Patellar tendon healing after removal of its central third. A morphologic evaluation in rabbits

The central third of the patellar tendon of the right knee was removed in 40 adult New Zealand White (NZW) rabbits. The animals were divided into two groups of 20 animals each. In group 1 the tendon defect was left open and in group 2 the tendon defect was closed. The peritenon was closed in all cases. The animals were killed after 2 weeks, 1, 3, 6 and 9 months. Operated and contralateral normal tendons of each knee underwent a gross, histologic, immunohistochemical and morphometric analysis. Gross examination revealed that in the closed group there was a greater shortening of the patellar tendon but the difference between the two groups was not statistically significant. The width, thickness and cross-sectional area increased progressively in both groups and were greater in the closed group. Histological examination showed that at 1 month in both groups the harvest site was filled with a loose unoriented fibrovascular tissue. In group 1, large fatty inclusions could be seen in the deep layer, that were still present in the 3-, 6- and 9-month samples. At 3 months in group 1 the newly formed tissue was immature and rich in elastic fibres, as confirmed by immunohistochemical analysis, whereas in group 2 it was more compact and oriented. In group 1 there was in many cases an ossification on the lower pole of the patella, which was not seen in group 2. At 9 months a well-organized fibrous tissue could be seen in both groups at the harvest site. Histomorphometric analysis confirmed the better orientation and the greater density of the newly formed tissue in the closed group as compared to the open group, and the greater number of elastic fibres in the open group, even after 9 months.