I would assume that titanium has a higher density than bone and rib bones aren't that large to begin with. Making them larger and thereby heavier would probably cause a few problems.
For example, there would have to be perforations for extracting the raw material from inside the hollow interior (probably titanium powder in this case) which means that you now have a hard to post process implant (titanium isn't exactly bioinert) with an exponentially greater surface area for possible autoimmune reactions or infections and an interior that is probably damn near impossible to sterilize (since you'd probably honeycomb the walls).