Perfectly smooth

High-tech implants present new challenges to surface finishing techniques. Finely adjustable technology and cooperative ventures between manufacturers are delivering economical and high-quality results. Implant manufacturers can concentrate on their core business, and prevent fixed costs from running out of control.
Joint replacements, artificial heart valves or tooth implants – patients expect customised products which alleviate their suffering for a foreseeable period of time. In order to fulfil the expectations made of them, medical implants must overcome two challenges: First, the material and its properties must mimic those of the original body part as closely as possible. Secondly, the surface quality of the prosthetic must not change while it is in the body.

For both manufacturers and mechanical engineers, this is a challenge in terms of shaping artificial implants and providing them with a final surface finish. Various procedures, such as microblasting or grinding, are setting the agenda in this challenging area. Take Sandmaster for example. This Swiss company has developed a microblasting satellite turntable unit which performs precision work on rotationally symmetrical parts such as tooth implants and bone screws. The most striking features are the very precisely dosed blasting medium made of high-quality corundum or a bio-compatible calcium mix and the process documentation required for medical technology applications.

The dry blasting system is connected to an industrial PC for control purposes and works with three independent blasting systems. The fine blasting jets have a diameter of 1.2 or 1.8 millimetres and are each connected to their own pressurised container. The jets are attached to brackets and can be aimed individually at the implants. They move as required on up to four linear units in an enclosed blasting chamber. The implants are rotated and processed in the three stations in the blasting chamber. In the next enclosed station implants are cleaned with compressed air. According to the company, the cycle time for each process is around 15 seconds.

While the internal processes are all controlled by PC, the parts can be loaded and unloaded in a manual operation or handled automatically by a robot. The manufacturer has included a dosing mechanism for delivering the abrasive medium which supports the automatic shift operation. The feed system is configured for both high-quality corundum and also bio-compatible calcium-based abrasive.

Another process is tumbling. This abrasive method has been used for many years in medical device technology to finish implant surfaces. Rösler in Untermerzbach in northern Bavaria has developed a special form of this process called drag finishing which is designed specially to deburr, grind and polish delicate and complex parts. The media used are precision tools for delivering a fine to very fine finish on delicate parts and can be tailored to the application in question. The range from Rösler numbers more than 6000 media, which vary in shape, size and composition.

Last year, the Lower Franconia-based company formed the Orthopaedic Implant Alliance with the mechanical engineering company Haas. The two companies have pooled their expertise to offer user companies a technically compatible configuration of systems delivering high-precision grinding and high-end surface finishing. The grinding expert Haas shapes the workpiece, while Rösler provides the surface treatment in its drag finishing plant. The objective of the collaborative venture is to combine the two complementary grinding processes to deliver a low-cost turnkey solution.

This not only relieves large implant manufacturers of engineering work but also reduces fixed and operating costs. For example, an important parameter in geometric grinding is surface roughness which can be achieved economically when the shape of the implant in question and quality of its surface are specified. Haas configures the lowest cost system variant for the whole finishing process taking into account all the essential considerations such as reproducibility and accuracy of any repetitions. Another plus point, particularly for automated manufacture, is the account taken of the various interfaces between the geometric grinding, handover of the workpiece and the drag finish. Other features of the cooperative venture are joint research and development activities with the aim of further optimising the processes.

Haas relies on a Multigrind CB grinding machine for the shaping process. This has recently been made available with an integrated belt sander. The Multigrind allows two implants to be worked on at the same time. After clamping, aligning and sprue removal, the functional surfaces of the joint such as the patella and the condyle are created. Then the external contours and, where relevant, other cartilage-covered surfaces of the joint are milled. After this, in the version with the integrated belt sander the surfaces can be evened out without changing the clamping position of the workpiece. The sanding belt is controlled by the same geometry data as the grinding wheels. Complete processing in just one clamping position not only improves quality, it also saves time. After geometric grinding, the average roughness in key areas of the implants is from 0.4 to 0.2 micrometers. The time required for each implant type is from 12 to 30 minutes for two implants.

The implant components are finished in the Rösler drag finishing system which, depending on the version of the system, can process up to 28 workpieces in a non-contact process simultaneously. The implants are clamped onto workpiece fixtures. Depending on the size of the machine and the number of spindles, from 12 to 28 components can be processed at the same time for knee implants (femur). The grinding and polishing media surround the particularly complex shapes completely and process them constantly. The media are developed and manufactured by Rösler and play a major part in the process. For example, special miniature grinding media achieve a perfect surface finish even on the most delicate parts.

Andreas Beuthner


German Summary

Hightech-Implantate stellen die Oberflächenbearbeitung vor neue Herausforderungen. Feinjustierbare Technik und Herstellerkooperationen ebnen den Weg zu wirtschaftlichen Ergebnissen in Top-Qualität. Implantathersteller können sich auf ihr Kerngeschäft konzentrieren, ohne dass die Fixkosten aus dem Ruder laufen. Der deutschsprachige Beitrag ist nachzulesen auf www.meditec-international.com/medi0511oft