Milan Horáček, Ondřej Charvát, Tomáš Pavelka, Josef Sedlák, Martin Madaj
Brno University of Technology – Institute of Manufacturing Technology,
The paper deals with the production technology of knee joint replacement by using Rapid Prototyping technology. The aim of the work is to outline the manufacturing technology intended for prototype production with the use of Rapid Prototyping and Investment Casting Technology in orthopaedics and surgery of knee joint replacement. The research results should make an effective contribution to the attempts of minimizing of the invasive surgical procedure, shortening of the production of knee joint replacement as well as cost reduction. At present, the research is focused on the preparation of STL dates from CT (computed tomography) and verification of the production technology of prototypes made of available RP technology and its evaluation.
Key words: Rapid Prototyping, Investment Casting, Knee Joint Replacement, Computed Tomography
Rapid Prototyping (RP) technology is a procedure of direct prototype production by means of gradual adding of individual material layers. The procedure based on data of CAD (Computer Aided Design) file is relatively known nowadays.
Rapid Prototyping is basically used for the production of prototypes and patterns as fast as possible. The production of prototypes and patterns using classical technologies is very demanding and time-consuming. 
Quality increase of patient care, saving of time spent by treatment, and prevention of possible complications is a current trend in the up-to-date medicine especially. It is important that a patient is treated as fast as possible and that he undergoes necessary examinations and interventions as little as possible.  A production of total knee joints replacements seems to be a very interesting area for the RP methods application. In principle, a total knee joint replacement is a replacement of a diseased joint by a suitable implant. Round 800 000 knee joint replacements are implanted all over the world yearly nowadays. Current replacements are designed so as to respect human anatomy. In principle, each individual has a unique knee shape. The target should be, therefore, to adopt the applied knee joint replacement to the given shape of the particular patient at the most. It is possible to produce a specific type of the knee joint replacement using data gained from CT or MRI in this way. 
2. Problems of a Total Knee Joint Replacement
Miscellaneous questions ensuing from partial experiments and measurements concerning the RP methods application in the area of foundry industry have emerged in the last two years of studying the problems of Rapid Prototyping technologies at the Faculty of Mechanical Engineering of Brno University of Technology. An application of combined RP technologies and precision casting for the production of prototype parts in several precision casting foundries has been the issue in particular. Following question arose in relation to that: How to make the best use of the promising results with the application of RP methods in the foundry industry up to now and which foundry product could benefit significantly from this specific know-how gained up to now in particular? It has been found out on the ground of several reasons that it could be the production of knee joints replacement and of other implants possibly.
3. Knee Joint Anatomy
A knee joint ranks among the most complex joints in the body. Joint areas of three bones contact here - of femur, tibia, and patella. The femur transfers the body weight through the knee joint to the tibia. Big muscles (quadriceps) running on the femur anterior side straighten the knee (extension). Big muscles on the femur posterior side (hamstrings) bend the knee (flexion). The patella acts as a lever for quadriceps, which increase its effect. Joint areas of tibia and femur glide on each other at the movement in the joint, the patella moves up and down in the groove on the femur anterior area. Front and side X-ray images of a healthy knee are shown at Fig.1 and Fig.2. The space between joint cartilages is called the joint cleft. 
Fig. 1 and 2: X-ray images of the knee joint in the sagittal and side view (1-femur, 2-tibia, 3-fibula, 4-patella)
There are quite a number of causes that can result in a knee joint disease. After all conservative treatment (physiotherapy, bandage treatment, baths, and analgesics or anti-inflammatory drugs) has been exhausted, the implantation of an artificial knee joint - called also a total knee joint replacement - helps to improve the patient's quality of life significantly (the mobility restoration, the pain relief and the like.)
4. Total Knee Joint Replacement
Specially treated components (prosthesis) produced from biologically compatible, metal and plastic materials of a high strength are used for the total knee joint replacement. Cobalt, chromium, and molybdenum alloys are used most frequently out of metals. Plastic materials are made from a high-molecular polyethylene. Total implants have been used for around 30 years and their tolerance in the body has been very good. High requirements are imposed on the components production, their surface must have identical properties all the time, and it must be smooth and glossy. 
