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Revista Española de Cirugía Oral y Maxilofacial

versión On-line ISSN 2173-9161versión impresa ISSN 1130-0558

Rev Esp Cirug Oral y Maxilofac vol.29 no.2 Barcelona mar./abr. 2007

 

ARTÍCULO ESPECIAL

 

Cranial reconstruction with biomaterials. Historical revision and current state

Reconstrucción craneal con biomateriales. Revisión histórica y estado actual

 

 

I. Zubillaga Rodríguez1, G. Sánchez Aniceto1, J.J. Montalvo Moreno2

1 Médico Adjunto.
2 Jefe de Servicio.
Servicio de Cirugía Oral y Maxilofacial. Hospital Universitario 12 de Octubre, Madrid, España

Correspondence

 

 


ABSTRACT

Craniofacial defect reconstruction is still a challenging procedure. A cranioplasty is the treatment of a cranial defect or a deformity.
Cranial injuries and neurosurgical procedures provide an important group of cases that allow the development of new materials and fascinating progress.
The aim of this article is a historic review of the different cranial reconstructive techniques used, describing advantages and drawbacks. The biomaterials applied in recent cases are presented.

Key words: Cranioplasty; Biomaterials.


RESUMEN

La reconstrucción de defectos craneofaciales es todavía hoy día un proceso desafiante. Se define craneoplastia como la reparación de un defecto o deformidad craneal.
La persistencia de traumatismos craneofaciales y procedimientos neuroquirúrgicos aporta un número importante de casos que permiten un fascinante progreso y desarrollo de nuevos materiales.
El objetivo de este artículo es la revisión histórica de los distintos métodos reconstructivos craneales empleados, comentando sus ventajas e inconvenientes según el tamaño y forma del defecto a reparar. Se presentan casos clínicos recientes reconstruidos con diversos biomateriales.

Palabras clave: Craneoplastia; Biomateriales.


 

Introduction

The reconstruction of craniofacial defects is a challenging process even today. Different techniques and materials have been used for restructuring the skull. The choice of material to be implanted has always been particularly controversial. This depends on the size of the defect and the conditions of the receptor area.

Cranioplasty has been defined as the repair of a defect or cranial deformity. The different techniques for cranial reconstruction or cranioplasties undoubtedly mix the concepts of art and surgical science. The different methods proposed for cranial reconstruction, from prehistoric times until modern medicine, have tried to solve cranial defects of multiple etiologies. The persistence of craniofacial trauma and neurosurgical procedures provides a considerable number of cases that allows fascinating progress and the development of new materials.

 

Historical revision

Skull trephines, the oldest surgical procedure known, were practiced by many ancient civilizations. In Peru there is ample evidence that the pre-Inca "surgeons" carried out this treatment 3000 years before Christ (Fig. 1). Trephined Inca skulls have been found along side gold and silver shells and plates. There is also evidence of skulls with these materials placed in situ covering the cranial defect. The most notable example is a skull dated 2000 B.C. found in Cerro Colorado (Paracas region, Perú).1 It had a left frontal defect that was covered with a 1 mm thick plate of gold. It has been mentioned that in these civilizations reconstruction with precious metals was reserved for the highest social class, while the use of weaker or more common elements was used for the lower social class.

In Celtic Neolithic culture, rounded or oval skull fragments have been found presumably from trephined skulls. On occasions they had a central orifice in the shape of a "ring", which had certain magical properties. A skull found in Crichel Down (United Kingdom,1938) had a large ring that had been placed in the original trephined cavity. The skull margins showed no signs of healing, to the extent that the cranioplasty could have been carried out postmortem. It shows, nevertheless, that the cranioplasty concept was not unusual in ancient European cultures.

Authors in ancient Asia, Egypt, Greece and Rome had very few innovations with regard to previous cultures. It was from the 16th century onwards that new "ideas" with regard to cranial reconstruction were raised. Fallopius proposed that trephined bone could be replaced after the initial procedure, if the duramater had not been broken. If there was a dural tear, the bone was eliminated and a gold plate was used. This is perhaps the first description made of a true cranioplasty. Some of his contemporaries such as Paré could not believe what had been carried out.

