CRANIOPLASTY: A REVIEW OF METHODS AND NEW TECHNOLOGIES IN IMPLANTS MANUFACTURING

CRANIOPLASTY: A REVIEW OF METHODS AND NEW TECHNOLOGIES IN IMPLANTS MANUFACTURING

Mishinov S.V., Stupak V.V., Koporushko N.A.

 Tsivyan Novosibirsk Research Institute of Traumatology and Orthopedics, Novosibirsk, Russia 

MODERN STATE OF THE PROBLEM

Surgery for closure of cranial bone defects has a thousand years of history. There are some findings of cranioplasty carrying out 7,000 B.C. [1]. This type of surgical interventions was common in representatives of different ancient civilizations: Incas, Brit, tribes of Northern Africa, Polynesians. Archeological artifacts suppose the operation with use of 1 mm golden plates which were conducted by Peru people 2,000 B.C. [2]. There is some evidence of successful trepanations in ancient tribes in the territory of modern Russia in V-VIII centuries B.C. During archeological excavations in Altay Republic, three skulls of Pazyryk culture were found. The skulls had some non-postmortal artificial bone defects in different parts of skull vault [3].
The success of such operations depends not only on surgeon’s skills, but also on used materials for defect closure. Each stage of development of civilizations and technologies is associated with search and improvement of materials used in medicine. All materials for cranioplasty can be divided into two main categories: own and foreign. The medical society has the uniform opinion that own tissues present the best material for realization of various reconstructive interventions. Therefore, maximally efficient salvation of bone fragments during the first surgery presents the most important principle of surgery. This approach is the gold standard for traumatic brain injury. For such cases it is appropriate not to remove the bone fragments, but to conduct the primary cranioplasty with own fragments of a broken bone with use of bone suture, cranial fixators and miniplates [4].
During cranioplasty, own flaps can be made by means of splitting the bones of cranial vault or by means of implantation with own bone flap which is prepared previously – during cranioectomy. The negative sides of this technique are lysis of bone fragments – 20-50 % according to data from different authors [5, 6, 7, 8]; infectious complications which reached 25.9 % in some series [9]. Moreover, the use of split flaps is impossible for complex, gigantic and cosmetically significant defects. The production of material is also possible with use of fragments of the rib or the iliac bone. These implants give even higher risk of resolution because of other (in comparison with cranial vault bones) way of development in fetal life, appearance of cosmetic defect in places of collection, difficulties of formation of the implant with the shape corresponding to lost structures [4]. As result, this approach is not used in the modern medicine.
The category of foreign materials can be divided into the groups: allo- and xenografts. The first group (the prepared cadaveric bone) is not used because of the range of causes: high number of infectious complications, high rate of lysis of the flap, legal complications for taking the material and risk of specific infections. The second group is the post popular in neurosurgical practice. It is presented by wide range of various materials: metals, polymer materials, hydroxyapatite, ceramic, synthetic tissues.
Metal and polymer implants are widely used in practice [4, 5, 10]. After long history of use of different metal alloys in reconstructive neurosurgery [11], the single real leader is titanium. It is strong, light weight, non-corrosive and biocompatible metal with minimal infectious complications as compared to strong metal implants [1, 2, 9, 11].
The most common representatives of the group of polymer materials are polymethyl methacrylate (PMMA), polyetheretherketone (PEEK), hydroxyapatite (HA) and domestic Repiren [12, 13]. Synthetic fibers [14] and polyetherketoneketone (PEKK), which appeared recently and is poor known in Russia, are used more seldom.
