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International PhD Foundry Conference PRODUCTION OF KNEE


									     Brno University of Technology
    Czech Foundrymen Society – CFS

International PhD Foundry Conference
              3rd June 2009


                         Ondřej Charvát, Josef Sedlák, Tomáš Pavelka
                                   Faculty of Mechanical Engineering
1.      Rapid Prototyping, Foundry and Knee Replacement – How they can find common

Rapid Prototyping is the automatic construction of physical object using solid freeform fabrication. The first techniques for
rapid prototyping became available in the late 1980s and were used to produce models and prototype parts. Today, they are
used for a much wider range of applications and are even used to manufacture production quality parts in relatively small
numbers. Some sculptors use the technology to produce complex shapes for fine arts exhibitions. [1]

Matters of the fastest and most economical way of producing the first prototype casting, eventually small series of verification
castings, are one of key activities of virtually all foundries nowadays. There are numbers of rapid prototyping technologies,
generally called RP technologies nowadays. For our topic are attractive Selective Laser Sintering (SLS), Direct Metal Laser
Sintering (DMLS) and Fused Deposition Modeling (FDM). These technologies can be used either for production of models as
finale products or as patterns for investment casting technology. [2]

Very attractive area for application RP technology appears to production of total knee replacement. Total knee replacement
(TKR), also referred to as total knee arthroplasty (TKA), is a surgical procedure where worn, diseased, or damaged surfaces
of a knee joint are removed and replaced with artificial surfaces. Materials used for resurfacing of the joint are not only strong
and durable but also optimal for joint function as they produce as little friction as possible. [3]

2. The last results about using RP technology and wax pattern at the Brno University of

The main objective of this part of the research was to verify the possibility of meeting the specified dimensional tolerances at
using the RP technology selected by us (producing the master pattern – making the silicon mould – making the wax pattern)
and to compare the achieved accuracies with those achieved at using the conventional technology (making the metal master
mould by machining – making the wax pattern). Moreover, these technologies were compared also from the viewpoint of
production rate and economy. Actual experiments were carried out partly in the laboratories of the Brno University of
Technology and partly also under working conditions in the FIMES foundry using the investment pattern technology. Only the
„intermediate product“ of the final casting making was compared, namely the wax pattern, as its accuracy and surface quality
is of cardinal importance for the final casting. Results were presented already at several past conferences including the
conference in Portoroz 2008. [4] [5] [6] [7]

Evaluation of the achieved dimensional accuracy of wax patterns

As has been said already, two technologies were used for the production of wax patterns: the RP and the conventional one
(filling of wax into metal master moulds). 20 wax patterns were made and measured (by casting wax into the silicon mould)
in total in the RP laboratory of the Foundry Department and the same number of wax patterns was selected then from the
series production of Fimes foundry (using the “conventional” method of making wax patterns by a low-pressure filling of wax
into metal master moulds). The three dimensions of wax patterns made by injection into the metal master mould and of wax
patterns made in the silicon mould were measured.

          Dimension A                                     Dimension B                                Dimension C

                               Fig. 1 Dimensions “A”, “B”, and “C” monitored at the wax pattern

Final evaluation of achieving the dimensional accuracy of wax patterns

It was found out by measuring the dimensional accuracy of wax patterns that tolerances specified in the drawing of a casting
can be met at using the production method of wax patterns with the help of the silicon mould and that this technology is
satisfactory from the viewpoint of satisfying tolerances specified by the customer. From the viewpoint of the process stability,
the values of capability index Cp exceeded 1, 33 and met thus the 4 Sigma capability condition.

However, the value of capability index Cpk of dimensions A and B, which deals with the process set up, did not achieve 1,33.
In other words, a correction of the master silicon mould dimensions and thus also of the master pattern used for the making
of the mould dimensions will be necessary. On that ground, it is not a stable process yet. However, the IT accuracy grades of
both dimensions satisfy the customers’ requirements. From the viewpoint of the surface quality, the wax patterns from the
silicon master mould achieved the quality that was very similar to the one from the metal master mould.

After the discussion with the customer, two methods of the subsequent “correction” of dimensions are possible:

1) by modification / approval of new final dimensions of a casting (by shifting dimensions A and B towards lower values – in
case of the dimension “A” from 58 to 57,6 mm and in case of the dimension “B” from 20 to 19,9 mm)

2) by modification of these two dimensions (A and B) at ABS patterns and by making a new silicon mould.

It can be stated in general that for small series of castings the production of wax patterns using the silicon master mould is a
fully satisfactory method both from the dimensional and surface quality viewpoints.