The first successful cranioplasty using a bone graft was recorded by Job Janszoon van Meekeren, a Dutch surgeon who treated a Russian noble who had lost part of his skull after being struck with a sword. The reconstruction was carried out with a fragment of bone from a dead dog that was adapted to the defect. When the competent authorities at the time heard of this, the patient was banished from the country but no measures were taken against the surgeon.1

At the start of the 19th century, the natives of the southern seas started to use coconut shell as cranioplasty material, and recently in the 20th century buffalo horn was used as reconstruction material.

A great variety of materials have been used throughout history for the reconstruction of cranial defects. As from the last century attempts have been made to find the ideal bone substitute and some characteristics have been described such as resistance, malleability, heat non-conductive, sterilizable, inert, radiolucid, non-magnetic, disposable and cheap.

Without any doubt, metallic bone substitutes are a great advancement with regard to the biomaterials available for cranial repair. Aluminum was the first metal in recent history to be used. Initially described by Booth and Curtis (1893),2 considerable tissue reaction occurred. It was epileptogenic and slow disintegration took place, to the extent that its use did not become generalized.

Precious metals such as gold and silver were also used at the end of the 19th century. Gerster3 and Sebileau,4 respectively described its use as a cranial substitute. Among the disadvantages were the high costs, weakness of the metal in its pure form and, with regard to silver, staining of adjacent tissue.

The use of lead (Mauclaire, 1908)5 resulted in some cases in systemic intoxication by this metal, which required plate removal, and its popularity decreased significantly. Other metals that were used were platinum, vitallium, ticonium, chromium, molybdenum, all characterized by a lack of malleability.

Tantalum was first described in 1802. It was proposed as a cranioplasty agent in 1942, after experiments in dogs showed no tissular reaction. It is radiopaque and the postoperative imaging tests were difficult to evaluate. Its use was substituted by acrylic components and by titanium.

At the end of World War II, Boldrey6 introduced steel mesh, which was recommended initially for small defects given its limited resistance in trauma. It shared many of the properties of tantalum and its great advantage was its low cost.

Current state

Titanium is an element that was first discovered in 1796, but it was not commercially available until 1946, after the reduction process of titanium tetrachloride. In 1965 Simpson7 was the first to use it in cranial reconstruction. Compared with tantalum, it is much more radiolucid and considerably less expensive. Other advantages are its biocompatibility and high mechanical resistance. It is inert, non-cancerous and non-allergic.

The reconstruction of cranial defects with titanium is carried out with mesh. It was originally developed during the Vietnam war. With the passage of time, refinements have been introduced such as a reduction in thickness, and increased resistance and malleability. It had previously been used for mandibular reconstruction and for atrophic maxillas. The good results obtained led to increased usage.

Its use is indicated in areas that cannot support loads and it is used in the following situations:8

1. Immediate reconstruction during primary treatment of comminuted cranial fractures with bone loss defects of up to 25 cm2. In these situations the miniplates do not generally permit the fixation of all bone fragments and, therefore, the best aesthetic results are not obtained.

2. Treatment of irregularities around the skull (Figs. 2 to 5)

Among the advantages of these reconstructive techniques, the high biocompatibility and applicability stand out (even when in direct contact with the paranasal sinuses), together with easy handling, possibility of combining with bone or other biomaterials, and minimum disturbance in postoperative imaging tests. The tridimensional reconstructions in complex anatomic structures are stable, there is no morbidity in the donor area, and there is low susceptibility to infection.

Janecka9 presented a wide experience in the use of titanium mesh in skull base surgery. The rate of complications was 5%, although this is partly due to the quality of the soft tissue.

As has been mentioned, a combination of titanium mesh with other biomaterials such as hydroxyapatite is possible (Fig. 6 to 9). The mesh provides structural support in the reconstruction, and the osteoconductive effect from the hydroxyapatite permits progressive bone growth in the cranial defect.10 A more precise tridimensional adjustment is in this way created, and stability and resistance to stress increases. And, the reconstruction of moderateextensive cranial defects that are even larger than 25 cm2 is possible.

Some authors11 defend the use of a titanium or resorbable mesh between the duramater and the hydroxyapatite cement in order to avoid any interference from the pulsations of the brain and the dura with the setting of the cement. Without the mesh, there would be small microfractures in the hydroxyapatite that would start local inflammation and infection, together with the posterior extrusion and failure of the implant. The mesh-hydroxyapatite combination does not have a negative impact on the biocompatibility, osteoconductivity and restructuring capacity of the cement.

