• Home
• Foundries
• Suppliers
• Newsletter
  • Content / Conditions
  • FAQ - Frequently Asked Questions
• Business Opportunities
  • Buy (Wanted)
  • For Sale
• Job Market
  • JOB OFFERS (vacancies)
  • RESUME
• News
• Technical Papers
• Events, Fairs, Courses, Training
• Forum

USEFUL LINKS

• Universities
• Associations, Institutes, Reseach Centers.
• Norms and Standards
• Quotations / Prices
• Other Links

Foundry Gate

• About Us
• Advertisement
• CONTACT US

following report investigates work being undertaken on the production technology of knee joint replacement using rapid prototyping technology. The aim of the work is to outline the manufacturing technology intended for prototype production with the use of RP and investment casting technology in orthopaedics and surgery. The authors - Milan Horáček, Ondřej Charvát, Tomáš Pavelka, Josef Sedlák and Martin Madaj of Brno University of Technology, Institute of Manufacturing Technology (Czech Republic) claim the research results should make an effective contribution to minimising the invasive surgical procedure, shortening the production of knee joint replacement and reducing costs. 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.

Rapid Prototyping (RP) technology is a procedure of direct prototype production by means of gradual adding of individual material layers. The procedure, based on CAD file data, is fairly well known nowadays and is used for the speedy production of prototypes and patterns as traqditional methods are very demanding and time-consuming⁽¹⁾.

An increase in the quality of patient care, saving treatment time, and prevention of possible complications is a current trend in medicine. It is important that a patient receives treatement quickly and with as minimal examination and interventions as is reasonably possible⁽²⁾. The production of total knee joint replacements is an interesting area for RP methods application. In principle, the procedure involves the replacement of a diseased joint by a suitable implant. Approximately 800,000 knee joint replacements are currently undertaken in the world. In principle, each individual has a unique knee shape. A futue target for the medical industry should be to adopt the applied knee joint replacement to the given shape of the particular patient, a move away from the present generic method. It is possible to produce an individually specific knee joint replacement using data gained from CT or MRI in this way⁽²⁾.  

Problems of complete knee joint replacement

Various questions from partial experiments and measurements concerning RP methods application in the foundry industry have emerged in the last two years 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 focus of attention. Knee joint replacements have been identified as an area where the use of foundry RP methods could make a significant difference.

Knee joint anatomy and replacement

A knee joint ranks among the most complex joints in the body. Joint areas of three bones contact here – femur, tibia, and patella. The femur transfers the body weight through the knee joint to the tibia. Large muscles (quadriceps) running on the femur anterior side straighten the knee (extension). Large 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 in figs. 1 and 2. The space between joint cartilages is called the joint cleft⁽³⁾.

There are many causes that 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 helps to improve the patient’s quality of life significantly with increased mobility restoration and pain relief⁽⁴⁾.

Specially treated components (prosthesis) produced from biologically compatible, metal and plastic materials of a high strength are used for total knee joint replacement. In terms of metals, cobalt, chromium, and molybdenum alloys are used most frequently. 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⁽³⁾.

The current trend is for damaged joint areas only, rather than entire knee, to be replaced at the total knee endoprosthesis. 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 figs 3 and 4. Polyethylene is then used to fill to the joint cleft⁽³⁾.

The metal femoral component is of the same size and shape as 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 also be cut off and covered by polyethylene. 

Components are frequently fixed to the bone by a special substance (polymethacrylate) - called ‘bone cement’. Alternatively, some components have a porous surface, into which the bone can grow in⁽³⁾.

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⁽⁴⁾.                                        

CT imaging technology

A CT image does not differ from a common x-ray image for the uninitiated, although tissues inside a body are also visible in them. 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 issue of making a 3D model from a CT image has not been finalised as yet, and a great number of experts, research teams, and companies are currently occupying themselves with the problem. The difficulty lies in the complexity of the 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)⁽⁵⁾.

