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1 Overall Objective

The reconstruction of craniofacial defects is a challenging task even for the experienced surgeon. For an optimal solution, the techniques and principles that have been described in the chapters before must be adapted to the individual situation. The challenges result from the patient’s individual situation, the preoperative planning efforts, from technical aspects as well as from the reconstructive means available. While the use of autografts has been the most widely recommended method, it does have its drawbacks, including long operation times, donor site morbidity, limited donor bone supply, as well as different anatomic and structural problems. The availability of autogenous bone grafts resembling the form of the skull is limited. Therefore, there is a need for alternative materials with adequate mechanical properties and biocompatibility (Blake et al. 1990; Eufinger et al. 1995; Eufinger and Wehmöller 1998, 2002; Klongnoi et al. 2006; Wiltfang et al. 2002, 2003; Schiller et al. 2004; Thorwarth et al. 2005; von Wilmowsky et al. 2008).

2 Patient-Related Conditions

The size of the defect, delayed, or inadequate debridement of the wound, delayed, or inadequate treatment of the patient or the relatively high risk of infections are the typical causes for complications in craniofacial injuries. In addition to primary or secondary treatment attempts, the morbidity of the patient and the risk of the planned reconstructive procedure may result in severe complications. Concerning the patient situation, the following questions have to be answered before an attempt for reconstruction is made:

  • Size and location of the defect

  • General health status

  • Neurological status

  • Patient’s wish/compliance

  • Treatment plan

  • Technical aspects

2.1 Size and Location of the Defect

The size of the defect plays an important role in the determination of the reconstructive procedure. Poukens et al. (2008) have developed a classification as a basis for the intended reconstruction. When designing skull implants, the sometimes complicated geometry of the defects is important to consider. Defects crossing the midline, or defects including the orbital rim or roof of the orbit, pose a greater challenge for design and manufacturing than a simple one-sided parietal bone defect. Apart from the size and extent of the defect, these anatomical factors ought to be included when classifying skull defects. The proposed classification (Table 16.1) is, therefore, based on three factors that determine the complexity of the reconstruction:

  • Size of defect in cm2

  • Defects crossing the midline of the skull

  • Involvement of the orbital rim/roof

Table 16.1 Classification of cranial implants based on the degree of difficulty in computer design and manufacturing
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The size of skull defects is important for obvious reasons—the larger the defect, the larger the span of curvature that has to be reconstructed.

The implant should be designed in a way that the contour of the surrounding skull bone is followed. Small defects of <5 cm2 have an almost flat surface, whereas large defects require a more sophisticated implant, which has implications on the design procedure.

Defects that cross the midline pose more problems when designing the implant, since information on the contour of the contralateral side of the skull is missing and, thus, mirroring of the contralateral curvature is not possible. Designing an implant that involves part of the orbit is more complicated because of the curvature of the orbital area and the need for mirroring of the other side.

The anatomical location of the defect is, of course, also important, since the bone thickness varies along the skull. The thickness of the bone has to be taken into account when designing the implant, especially the part that will be fixed to the skull bone. The proposed classification divides skull defects into six distinct classes, based on the difficulty of implant design. A skull defect is described by the class it belongs to and its anatomical location.

2.1.1 Examples

Patient 1 had undergone previous surgery for removal of a meningioma, with creation of a defect at the left parieto–occipital area of the skull (Fig. 16.1a–d). The skull defect was classified as a Class II occipital skull defect. Design and manufacturing was based on a multispiral computer tomography (CT) scan (Toshiba, Japan). The scan data were converted into a stereolithographic (STL) data set using the computer program Mimics (Materialise, Leuven, Belgium). The STL data were imported in 3Matic (Materialise, Leuven, Belgium) and further processed on a Pentium IV workstation. It was decided to manufacture a titanium implant, using high-speed milling (HSM).

Fig. 16.1
figure 1

(a) Impression at the left parieto-occipital area from previous surgery for removal of a meningioma. (b) Rapid prototyping model shows the large skull defect. (c) Implant designed for the left parieto-occipital defect. (d) Implant placed in the defect

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Patient 2 had undergone surgery for decompression of the brain after severe trauma. A part of the skull bone became necrotic, leaving a large skull defect at the right side (Fig. 16.2). The skull defect was classified as a Class III temporo-parietal skull defect. The computer-aided design resembled the procedure in case 1. The implant, however, was manufactured by selective laser melting with titanium powder (SLM).

