THE MORPHOLOGY OF LIGAMENT INSERTIONS AFTER FAILURE AT LOW STRAIN VELOCITY - AN EVALUATION OF LIGAMENT ENTHESES IN THE RABBIT KNEE

The morphology of failed rabbit knee entheses is described after in vi tro load to failure testing at low strain velocity. Avulsion fracture was the dominant failure mode both for the cruciate ligaments and the medial collateral ligament. The patellar ligament became avulsed in mo st cases from the patellar insertion. The ligamentous anterior attachm ent of the medial meniscus failed by a midsubstance rupture and the po sterior fibrocartilaginous attachment by a rupture near to the menisca l horn. On histological inspection the failure characteristics usually appeared more elaborate, involving to different degrees all portions of the bone-ligament-bone complexes. Avulsion fracture through subchon dral bone was often combined with a partial ligament midsubstance rupt ure. In few cases avulsion from the cement Line was combined with a ru pture between the uncalcified fibrocartilage and the ligament. The med ial collateral ligament failed in one case entirely at the cement line of its femoral insertion. Horizontal rupture through the calcified fi brocartilage and a vertical cleavage crossing the tidemark were also o bserved. The superficial portion of the patellar ligament failed with a midsubstance rupture, and the deeper part with an avulsion through t he calcified fibrocartilage or an avulsion fracture. The complex failu re characteristics may be attributed to uneven loading, nonuniformity of the structure, and specific anatomical location. Subchondral bone b eneath femoral and tibial insertions seems to be weaker than the trans itional zone between soft tissue and hard bone at the enthesis. The ov erall inferior structural quality of a ruptured ligament has to be tak en into account when parts of the original structure are used for sutu ring or reconstruction.