Cranioplasty is done for making contouring changes to the cranial vault, which is defined as the skull, forehead and brow bones. Common causes for the need for cranioplasty are congenital deformities (after primary reconstruction), neurosurgical bone flaps, and traumatic injuries. When done for these reasons an open approach is always used as there is usually a pre-existing scalp scar/incision from a prior procedure. This makes it very easy to apply the traditional synthetic cranioplasty materials such as PMMA (acrylic) and HA. (hydroxyapatite)
However, some skull shape problems may be relatively small or may not be associated with any pre-existing incision for access. When balancing the trade-off of a new scalp scar versus keeping the existing skull concern, many patients (particularly men) would consider the scar as more undesireable. Cranioplasty would be more appealing in this circumstance if the cranioplasty material would be able to be delivered from small and remote incisions. In essence, a cranioplasty material that could be delivered by an injection process.
The current craniplasty materials are far from ideal to be delivered through any form of remote access. PMMA, polymethylmethacrylate, is an initial liquid which can be delivered through a tube but it is very runny on delivery and sets up with a very high heat from an exothermic reaction. The numerous forms of hydroxyapatite (e.g., Mimx) create an initial viscous slurry which has no material flow at all. This makes it not only undeliverable by injection but its sensitivity to fluids and its easy fragmentation on setting make external digital molding unpredictable.
Kryptonite, the newest FDA-approved cranioplasty material offers some real promise as an injectable skull-shaping technique. It is a bone cement that is created through the combination of a calcium carbonate powder and two fatty acid liquid derivatives from castor oil. When combined together this makes a final hardened material that is porous and adhesive with bone-like properties. While this is as favorable to bone as any of the other cranioplasty materials, its physical features in the set-up process make it potentially useful to be delivered by an injection method.
Once the three ingredients are mixed, a very flowable liquid is created. During the intial polymerization process (up to 4 minutes), the material can be loaded into a syringe. Once in the syringe, it remains in a thick but flowable liquid phase up to 8 minutes are mixing. This provides the opportunity for delivery by injection. Once it passes the 8 minute time period, it enters a sticky taffy phase where it becomes very adhesive and is no longer injectable. A moldable phase will exist then up to 25 minutes in which further shaping can be done.
To pass the injectable cranioplasty test, it first must have enough flow to be delivered through a small enough tube that has sufficient length. A true injection method is delivered percutaneously through a needle that is no larger than an 18 gauge. But that is not the type of injection to which I refer when being used for cranioplasty. Because instruments must be used to develop the subperiosteal pocket, either through an endoscopic or blind technique, one or two small remote incisions (less than 1 inch) would be used. Therefore the internal diameter of the injection method can be larger. But how large does it need to be? (or how small can be that it still works) In bench top testing, I have determined that it flows very nicely through an internal diameter of 2.7mms. (8 French catheter) This makes it possible to use long catheters for remote access. Some material will be lost in the tube during delivery which is 1cc per 13cm of length at 2.7mms diameter. Most injectable cranioplasties will not need more than 5 or 6cms of tube length for delivery. The indwelling .5cc of material can be pushed through with a saline fluid bolus behind it.
The other injectable consideration is can the material be effectively molded by external manipulation. Is it able to be pushed around and molded into fine edges without fracture or separation of the material? In testing on a pig’s head (from the butcher), Kryptonite was injected and externally molded to the back of the skull from an anterior incision. On dissection after setting, it was adherent to the bone, did not stick to the overlying soft tissue and had nicely contoured edges. Its sticky taffy phase which is between 5 and 15 minutes after mixing gives it a texture which really molds and shapes well.
Kryptonite bone cement appears to offer physical properties that would make it the first truly injectable cranioplasty material. Its use in this manner is for partial-thickness contour deformities of the skull and forehead.
Dr. Barry Eppley