The use of metals for cranioplasties and the previous difficulties in obtaining radiographies that were easily interpreted, was a great handicap, and it was one of the main reasons behind the attempts by many surgeons to discover and use new bone substitutes that were not metallic.

The development of our specialty and of cranial reconstruction in particular, is closely related with wars on a worldwide scale. During World War II, a great number of cranial defects were produced. Although tantalum was one of the materials of choice in this era, there was growing interest in acrylic resins. Just after the end of World War II these resins were used as material for dental prostheses with good results. It was then suggested that they could play an important role in cranial reconstruction. One of the initial advantages was a greater radiolucency.

Methyl methacrylate was then discovered in 1939. Among its qualities was a great resistance to stress and heat. It is an inert metal and it was first used in an experimental way as cranioplasty material in a rat in 1940. Zander1 was the first to use it in humans in October of the same year. As it has evolved, preformed and sterilized plates have been developed which have facilitated its use.

It is currently one of the biomaterials most used in cranial reconstruction, perhaps because of its high resistance to external stress, its bioavailability and low cost. It has excellent biocompatibility with adjacent tissues. However, strange body reactions have been described after its use, with the resulting infection in the surgical area of the implantation. 12 At an intraoperative level, polymerization of polymethylmethcrylate gives rise to a significant exothermic reaction and temperatures of up to 80º have been reached. There is thermal reaction that can damage adjacent tissues, but this can be avoided by using the previously mentioned preformed and molded plates (computerized tridimensional design). It is an inert material (absence of bone substitution) and there is no real integration with the receptor bone. There is a longterm bone-implant interphase. Some studies show an improvement in osseointegration after submerging the implant in polylactic-glycolic acid gel.13

The rate of global infection is around 5% according to different studies. However Manson’s14 series of 42 cranioplasties with methyl methacrylate was completely successful and there were no infections. Patients with simultaneous cranial, orbital or nasal reconstruction had an infection rate of 23%. The patients that developed implant infection had experienced previous infection of the area where the methyl methacrylate had been placed. This is the material of choice, according to Manson, for those adult patients with good quality soft tissues and with no previous history of local infection.

HTR (Hard Tissue Replacement) is a polymer of polymethyl methacrylate-polyhydroxymethyl methacrylate. It is highly resistant to the forces of compression and it is biocompatible. It has micropores in its structure of 250-300 microns, which theoretically allow the initial invasion of fibrovascular tissue and posterior bone substitution. It has a layer of hydrophile on its surface with negative charges that avoids bacterial adhesion and therefore reduces the risk of infection.

A high resolution tridimensional CT scan (with slices every mm) is carried out to define the defect in a precise fashion, in order to permit the design of the implant. Software that is compatible with that of the company manufacturing the product is required, and until the implant is received there will be a 6- 8 week waiting period. It is fixed to the skull with titanium or resorbable plates.15 Cranial defects of more than 100 cm2 can be reconstructed (Figs. 10 to 13).

Many biomaterials have been used over time for reconstructing the skull. Looking for the ideal biomaterial has been the object of numerous clinical studies. It would appear logical that progressive bone substitution of the implant is an indispensable condition for procuring suitable biomaterial.

Over the last decades hydroxyapatite has been introduced as beneficial material in different cranioplasties. It is the primary mineral component of bone, and it is composed of interconnected calcium phosphate molecules that form a hexagonal structure.

It was initially available (in the middle of the 20th century) in a ceramic form that had been created after the calcium phosphate preparations had been subjected to high temperatures. They were of limited porosity and their great disadvantage was that they consisted of preformed implants.16

As a result there was a drive to develop non-ceramic forms of hydroxyapatite (cement) that could be molded intraoperatively until the right size was achieved. They were developed in 1986 by the American Dental Association and approved by the FDA in 1996 for human use. These implants were not subjected to high temperatures when created (hydroxyapatite crystallization). Initially they could withstand lower forces of compression than PMMA. Their use in areas that cannot take loads is not recommended, as the resistance to stress is low. The micropores favor a progressive tendency to be incorporated and later substituted by the native bone surrounding it. It acts as osteoconductive material, and it should be in contact with the healthy bone surrounding it. Its composition implies the absence of strange body reactions, and the risk of infection and posterior extrusion is reduced. It can be used for repairing complete cranial defects or as an onlay graft on the internal cortical or remaining diploe. (Fig. 14). If the cranial reconstruction is of total thickness, using mesh to avoid the fragmentation of the hydroxyapatite is necessary in the setting process because of the pulsations of the dura. In addition, the process of guided bone regeneration is facilitated. Infection rates of up to 5% have been described.

