Cartilage and bone neoformation in rabbit carotid bifurcation aneurysms after endovascular coil embolization

Occurrence and histomorphology of cartilage and bone neoformations was retrospectively evaluated in rabbit experimental aneurysms after endovascular coil embolization. During product development, 115 carotid bifurcation aneurysms were treated with hydrogel-containing devices (HydroCoil or target, n=77; HydroSoft or target, n=28; prototype Hydrogel-only, n=10; MicroVentionTerumo, Aliso Viejo, CA). Additional 29 aneurysms were treated with standard (n=22) or with degradable polymer-covered (n=7) platinum coils. After 4 to 52 weeks, the retrieved aneurysms were methylmethacrylate embedded, and ground sections were surface-stained with Rapid Bone Stain and Giemsa solution. Cartilage and/or bone tissue was assessed by light microscopy; respective tissue areas in the aneurysms were determined by computerized histomorphometry. Cartilage neoformation was observed from 26 to 52 weeks. Single chondrocytes to hyaline or fibrous cartilage areas, occupying up to 29% of the aneurysm cavity, were found in 6 aneurysms, treated with HydroCoil (n=4), Hydrogel-only (n=1), and resorbable polymer (n=1) devices. Chondral ossification associated cartilage neoformation in 2 of these 4 HydroCoil-treated aneurysms. Membranous woven and lamellar bone ossicles were observed from 13 to 52 weeks in 7 aneurysms, treated with HydroCoil (n=3) and platinum coil (n=4) devices. Altogether, cartilage and/or bone neoformation was observed in 13 (9%) of 144 rabbit bifurcation aneurysms treated with various embolic devices. Incidence was low until 26 weeks, but increased at 52 weeks in both, HydroCoil and standard platinum coil treated aneurysms. As the neoformations were predominantly located in proximity to the aneurysm neck, they could be related to the long-term mechanobiology of cell differentiation during fibrovascular healing of blood flow-exposed embolized aneurysms.