absence of focal and diffuse chondroid metaplasia, and differ entiation of the dorsal (DB) and palmar (PB) borders of the tendon. Re sults-Fiber undulation and cellularity significantly decreased with ag e. The proximal and middle metacarpal segment fibers were significantl y less undulated and their ICS were smaller than those of the other se gments, especially in old horses. Focal chondroid metaplasia developed from 5 years onward, mainly in the sesamoidean segments. Diffuse chon droid metaplasia was characteristic of the digital region in horses > 6 years old. The DB of the metacarpodigital region tended to different iate into fibrocartilage in association with age. The PB was generally differentiated as nonfascicular dense connective tissue. Activity ind uced a decrease in the number and size of the ICS. Conclusions-The les ser undulation of the proximal and middle metacarpal segments fibers c an be correlated to their mechanical behavior (stress-strain curve) an d relative weakness within the SDFT. Focal chondroid metaplasia and fi brocartilage on the DB are normal features, related to the compression stresses undergone by the sesamoidean region of the tendon.