The main causes for craniectomy are traumatic brain injury, ischemic and hemorrhagic stroke, surgical interventions for fibrous dysplasia and various tumors. After the analysis of the literature and web sources, we did not find any clear statistical data on number of patients with cranial bone defects in the territory of the Russian Federation. Also we should note that we did not find such statistics for other countries. This fact is explained by complexity of recording of such patients and by absence of a uniform electronic database. S. Yadla et al. [15] conducted the systematized analysis of literature and summarized the causes of craniectomy carrying out for 2,254 patients and distributed them into the groups in dependence on pathology. The proportion of injury was 37.2 %, vascular pathology (strokes, aneurysm rupture) – 31.7 %; craniectomy for tumors – 11.2 %, for inborn abnormality – 5.7 %; removal of cranial fragments after infections – 5.5 %. In 8.7 % of cases, the cause of craniectomy was associated with other causes (deformations after radial exposure, intrasurgical bleeding, pseudotumor, arachnoidal cysts). According to a randomized controlled multi-center study ACTRN12612000353897 [16], the patients were distributed as indicated below: 67 % – consequences of TBI; consequences of decompressive operations for strokes: 16 % – ischemic, 22 % – hemorrhagic. The proportion of patients with tumors was 3 %. H, Joswig et al. presented in their study the following distribution of patients who received cranioplasty: 52.4 % – consequences of head injuries, 13.6 % – subarachnoidal hemorrhage, 6.8 % – ischemic stroke, 5.8 % – intracerebral hemorrhage, 21.4 % – others [17]. One should note that the indicated data does not reflect the real distribution of the abnormality in the population according to nosologies, but the leading position of traumatic brain injury does not make doubts.
There was a previous opinion that reconstructive neurosurgical interventions are directed only to closure of a defect for protective and esthetical purposes, However the recent studies demonstrated the improvement in liquor circulation, normalization of intracranial and cerebral perfusion pressure and improvement in cognitive functions after cranioplasty [18-23]. Despite of this fact, the indications for surgical interventions have not been defined clearly [5]. Usually, surgeon orient to clinical picture and complaints indicating the presence of trepanation syndrome, location of a defect and its cosmetic significance and directly to sizes of a defect. Concerning the last mentioned fact, there is a still disputable question about need for closure of small defects (up to 10 cm2) in absence of complaints and about cosmetic significance and a clinical picture relating to a defect.
After realization of necessary examinations, determination of indications for surgery and making a decision on cranioplasty, surgeon faces a question of selection of a method and an optimal material for the indicated intervention. Previously, we considered the main groups of materials, analyzed the literature and our experience, and concluded that titanium, PMMA and PEEK implants are the most popular in Russia [24-27].
The cranioplasty techniques can be divided into two main groups: with use of individual and timely produced implants, and with use of standard titanium meshes – templates or polymer mixes, which are modeled and formed immediately during surgery. The used of the last ones increases the time of a surgical intervention as compared to prefabricated items since a surgeon needs time for making the appropriate shape and curvature of the implant. Some authors offer the intrasurgical navigation for verification of required curvature of the implant for improvement in esthetic characteristics [4, 5]. However this technique increases the duration of surgery. The use of individual items prevents the time consumption, as well as simplifies and accelerates the surgeon’s activity.
The first adopter in development of individual implants in Russia is A.A. Potapov, MD, PhD, professor [5, 25-27]. The technique consists in production of implants with use of a digital model of the skull with defect(s) of cranial bones, and subsequent use of stereolithographic anatomical models of the skull and moulds. The individual implant is produced by means of casting the polymethyl methacrylate into press-forms. After hardening and matching with the defect in the stereolithography model, the implant is sterilized and then it is ready for surgical use.
