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					       Design of Customised Medical Implants by Layered
                       Manufacturing.
                              S F Khan, K W Dalgarno
                   School of Mechanical and Systems Engineering,
                       Newcastle University, NE1 7RU, UK

                                     Abstract
     The utilisation of Layered Manufacturing (LM) and CAD for medical
     applications had been reviewed in this paper. It highlights a number of
     researches describing the implementation of LM and CAD processes in
     medical applications and also described the design process for
     customised medical implants by layered manufacturing. The paper then
     examines the feasibility of designing of new implants in mandibular
     bone based on bio ceramics and new fixation methods. It describes the
     requirements for these bone implants and the processing steps required
     to develop the implants. This paper then concludes by outlining future
     work.

1. Introduction

Layered Manufacturing is a technique of fabricating parts by additive method. A 3D
model generated in a CAD system is sliced into 2D profile by the software in the LM
machine. The sliced-layers of the model are then added one layer at a time onto the
build platform by the LM machine until a 3D part is produced. Main LM technologies
used are stereolithography (SLA), laser sintering such as Selective Laser Sintering
(SLS), Direct Metal Laser Sintering (DMLS) and Selective Laser Melting (SLM);
Fused Deposition Model (FDM); 3D ink jet printing (3DP) techniques such as
Sanders ModelMaker™, Z-Corp Ink Jet System™, 3DS Multi-Jet Modeling™, and
3DObject PolyJet™; and electron beam melting (EBM). LM is largely being used to
produce prototypes and functional parts in the engineering and manufacturing
industries. Since it inception in 1986, numerous engineering and manufacturing
applications utilizing LM have been documented and researched. In LM technologies,
various applications in fields which are not traditionally associated with engineering
and manufacturing have opened up such as in architectural and medical modeling,
artistic creation [10, 36, 48] and historical restoration work [4]. The main applications
are in the evaluation, visualization, validation, form fitting and functional testing in
the early stage of product development process as well as tooling aids (refer to Fig 1)
[39]. Only about 12% are in the Rapid Manufacturing application with growth
expected in its application for customised parts. This will be particularly relevant to
fields such as the medical and healthcare industry.

The principle pulling point of utilizing LM for fabrication and manufacturing parts in
this field is that LM systems can produce parts of almost any geometrical complexity
with relatively minimal tooling cost and time as well a significant reduction in
requirements of technical expertise. The removal of tooling will reduce the cost at
early stages of the product development process and avoid the lead times imposed by
tooling. Minor or substantial changes to part geometry during the course of design
will not incur the times and costs of producing new tooling [11, 26].




         Fig. 1 LM Applications [39].

2. LM technologies in Medical Applications

Numerous researches have documented the use of LM in medical applications. In
general LM technologies for medical applications can be categorized as follows:-
    visualisation and surgical planning
    customized orthoses and protheses implant/replacement
    scaffoldings and tissue engineering
    drug delivery and micron-scale medical devices

   2.1 Visualisation and surgical planning

LM systems can be used to produce physical models of parts of the human anatomy
and biological structures to assist in surgery planning or testing as well as for
communication. 3DP system can be used to produce coloured medical models (Fig.2)
to enhance learning for students in the classroom as well as for researchers. It can be
used to better illustrate the anatomy, allow viewing of internal structures and much
better understanding of some problems or procedures [11, 26]. The possibility to mark
different structures in different colors in a 3D physical model not only well suited for
teaching purpose but can be very useful for surgery planning. The medical models
generated using LM systems plays a vital role in providing tactile interaction for the
surgeon with patient anatomy prior to the operation. This facilitates the surgeon in
planning and performing complex pre-operative surgical procedures and simulations(
Fig. 3). When using medical models, the surgeon will have a chance to study the bony
structures of the patient before the surgery, to increase surgical precision, to decrease
time of procedures and risk during surgery as well as costs and also to predict possible
problem that may arise during operation [11]. As compared to conventional MRI
assessment, medical models allow a more acceptable judgement over the feasibility of
diagnosis and procedural planning [1].
  Fig. 2 Left: CAD image - Right: Medical                  Fig. 3 Biomodel use in pre-
  model [47].                                              surgery planning [46].


    2.2 Customized orthoses and protheses implant/replacement
Since every patient is unique, LM systems are used in fabrication of personalised
implants for reconstructive and plastic surgery. Due to the inherent strength of LM
technologies to fabricate complex geometry, it is very easy to manufacture custom
implants. The model can be used as the custom implant itself or of the implant.
Winder et al. [37] and D’Urso et al. [7] successfully used implants model fabricated
using LM systems as a master model for reconstructive surgery of a skull defect. They
claim there is a reduction on operating time and excellent outcome at ‘reasonable’
cost. The capability of LM to customise implants to quickly fit into a patient's unique
size is a great advantage. Hip sockets, knee joints and spinal implants could greatly
benefit from this. He et al. [15] claims that the composite hemi-knee joint prosthesis
(Fig. 4) reconstructed using LM are accurate to within a maximum tolerance of 0.206
mm. It fitted well and matched with the surrounding tissues, in particular to the lower
tibial knee joint. Chang et al [4], Eggbeer et al [9] , Kruth et al [21] and Bibb et al [2]
had demonstrated the use of LM technologies in dental applications to be viable as it
can improved the speed, quality and efficiency.




