Ultrastructural modifications of patellar tendon fibres used as anterior cruciate ligament (ACL) replacement.

Using transmission electron microscopy, patellar tendons that had been autografted to replace the ACL were studied. Follow-up biopsies, performed at 6, 12 and 24 months after reconstructive surgery demonstrated that the autograft patellar tendons had undergone considerable change. After 6 months, the large collagen fibrils were less numerous and more widely spaced. The spaces were occupied by dense colonies of very small, newly-formed collagen fibrils and were rich in glycosaminoglycans. Small elastic fibres, fibrocytes, round cells with abundant rough endoplasmic reticulum and macrophage-like cells were scattered among the collagen fibrils. At 12 months, sporadic clumps of large collagen fibrils were seen between the compact bundles of small, uniform collagen fibrils. Both elastic and oxytalan fibres were observed; round cells with abundant rough endoplasmic reticulum were the most prevalent cell type. At 24 months, the tissue had the appearance of a normal ligament. The small uniform collagen fibrils had formed large, compact bundles that were prevalently arrayed parallel to the long axis. Both elastic and oxytalan fibres were observed and the numerous cells present exhibited the same morphological features as those observed at 12 months after surgery. The appearance of compact collagen bundles, the abundant elastic system and the abundance of active cells observed in the patellar tendon autograft 24 months after implantation, suggest that this type of tissue is a valid functional ACL substitute of a knee required to perform normal mechanical activity.