2.1.        Comparison of time demands of both production methods

                      Building the ABS master pattern: 3 hours

                     Puttying and grinding the pattern: 15 hours

                     Making the silicon mould: 30 hours

                     Casting the wax pattern: 4 hours

                                                                                     The first wax pattern is ready in c. 52

                   Fig. 2 Wax pattern production using the method RP from the viewpoint of time demands
The total production time from the ABS master pattern building to the wax pattern casting lasted approximately three days in
practice. The master pattern building itself took less than three hours, but its completion took longer. It takes one day to
spray the pattern with the putty, its perfect hardening lasts 12 hours, and the subsequent grinding 3 hours. The production of
the entire silicon mould took approximately 2 days. This time is influenced by the silicon material solidifying for approximately
15 hours after casting above all. The casting of the wax pattern itself in the silicon mould lasted several minutes. The total
time of 4 hours corresponds to the real time, which includes the wax melting, the casting itself, and the wax cooling
(solidifying) in the mould. It is possible to disassemble the mould afterwards and to take out the ready prototype.

We came to the conclusion after the consultation with the investment casting foundry Fimes, a.s. that the resulting time (52
hours) of the first wax pattern production at using the silicon mould is much shorter than at using the conventional
technology, that is to say at injecting the wax material into the metal master mould. The foundry is capable of producing the
first wax pattern within c. 5 days.

2.2.        Comparison of economic demands of both production methods

Production costs of the first wax pattern from the economic viewpoint can be estimated as follows:

                      Building ABS master pattern: 60,- EUR

                     Puttying and grinding the pattern: 40,- EUR

                     Producing the silicon mould: 90,- EUR

                     Casting the wax pattern: 10- EUR

                                                                                                         The price of the
                     first wax pattern

                                                                                              amounts to 200,- EUR
                      Fig. 3 Wax pattern production using the RP method from the economic viewpoint

It was established from the documents of the foundry Fimes, a.s. that the price of the first wax pattern production using the
conventional technology, that is to say the injection into the metal mould, exceeds 1250,- EUR. It involves costs spent for the
production of the metal master mould above all.

From the economic viewpoint, a considerable cost reduction of the first wax pattern production (from 1250 to 200 EUR) at
using the silicon mould is evident clearly.

3. Production of Total Knee Replacement by using RP technology

During the last two years of studying Rapid Prototyping technology at BUT, there have come up different questions that kept
arising from partial experiments and measurements concerning usage of RP methods in foundry. Recently it has been
particularly application of RP technology and investment casting combination for production of pattern prototypes in several
investment casting foundries. Referred to these issues there has come up a question: How to use the best the existing good
outcomes in applying RP methods in foundry and to which particular foundry product could these acquired pieces of
knowledge help in some significant way. From several reasons, there has been discovered that it could be the production of
total knee replacements, eventually other implants.

3.1.Anatomy Knee

The knee is not a simple hinge or pivot joint; when one walks, the surfaces of the knee roll and slide against each other. With
every step, the knee experiences compressive forces several times that of the body weight.

The knee joint is comprised of several bones, which can be seen in Figure 4. Two bones, the femur and tibia, bear the weight
of the body while the patella and fibula provide support and allow for greater mobility. The thighbone, or femur, is the longest
and strongest bone in the human body. Articulating with the femur is the tibia, or shinbone, which is located at the front of
the leg and serves as the weight-bearing bone of the lower leg. For protection, the articulation between these two long bones
is covered in a shock-absorbing, low-friction layer of articular cartilage. The patella, commonly known as the kneecap, is a
small, triangular, disc-like bone that sits at the front of the knee. This bone protects the knee joint and provides leverage for
the muscles of the leg. The fibula is a long slender bone located laterally and posteriorly to the tibia. It is not a part of the
knee joint itself; instead, it articulates with the tibia and helps to stabilize the ankle and acts as an anchor for muscle
attachment. [8]


                                         3                                     3

                            Fig. 4 A frontal (left) and lateral view (right) of the bones of the knee
                                             (1-femur, 2-tibia, 3-fibula, 4-patella)

A fibrous tissue capsule surrounds the knee joint. Depending on a person's activity level, the knee capsule contains 0.3 to 1
mL of synovial fluid. The viscous synovial fluid serves multiple purposes. It acts as a lubricant to reduce friction between the
knee joint surfaces much like motor oil lubricates engine components. It also behaves like a shock absorber by distributing
stresses in the joint due to impact and motion. Finally, synovial fluid serves as a transport medium by removing waste and
carrying nutrients to the articular cartilage.