            a                            b                              c

Fig. 4 SL pattern of a hemi-knee joint (a) as a master for the titanium joint (b)
which implanted in the femur bone (c) [15].
    2.3 Scaffoldings and tissue engineering
With it ease of fabricating internal structure, LM technologies are ideal for generating
implants with special geometrical characteristics, such as scaffolds for the restoration
of tissues [18] as shown in Fig. 5. Scaffolds are porous supporting structures used as
transplantation of tissue cells for the rapid and guided growth of new tissue in
damaged or defective bones of the patient. 3D scaffold fabrication techniques for
tissue engineering has been used for the last 30 years with less favourable success due
to the lack of mechanical strength, no assurance of interconnection channels and
uncontrolled pore size [18, 40]. Hence in order to for tissue reconstruction, scaffolds
must have interconnected macro and micro-channels with high porosity and adequate
pore size to facilitate cell seeding and diffusion of both cells and nutrients throughout
the whole structure. These characteristics are required in producing scaffold for tissue
engineering. LM has been studied by numerous researchers as a fabrication choice in
constructing 3D scaffolds to guide the development tissue culture both in vivo and in
vitro [13, 17, 33]. Armillota et al. [1], Hollister [17], Hutmacher et al [18], and Yang
et al [40] have demonstrated and rationalised the use of LM technologies in scaffold
design for tissue engineering to be viable, cost effective and practical.
Yang et al [24] state the advantages of using this technique in designing and




             a                                b                                c

Fig. 5 LM femur with scaffold (a) [27], (b) [17] and mandibular scaffold implant (c) [45]


fabricating scaffolds. Fig. 6 illustrates a scaffold tissue engineering proposed for
reconstruction of a mandibular implant. LM systems like SLS, FDM, and 3DP have
proved to be suitable for fabricating controlled porous structures for use in tissue
engineering. LM technologies have contributed significantly to the field of
scaffoldings and tissue engineering through the use of biomaterials [20].




               a                                    b
Fig. 6 Layered manufactured scaffold tissue engineering (a) used in reconstruction of
mandibular implant (b). [42]

    2.4 Medical devices and Drug delivery systems

Another application of LM techniques is in fabricating medical devices and drug
delivery systems. Skull defect and dental implant are restoration process that requires
detailed planning and high accuracy in implant placement. Hence surgical guidance
aids are required. Sarment et al. [27] and Di Giacomo et al. [6] investigated the use of
SL surgical guides (Fig. 7) to accurately place dental implants and concluded that
there is a significant improvement in implant placement. Ruppin et al [29] claim that
LM fabricated surgical guides are comparable to optical tracking system and in
agreement other researches on accuracy in computer aided surgery for implant. Bibb
et al [2] studied the use of LM to fabricated removable partial dental (RPD)
framework for retaining artificial replacement teeth in the oral cavity. The patterns
produced were deemed by a qualified and experienced dental technician to be a
satisfactory fit and comparable with those produced by expert pattern technicians. In
the study, the stiffer patterns produced by SL were easy to handle, were accurate, and
produced satisfactory results. And Tay et al. [35] claim that an actual prosthetic
socket fabricated using an FDM system (Fig. 8) provides an acceptable degree of
comfort, and clinical trial confirmed the viability of fabricate prosthetic socket using
FDM technology. Besides medical devices, LM methods are also use to produce drug-
delivery system like oral tablet. Rowe et al [28], Leong et al [22] and Low et al
[24]demonstrated the possibility of building oral tablet that controlled specific and
precise drug delivery by using SL.




               a                        b                           c
  Fig. 7 SLA surgical guide (a) and the constructed surgical guide in CAD (b)
  [29] fabricated guide (c) [43].




                             a                                           b

   Fig. 8 Below-the-knee Selective laser sintering prosthetic (a) [44] and FDM
   total knee replacement (b) [41]
3. Implementation of LM and CAD in medical applications

The development of medical images into 3D models as a tool to help practitioners
visualize 2D images has contributed to the development of a new methodology in
fabricating medical parts. Medical imaging provides important data of various body
structures for diagnostic reasons. These data can be used to obtain geometrical
information of the body structures for three-dimensional modeling. CT and MRI
images of various structures from conventional hospital scanners provided the input
data for commercially available software packages. The image data are visualised,
segmented and three dimensionally reconstructed. Solid models can then be generated
for use in CAD systems. The generated models incorporating tissues of interest can be
imported into a CAE environment for further CAD modelling and finite element
analysis. That environment also serves as a platform for conversion to a readable
format by rapid prototyping systems. LM systems are then used to produce the
physical medical models.
The integration of Medical Imaging, CAD, FEA and LM has been presented as a
realistic method for modeling and designing various body structures in medical
applications. Hieu et al [38], Gopakumar [14] and Lohfeld et al [23] in the study of
designing cranial and maxillofacial implant claim that there is a reduction time in
implementing the integrated approach of Medical Imaging, CAD, FEA and LM for
fabricating personalised medical implants. The common theme in Hieu et al [16],
Gopakumar [14] and Lohfeld et al [23] methodology of this approach is illustrated in
Fig. 9.


