Meshmixer is one of the most helpful software tools to help modify, repair, and prepare 3D geometry for 3D printing. This new Meshmixer 3.0 update from Autodesk Research’s Ryan Schmidt was released January 19, 2016 at SIGGRAPH and adds a major new toolset for designing discrete-region multi-material objects,developed by Autodesk Research’s Tyson Brochu  This is the first-ever tool of its kind, making it much
easier to design for the multi-material FDM & Stratasys Objet Connex printers available today.

Meshmixer is free and currently available for Windows, OS X, and Linux.

Check out all the good things done for health related objects such as prosthetics and orthotics using Meshmixer.
http://meshmixer.com/health.html 

Meshmixer 3.0 quick overview: https://www.youtube.com/watch?v=PCgP8VLFaz4
AU 2015 Meshmixer demo reel: https://www.youtube.com/watch?v=ADdb9SCp9zQ