The effect of tension on tendon healing was evaluated in vitro using c
yclic tension and no tension groups of chicken flexor digitorum profun
dus tendons and histologic and immunohistologic techniques. A Vitrodyn
e force-loading machine was used for application of cyclic tension on
partially lacerated chicken flexor tendons in culture media. Laceratio
n sites under cyclic tension after 14 days were covered by newly proli
ferated fibroblasts, aligned in the direction of tension. This new gro
wth was much thicker than that seen in the no tension group at the sam
e time interval. Procollagen synthetic activity began at 3 days of cul
ture in both groups. At 21 days, newly formed fibroblasts in the cycli
c tension group were stained positive more strongly at the surface lay
er than in the deeper layers. In the no tension group, the staining wa
s primarily in the surface layer. Cyclic tension stimulated the intrin
sic response of lacerated flexor tendons significantly more than no te
nsion did by enhancing proliferation and migration of fibroblasts, as
well as stimulating collagen synthesis.