   Capture Patient Data                   Process Scan data
                                                                    Export data in suitable format
                                  Medical Modeler Software          IGES, SSL, STL, DXF, 3DS
Scan Images from CT, MRI




                                                              Modeling in CAD and FEA
                    Export data in RP format
                                                         Commercial CAD/CAE Solid or NURBS based
                  STL (Binary or ASCII)                  parametric Modeler for design and analysis.



                LM of Medical Implant
                SLA, SLS, SLM, FDM,3DP.EBM




   Direct Method                                Indirect Method


  Fig. 9 LM and CAD methodology adapted to design and manufacture of implants

However Starly et al [33] highlight a more comprehensive approach that generate
CAD models from scan images. Integrated slice software in LM is used as an
interface between the STL file generated from CAD modeled implants and the LM
machine. It allows the user to specify the attributes for the LM to build the physical
model.

4. Implants design for mandibular bone based on bioceramics and new fixation
methods

Bone is a living tissue; considered as a composite material, it comprises of trabecular
(cancellous) bone and cortical (compact) bone. It has the capability of healing and
remodelling. It will respond with adaptation in its structure to loading stress or injury
such as fracture. However, bone is unlikely to remodel itself in major losses cause by
trauma, cancer, congenital abnormalities or bone deficiency. Most of these types of
major bone repairs are treated by grafting which uses the patient’s own bone
(autografts) or donor bone (allografts). The need of further surgery, risk of transmitted
disease and limited material from donor site pose some limitations to the current
practices [25, 34, 32]. Synthetic substitutes using metal, ceramic, polymer and
composites are currently been use to overcome these limitations. Bioceramics are
most frequently used in scaffold manufacturing and hard tissue implants within bones,
joints and teeth. Bioceramics have the basic chemical composition akin to natural
bone. Goodridge et al [13] and Dyson et al [8] have demonstrated the feasibility of
using bioceramics for use as bone substitute. The design of the implant must take in to
consideration     biocompatibility,      mechanical     properties,     cost     effective
manufacturability and process [19] as well as an accurate fit that requires minimal or
no healthy bone removal. Furthermore, the bone variations in material and mechanical
properties are dependence on location and function [12, 31]. Hence a promising
method of manufacturing implants is through LM. This paper proposes a LM and
CAD methodology to design customised implants of mandibular bone. The process
flow is as shown in Fig 10.
     Capture patient data


    Process Scan data


    Export data in suitable format                 Create Library of 3D cellular microstructure


    Modeling in CAD and FEA                        Selection of microstructure



                                     Boolean Operation


          Dense Implant              Implant with Microstructure




                     Export data in RP format


        Verify data RP file before uploading into LM system



                     LM of Medical Implant


                            Post process


                 Customised Bioceramics Implants


  Fig. 10 LM and CAD methodology for implants design in mandibular bone based
  on bioceramics and new fixation

Normally most of the fixation methods for mandibular reconstruction and fracture
system consist of drill bits, plate bending forceps, plate holding forceps, plate cutters,
cannulae, taps, countersinks, plate bending pliers, plate cutters, drill guides and
screwdrivers to facilitate the placement of screws and modification of plates. The
implant for reconstruction is secured in place by plate and screw. However, this paper
proposed a design that integrates the fixation method into the implant. This is derived
from the advantages of LM. A lower jaw bone model in STL format was imported to
the CAD to design an implant. The lower jaw is non-defective and was obtained from
a secondary source. A simulated defective section was created on the jaw by cutting a
section of it in the CAD software (Fig. 11).
                  Fig. 11. A section cut out for implant.



Several joints are proposed as shown in Fig 12. Type (a), (b), and (c) joints rely on
rapid bonding to secure the implant while type (d) uses conventional screws.




        a                      b                    c                      d

Fig. 12 Different types of joint under consideration.


The design of the implants will need to incorporate interconnected controlled channels
in order to promote bone growth. Bignon et al [3], Woesz et al [38] and Chu et al [5]
indicated that there are significant correlation between micro- and macro-porosity as
well as controlled channels with bone response and mechanical properties.

5. Conclusion and Future work

The LM and CAD approach for medical applications had proved to be viable and
promises potential benefits as demonstrated by the numerous researches conducted.
Precise customised bioceramic implants can reduce the removal of healthy bone,
eliminate the need for bone grafting, and promote effective planning of implantation.
The future work to be undertaken should be related to finding suitable bioceramics’
material and mechanical properties that adhere to the requirements of successful bone
growth. In addition, further work will be conducted to propose new fixation methods
and assembly. This will involved FEA to determine the strength of the propose
fixation joint as well as of the implant itself.



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