Orthopaedics Machining Case Study
The machining issues involved in successful manufacturing of orthopaedic implants are examined together with some of latest machining techniques being employed.
Innovation is the driver of industrial growth and doctors, consultants and surgeons, who are always seeking better treatment for their patients, are driving the orthopaedic implants market. The latest stand-alone anterior lumbar fusion cage implant used in spinal surgery from Surgicraft is manufactured from biocompatible polyetheretherketone (PEEK OPTIMA, Invibio Ltd, Thornton, UK). The benefits this implant offers include reduced operating times, better bone fusion, less shrinkage and loss of height, and improved spinal alignment. These features are achieved in a number of ways.
The implant can be inserted between vertebrae where it serves as a substitute for degenerative spinal discs. In first time spinal operations, it can simply be inserted into place and the grip-like tread on its surface and screws hold it in place.
The Young’s Modulus of PEEK is similar to that of cortical bone, therefore, it offers more elasticity than metal. It can absorb energy, handle the normal weight of the body and minimise stress on adjacent levels. The material is also radiolucent (transparent to X-rays) and thereby allows an improved view of the fusion mass that is taking place. However, to be able to offer X-ray (computer tomography or magnetic resonance) imaging for optimal positioning and postoperative assessments, titanium trace wires are press fitted into the implant.
Recovery for the patient is faster in many cases. Some patients need to be operated on from the back. However, the implant can be inserted through the patient’s stomach where reconstructing the spine is much less invasive than through the back. Some only need a small incision from the front or side, and in these cases recovery can be rapid, requiring only a four or five day hospital stay followed by a period of recuperation to allow the fusion to knit.
The following issues are inexorably linked with the machining of orthopaedics and a good machining partner must provide some critical resources.
Manufacturers of orthopaedic implants need a machining partner that is able to examine and interact with the concept for a new or modified implant. It must be able to give expert advice on the best design to meet the needs of manufacture and production, while bearing in mind those of the surgeon and patient.
Once the design has been given final approval, the machining partner must then be able to produce variable batches of complex, high tolerance medical implants from "exotic" materials. As well as PEEK and titanium, these include carbon composites, manganese alloys and ceramics. A material’s characteristics change during machining because the process introduces internal stresses into its molecular lattice. If these stresses are not removed, there is a risk that the implant will fail during its period of implantation. Post-operation stress relieving (annealing) involves drying the components for a minimum of 3 hours at 150 deg C. The components are then heated up at 10 deg C per hour until an equilibrium temperature of 250 deg C is reached. Then the components are allowed to cool at 10 deg C per hour until reaching below 140 deg C, and subsequently allowed to cool down to room temperature.
The machining partner must be totally committed to supplying products of high quality that are fully traceable. It needs to be able to consistently conform to the rigorous quality standards demanded by ISO 9001:2000 and ISO 13485, Medical Devices, Quality Management Systems.
Order lead times for implants are short, sometimes only one day. Consequently, the machining partner must able to guarantee product availability at all times and, ideally, hold two month’s supply of stock at all times.
The production equipment used in the manufacturing process of orthopaedic implants involves computer numerical controlled (CNC) multi-axis machine tools that are able to produce high quality complex components in a consistent way from raw plastic and titanium material billets. Novel work holding techniques have been developed for two axis and three axis CNC machine tools to address the following issues:
- The need to keep wastage of high cost materials to an absolute minimum
- The complexity of the shapes being machined
- The ability to handle components with unusual material characteristics
- The need to maintain high accuracy at all times
- Specialist applications that required standard machine tool technology to be adapted.
To produce the spinal implants, a CNC vertical machining centre with a fully integrated fourth-axis capability is required. The main operations are drilling, tapping and surface contouring components manufactured from titanium and plastic. In addition, an inclined fixed fifth-axis configuration is required. Combining this with the CNC machine tool will enable the machining cycle time to be kept to a minimum. Using CNC programs that contain high levels of parameter programming capability, it is possible to machine components 8-10 times faster than using manual machine tool technology. Coordinate measuring machines (CMM) with bespoke probing systems and statistical process control help ensure accurate and consistent manufacture of quality products. In addition, the CMM software, which has been modified for this application, is able to export inspection data directly into inspection sheets written in MS Office Excel spreadsheets.
