Achilles tendon rupture: experimental results on spontaneous repair in a sheep-model

We studied the course of spontaneous healing of Achilles tendons in a sheep model after transection and partial resection of the tendon by means of hi stological and biomechanical analyses. In 18 sheep used for another animal study the operated knee had to be excluded from weight-bearing; therefore t he Achilles tendons were transected and for 1.5 cm partially resected in th e middle substance of the tendon. For evaluation these spontaneously healed tendons (n=18) were compared with the contralateral noninvolved tendons (n =18). Specimens were analyzed 3, 6, and 12 months postoperatively by means of histology, polarized light, angiography, and mechanically analyzing the specific tensile strength and absolute tensile strength. We found that in a ll animals the resected tendon healed spontaneously. All animals exhibited a normal gait pattern at least 6 weeks postoperatively. Histologically, the tendinous area of healing demonstrated after 3 months a fibrous collagenou s tissue with a loose fiber orientation. The cross-sectional area had at 3 months increased to maximum but decreased later. Concomitantly the fiber or ientation improved with time and 12 months after transection the specimens showed a nearly normal histological structure of the healed tendon. Biomech anically the specimens exhibited a rather low specific rupture farce after 3 months (28.8% of normal tendons) and 6 months (30.2%) but increased after 12 months (56.7%). In regard to the resulting total rupture force the decr ease in the spontaneously healed tendons was less (75.6% after 3 months, 56 .1% after 6 months, 81.18% after 12 months), because the cross-sectional ar ea of the healing tendon had significantly (P<0.05) increased to maximum af ter 3 and 6 months. Sheep Achilles tendons thus healed spontaneously withou t any immobilization. The initial healing mechanism is thickening of the sc ar tissue with improvement of the fiber orientation towards a tendonlike st ructure within 1 year. Parallel to this, the specific rupture farce increas ed and the thickness of the newly tendonlike area decreased.