**6. Applications**

160 Biomedicine

Fig. 14. Results of bone ingrowth testing of porous Ti64 in New Zealand rabbits (Courtesy of Instituto de Biomecánica de Valencia): a) excised sample for pull-out test; b) CT image of EBM sample; micro CT reconstruction of EBM (c) and conventional (d) Ti64 sample.

As explained in Supply Chain section, AM technologies manufacture directly from digital information of the part (digital files with 3D geometry) and do not need any kind of auxiliary tooling during the manufacturing process. Normally, the use of tooling (moulds, machining tools, etc) makes crucial influence on the product geometry, since desirable product features cannot be produced.

These manufacturing constraints are not present in AM processes. Using AM processes, designers are not limited or conditioned by conventional manufacturing constraints and can focus only on the optimum design of the product according to its application. AM technologies permit greater freedom in product design, enabling the manufacturing of much more complex geometries and in many cases, geometries that are impossible to manufacture with another fabrication method.

As a matter of fact, EBM has few manufacturing constraints, in terms of producing complex geometries and scaffolds structures and also offers the highest production speed (AIMME, 2009). Its high productivity makes economically viable the fabrication of high added value implants. Although it must be said that due to high processing temperature in EBM process, unfused powder is sintered around the part or scaffold. In certain geometrical features the cleaning process may be difficult, especially in large scaffolds with very small pore size.

The main advantages of using EBM as a Manufacturing technology for implants consist in:


For the time being, three titanium (Ti64, Ti64 ELI and Ti grade 2) and one cobalt chromium (CoCr ASTM F75) alloys are being commercialized by the EBM technology provider and widely used for medical implants. There is a big number of case studies of customized implants that have been implanted in human body. There are also standard implants with added value certified for sale in EU and worldwide. In this section, the authors demonstrate the advantages of AM through different types of application, such as:

two aims:

bone stiffness.

Additive Manufacturing Solutions for Improved Medical Implants 163

component of a hip prosthesis). According to Wolff's law (Wolff, 1986), bone in a healthy person or animal will remodel in response to the loads it is placed under. Therefore, if the bone load decreases, bone will become less dense and weaker because there is no stimulus for continued remodelling that is required to maintain bone mass. During the design process of the customized implant stress shielding can be taken into account and minimized by means of different designs and MEF structural analyses in order to achieve

 The implant should transfer bearing loads to the bone in order to avoid bone resorption. Decrease the stiffness (Young modulus) of the implant in order to make it similar to

In case of standard implants, the size chosen by the surgeon doesn't adapt perfectly to the biomechanics of the patient and there will be a higher probability of bone resorption since

this standard implant doesn't transfer loads properly to the nearby bone tissue.

Fig. 15. Customized hip prosthesis developed in Project FABIO (Delgado, 2010). This design was manufactured by means of EBM including a porous region for

In conclusion, the use of standard implants not only implies to remove more bone tissue than with tailor-made implants, moreover there is a also higher probability of bone resorption. Therefore, the future revision surgery might be more complex since the patient

Another very important benefit of using customized implants is that surgical operations are shorter. During the operation, surgeon must adjust standard implants manually to the patient's anatomy and pathology in order to be able to place and fix them. This cannot be

improving osseointegration and implant fixation in the body.

**6.1.3 Avoiding manual adjustment of standard implants** 

done until the surgeon has intervened the damaged zone (Figure 16).

(Courtesy of AIMME, IBV, ASCAMM, TECNALIA).

has lost more bone tissue in the damaged area.


## **6.1 Customized implant**

Currently, CT imaging has improved, in terms of resolution and 3D details, obtaining very accurate information from the patient. With this information, implants can be designed taking into account patient's anatomy, type of injury, surgical technique, etc. As previously mentioned, the design process starts from scanned information of the patient (Computed Axial Tomography (CAT), Magnetic resonance imaging (MRI) or Radiography). The implant design and development process are also supported by the previous experience of the design team (scientists, engineers, surgeons, etc). The new customized implant design is commonly validated by Finite Elements Analysis (FEA or FEM). In the case of a structural analysis, the solution shows a 3D map distribution with the stress level (strain, displacement, etc) along the geometry (Figure 15).

