Determination of the 3-D morphology of degradable biopolymer implants undergoing in vivo resorption.

A technique is presented that allows the visualization of the degradation process of biopolymers. Serial thick-section specimens containing cross-sections of polydioxanone implants (PDAs) were realigned for 3-D reconstruction using a double-embedding technique. The outlines of the cross-sectioned implants were traced using an automatic image analysis system and converted into x, y, and z coordinates. The reconstruction of the implant body was achieved by the stepwise analysis of vertical relationships between two adjacent section levels. The surface was approximated by triangulation and calculated from the surface triangles. Visualization was achieved by introducing the data into commercially available software using z-buffering and Gouraud shading. The geometric precision of the reembedding technique was found to be 0.8-1.5% of the screen width; recovering experiments showed a good approximation to the actual physical volume of unresorbed implants (+/- 2.6%). The error of volume determination due to the limited resolution in the z direction was calculated to be acceptable (< 5.0%) in isotropic objects where the ratio between the radius of the surface curvature and the distance between the sections is > 4. Results indicate that large devices of PDAs are degraded in vivo in a fashion similar to that previously described for in vitro degradation of PLA molded screws by a surface/center differentiation with formation of hollow residuals after 17 weeks and complete degradation with phagocytosis of microparticles after 26 weeks.