For the titanium neck implant, a two-axis CNC turning centre/lathe is used. A novel work holding design replaces the need for an expensive three or four axis turning centre and achieves a less costly component. As above, a critical requirement is the ability of the technology to ensure accurate and consistent manufacture of quality products. For the company’s adjustable neck implant, which is currently under development, in addition to the CNC vertical machining centre, electrical discharge machining (EDM) CNC wire cutting machine tool technology is used. EDM enables fine accuracies of 5 um Ry surface roughness with only two cuts to be achieved while balancing fast cutting speeds of more than 4 mm/min with the best surface finish. In addition, the EDM process does not induce any heat into the work piece, which reduces the likelihood of any stress-related defects in the components manufactured using this technology.
The EDM machine tool has the ability to manufacture titanium components to a high level of accuracy. This requires micromachining, a five-axis cutting capability, and compound angular work. The EDM CNC machine also has its own computer aided design (CAD) and manufacturing system that is capable of accepting CAD files directly from a PC to speed up the manufacturing process. The machine can operate in a "lights out mode," that is, remote monitoring of production processes from off-site locations using the Internet.
Work holding considerations are challenging and require innovative bespoke solutions. The complex design of the surgical implants requires the machining partner to apply its innovative capabilities to the full. These include jigs and fixtures that are capable of handling small batch production runs using four and five-axis machining capability.
The spinal implant has 68 design variants, the current neck implant has six variants, and the new adjustable neck implant will have at least 20 variants. Each work holding solution must be capable of fast raw material set up and finished component change over. Also, because of the high cost of the raw materials used and the need to maintain consistency of quality and accuracy, the work holding must be robust. It must also be designed to ensure that cross contamination between different material types does not occur.
Tooling considerations are influenced by a variety of factors including the material being machined, the surface finish that is required and the wear characteristics of the material. For plastic components manufactured on vertical machining centres, purpose-made solid carbide tooling is used. Although aesthetics are not necessarily an issue with all the implants, surface finish is dependent on the application. For example, the neck implant is in physical contact with the voice box and requires a zero friction surface finish. EDM machining uses brass wire that is designed specially for medical component applications.
Materials and machining
The machining of the materials used to fabricate medical implants presents production engineers with many challenges. Titanium requires a different cutting technology from the more traditional steels and a wide range of carbide tipped tools are employed. Because of the high cost of the raw material and its conductive properties, great care must be taken when machining thin sections to ensure rejects are kept to a minimum. The solution lies in the unique work holding, which helps to avoid component distortion caused by heat generated during the machining process. Also material removal must be performed to ensure the minimal generation of heat. For example, trochoidal machining can be used: the rotating cutting head is moved in a particular way during metal removal to minimise heat input. For plastic, additional heat treatment needs to be performed post-machining to remove any inherent stress created, as described earlier.
Full traceability is mandatory. A production control system needs to be put in place to ensure complete component traceability. This includes raw material certification, batch segregation, laser marking and final inspection prior to despatch for further processing, involving sterilisation and sealed packaging.
Relevant quality standards include BS 6001-0:2006, Sampling procedures for inspection by attributes, and ISO 9001:2000 certification. Sampling is used as standard with components subject to three-axis CMM inspection at various stages during the manufacturing cycle. Fully documented quality control procedures need to be maintained and any change to the manufacturing process must be re-evaluated with a capability study that achieves 1.66 Cpk. deviation from acceptable norm.
Selecting the correct material for an implant is not a straightforward exercise because there are a number of factors that have to be considered, including biocompatibility, static and dynamic mechanical properties, ability to machine to specifications without changing the material properties and the way in which the surgeon will use the implant. When selecting the optimum manufacturing process, it is vitally important that the final implant will meet its design criteria.
Surgicraft designs and markets spinal surgery, orthopaedics and obstetric disposable devices. Its core area of expertise is the STALIF medical implant device used in spinal surgery. It is in this area where it has worked in successful partnership with Precise Component Manufacture Ltd, March, UK.