Principal benefits of customized implants are:


### **6.1.1 Design adapted to the patient**

Customization of implant geometry is especially important in long duration prostheses (between 10-15 years), i.e. hip and knee prostheses, since the implant geometry adapts perfectly to the anatomy and injury of the patient (Figure 15). In other words, the size and weight of the prosthesis should be the strictly necessary for every patient, increasing the level of comfort. These benefits can be better understood comparing with standard prostheses and usual surgical procedure. When a standard prosthesis is implanted, the surgeon must decide which implant size could fit in the patient. It may be necessary to choose the bigger size. In this case, the surgeon has to create sufficient space for putting in and fixing the prosthesis. It implies to cut and remove more bone tissue and the patient has to carry with a bigger prosthesis. The removal of bone could be critical in the case of younger individuals (< 60 years) (Ratner, 2004), where it might be necessary to perform a revision surgery. Revision prostheses are bigger, causing further increase of fitting place and therefore the removal of more bone tissue. In contrast, the use of customized implants implies that the surgeon has just to create the minimum necessary space in order to fit the implant and leaves intact much more bone tissue for future revisions.

#### **6.1.2 Diminishing of stress shielding effect**

It is worth mentioning that customized implants could diminish the negative effect of stress shielding. Stress shielding refers to the reduction in bone density (osteopenia) as a result of removal of normal stress from the bone by an implant (for instance, the femoral

Currently, CT imaging has improved, in terms of resolution and 3D details, obtaining very accurate information from the patient. With this information, implants can be designed taking into account patient's anatomy, type of injury, surgical technique, etc. As previously mentioned, the design process starts from scanned information of the patient (Computed Axial Tomography (CAT), Magnetic resonance imaging (MRI) or Radiography). The implant design and development process are also supported by the previous experience of the design team (scientists, engineers, surgeons, etc). The new customized implant design is commonly validated by Finite Elements Analysis (FEA or FEM). In the case of a structural analysis, the solution shows a 3D map distribution with the stress level (strain,

Customization of implant geometry is especially important in long duration prostheses (between 10-15 years), i.e. hip and knee prostheses, since the implant geometry adapts perfectly to the anatomy and injury of the patient (Figure 15). In other words, the size and weight of the prosthesis should be the strictly necessary for every patient, increasing the level of comfort. These benefits can be better understood comparing with standard prostheses and usual surgical procedure. When a standard prosthesis is implanted, the surgeon must decide which implant size could fit in the patient. It may be necessary to choose the bigger size. In this case, the surgeon has to create sufficient space for putting in and fixing the prosthesis. It implies to cut and remove more bone tissue and the patient has to carry with a bigger prosthesis. The removal of bone could be critical in the case of younger individuals (< 60 years) (Ratner, 2004), where it might be necessary to perform a revision surgery. Revision prostheses are bigger, causing further increase of fitting place and therefore the removal of more bone tissue. In contrast, the use of customized implants implies that the surgeon has just to create the minimum necessary space in order to fit the

It is worth mentioning that customized implants could diminish the negative effect of stress shielding. Stress shielding refers to the reduction in bone density (osteopenia) as a result of removal of normal stress from the bone by an implant (for instance, the femoral

Production of small-medium series of value-added biomedical products.

Production of standard value-added biomedical products.

Customized implants.

**6.1 Customized implant** 

Research in the biomedical field.

Scaffolds with controlled designed porosity.

displacement, etc) along the geometry (Figure 15).

Implant design adapted to the patient's anatomy and pathology.

implant and leaves intact much more bone tissue for future revisions.

Avoiding of manual adjustment of standard implant during surgery.

Principal benefits of customized implants are:

Diminishing of the stress shielding effect.

**6.1.2 Diminishing of stress shielding effect** 

**6.1.1 Design adapted to the patient** 

component of a hip prosthesis). According to Wolff's law (Wolff, 1986), bone in a healthy person or animal will remodel in response to the loads it is placed under. Therefore, if the bone load decreases, bone will become less dense and weaker because there is no stimulus for continued remodelling that is required to maintain bone mass. During the design process of the customized implant stress shielding can be taken into account and minimized by means of different designs and MEF structural analyses in order to achieve two aims:


In case of standard implants, the size chosen by the surgeon doesn't adapt perfectly to the biomechanics of the patient and there will be a higher probability of bone resorption since this standard implant doesn't transfer loads properly to the nearby bone tissue.

Fig. 15. Customized hip prosthesis developed in Project FABIO (Delgado, 2010). This design was manufactured by means of EBM including a porous region for improving osseointegration and implant fixation in the body. (Courtesy of AIMME, IBV, ASCAMM, TECNALIA).

In conclusion, the use of standard implants not only implies to remove more bone tissue than with tailor-made implants, moreover there is a also higher probability of bone resorption. Therefore, the future revision surgery might be more complex since the patient has lost more bone tissue in the damaged area.
