Background: Unlike skull reconstruction elective aesthetic skull augmentation has a much more limited history. Due to an historic low demand and limited options for performing it building up areas of the skull has relied on the use of bone cement, mainly PMMA. (acrylic) By history such bone cements were initially made for reconstruction of skull defects which have well defined borders. Like concrete it is a separate liquid and powder which is mixed intraoperatively to create a putty which hardens in less than 12 minutes. Thus it is easy to see how this putty application into a skull defect can be easily applied, contoured and allowed to harden.
When PMMA bone cements are used as an onlay skull augmentation material, however, it has some very different application issues which impact its aesthetic effects. The most obvious plication issue is incisional access. In skull reconstructions there is always a larger scalp incision so wide open exposure exists. But the use of such large scalp incisions is rarely permitted by the patient in aesthetic skull contouring. As a result limited incision applications techniques have been developed which basically consists of pushing through the initial putty mixture into a created subperiosteal scalp pocket and then blidndluy molding it externally as it sets.
While this aesthetic bone cement skull augmentation technique can be effective in some cases, it has multiple limitations. The most significant one is also the least obvious….an initial soft putty mixture has limited to no ability to push the tight scalp outward. The tight scalp pushes back and flattens the soft material which is magnified by externally molding it by hand. Thus the thickness of the augmentation and extent of surface area coverage is going to be limited.
Another PMMA bone cement limitation is getting an exact shape of the desired skull contour. Other than some gentle limited rounding there is no way to get a very specific controlled skull augmentation even if one has wide open access…as the following case will illustrate.
Case Study: This adult male with a scaphocephalic head shape underwent an open transcoronal skull augmentation procedure well over a decade ago. In an effort to improve the backward slope of the skull the PMMA bone cement can be seen to have had a limited effect. (green color)
As a replacement for the bone cement a custom skull implant was designed with a shape that better corrected the backward slope of the skull as well as provided some needed width with that sagittal contouring effect.
With an increased projection of 16mms and a volume of 200ccs (the bone cement was 90ccs) the improvement in the skull shape could be appreciated by comparing the two in profile.
Through his existing coronal scalp incision the bone cement was removed by fracturing it in pieces. It does not actually bond to the bone as it name applies, it still forms a capsule between it and the bone since it is an implant material so it separates fairly easily. It needs to be fractured because of its convex shape over the bone to remove it.
After the capsule was removed from the bone the pocket was expanded aneteriorly, the new custom skull implant was placed with screw fixation and the scalp closed over it with two drains.
When seen the following day for head dressing and drain removal the change in his head shape could be fully seen…as would be expected based on the implant design.
Bne cement, unlike its name, does not bond to the bone at least in PMMA composition. The body simply views it as another type of implant and encapsulates it. Given its numerous reshaping limitations and the more contemporary and versatile aesthetic skull augmentation technique of custom skull implant designing improved skull reshaping results are now possible.
Key Points:
1) PMMA bone cements provide inadequate skull augmentations when significant onlay volume additions are needed.
2) A custom skull implant provides more augmentation volume in a controlled predetermined shape
3) The prior bone cement provided some scalp expansion which permit more implant volume to be added secondarily.
Dr. Barry Eppley
World-Renowned Plastic Surgeon