Fig. 16.2
figure 2

(a) Rapid prototyping model shows the large parieto-fronto-temporal skull defect. (b) Implant designed for the reconstruction of the defect. (c) Selective laser melting (SLM)-made implant. (d) SLM-made implant placed in the defect

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The implant in patient 1 was designed in 3 h with the help of surface guidelines and mirroring of the contralateral side (Fig. 16.1b) (Hutmacher et al. 2004). Four fixation lips on the implant were designed to allow fixation to the skull. The implant (Fig. 16.1c) was milled out of medical grade 5 titanium block in a five-axis HSM machine in 48 h (IDEE, MUMC Maastricht University Medical Centre, Maastricht, Netherlands). The implant was fixed with four titanium screws (length 3 mm, diameter 1.5 mm, KLS Martin, Tuttlingen, Germany) to the remaining skull (Fig. 16.1d).

The implant of patient 2 was designed in 16 h with the help of surface guidelines and mirroring of the contralateral side. Five fixation lips on the implant were designed to allow fixation to the skull. The implant (Fig. 16.2b, c) was manufactured out of medical grade 5 titanium alloy by Electron Beam Melting (Arcam, Sweden) in 12 h. The implant was fixed with five titanium screws (length 3 mm, diameter 1.5 mm; KLS Martin, Tuttlingen, Germany) to the remaining skull (Fig. 16.2d).

2.2 General Health Status

The medical history of the patient as well as size and localization of the defect are crucial elements for the determination of the treatment plan. The anamnesis must consider the exact medical history of the patient, as different defect causes—trauma, tumor, infection, neurological diseases, apoplectic insults, malformation—may lead to a different evaluation and treatment plan. Coexisting medical problems may even prevent any further attempt at reconstruction:

  • Age, life expectancy, and prognosis

  • Severe medical disorders

  • Multiple treatment failures due to infections or others

  • Anticoagulative medical treatment

  • Deficits in cooperation

  • Alcohol or drug abuse

The duration of the operation plays a considerable role in the reduction of risks. It reduces the blood loss and the risk of brain swelling, the infection risk, and the risk of tissue damage. The indirect reconstruction of defects using prefabricated allogenic implants reduces the operation time, whereas a direct reconstruction with autogenous bone material causes higher morbidity and is more time intensive due to harvesting the bone specimen.

There are, however, patients where no further attempt at reconstruction should be considered. These are patients with a clearly reduced life expectancy or patients that do not accept any reconstruction. Drug or alcohol abuse may be further contraindications for reconstruction. In patients suffering from apoplectic insults or intracranial bleeding with a high risk of thrombosis or embolism, the surgical risk and the method of reconstruction must be calculated with respect to the anticoagulative therapy.

In this group of patients, allogenic implants are advantageous as the surgical intervention should be limited to the defect reconstruction. A limited surgical intervention with an acceptable duration of the operation will help to reduce bleeding to a minimum. Nevertheless, the risk of thrombembolism must be respected and evaluated as a surgical intervention is planned. A preventive low-dose medication with NSARs should not be stopped.

A second intervention (e.g., the harvesting of autogenous bone material) should be avoided, however. Small defects (class I) as the result of skull bone trepanation or trauma, for example, may not always be reconstructed. The decision depends on the localization of the defect, the age of the patient, and the risk of injury in the affected region. In young, nonhandicapped, active patients there may be a higher need to reconstruct defects as they may limit an individual’s freedom and quality of life. Also, aesthetic aspects—especially in the forehead region—are considered as a serious indication for reconstruction. In older patients with high comorbidity risks due to medication or age, the necessity to reconstruct skull bone defects may be evaluated differently, as the possible improvement must be seen in relation to the general risk of the operation.

There is also a group of patients where other solutions should be considered than the methods mentioned above. These are patients, where multiple attempts using different techniques of reconstruction have failed, mostly due to previous infections.

Multiple infections in the region of interest result in a relative contraindication for PSI placement.

As soft tissue coverage is essential for success with allogenic and autogenous materials, it may in some rare cases not be advisable to reapproach these patients again with a surgical solution. For these patients, helmets and epithetic applications help to reduce the risk of injury and improve the aesthetic appearance.

2.3 Neurological Status

Neurological deficits may be the consequence of skull bone trauma, intracranial tumors, or apoplectic insults. Besides the general medical status of the patient, the necessity, urgence, and method of skull bone reconstruction depend on the patient’s neurological status.

