CASTING OF HIP RESURFACING Most of the Resurfacings are produced by a casting technique.
Investment casting has become a worldwide and distinctive industry. Casting gives a possibility for close accuracy of shape and dimensions in materials.
The term investment casting derives from the characteristic use of mobile ceramic slurries, or investments to form moulds with extremely smooth surfaces.
The long established lost wax process is used. “Lost wax” or “cire perdue” casting has been known for well over six millennia!
HOW IS A HIPRESURFACING CAST MADE?
(pictures 1-7 from Wright Medical)
1. Molten wax is injected into a die. 
Figure 1: WAX “TREES” READY FOR COATING
2. The wax is clothed in ceramic slurry. 
Figure 2: CERAMIC COATING
3. The wax is melted out and a ceramic mould is created 
Figure 3: COATED TREES
Figure 4: HEATING OF THE TREES
4. The hard ceramic mould is ready to receive the molten metal.
5. A chrome cobalt alloy is introduced. 
Figure 5: PRODUCTION OF CASTINGS
6. After cooling down the ceramic mould is removed. 
Figure 6: “TREES” WITH MOLTEN METAL
Figure 7: Machined Products (Conserve Plus Resurfacing Cups)
METALURGY
The metal in metal-on-metal prosthesis is made by a chrome, cobalt and molybdenum alloy. The “as cast” state (means that no other treatments to the metal took place) is characterised by a heterogeneous cored structure, with interdendritic regions containing block carbides. The carbides are M23C6 complexes, a conglomerate of 23 metal ions and 6 carbon ions. 
Figure 8: Carbide Structure
In this “high” carbide structure the carbides are like “icebergs” in the surface functioning like a ceramic layer on the metal. The wear that occurs in this system is an abrasive wear.
(These block carbides are found everywhere in the Chrome Cobalt Matrix.)
In the old, good working metal-on-metal prosthesis (Ring or McKee Farrar prosthesis) we can find these same carbides.
HEAT TREATMENT
HIP-treatment is hot isostatic pressing of the metal which often is used to increase the mechanical properties, like in Titanium
casting in aerospace products. Hot isostatic pressing (also called HIPping) exposes castings to high pressures and temperatures in order to minimise or eliminate internal porosity in the casting. It is also used because of the lower cost of production and the production of less expensive castings. 
Figure 10: Example of Surface Porocity
History and retrieval studies show that a high carbon, high carbide CoCrMo alloy is necessary for good “in vivo” wear properties.
Low carbide content can result in higher wear of the metal and in the extreme condition (NO carbides) it will lead to adhesive wear, which increases exponentially in time.
The discussion about best material “heat treated” or “not heat treated” is still going on. If this is only a philosophical discussion, time will tell, but more and more published papers show there is no difference as long the metal is high carbide and not “low” or “no=zero” carbides!
In vitro studies performed show no statistical difference as long the carbide content is high and not low.
(see study Nevelos e.a. in Hip International / Vol.14 no. 1)
Heat treatments where the carbide content stays high have no importance.
Of course clearance, mechanical properties of the prosthesis, roughness and roundness can affect the wear to.
The surface finish and diametrical clearance of the prosthesis will contribute far more to potential wear of the implant then metallurgy.
Forging of wrought materials, if high carbon and high carbide content, can work well, certainly in the normal head sizes of 28
and 32 mm. The Centerpulse Metasul metal-on-metal couple has proven the in vivo behaviour of this friction couple. The carbides are smaller M7C2 (7 metal ions and 2 carbon ions) and typical oval or round when looked at it with back scanned electron microscopy. 
Figure 9: Metasul Cup
The higher wear of such a low carbon-low carbide device was also illustrated by a comparative study between a Endoplus (low carbon) and the Metasul (Centerpulse – Zimmer)(high carbon) components.
That a high carbon initial metal is needed to obtain a high carbide structure Chrome Cobalt alloy is certified. A high carbide content metal-on-metal stands for good wear properties in friction.
The role of heat treatments in this matter is still discussed where some studies are favourite for heat treatments or others really reject these as bad.
For stems in total hip implants and jet engine blades, porosity is more important, and heat-treatments are really necessary.
Figure 11: Casting in AerospaceIt is certain that clearance and other factors are more important! What the perfect clearance is for a defined diameter is not yet known.
In vivo experience only can clear this out. Impaction of the cup with deformation is also a cut back in making clearances too small. Last updated: 15-10-2005
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