Case Studies

Hip Joint

Client: Major Implant Manufacturer

Background:
Artificial hip joints have historically been manufactured from various titanium alloys or a cobalt-chrome material called Vitalium. Both materials offer excellent bio-compatibility and strength characteristics. In use, the component is inserted into the femur (the thigh bone) causing that portion of the bone to become more rigid. On some hip joint designs it was determined that this increased rigidity caused unnecessary post operative discomfort. To alleviate this problem a slot was added to the distal end of the component.

Manufacturing Challenge:
Both materials used for hip joints, titanium and Vitalium, are extremely difficult to machine using conventional techniques. Abrasive grinding or milling is costly and slow. Non-conventional techniques such as EDM are effective but are also slow and create a secondary problem. A recast layer is formed at the electrode/part interface which is not bio-compatible and must be removed by some secondary process.

Solution:
Electrochemical Grinding (ECG) overcomes all of these obstacles. Electrochemical Grinding is very productive compared to EDM and does not introduce unwanted bio-compatibility issues. Electrochemical Grinding is also much more effective than conventional methods (abrasive grinding or milling) on exotic materials. The process does not generate excessive heat, creates no burrs eliminating secondary operations, and machines well on electrochemically reactive materials such as Vitalium. The internal slot finish on a Vitalium hip joint is about 4 - 8 Ra.
 

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