The effect of this biomaterial is controversial in cranial growth. Although studies with animals have not demonstrated a negative effect in skull growth, avoiding its use during the first 4-5 years of life would seem appropriate (the brain reaches 80% of its adult size at the age of 2-3), the period with the fastest cranial growth.16

Although hydroxyapatite plays an important role in cranial reconstruction, the ideal bone substitute has not been found. There is to date no histological evidence of significant human growth after a follow-up period of 3 years. Studies are being developed that increase the osteogenic potential of hydroxyapatite (osteoinduction) by combining it with other biomaterials such as morphogenetic bone proteins or with tricalcium beta phosphate.17

The future of cranial reconstruction undoubtedly lies in molecular biology techniques, and in attempting to develop osteoinduction processes using morphogenetic bone proteins. In the future these proteins will impregnate the implant, providing osteoconductive-osteoinductive properties.

 

Discussion

Cranial defects can be divided into congenital or acquired defects, with the latter being the most common. In most cases these are post-traumatic defects. Among the congenital pathologies to be reconstructed are meningoceles, encephaloceles, massive parietal foramen, aplasia cutis with skull agenesis, and cranium bifida. However, reconstructive surgical techniques are used more frequently for correcting the cranial defects that may appear after surgical correction of craniosynostosis. Post-traumatic cases should be included within acquired pathologies (decompressive craniotomies), together with oncological cases (after exeresis of cranial bone or for filling the temporal fossa after using the temporalis muscle) (Fig.15), cranial osteomyelitis... The cranial region most commonly reconstructed is the frontal region.

In general, with the exception of cranial defects in children under the age of 3-4, there is no cranial osteoneogenesis so to speak. Small defects, particularly those covered by muscle (except in the frontal region) may not require cranioplasties. Neither is it indicated for areas supporting loads. In the remaining cases, skull reconstruction techniques are proposed for two clear motives:

1. Aesthetic considerations.
2. Protection against trauma

Many factors can delay surgical correction of the skull. Nevertheless, a minimum delay of 3 months is recommended in post-traumatic cases, and a minimum of 6 months for cases with local infection in order to establish chronic antibiotic treatment.

Many desirable properties have been described with regard to biomaterials, although the ideal characteristic has not been found. Among these there is biocompatibility, strength, resistance to fractures and to thermal conduction. It should also be inert, sterilizable, low cost, radiolucent, malleable and it should have osteoactivity.18

There are also advantages over autogenous grafts, such as the absence of morbidity in the donor site, availability, absence of resorption, easy contouring, superior cosmetic result. The greatest disadvantage is that it is still a strange body, and on occasions local inflammatory reactions will start, that may result in local infection, which will then lead to extrusion and implant failure.

Attributing many of the complications just to the material to be implanted is difficult, as there is a close connection with surgical technique, host response and local conditions in the area to be reconstructed.19 Perhaps the biomaterial used is not that important, and what is important, is the quality of the soft tissue in the receptor area of the implant and the adequate management of the paranasal sinuses that may be involved.

An intracranial dead space may be created after carrying out a cranioplasty, either because of dural retraction because of healing, or because of a loss of brain tissue. How this situation should be handled is controversial. The strict isolation of the paranasal sinuses with local flaps can be chosen20 as opposed to the complete obliteration of the dead space with microvascularized free flaps21 (Fig. 16).

 

Conclusions

Obtaining a satisfactory clinical result in cranial reconstruction depends on:

1. The selection of a biomaterial that reproduces the rigid cranial structure according to the size and shape of the defect.

2. Having an optimum receptor bed that favors the longterm stability of the reconstruction, and the vascularization of the implant, while in keeping with the desired aesthetic result.

3. Adequate management of the paranasal sinuses (Fig. 17).

 

 

Correspondence:
Ignacio Zubillaga Rodríguez
Servicio de Cirugía Oral y Maxilofacial.
Hospital Universitario 12 de Octubre
Avda. de Córdoba s/n.
28041 Madrid, España
E-mail: ignaciozubillaga@yahoo.es

Recibido: 19.01.05
Aceptado: 06.10.06

 

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