Segmental variations of in vitro mechanical properties in equine superficial digital flexor tendons.

OBJECTIVE

To determine the local variations of mechanical properties of the equine superficial digital flexor tendon (SDFT).

SAMPLE POPULATION

10 SDFT of adult horses, selected for absence of abnormality.

PROCEDURE

Needles (with a dark marker at both ends) pinned perpendicularly through each tendon to delimit 7 segments. Each tendon was tested in traction until rupture; test was filmed, using an 8-mm video camera. For each image, the coordinates of the center of mass of each marker and the corresponding force were registered. The third-degree polynomial that best fits the stress-strain curve thus obtained was calculated by a least squares approximation. The modulus of elasticity (Emax) of each segment was evaluated as the maximum of the derivative of this polynomial.

RESULTS

Mean rupture load of the 10 SDFT was 12,356 +/- 1,333 N. The strain at tendon rupture and Emax varied, respectively, from 8.1 +/- 1.3% and 1,002 +/- 161 MPa (sesamoidean region) to 12.5 +/- 1.7% and 1,189 +/- 63 MPa (metacarpal region). The values of the strain corresponding to Emax were remarkably similar along the SDFT (approx 5%).

CONCLUSIONS

Emax appeared early homogeneous along an SDFT, although being slightly higher in the metacarpal region. The 5% strain corresponding to Emax could be the limit strain beyond which microlesions of the tendinous fibers begin to appear, this threshold being first reached during the traction test by the metacarpal segments.