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This study compared for the first time the osseous healing, the intraosseous remodeling and the bone remodeling between soft tissue autografts and fresh-frozen allografts with extracortical fixation in a sheep model.
Altogether 54 female Merino-Mix sheep, aged 2-3 years, were included in the study. In these animals the tendons of M. flexor digitalis were removed and inserted during the same operation as cruciate ligament substitute, or they were kept for at least one week at a temperature of -80°C and were then inserted in another animal as allografts. The experimental animals were divided into 2 groups (autologous/allogeneic), each of them with 3 different periods of observation (6, 12 or 52 weeks, resp.). In order to improve the judgement on boneremodeling 3 fluorescence markers (calcein-green, xylenol-orange and tetracycline) were applied additionally at 3 different times. After killing the sheep microscopic sections were stained, using Masson-Goldner-trichrome, safranin-O ad modum Kossa or alcian blue, respectively. The quantitative microscopic examination of the preparations was done partly by the help of a polarization microscope. To compare the results of descriptive statistics the grade of direct ligament insertion was determined by the semiquantitative score according to Unterhauser et al. (2006).
It was shown that extracortical indirect fixation of the soft tissue graft facilitates an anatomical neoinsertion after 52 weeks. In the literature this was unmentioned till date.
Osseous tendon-to-bone-healing as well as intraosseous remodeling of soft tissue grafts from human donors, so-called allografts, require more time than autologous grafts. In the autograft group first signs of the development of a direct ligament insertion at the tibial tunnel entrance were seen after 6 weeks already. Remodeling activity at the anterior tibial tunnel aperture site (tensile site) (grade II) was more pronounced than at the posterior site (grade I). In allografts only a few chondroid cells were seen. Bone-incorporation and ligament remodeling of autografts were more advanced than in allografts. A typical direct ligament insertion was already found in autologous reconstructions after 12 weeks, especially at the tibial tunnel entrance. All maturity levels of a direct ligament insertion were found along the other tunnel sections. Indications of a mature direct ligament insertion could also be observed after 12 weeks, but overall maturity was less in allocompared to autologous animals.
A neoligament inserted with a mature direct ligament insertion at the bone 52 weeks after reconstruction in all auto- and allograft reconstructions. In comparison to the native cruciate ligament no histological differences could be found.
This study focused on the bony incoporation and development of neoinsertion of an ACL graft. More studies are needed to assess the intraarticular healing of auto- and allografts, which ensues parallel to the bony incorporation of the grafts. These results will provide a complete overview of the differences of auto- and allograft in ACL reconstruction and help to design optimal rehabilitation schemes to address the differences and eventually motivate future studies to compensate for possible disadvantages of either type of graft.