Damaged joint areas only, not the entire knee, are replaced at the total knee endoprosthesis in the present up-to-date medicine. In principle, the surgery lies in the replacement of the joint surface and joint cartilage only. Only a small part of the bone is removed, original ligaments, tendons, and muscles are retained and re-fixed. Various axial deviations (bandy or knock knees) can be corrected by a correct bone cutting off, removing osteophytes, and adjusting the ligament lengths at a surgery. Front and side knee views after the total knee joint replacement are shown in figures below. Polyethylene is used to fill to the joint cleft then. 
Fig. 3 and 4: X-ray images of an implanted knee joint replacement in the sagittal and side view
The metal femoral component is of the same size and shape like the femur end. The tibial component placed on the tibia apex has a metal base but the upper surface is always made from polyethylene. Part of the patella surface may be cut off and covered by polyethylene, too.
Components are fixed to the bone by a special substance (polymethacrylate) that is called "bone cement" frequently. Alternatively, some components have a porous surface, into which the bone can grow in. 
There is a wide range of models produced in different sizes for all prosthesis types. The bone shape, the weight, the physical activity of the patient, and the surgeon's experience and philosophy determine the selection of prosthesis. 
Fig. 5 and 6: Examples of joint replacement configurations
4. CT Imaging Technology
A CT image does not differ from a common X-ray image for the uninitiated. Tissues inside a body are visible in them, though. Hard tissues (bones, cartilages) are displayed in white and light grey colours in the images. Softer tissues (muscles, brain) are of a grey up to dark grey colour, lungs, bowels or ventricles are almost black. The tissues both on X-ray and CT images have the same colour scheme, because the computed tomography method uses X-ray as its base.
The problem of making a 3D model from a CT image has not been finalised as yet, the proof of which is a great deal of experts, research teams, and companies occupying themselves with it. It is due to the complexity of a human body, which contains many structures of irregular shapes and sizes that are positioned differently often and to the presence of foreign bodies at scans (fillings, implants).
Fig. 7: CT data conversion
5. Materials for the Production of Implants
Metals prevail absolutely as initial materials for the implants production at present. Even though the research has been looking for an optimum use of composites and plastics in the implants production for decades already, these substances have not been used widely not counting using polyethylene as an articular inserts. Out of non-metallic materials, only ceramics is being used in the day-to-day work, e.g. hip implants heads, not even this material has predominated yet and problems arise now and again.
Implant materials of all three world producers are derived from three basic metals: iron, cobalt, and titanium in the form of alloys that have the needed mechanical and anticorrosive properties. The most widely spread alloys are chromium-nickel austenitic corrosion resisting steels, a chromium-molybdenum alloy of cobalt, and an aluminium-vanadium alloy of titanium. These metals predominate and will keep on predominating in material base of artificial joints at most producers for a long time.
Out of many viewpoints that are applied at the selection of a metal material for an implant, the biological compatibility aspect is being stressed more and more. If we assess the biological compatibility pursuant to the behaviour of the bone tissue to the implant material, we can classify the known materials roughly to three groups listed in the Table below. 
BIO - PROPERTY
Corrosion - resisting- steel
Ti - alloy
carbon, Al- oxide, zirconium-oxide and Ti-oxide, TiN, Si3N4
Bio glass, bio ceramic,
Tab. 1. Bio-properties of materials
The biological compatibility is a property that is verified at the implant surface and live tissue interface. That is why the zone of interface between an implant and surrounding tissues is the most important place for determining the biological reaction to the implant and reaction of the implant material to the body environment. The metal material, out of which the implant is produced, does not make this interface all the time. Its surface is treated in various ways namely. Only some biologically inert and no biologically active materials can be used for the production of implants so that these chemical compounds must be coated to the implant surface. That is the reason why so many works are carried out all over the world that investigate how coatings of miscellaneous layers influence the parent base metal quality of the piece. 