The neurological deficit can result from the disease leading to the skull bone defect, but also from the defect itself. To detect and evaluate the skull bone defect as primary or additional cause for a neurological deficit, however, is difficult. Nevertheless, there is a need for reconstruction of skull bone defects to improve the individual’s neurological situation and protect the brain from further damage. The neurological deficits can be temporary, long lasting, or even progressive. Traumatic brain injuries may heal without consequences, but in 35% of the cases with open brain traumas traumatic epilepsy develops (Poeck 1987). Nearly 95% of these traumatic epilepsies develop in the first 2 posttraumatic years (Poeck 1987).

The most obvious indication for skull bone defect reconstruction is to prevent further damage to the patient. Sensory or motor deficits, hemi-, or paraplegic paralysis, epileptic attacks, or infections will lead to a progressive loss of motor and sensory function and control. Medical treatment and rehabilitation will result in a sensory-motor improvement or stabilization in many patients.

The better the functional results are, the more urgent is the need for a skull bone reconstruction, especially in large bone defects. This implies that the reconstruction of skull bone defects is necessary to avoid any further traumatic brain damage resulting from a direct trauma to the brain (Poeck 1987).

In patients with long-persisting neurological deficits, the rehabilitation efforts may be supported by the reconstruction of skull bone defects in two ways:

  1. 1.

    The integrity of the skull lowers the risk of a further injury.

  2. 2.

    The skull bone reconstruction itself may improve the neurological status.

Aesthetic rehabilitation due to skull form may be another indication for reconstruction, especially in patients with no or few neurological deficits (Blake et al. 1990; Eufinger and Wehmöller 1998, 2002; Poukens et al. 2008).

A recently published study (Zegers et al. 2017) showed a statistically significant improvement in quality of life after PSI placement in skull bone defects. Furthermore, it decreased pain and headaches and gave aesthetically good results.

2.4 Patient’s Wish

Suboptimal outcomes are not as uncommon as they should be. Careful assessment and analysis of the existing treatment deficit including three-dimensional CT scans and soft tissue evaluation, as well as careful follow-up and audit will establish their full extent. Inadequately treated craniofacial defects can result in significant cosmetic deformity and functional disabilities, which are extremely difficult to correct. If possible, a simulation of the intended reconstruction should be performed to compare different treatment options.

To re-establish a maximum of quality of life, function, social acceptance, and self-esteem, every reasonable effort should be undertaken to correct and improve a patient’s situation and individual cranio-maxillofacial defect. In some cases, a craniofacial implant must be inserted as a prerequisite for further epithetic reconstruction as shown below.

A serious evaluation and discussion with the patient may in some cases lead to the conclusion that no reconstruction should be attempted. This should be considered, if it is the patient’s wish and the technical possibilities cannot be harmonized to reach a treatment goal which is acceptable to the patient (Fig. 16.3)

Fig. 16.3
figure 3

(a) Rapid prototyping model shows the large craniofacial bone defect. (b) Implant for the reconstruction of the latero-orbital defect. (c) Control X-rays after implantation. (d) Patient before and after reconstruction with titanium implant and epithetic reconstruction of the orbit

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2.5 Treatment Plan

A team of specialists has to be at hand to perform the planning, designing, and manufacturing of the implant and reconstruction of the defect. As each step of the procedure has to be validated, a close cooperation between the partners is an absolute necessity for a good result.

The ideal team consists of:

  • Cranio-maxillofacial reconstructive surgeon/neurosurgeon

  • Radiologist

  • Engineering team for design and preparation of data sets

  • Engineering team for manufacturing of the implant

  • Sterilization unit

  • OR capacity with experienced anesthetist and nursing staff

  • Intensive care unit

In an ideal setting, the cranio-maxillo-facial surgeon and neurosurgeon work together as a team to perform the reconstructive surgery. The team leader is responsible for the data acquisition, product planning, and production of the implant. The leading reconstructive surgeon has to validate the production process step by step, as the engineering teams usually have no direct contact with the patients. He has to organize the whole procedure with respect to the operation date. The production process, the material used, the methods of manufacturing, sterilization of the implant, and the transport of the sterilized implant to the operation room must follow a certified pathway. It must be guaranteed that each step of the production process is documented according to internationally accepted quality standards. The whole process from data acquisition to the operation may take at least 6 weeks. Figure 16.4 shows a diagram of the treatment plan.

