A histomorphometric comparison of the muscular tissue reactions to high-density polyethylene in rats and rabbits

In order to bring quantitative elements into the rational selection of animal species for the evaluation of in vivo compatibility of biomaterials, a comparison between the muscular tissue response of rats and rabbits was made using quantitative histomorphometry. The experimental model was based on the quantification of the tissue reaction to specimens made of ultra-high-density-polyethylene (UHDPE) implanted in the paravertebral muscles, according to ASTM F-469 standard. The tissue reaction was evaluated by digitization of both the encapsulating membrane contours and the different cell types located within it. In each animal species, the cell distribution from the interface exhibited an asymetric bell-shape curve with a maximum close to the interface, corresponding to a three-parameter statistical model. The membrane thickness, a fibrocyte-related parameter, was larger in the rabbit, but its time-course was identical in each species, showing a late and significant increase. Few differences were observed for most cell types due to the low level of foreign-body-reaction induced by HDPE implants. Although there were interspecies differences for fibrocytes and, to a lesser extent, macrophages, between 2 and 8 weeks, there was no statistical differences for any of the computed parameters either at 2 weeks or between 26-week rats and 52-week rabbits. The quantitative histomorphometry picked up differences between close tissue responses, whereas the qualitative observation was unable to demonstrate distinguishing differences between rats and rabbits.