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Background: Aesthetic skull augmentation have been done by two types of implantable materials, bone cements vs performed custom implants. These two skull augmentation materials have very different handling properties which not only affects their intraoperative placement but leads to major differences in what either one can aesthetically achieve. Failure to understand these differences leads many patients and surgeons as well to conclude that while the materials are different their aesthetic outcomes are similar. Nothing could be further from what actually happens.

Bone cement has a long history in skull surgery as a bone flap replacement in neurosurgery going all the way back to the 1950s. The original and present bone cements are PMMA, a plastic composite that is used in orthopedic surgery for cementing in joint replacements, hence the cement name. Because it is a composite powder and liquid material that is mixed together intraoperatively and an initial putty is created it can readily be applied to a partial or full thickness skull inlay defect where it can be placed into it, shaped to  a smooth outer surface to the surrounding bone and then allowed to harden. The intent of its use is different between the skull and joint replacements but it is still a thermoplastic acrylic material.

While PMMA bone cements can be used for aesthetic skull augmentation it was never meant to be used this way and its material properties are not ideally suited for it. Its moldable properties, which seem appealing, are exactly what makes it ill-suited for skull augmentations. Onlay skull augmentations, unless they are performed through a wide open bicoronal scalp incision approach, requires a strong push on the tight overlying scalp tissues to create their augmentation effect. A moldable putty material placed under a tight scalp cover is going to be displaced/spread out by the pressure of the overlying scalp. Worsening that displacement effect is the surgical technique for their placement through a small scalp incision is to push the scalp down around it to mild it and smooth it out…further decreasing any significant augmentation effect.

HA or hydroxyapatite cements, while offering improved biologic effects over an acrylic material, have the same moldable features and limited skull augmentation effects as PMMA. And they offer much lower volumes of material at a much higher cost than PMMA.

When using a small incision approach in aesthetic skull augmentations, which is an important element of any aesthetic implant procedure in the face or body, what is placed underneath must not be displaced or compressed by the scalp. Besides the performed design this is the major reason that custom skull implants provide far more significant aesthetic effects than any type of bone cements.  

Case Study: This male had congenital plagiocephaly on the right back of his head. He had a prior skull implant augmentation procedure with HS bone cement which provided no appreciable improvement. His 3D CT scan show the cement which had a similar appearance to bone due to calcium phosphate composition. It is identified on the scan due to its lumpy appearance.

A custom skull implant of 150cc volume was designed to more ideally correct the right plagiocephaly and give it a better match to the normal left side of the back of his head.

Under general anesthesia and through a small low horizontal scalp incision the scalp was elevated initially exposing the white HA bone cement. The custom skull implant was passed through the incision and positioned as per the implant design. The implant fit was so tight that no screw fixation was needed.

Besides improved material characteristics to more effectively push out the scalp, custom silicone skull implants provide two other very important features that are important in aesthetic head augmentations. The first is the amount of volume it provides. As well illustrated in this case the custom implant provided 150ccs of volume while the bone cement was less than 15ccs in volume, a ten fold difference. Secondly a custom implant can augment a skull surface that is not bone (temples which are muscle) while bone cements can not. This is important in plagiocephaly where part of the skull deficiency extends and wraps around the back of the head onto the recessed temporal muscle area on the side of the head. 

Key Points:

1) Hydroxyapatite cement works poorly for many aesthetic onlay skull augmentations due to the inability of the material to push the scalp outward as it sets.

2) A custom implant provides a better skull augmentation effect because it is a non-compressible material. 

3) In this case of skull augmentation for plagiocephaly a custom skull implant provided a 150cc vs less than 15cc bone cement displacement effect. 

Dr. Barry Eppley

World-Renowned Plastic Surgeon

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