Currently, the wider use of individual implants is realized in conditions of specific medical facilities. Items are delivered to a clinic and are ready for implantation after sterilization. There are both Russian and foreign companies working for this direction. The basic materials are lighted polymers (PMMA, PEEK, PEKK) and titanium meshes. The working process is sufficiently similar with the above-mentioned. At the first stage, a patient with cranial bone defects receives the multi-slice computer imaging of the head. The examination gives sliced images of the skull, which are exported as DICOM digital images to the program for 3D modeling. The second stage with specialized software includes the 3D polygonal model of the skull. Then the 3D modeling operator creates a virtual implant for closure of a cranial bone defect. The third stage includes the physical creation of the implant which is realized with different ways: with use of dense silicone press-forms for making the polymer material. The cutting machines are used for cutting the polymer form, resulting in ready product. Titanium mesh individual implants are produced with sheet titanium alloy, which is formed with 3D anatomical model of the skull, which is made with 3D printer in 1:1 scale. This approach is used for creation of individual titanium implants produced in Russia.
Significant amount of researchers prefer the individual implants. So, Schwarz et al. [28] reported on unsatisfactory cosmetic results of reconstructive interventions for big cranial defects, whereas their closure is performed with hand-made implants from polymer implants immediately during a surgical intervention.
S.A. Eolchiyan [24] reported that the use of individual implants produced with CAD/CAM technologies with titanium and PEEK-Optima material demonstrated their evident advantages in view of high preciseness, low traumatic potential, reduced time of surgery and, finally, in achievement of predicted stable functional cosmetic result.
M. Cabraja [29] demonstrated that cranioplasty with CAD/CAM individual titanium implants was appropriate for any bone defects regardless of sizes and complexity, showed the minimal percentage of complications and did not hinder subsequent control tomographic examinations. Titanium implants present the material of choice for secondary cranioplasty in patients with subsequences of decompressive trepanations after traumatic brain injuries or other urgent neurosurgical conditions.
J. Höhne [17] et al. compared the results two different techniques of cranioplasty performed in 2006-2013. The first group (60 cases) included the patients who received interventions with implants which were produced from PMMA intrasurgically. The second group (60 cases) received titanium meshes, which were formed according to anatomical models. The surgery time was reliably lower in the second group. The patients of the second group showed the lower amount of complications and the best cosmetic results.
J.M. Luo et al. [30] conducted their study in 2005-2011. 161 patients were distributed into two groups: with use of titanium mesh implants modeled during surgery (78 cases), and with use of titanium mesh implants preliminary modeled with CAD/CAM before surgery (83 cases). The authors showed that the use of implants, which were made with 3D skull model before surgery, allowed using less screws for fixation of the implant, decreased the rate of postsurgical complication and gave the best esthetical results.
  J. Kwarcinski et al. [31] conducted a systematic review and concluded that the risk of postsurgical infectious complications was reliably increased by surgery time and recurrent surgical interventions [32], whereas the comparison of implants made of different materials does not find the ideal material with minimal infectious risks. Also the authors present a hypothesis that the implant structure promoting the better integration into surrounding tissues (porosity, roughness) can influence on the decrease in postsurgical trophic disorders and, as result, on the decrease in the rate of implant infectioning.
D.J. Bonda et al. [31] reported in their review that the use of individual implants on the basis of 3D modeling techniques and printing is the most evident perspective in reconstructive neurosurgery. 