Fig. 16.4
figure 4

Treatment plan

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2.6 Technical Aspects

The individually made implant should ideally be fixed with standard titanium screws of 2.0 mm diameter and a variable length. The designing engineer has to consider fixation elements, such as lips or tangential screw canals for the fixation of the implant. Both elements can be combined.

The fixation lips have to be long enough and may host two drill holes. The reconstructive surgeon has to discuss the design of the implant and the position and number of fixation elements with the engineers. The bicortical layer of skull bone is ideal for a monocortical implant fixation, whereas the region of thin bone from the infratemporal region should be avoided. The length of the screws needed can be planned virtually and be indicated on the patient-specific implant (Figs. 16.5 and 16.6).

Fig. 16.5
figure 5

Milled titanium implant. L fixation applications, K screw canals tangentially designed for direct screw fixation, P perforations for tack-up sutures

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Fig. 16.6
figure 6

X-ray control after skull reconstruction with the titanium implant displayed above. Absolute exact fit. Fixation applications are clearly visible

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The patient’s head has to be fixed in a Mayfield clamp or headrest for an absolute stable fixation. The surgical treatment plan has to be discussed with the responsible anesthesiologist to control and lower the blood pressure to a reasonable level to avoid an unnecessary blood loss. After the operation, the patient should be transferred to the intensive care or recovery unit.

3 Recent Developments

Since the mid 1990s allogenic implants were applied to restore skeletal defects in the cranio-maxillofacial region with varying success. Due to incorrect planning, manufacturing, or application, these implants found limited acceptance.

Today, the higher life expectancy and mobility of patients, as well as an increasing number of younger patients in need of such implants, require better solutions than in the past. The highly sophisticated data acquisition and better computer programs have improved the precision and acceptance of the implants.

An improved integration of titanium implants at the titanium-bone interface could be realized through inducing better bone ingrowth by producing a more porous surface structure. The positive effects of increasing the surface area are well known from dental implants (Hattar et al. 2005; Klein et al. 1994; Li et al. 2005).

The manufacture of implants with graded mechanical properties such as reduced weight, full freedom of form and stiffness is possible with the current state-of-the-art technology.

3.1 Additive Manufacturing

With addivitve manufacturing (AM), it has become possible to produce complex, three-dimensional implants directly from serial materials. Titanium powder, e.g., is brought onto a work platform in layers of 0.03–0.1 mm thickness.

According to three-dimensional computer data sets, a layered implant construction is possible. A focused laser beam of high intensity delivers the energy to melt the powder particles to form a solid titanium implant (Hon and Gill 2003) (Figs. 16.7a, b and 16.8).

Fig. 16.7
figure 7

(a) Manufacture of AM implants: melting titanium powder. (b) Manufacture of AM implants: layered technique

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Fig. 16.8
figure 8

AM manufacture of a cranial implant with hollow and grid structures

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Implants manufactured according to the AM process yield densities of approximately 100% without postprocessing steps. Postprocessing procedures, however, are necessary to stabilize titanium molecules of the outer layers to prevent them from dissolving after implantation.

The main advantages of the AM process are:

  • Manufacture of complex geometries

  • Direct rapid production of customized geometries

  • Additive manufacture without loss of unprocessed material

  • Fabrication of:

    • Defined roughness

    • Graded porosity

    • Lattice structures to realize adapted stiffness

In addition to the medical aspects, the economic aspects are of great importance for custom-made implants. The construction and manufacture of individual implants for craniofacial application are time-consuming and costly. The cost-dominating factors are the expenses for design, construction, and manufacturing, particularly for large defects. Design and manufacturing comprise several processing steps:

  • Tooling

  • Forging

  • Casting

  • Forming

  • Machining

  • Finishing

Costs are mainly determined by speed and effort for design and manufacturing, and both processes need exact tuning and optimization in order to meet the demands of the surgeons. In comparison with conventional implant manufacturing, the production time needed for an AM implant can be reduced by about 50%. Potential advantages are reduced operating costs due to shorter operation times and shorter hospitalization. Considering the overall objective of specialized medical centers, an implementation and certification of an effective and integrated process chain to plan and manufacture customized implants within 48 h seems to be possible today (Fig. 16.9). On the basis of the SLM process, new designs and material applications will become possible. Porous surfaces with a drastic reduction of the metallic content of the implant by creating three-dimensional spongious structures will allow material compositions with biomaterials without reduction in stability and precision.

