Regeneration of the semitendinosus tendon after its use in anterior cruciate ligament reconstruction: a histologic study of three cases.

We describe the histologic course of regenerated tissue simulating the semitendinosus tendon in three patients after anterior cruciate ligament reconstruction with doubled semitendinosus and gracilis tendons. Specimens were retrieved during revision surgery for removal of tibial hardware at 6, 24, and 27 months, respectively. Resisted flexion during physical examination in all patients revealed a "string" on the posteromedial aspect of the knee that was quite evident, suggesting the presence of a regenerated semitendinosus tendon. At surgery, a well-defined fibrous band was identified about 3 cm proximal and medial to the pes anserinus, reproducing the pathway of the native semitendinosus tendon. In the specimen retrieved 6 months postoperatively, a prominent fibroblastic proliferation was observed together with a few vessels surrounded by fibrous tissue; only a few bundles of well-oriented collagen fibers were present, together with scattered rows of spindle-shaped cells. In both of the specimens retrieved 2 years postoperatively, all of the central, thicker portion of each specimen was occupied by well-oriented tendon-like fibers together with uniformly distributed spindle-shaped cells that had the appearance of mature tenocytes arranged in orderly rows. These findings help us to understand the progression of the healing process at the harvest site and the nature of the regenerated semitendinosus tendon, which has previously been investigated only by studies of clinical and functional outcomes and by imaging studies.