DISCUSSION

After analysis of the literature and our experience with various variants of implants for cranioplasty we concluded that the use of individual implants was correct in all cases of cranial defect closure. Such point of view will certainly raise a lot of discussions, but this fact is quite justified if the problem is reviewed from the perspective of rational use of resources. As compared to standard templates for cranioplasty, the use of individual items reduces the time for formation of the implant during surgery. The use of individual implants precludes a possible looseness and interval between the plate and a bone. It requires for fewer screws and warrants the good contact and fixation to the skull. After surgeries with standard titanium plates, some uncut fragments remain which are not usually used later. If this circumstance is analyzed in condition of a big city, a big amount of the material appears within a year, but it is completely utilized after secondary preparation.
The techniques for production of individual implants with use of anatomical model of the skull and the press-forms have some disadvantages also. Along with realization of a surgical intervention, some things remain which are not used and require for utilization. Currently, a direction in three-dimensional printing, which is the most economic from perspective of use of materials, actively develops in medical industry. We reviewed the most popular techniques of additive technologies in the territory of the Russian Federation: fused deposition modeling (FDM), stereolithography (SLA), selective laser sintering (SLS) and direct metal laser sintering (DMLS) [34]. So, SLS and SLA printing techniques are compatible according to accuracy of models. Moreover, the items demonstrate the higher strength characteristics as compared to analogues made with FDM printing. However the above-mentioned techniques do not use any biocompatible materials, which are allowed to use in medicine and implantation, and it does not allow immediate production of the implant. Therefore, these techniques allow producing the prototype of the implant, but the development of the medical item requires for production of a press-form on the basis of the prototype with subsequent filling with hardening medical polymer, or formation of the implant with use of the prototype as the anatomical model.
DMLS (Direct Metal Laser Sintering) allows direct production of the titanium implants without any intermediate items (press-forms, anatomical models). Production of individual titanium implants with this technique allows recurrent use of non-sintered material after removal from the operating camera, resulting in minimization of material losses. Production is almost wasteless that differs DMLS from subtractive technologies such as cutting and allows producing several models at the same time, with the only limitation of size of the operating camera [35]. Construction of models takes hours that is more efficient than the casting process, which can take up to several months with consideration of the whole production cycle.
We selected DMLS as the optimal technique for development of individual implants for reconstructive interventions for cranial bones. The production process with DMLS differs from the above-mentioned one according to only a way of production of the item. 3D printing of the implant with use of titanium (Ti64) powder is conducted at the third stage in virtual medium after creation of the individual implant. As double control at the same stage, we created a fragment of the patient’s skull in the field of a defect with use of SLS printing with polyamide (Fig. 1). Creation of the anatomical model confirms the congruity of the implant before surgery. After this, the implant is sterilized in the autoclave. Then the item is ready for implantation [34, 36, 37]. Production of implants with this technique gives the possibility for physicians to participate in development of design: from selection of surface texture and fixation techniques (Fig. 2) for creation of additional elements (reinforcing plate, aligning guide, additional holes for draining of subimplantation space, additional fixation points for soft tissues and others).

Figure 1. Individual titanium implant produced with three-dimensional printer with DMLS technology, with anatomical model of the patient’s skull




Figure 2. Main types of fixing of individual implants: a) with loops; b) overlapped; c) into the end

     

Currently, some authors present their works dedicated to search for methods for development of cheap individual implants since the mean price of such products in foreign countries is 3,000-7,000 Euro depending on the used material, and the price is quite high even for developed countries [24-38].
So, the study by Eddie T.W. et al. [38] shows that the use of individual implants can be realized by a surgeon who has experience with simple computer modeling. The authors offer using the cheap desktop 3D FDM printer for production of PLA press-forms, which are used for casting the biocompatible polymer (the authors used Surgical Simplex P Radioopaque bone cement by Styker Corporation). Therefore, a surgeon can independently make the required individual implant without any additional assistance. According to the authors’ conclusion, the price of such implants is several times lower than the analogues in Europe and Northern America. According to Pham B.M. et al. [39], the decrease in price of individual implants for cranioplasty can be achieved with three-dimensional modeling of the basic product performed by surgeons directly in a clinic. However the authors mention that it requires for special knowledge of CAD/CAM modeling. This approach to production of individual implants was tested and compared to products from specialized medical factories in out clinic in 2014-2015. According to our opinion, optimization of working time of surgeons and quality of final products are priority tasks. Therefore, production of individual surgical implants is to be realized on the basis of licensed medical industrial enterprises with use of special equipment. Whereas the price of individual items can be decreased not by means of realization of 3D modeling by physicians, but by means of development of software for modeling the implant in automatic or semiautomatic mode [40]. The actual issue is also study of biocompatible polymers (PEEK-FDM, PC-ISO, ABS-M30i, FDM Nylon 12), which are already used in 3D printing, for issue of their safety for production of implants. The indicated approaches will increase the availability of medical care with use of individual products produced with three-dimensional printing. 

CONCLUSION

1. The use of individual implants is appropriate for neurosurgical practice for treatment of bone defects of any size and locations.
2. The technique of direct metal laser sintering is the optimal technique for production of individual titanium implants in Russia at the present time.
3. For wider coverage and timely realization of surgical care for patients with cranial bone defects it is necessary to create the uniform registry and registration system for patients with this abnormality. 

Information on financing and conflict of interests

The study was conducted without sponsorship.
The authors declare the absence of any clear or potential conflicts of interests relating to publication of this article.

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