I received this question in the comments section of one of my posts:

I have a question. For surface piercings, which kind of barbells are suitable?

My reply: On some types of tissue and on certain areas of the body, it won't matter WHAT style of jewelry you use--the body simply won't accept jewelry in all locations. Here is an excerpt from The Piercing Bible about surface piercings:

The following factors affect the success of surface piercings even more than they do in the traditional spots:

•     Pliability    of    the    skin

•     Placing    sufficient    tissue    between    the    entry    and    exit of the piercing

•     Jewelry    style,    material,    and    size

•     The    consistency    and    appropriateness    of    aftercare

•     Minimizing    trauma    during    healing

Technical aspects aside, some people are predisposed to healing surface piercings. If you are not among the lucky few, then migration, rejection, and significant scarring are all probable consequences. If you can’t live without attempting a surface piercing, be prepared for a lengthy healing period (usually six to nine months or longer) and the possibility you will be wearing a scar instead of jewelry by the end of it all. Different piercers have different methods. I have favored using Tygon, which is a flexible, inert plastic. Some piercers like it and others do not. Some piercers use special "surface bars."

Here is an excerpt from The Piercing Bible about them:
Surface Bars
This modified barbell is designed for piercings on flat areas of the body. It is shaped like an open staple, with a straight bar post between two short legs or uprights. Often the legs  are at right angles to the bar, but for some areas, one or both may have a different angle. The bar post should rest at a uniform depth under the surface with the uprights at 90-degree angles to the tissue. This should reduce pressure, distortion, and irritation during healing. Bars used for Christina piercings have only one upright leg. Discs, gems, or other threaded pieces screw onto the ends of the bar. An accurate fit is crucial; the jewelry must be the perfect length to encompass the tissue between the entry and exit of the piercing, and the barbell ends must not sink into the skin, nor should the legs protrude more than a millimeter beyond the surface.