Articular cartilage is a specialized form of cartilage that reduces joint friction, resists wear, and distributes forces to the
underlying bone. The cartilage has a specialized structure with a relatively hard outer surface of collagen fibers on an
underlying porous collagen structure. This structure allows the cartilage to have a wear-resistant surface on a flexible, shock-
absorbing substrate. The cartilage of the knee is living tissue but is avascular and thus does not have its own blood supply. It
receives all of its nutrients from the surrounding synovial fluid. Once damaged through arthritis or injury, articular cartilage
has limited regenerative capability. [8]

3.2.Total Knee Replacement

 Total knee replacement employs specially designed components, or prostheses, made of high strength, biocompatible,
metals and plastics, to replace the cartilage in your knee. The metal that is most commonly used is an alloy of cobalt,
chromium and molybdenum. The plastic is ultra-high molecular weight polyethylene. These materials have been used in joint
replacement for about 30 years and their behavior in the body is well-known. The components are very precisely
manufactured and the surfaces are congruent, smooth and highly polished. In this manner, congruent, smooth, low-friction
surfaces are restored to the knee. [9]

In modern total knee replacement surgery, only the worn-out cartilage surfaces of the joint are replaced. The entire knee is
not actually replaced. The operation is basically a "re-surfacing" (or "re-tread") procedure. Only a small amount of bone is
removed, the collateral ligaments are left intact, and the muscles and tendons are left intact. Alignment abnormalities can
usually be corrected during the operation by adjusting the direction of the cuts of the bones, removing bone spurs
(osteophytes), and lengthening tight ligaments. Front and side views of a knee following total knee replacement are shown in
Figure 5. Note that the smooth surfaces of the joint are restored. The joint space is now comprised of polyethylene. The
operation only replaces the worn surfaces of the joint. The ligaments, tendons and muscles are retained.

                            Fig. 5 Front and side views of a knee following total knee replacement

The femoral component is metallic, and is similar in size and shape to the end of the femur bone (thigh bone). The tibial
component, which goes on the top of the leg bone (or tibia), may have a metallic base, but the top surface is always
polyethylene. The undersurface of the knee cap (patella) is cut flat and covered with another polyethylene component. Since
metal covers the surface of the femur (thigh bone) and polyethylene covers the surfaces of both the tibia (leg bone) and
patella(knee cap), total knee replacement involves metal-on-plastic articulation.

The components are attached to the bone with a specialized polymer (polymethylmethacrylate), commonly referred to as
"bone cement". Alternatively, some components have a porous texture on their under-surface, into which the bone can grow.
This method of attachment is referred to as "porous ingrowth". [9]

                                          Fig. 6 Examples of Total Knee Replacement

4.          Using Computed Tomography (CT) for STL File

Computed tomography (CT) is a medical imaging method employing tomography. Digital geometry processing is used to
generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken
around a single axis of rotation. A form of tomography can be performed by moving the X-ray source and detector during an
exposure. Anatomy at the target level remains sharp, while structures at different levels are blurred. By varying the extent
and path of motion, a variety of effects can be obtained, with variable depth of field and different degrees of blurring of 'out
of plane' structures. [10]

                                                  Fig. 7: CT scan illustration

After saving CT or MRI image data, they should be transferred to RP or RE laboratory. The next step is processing these data,
which is a very complex and important step that the quality of the final medical model depends on.

For this step engineers need software package (Mimics, 3D Doctor) in which they can make segmentation of this anatomy
image, achieve high resolution 3D rendering in different colours, make 3D virtual model and finally make possible to convert
CT or MRI scanned image data from DICOM to .STL (Stereolithography) file format, which is universally accepted RP file
format. These software packages allow making segmentation by threshold technique, considering the tissue density. In this
way, at the end of image segmentation, there are only pixels with a value equal or higher than the threshold value.

The virtual model of internal structures of human’s body, which is needed for final production of 3D physical model, requests
very good segmentation with a good resolution and small dimensions of pixels. [11] [12]

                                          Package of CT


                                       Making of 3D model

                                                   Fig. 8: Transfer CT data

5.          Materials for Production of The Knee Replacement

Metal parts of the implant are manufactured of Cobalt-chrome or Titanium. There is no agreement as to which is the better
metal. But there is universal agreement that it is better if the metal part that moves on the plastic is made of cobalt chrome.
The most important problem in the complex field of implant design is the issue of metal and plastic wear (resulting from parts
moving on each other) and the tiny particles produced by such wear. These particles may cause adverse responses in the
surrounding tissues and bone, resulting in loosening of the implant. The greatest amount of particles is produced by a
titanium metal part moving against a plastic part. It is an area of continuing research.