Materials for the production of implants

Metals prevail as initial materials for implant production at present. Although research has been looking for an optimum use of composites and plastics in implant production for decades, these substances have not been used widely (not counting using polyethylene as an articular insert). Out of non-metallic materials, only ceramics are being used in the day-to-day work, e.g. hip implants heads, however, this material has not predominated yet and can be problematical⁽⁶⁾.

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 increasingly being stressed. 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 Table 1⁽⁶⁾.

Table 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. 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, which is 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 foundry industry

There are several companies producing knee joint replacements in the Czech market at present using mainly titanium alloys. This production is carried out predominantly by machining where up to 80% material losses occur. At present cobalt alloys are mostly used in knee joint replacement production in the foundry industry. A particular problem for foundries is the very strict certification regarding the material quality. 

Joint replacements are produced in six sizes, for the left and right knee joint separately. Producers try to satisfy each individual patient’s needs at a surgery in this way.

The research activities of the 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. It will be produced for the individual specifically. The data for the production of such an original would be based on the CT data of the particular patient concerned.

Detailed analysis of individual stages

Edited CT data acquired during the treatment of a patient after a serious accident are at the Faculty of Information Technologies disposal thanks to a longer-term cooperation with St Ann’s Hospital in Brno.

The faculty has been engaged in CT and MRI data editing and 3D models for some time. 3D data acquired in this way should facilitate an essentially different view of using RP technologies in the construction of knee joint replacements.

The objective is 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.

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 wax patterns cast in the vacuum chamber. Precision casting technology is applied hereafter. A knee joint replacement is obtained from the required biologically compatible material after casting in both cases.

A subsequent verification, measuring, and financial evaluation should help to determine whether this method of knee joint replacements production is feasible.

The technology of wax pattern production using a silicon mould and ABS pattern will also be verified for other artificial replacements 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 characterising the quality of production as precise as possible. Very promising results have already been achieved in this area in the Institute of Manufacturing Technology (Dept of Foundry Technology) from the viewpoint of dimensional accuracy of wax patterns made in this way⁽⁸⁾.

Some castings or wax patterns made using the reverse engineering method will be digitalised as a back checking of total changes not only in dimensions but also in shapes. The above-mentioned technology will also be applied 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) is being solved in cooperation with St Ann’s Hospital.

Conclusions

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 thanks to specific CT data of a particular patient being a base of the procedure. The target is therefore 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 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⁽⁸⁾.

References

1. Charvát O. ‘Možnosti aplikace metod RP s použitím technologie vytavitelného modelu’. Diploma thesis, Brno University of Technology, Faculty of Mechanical Engineering, Department of Foundry Engineering, April 2006. pp119.

2. Braun B. ‘Miniinvazivní operační přístupy a počítačová navigace’. [online]. Medical s.r.o. Czech Republic. February 2006. www.bbraun.cz/braunnoviny/HI-TECH/hi-tech_2006_02b.htm

3. Orthes, s.r.o. Totální endoprotéza kolenního kloubu. [Online]. www.orthes.cz/tkr.htm> [cit. January, 20. 2009].

4. Centrum Prof. Čecha s.r.o. Anatomie koleního kloubu.[online]. www.ortopedie-fyzioterapie.cz/ortopedicka-ambulance/umely-kolenni-kloub.html>[cit. January, 20. 2009].

5. Campr P. ‘Získávání 3D modelů lidských tkání z obrazových dat CT’. Diploma thesis. Pilsner, the University of West Bohemia. Faculty of Applied Sciences May 2005. pp58.

6. Beznoska s.r.o. Nauka materiálu pomáhá ortopedům. [Online]. www.beznoska.cz/indexm.php?a=text&id=8&lan=cz> [cit. January, 20. 2009].

7. Pavelka T. ‘Přehled pokročilých technik Rapid Prototypingu a jejich využití v oblasti lékařství’. Bachelor thesis, Brno University of Technology, Faculty of Mechanical Engineering, 2006. pp36.

8. Horacek M, Charvat O, Smrcka V ‘Rapid wax patterns obtained by RP and silicone mould technologies’. Proceedings of the 48^th Conference Portoroz 2009, pp.5.