Fig. 16.9
figure 9

Process chain for individual implant manufacture

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3.2 PEEK-Implants

Polyetheretherketone (PEEK) is a semi-crystalline thermoplastic polymer with biophysical properties similar to that of human bone (Jahur-Grodzinski 1999). The physical characteristics of the material combine strength and stiffness similar to that of bone with excellent thermal and chemical properties. Its excellent biocompatibility makes this material ideal for long-term medical implant applications. With a continuous use temperature of 260 °C, this polymer is suitable for every clinical sterilization method.

PEEK is a fine powder and commercially available. Materials with low melting viscosity and a homogenous distribution of particle size are best suited for the additive manufacturing process. The average particle size varies between 50 and 150 microns with irregular, edged particles. The irregular particle size makes it necessary to sieve the material, as layers of 100–150 microns seem to be ideal for the layer-by-layer sintering technique.

To improve the biological and physical properties of the material, other allogenic and xenogenic materials can be added. Addition of nano-sized carbon black improves the flow characteristics of PEEK for modeling the implant. Organic and inorganic biological materials, such as tricalcium phosphate (TCP) can be added to improve the biological response of tissues to nondegradable polymers as in vivo application of PEEK results in an encapsulation of the implant with fibrous tissues, isolating the material from the surrounding bone (Balani et al. 2007).

It has been shown that addition of bioactive ceramics such as Bioglass and sintered hydroxylapatite enhances osteoblast proliferation. In vivo experiments with biologically altered PEEK basis material resulted in bony ingrowths into the implants based on a bone-like apatite layer on the surface of the implant (Rodil et al. 2005; von Wilmowsky et al. 2008).

PEEK combines the excellent manufacturing properties of titanium implants with the advantages of an outstanding material with chemical and physical properties with a close resemblance to bone. The general problem of PEEK materials is fibrous encapsulation.

As promising studies have shown, this might be overcome in the near future. Therefore, for the time being additive manufactured PEEK-implants appear to be ideal as bone substitutes for various indications.

3.3 Outlook

Based on the experience with complex three-dimensional data sets, the development of implants with completely new features is close at hand. The features are:

  • Well-defined porosity for improved bone ingrowth

  • Adapted stiffness and elasticity close to that of bone

  • Maximum reduction of allogenic material

The realization of these features requires the following work items:

  • Dimensional accuracy <0.1 mm

  • Fabrication of thin lattice structures with a detail resolution of 150–200 microns, made of different materials

  • Smooth transfer of biomechanical loads from the natural bone into the implant based on Finite Element Method (FEM) implant design

  • Fabrication of graded surface porosity

  • Fabrication of surfaces with defined roughness (RZ = 15 microns up to 100 microns)

  • Implant manufacture for all kinds of bone defects

The AM technique, based on titanium and PEEK polymeric materials, is available for clinical use today. However, as described before, the technical process of transforming a virtual three-dimensional data set into real existing implants is challenging. The process chain demands a close cooperation between engineers, the manufacturing team and surgeons to guarantee a successful reconstruction. Today, this ideal setting is only available in a few medical centers.

A major step in the close future will be the possibility to stimulate cell ingrowth in individually designed implant structures (biologization). For this goal, new materials are necessary.

There is a research focus on medical grade thermoplastic polymers, such as poly(lactic acid) (PLA) and poly(lactic-co-glycolic) acid (PLGA). These biodegradable polyesters offer strong bulk mechanical properties combined with tunable physico-chemical properties, such as surface energy and degradation rate. From a processing point of view, however, they are strongly prone to degradation, especially when subjected to thermal processing as in the case of fused deposition modeling or extrusion-based 3D plotting. The aim is to enhance both processing and product properties to study the thermo-mechanical behavior of functionalized PLGA and optimize the printing settings or conditions accordingly. Furthermore, additives will be investigated to solve three main functions:

  1. 1.

    Act as foaming agents after plotting so as to create customized internal porosity, morphology, or gradients in each single fiber of the scaffolds, which are known to influence both mechanics and cell differentiation

  2. 2.

    Act as plasticizers, nucleating agents, stabilizers, or chain modifiers in order to tune processing behavior and product properties

  3. 3.

    Display a bioactive characteristic, so that when leached out of the polymers the additives could help in the process of cell differentiation

The process of biologizing implants will lead to a new class of implants for application in the human body.