CRANIOPLASTY TODAY, ISSUE 1

Adopted from Coulter et al, Acta Neurochir (2014) 156:1361–1368.

HA – hydroxyapatite, PE – polyethylene, PEEK – polyethylethylketone, PMMA – polymethylmethacrylate, NR – not recorded

With the growing number of cranioplasties performed every year as a result of an increased use of decompressive craniectomies, experts from leading centers in both Germany and the UK have de- cided to shed light on the true situation regarding techniques, products and patients.This will not be the subject of a randomized clinical trial but rather elucidated using registries.

Clinical registries have increased in popularity over the last ten years as technological advancements in handling information have taken place.A well set-up registry has the potential to provide realistic figures on the outcome of any procedure and how it relates to techniques and patient specific factors.

In Germany, the effort goes under the name of the German Cranial Reconstruction Registry (GCRR) and is organized by a consortium of individual founding members.All centers perform- ing cranioplasty in Germany will be invited to join the registry and report their data to a prospective multicenter database through a questionnaire that covers all aspects of the procedure and follow-up period. Data interpretation and ownership are exclusive to the consortium and the participating cen- ters, and will continuously be presented at the meetings of the German Society of Neurosurgery.

In the UK and Ireland, a similar effort was presented in June in the British Journal of Neurosurgery. The UK Cranial Reconstruction Registry (UKCRR) is currently just waiting to start, with 15 centers having signed up for participation already.The registry founders expect that all neurosurgical centers in the UK eventually will participate.

The initiatives in Germany and in the UK have a great deal in common, first and foremost that their long-term success will increase the quality of available data and also of the care given to patients undergoing cranioplasty.

IN DEPTH

THE EVOLUTION OF CRANIAL IMPLANT MATERIALS - FROM REPAIR TOWARDS REGENERATION

Is it likely that anyone who drills, burrs or with whatever means creates a hole somewhere in the hu- man body also at some point in time gets an urge to plug, pad or fill said hole? If that is the case, the history of cranioplasty goes as far back as the first records of another surgical procedure, the trepana- tion.At a French burial site from the Neolithic era dating back to 6500 BC, 40 of 120 skulls had trepanation holes in them and at roughly the same time, the first archeological evidences of actual cranioplasty appears. It seems as the material choice of the time was a reflection of the patient’s social status – gold plates were used for the nobility, while the common man had to make do with gourds. As for anesthetics, coca leaves were placed on the scalp edges.

Skull Trepanated skull, Neolithic. The perimeter of the hole in the skull is rounded off by ingrowth of new bony tissue, indicating that the patient survived the operation. (Natural History Museum, Lausanne)
The first documented thoughts on cranioplasty bears the sign of Italian anatomy master Gabriele Fallopio (1523-1562), who despite getting his name connected with other bodily regions actually performed the majority of his work on the anatomy of the head. He too suggests gold as a replace- ment material, but only in cases where the dura has been injured, if the dura is intact, bone is the Fallopian material of choice.

Now follows the quest of finding the appropriate piece of bone for the matter; throughout modern history, starting with Job van Meekeren (1611-1666), attempts have been made to transplant bone using dog, ape, goose, rabbit, calf and even eagle as host. Not seldom the transplants hade been modified and even perforated to improve outcome.Another xenograft track deals with the use of horn material instead. And again, the experimental fauna is plentiful with ox, buffalo and ivory pro- ducing satisfactory results according to German pathologist Marchand (1846-1928), perhaps more known for the coining of the term atherosclerosis.

In parallel with the xenogenic attempts much work was also done using cadaveric materials.These were often treated with alcohol and heat sterilized but still failed to produce acceptable rates of infec- tion and bone resorption, and meanwhile, better results had been achieved using autologous bone.

The first use of an autologous bone graft for cranioplasty is credited to German surgeon Philip Franz von Walther (1782-1849) and dates back to 1829.The more widespread use of autologous bone became popular as we progress into the 20th century and is still a method of choice at many centers in the world where bone flaps are preserved either by cryopreservation or in a subcutaneous pocket of the abdominal wall. Despite the good aesthetic outcome and cost-efficiency of using autologous grafts, the issues of high infection rates and problems with bone resorption has led to the develop- ment of synthetic grafts of various origins.

Metals were the first class of materials to be tested and as previously mentioned, the precious ones were initially most popular.They made a comeback after aluminum was discarded due to high infec- tion rates but gold and silver were abandoned in favor of tantalum around World War II.Tantalum however, is a scarce compound, which also has good heat conductive qualities and when applied in cranioplasty had the drawback of heat-generated headaches.

So the search for an improved cranioplasty implant material continued, now amongst acrylic materials and the rise of methyl methacrylate (MMA) pushed the metals back until the metallic alloy titanium entered the scene. MMA and poly-MMA (PMMA) are strong materials but with a susceptibility for fracture and with weak incorporation properties.

A step towards better implant integration was taken in the form of hydroxyapatite. It is a natural mineral and a constituent of bone and is also used in archeology to elucidate what diets were popular among our forefathers.While hydroxyapatite represented a step in the biological direction, the use of titanium and polyetheretherketone (PEEK) could be said to take two steps back as they are both inert materials used mostly for their high tensile strength and ease of processing.

It is not the purpose of this short text to weigh the advantages and drawbacks of contemporary ma- terials such as autologous bone, titanium, PEEK and PMMA against each other, as this has been done by others with much more precision. However, it could be interesting to invite you to a discussion on where the field will be heading in the future. Some authors claim that we are seeing a shift where molecular biology will drive the development of new materials and implants.Will the next genera- tion implants have osteoinductive characteristics and really aid with regeneration of a scull defect, instead of just aiding the repair of it.What’s your opinion?

WHAT’S AHEAD?

May 2-6, 2015 – American Association of Neurological Surgeons Annual Meeting, Washington DC. www.aans.com 

June 7-10, 2015 – German Society of Neurosurgery Annual Meeting, Karlsruhe, Germany. www.dgnc.de/2015