Substantial legal requirements result in complex functional specifications for implant manufacturers. From the design, including materials, through production, the complete process chain must be documented and validated. Computer numerical controlled (CNC) high-tech machines support prosthetics manufacturers and allow highquality products to be manufactured, despite continuing high cost pressures.

Fig. 1 – Whether one-off manufacturing or serial production, the highest demands are placed on each individual medical engineering workpiece.
The idea for a new medical implant normally arises to meet a practical need. It is a long process before the implant can be produced in quantity. It can take approximately 12 to 18 months from the first drawing, including the design and job planning, through the completed, approved implant.

“After receiving, for example, a request for a new product from an orthopedist, we first design and develop a prototype,” explained Hans-Joachim Mahr, production manager at implantcast GmbH, Buxtehude, Germany, which produces primary and revision prostheses and tumor endoprostheses. “The CNC controller assumes an important function for the transfer of the CAD/CAM data to the actual production. It supports the idea and the virtual model in a workpiece that the orthopedist then can hold in his hands for the first time.”

Artificial bones made of titanium are mainly manufactured using a stock-removal process. At implantcast, for example, the production of hip implants runs on DMG turning-milling centers equipped with the Siemens CNC Sinumerik 840D sl automation system that produces complex single parts as well as large production runs on the same machine. Fast data processing is a particular advantage for the standard implants produced in quantity.

Each implant has an associated, specially-tailored set of instruments required for the implantation. This includes, for example, a surgical rasp used to prepare the bone (See Figure 1). implantcast also offers, for particularly complicated applications, such as cancer patients, the manufacturing of patient specific implants. In this case, both implants and instruments are cut to match the patient’s associated bone structure with the help of a CT or MRI scan.

The CNC user interface employed in this case, features many intelligent functions that assist the measuring of tools and workpieces. These measuring functions can be used both for the setup and process measurement, that is, the overall quality assurance during the machining.

Irrespective of the size of the run, the highest demands are placed on each individual medical engineering workpiece. It is obvious that no deviation may be permitted from rigorous quality and precision standards. Furthermore, the manufacturer is responsible for validating the complete production process, which means that even prototype construction must satisfy the same conditions as subsequent volume runs. (See Figure 2)

Implants Continue to Increase

Fig. 2 – Bone rasps are supplied to orthopedists along with the joint implants for the proper preparation of the bone.
The number of artificial bone implants is increasing continually, while operations such as the use of a replacement hip made of titanium also belong to the standard daily program in orthopedic surgery today. In 2010, one million artificial hip and knee joints were implanted in the US. Forecasts expect this number to exceed 4 million by 2030. This large number, however, does not obviate the fact that the highest quality and safety demands are placed on each individual medical implant.

Standard implants are now available in various sizes as required to match the physique of the patient. The CNC programming that defines the functions for machining the workpiece and all associated details concerning the form and technology, is very fast. Having a user-friendly CNC machine can save time and trouble.

Axel Robiller, manager of the stock removal department at implantcast, explains that “The human-machine interface has the familiar Windows style known to us from the PC world.” When all operator functions and cycles are supported with animated elements, the operator intuitively knows how the function is to be used. Dynamic vector graphics can be used when the animated elements do not sufficiently explain the purpose of the individual input values, such as the parameterization of complex cycles. Graphics reflect the current input values with their proportional representation.

The increasing complexity of turning-milling machines and production programs can increase the risk of programming errors that, under some circumstances, can cause machine damage. For this purpose, some systems offer a program simulation that shows the stock removal process in virtual 3D. In addition to the complete representation of the cutting actions, the expected machining time corresponding to the programmed technology values can also be displayed, before producing a single workpiece.

A short machining time, however, is critical for the economy of quantity production. The turning-milling center used at implantcast, with cross slides for Y and B axes, permits five-axis simultaneous machining and so significantly reduces the processing time. (See Figure 3)

Fig. 3 – Dual-channel representation in the CNC simulation brings transparency to the programming and the setup, even for complex workpieces.
The implants are manufactured in a single operation, with parallel machining on the main and counter spindles. For the complete machining, the system provides turning-milling functions that can be used in combination. The complete scope of all milling functions is available,from the cycle technology to the simultaneous free surface machining.

Simple Parts Don't Exist

Bone rasps are very complex workpieces because their basic form represents a significant challenge in machining, namely, a rasp body with many rasp teeth located on the contoured outer surfaces. The technology package used must ensure the required surface quality at high machining speed, even for difficult materials. And the motion control must produce milling results that meet the stringent requirements placed on medical implants and instruments.

Each individual part is then subjected to a comprehensive quality control. Sensing probes check the accuracy of the contour and the peak-to-valley height and scan the surface for even the smallest damage. To check the material structure, Xray examinations are also made on some parts. Only when all these hurdles have been overcome are the implants passed on for sterile packaging and subsequent shipping.

Never Forget the Patient

A patient with bone prostheses receives a prosthesis pass. This document allows the complete manufacturing process of the prosthesis to be reconstructed at any time. Whereas in other industry sectors, such as the automotive or food processing industry, products must be able to be tracked batch-related, this tracking requirement in the orthopedic world is specific to each individual patient’s medical implant. This is a major challenge, considering implantcast produces approximately 3,000 items each week.

Despite the extensive use of highly sophisticated technology, at the end of the process chain there is a suffering patient who must be helped. This difference between the manufacturer of turned, milled, and ground parts used in the medical technology and one that produces parts for machinery construction makes itself apparent, for example, when Mahr, the production manager, says that he himself delivers packages to the post office in the late afternoon. The trip to the post office is not actually part of his job. “The patient whose prosthesis is in the package would need to wait one day longer for his/her operation, were we not to act immediately,” he explains.

This article was written by Ryan Legg, Product Manager for CNC, Siemens Industry, Inc., Elk Grove Village, IL. For more information, Click Here .