One of the techniques for contouring of the skull is augmentative cranioplasty. This is where the skull or forehead is built out through the use of a variety of synthetic materials. While bone would seem to be a logical choice, it is associated with resorption when used as an onlay not to mention the need for a donor site. Synthetic materials offer contour stability, off-the-shelf ease of use and the elimination of a donor site. Even though there is a risk of infection with synthetics, their benefits far outweigh this very uncommon cranioplasty risk.
One of the most common cranioplasty materials is acrylic or PMMA. (poly methylmethacrylate) It has been used successfully for decades and offers a material strength that is similar to bone and is virtually impact-resistant. It is used in orthopedics as a bone cement for joint prostheses but on the skull it does not have ‘cement’ capabilities. This means that the material does not bond to the bone or the overlying soft tissues. Rather it forms a scar capsule around the material, particularly between the material and the underlying bone. This means that there can be small amounts of motion or instability of the cranioplasty material. In revision of pure onlay cranioplasties, it can be very easy to lift the material off of the bone. This material looseness may or may not cause any long-term problems.
Since a PMMA cranioplasty does not bond to the bone, I have always used an anchoring method for the material. The simplest method is to preplace small titanium crews into the outer table of the skull and leave them raised several millimeters. When the cranioplasty material is applied, the initial putty phase of the material wraps around and grabs the screwheads. Once cured, there is a rigid lock of the material to the bone. This prevents any chance of instability or shifting of the material on the skull’s surface.
This screw anchorage of cranioplasties can be used at any location on the skull. It is of particular value on the occipital (back of the head) region where the skull is exposed to the greatest amount of regular stress. (laying on the back of your head)
This cranioplasty fixation method is not as important with the use of other materials such as hydroxyapatite. While these materials are far weaker and minimally impact-resistant, they do have the ability to bond to bone so they have no risk of material movement or instability.
Dr. Barry Eppley