Disposition of collagen fibrils in human tendons

Fixed and unfixed human tendons originating from cadavers and postoperation specimens were examined using inclined parallel beams of light in a reflecting mode. Along the tendon, numerous planes, constantly inclined to the axis, were observed edge-on at the surface and within the interior. Their angle of inclination, with respect to the distal end was very nearly +/- 50 degrees. The planes consisted of individual segments arranged in steps which were on average 190 x 50 microns. Similar configurations were also observed with the scanning electron microscope. Using this technique, the segments were identified with collagen bundles turning at a sharp angle with respect to the axis of the tendon at the level of the inclined plane. Crimped planes were found to be irregularly distributed along the tendons. On longer flatter tendons the average distance between planes was in the range of 1-12 mm. On stretching, the inclined pattern disappeared and was rapidly reestablished in the previously observed position when the strain was released. It is suggested that the observed structure forms a mechanism which is responsible for the appearance of the first part of 'foot' region of the tendon's stress-strain diagram.