Freezing causes changes in the meniscus collagen net: a new ultrastructural meniscus disarray scale

Alterations in meniscal permeability leading to nutritional deficit have been suggested as a cause of shrinkage in meniscal transplantation. The purpose of this study was to ascertain how freezing, one of the most common procedures used to preserve meniscal allografts, alters the collagen’s architecture. Twenty-six fresh human external menisci were analyzed with transmission electron microscopy. Thirteen of them were previously frozen at −80°C while the rest were used as controls. A new scale of the collagen meniscal architecture was proposed according to the collagen’s periodicity and degree of disruption, loss of banding, degree of collagen packing, fibril size variability and its intrafibrilar oedema. Each meniscus was scored from 0 to 7. Subsequently they were classified in grades ranging from a normal state (grade I; 0–2 points) to severe disarray (grade III; 5–7 points). The fibril collagen diameters of those menisci which had been previously frozen showed an average size in the longitudinal section of 14.26 nm, whereas it was 17.28 nm in the menisci used as controls (p = 0.019). In the transverse section, the frozen menisci averaged 13.14 and 16.93 nm in the controls (p = 0.003). Samples of the 13 previously frozen menisci were classified as grade III in 61.54% of the cases. In the control group, all the menisci were classified either as grade I or II. The frozen menisci averaged 4.85 points, whereas the control group did so at 2.46 (p < 0.001). The fibril diameters in frozen menisci showed a thinner diameter and had a higher degree of disarray. Therefore, the results suggest that the freezing process alters the menisci’s collagen net. This could partially explain the pathological changes found in shrunken menisci after transplantation.