The plastic parts of the implant are made of high-density polyethylene which has proved very acceptable over the years.
Efforts are underway to develop “improved” polyethylenes. [13]

         BIO - PROPERTY                  METALS                                CERAMICS, PLASTICS
                                Corrosion – resisting- steel
       Biological tolerance            Cobalt alloy
          Biological inert                                     carbon, Al- oxide, zirconium-oxide a Ti-oxide, TiN, Si3N4
                                         Ti - alloy
         Biological active                                                     Bio glass, bio ceramic,
                                            Tab. 1 Biological property of materials

Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a
material without specifying where or how the material is used (for example, that it elicits little or no immune response in a
given organism, or is able to integrate with a particular cell type or tissue), or to more empirical clinical success of a whole
device in which the material or materials feature. The ambiguity of the term reflects the ongoing development of insights into
how biomaterials interact with the human body and eventually how those interactions determine the clinical success of a
medical device (such as pacemaker, hip replacement or stent). Modern medical devices and prostheses are often made of
more than one material so it might not always be sufficient to talk about the biocompatibility of a specific material

6.          Proposal of a new approach to the production of knee replacement

Production of knee replacement is very often carried out by machining nowadays. Speially knee replacement from Ti-alloy.
During machining is lost up to 80 % of material. In the investment casting area is more often used cobalt-alloy. Total knee
replacements are produced in several sizes (5-7) by the size of bad bone.

At the Foundry Department is solved idea how to gain and produce new type of knee replacement for a particular person
(patient). Data for this new replacement should by gain from CT of the particular person.

Next steps contributing to gain new piece of knowledge are illustrated in the following table.

                                                                CT data

                                                       Transformation CT data
                                                               to STL

                                                      Making STL file of knee

                            Printing of 3D model by                               Printing 3D model by
                                   using FDM                                            using SLS

           Production of silicone                Production of knee              Knee replacement from
                  mould                         replacement castings                    Ti-alloy

             Production of wax               Finishing and evaluation            Finishing and evaluation

             Production of knee
            replacement castings

          Finishing and evaluation

                                      Fig. 9 Graphic illustration of expected experiments

We are solving theme about producing new type of total knee replacement by using new technique than standardly used. The
most important is fact that they are used CT data from real patient. We want to find the knee replacement which will be
better for patient from medical view point. In other words we would like to produce the concrete knee replacement for the
concrete patient. Of course, it is necessary to keep the same quality of material and mechanical properties as at knee
replacements which are produced by machining. But if it would by able to keep this standard quality, we would have a new
way how to produce the knee replacement very fast and very cheap (low cost). This theme take into consideration very fast
development of RP machines therefore we want to utilize all potential of RP technologies nowadays.

In next figures are illustrated first experiments about construction new type of the total knee replacements. For first one was
used standard knee replacement from Beznoska Company. Second one has presented construction of shell replacement. For
experiment is used software CATIA.

                                   Fig. 10 Total knee replacement in the design stage

7.        Literature

[1]    Rapid prototyping. World Wide Web: 15.3.2009
[2]    Horáček, M., Charvát, O., Smrčka, V. Rapid wax pattern obtained by RP and silicone mould technologies. The 48.
       International Foundry Conference Portoroz. Conference proceedings. 10.-12.9. Portoroz 2008, ISSN 1318-9123
[3]    What is knee replacement? World Wide Web: 15.3.2009
[4]    Horáček, M. Cileček,J. Capabilities of Investment Casting Technology – Zmožnosti tehnologije precizijskega lii.ea,
       Livarski Vestnik, Volume 54, 4 / 2006, pp. 175-186
[5]    Horáček, M. Cileček, J. Accurate and Complex NET-SHAPE Castings for Challenging Markets”, Foundry Trade Journal,
       U.K., Volume 180, Nr. 3641, 2007, pp. 32-35
[6]    Charvát, O.-Horáček, M. Possibilities of Using Rapid Prototyping Methods with Investment Pattern technology, 13th
       International Conference, April 25th to 27th, 2007, Tatranské zruby, Vysoké Tatry
[7]    Horáček, M.-Charvát, O.-Michalec, P. Combination of Rapid Prototyping and Investment Casting Technologies- a
       route to “Rapid Castings”, 47th International Foundry Conference, Portoroz, Slovenia, September 12th to 14th, 2007
[8]    Bern Jordan. J., Comparison of Four Treatments for Patients with Severe Knee Cartilage Damage. World Wide Web: 15.3.2009
[9]    What is Total Knee Replacement? World Wide Web: 15.3.2009
[10]   Computed tomography. Word Wide Web: 15.3.2009
[11]   Milovanović, J.Trajanović, M. Medical applications of rapid prototyping. World Wide Web: 15.3.2009
[12]   Campr, P. Získávání 3D modelů lidských tkání z obrazových dat CT: Diploma thesis. Pilsner, The University of West
       Bohemia. May 2005. 58 p.
[13]   Arthritis of the knee joint. World Wide Web: 15.3.2009


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