Mitral repair with the Evalve MitraClip device: histopathologic findings in the porcine model

Percutaneous mitral repair with the MitraClip device ("clip") is currently being evaluated in a Phase II clinical trial (EVEREST II). This device was evaluated in an animal model prior to use in humans.

Twenty-one excised clips with accompanying leaflet tissue from pigs were examined at 4, 12, 17, 24, and 52 weeks. Sixteen specimens were available for hematoxylin and eosin and Movat pentachrome staining, and five were sent for scanning electron microscopy. The devices were examined grossly for tissue growth on flow and nonflow surfaces, thrombus, and vegetations. Microscopic evaluation focused on the presence of tissue growth around the device, the inflammatory response, and the presence of thrombus, infective endocarditis, and hematoma.

Tissue growth on both flow and nonflow surfaces was seen in all specimens with variation of tissue thickness proportional to the duration of device implantation. Evidence of endothelialization, fibrous encapsulation, and organization of tissue between the aortic and mitral leaflets was observed. Adjacent chordae tendinae were incorporated into the healing tissue growth around the device as early as 4 weeks, in 33% of clips implanted for that time period, increasing to 67% of clips at 12 weeks, and 100% of clips at 17, 24, and 52 weeks. Two animals were diagnosed with infective endocarditis during life.

Mechanical coaptation of the mitral leaflets in an animal model demonstrates adequate tissue response and healing with complete encapsulation of the device by 12 weeks and ongoing healing response proportional to duration of implantation. Infective endocarditis remains a potential complication in the animal model and for all implanted prosthetic devices.