6. Proposal of a New Approach to Problems in the Area of Foundry Industry
There are several companies producing knee joint replacements at the Czech market at present using titanium alloys mainly. This production is carried out by machining predominantly. Up to 80% material losses occur during the machining. Cobalt alloys are used more in the area of knee joint replacements production in the foundry industry at present. A very strict certification regarding the material quality mainly causes problems to foundries.
Joint replacements are produced in 6 sizes, for the left and right knee joint separately. Producers try to satisfy each individual patient's need at a surgery in this way.
The research activities of foundry engineering branch at the Brno University of Technology focus on finding a metal implant (original) of a knee joint replacement designed for a particular patient at present. It will be produced for him specifically. The data for the production of such an original would be based on the CT data of a particular patient.
Detailed Analysis of Individual Stages
Edited CT data acquired during the treatment of a patient after a serious accident are to the disposal thanks to a longer-term cooperation with St. Ann's Hospital in Brno.
We have commenced a close cooperation with the Faculty of Information Technologies at the Brno University of Technology presently, which has been engaged in CT and MRI data editing and 3D models creating long-term already. 3D data acquired in this way should facilitate an essentially different view of using RP technologies in the construction of knee joint replacements.
Fig. 8: 3D data of knee joint
It is the aim to make a new type of a knee joint replacement of its femoral part in the main in the framework of creating an STL file of knee joint replacements. It should preserve partially the shape of a total knee joint replacement of SVL type produced by Beznoska Company as standard and get a new surface shape partially in the area of the contact of the femoral part of the implant with an affected bone. It concerns cases above all when a bone is destructed fundamentally due to a car accident or a tumour (an oncogenous disease). This part should be adjusted according to the specific shape of the femur end depending on a CT image of a particular patient.
Fig. 9 and 10: Reconstructed STL model of knee joint replacement (SVL Beznoska)
The 3D file (STL) made and edited in this way is used for production ABS pattern by applying the FDM (Fused Deposition Modelling) method. The use of this semi-finished product is twofold. It is possible to use this pattern directly for the precision casting technology on the one hand. Basically, it is a wax pattern replacement. A risk of the ABS pattern or of the subsequent shell mould destruction is a disadvantage because it is necessary to reprint the ABS pattern in such a case, which prolongs the production process and makes it more expensive. Alternatively, it is possible to make a silicon mould using the ABS pattern, with the help of which wax patterns are cast in the vacuum chamber. Precision casting technology is applied hereafter. We get a knee joint replacement from the required biologically compatible material after casting in both cases.
Fig. 11 and 12: Printed ABS model of knee joint replacement a silicon mould
A subsequent verification, measuring, and financial evaluation should help to find out, whether this way of knee joint replacements production is feasible.
The technology of wax pattern production using a silicon mould and ABS pattern will be verified also for other artificial replacements than a total knee joint replacement with respect to very strict requirements concerning the use of new technologies in medicine. Patterns produced in foundries as a standard will be tested, too, to get data characterizing the quality of production as precise as possible. Very promising results have been achieved already in this area in the Institute of Manufacturing Technology (Dep. of Foundry Technology) from the viewpoint of dimensional accuracy of wax patterns made in this way above all .
Fig. 13: Prototype of wax pattern for investment casting technology
Some castings or wax patterns made using Reverse Engineering method will be digitalised as a back checking of total changes not only in dimensions but also in shapes, too.The above-mentioned technology will be applied also in the production of other total replacements, as an example of an acetabulum. This problem (the issue of shape and implant fixing to the bone above all) is being solved in cooperation with St. Ann's Hospital at present.
Fig. 14 and 15: Prototype of a new matal knee joint replacement
The main aim of the work was to make a total knee joint replacement using new processes other than those applied as a standard at present. The work endeavours to get such an implant that will be more suitable for a patient from the medical standpoint especially thanks to specific CT data of a particular patient being a base of the procedure. It is true to say that the target is to produce an implant "tailor-made" for the patient. If material and mechanical properties of implants made in this way were comparable with implants produced using standard technologies, it would mean a new way of producing a total knee joint replacement (any replacement) fast enough obviously for a particular patient under very advantageous financial conditions. This work is also taking into account the very fast development in the RP area and tries to make use of the resulting potential, which this technology offers and will keep on offering in the future. 
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