Low Plasticity Burnishing Enters the Operating Room
Exactech and Lambda Technologies Partner to Produce Enhanced Hip Replacement Components
In early 2004, Exactech Inc. and Lambda Technologies (Lambda) began implementation for production of Low Plasticity Burnishing (LPB) processing that hit the manufacturing floor in January 2005. In meeting our goals, Lambda and Exactech forged a partnership that broke new ground for both companies. With this project, Lambda initiated the first commercial application of LPB to medical implants, and Exactech applied this unique technology to improve the fatigue strength of an important hip implant by at least 33%.
Prior to this effort, LPB applications were developed for defense and aerospace components including turbine engines, landing gear, and other aircraft structures. For example, Lambda contracted with the U.S. Navy through the Small Business Innovation Research (SBIR) program to enhance engine parts for the AV-8B Harrier jet fighter used by the armed forces of the U.S., United Kingdom, Spain, and Italy. One of the main objectives of the SBIR program is to support, encourage, and fund small businesses in their efforts to commercialize technologies developed for military uses. While the diversification into commercial applications was part of Lambda’s strategic vision, working with Exactech provided an excellent opportunity to jump-start those plans. Lambda’s work with the military and aerospace continues to expand.
The Exactech M-Series Modular Hip Prosthesis is comprised of three major components that, when assembled, form a functional hip implant or prosthesis. These components include the femoral stem, metaphyseal and neck segments, and are held together with taper interfaces and a locking screw.
In service, the neck segment tapers experience high loading conditions in a fretting environment. Since every step taken by a patient represents a single loading and unloading cycle that accumulates over years of usage, high cycle fatigue (HCF) performance was a critical factor. Exactech had explored a number of alternative solutions to increase performance including laser peening and roller burnishing. Laser peening was quickly discarded due to prohibitive cost and dimensional factors. The roller burnishing process was abandoned due to a lack of control during application and erratic testing results. It was at this point that Exactech contacted Lambda Technologies.
Edmund Loftus, Exactech’s Development Engineer for the project relates, "At our first meeting, it became apparent that Lambda’s approach was innovative and unique. Their focus was on understanding our application and then demonstrating how the LPB technology could substantially improve fatigue performance. This commitment provided Exactech with a comprehensive solution, as opposed to simply delivering a piece of equipment."
Doug Hornbach, Lambda’s Vice-President and Director of Laboratory Operations had his own views from the initial meeting. "Exactech was looking for a partner to assist in meeting this challenge. They were open to new ideas and worked with us from both an engineering and manufacturing perspective. But above all, they emphasized that their objective was improved implant fatigue strength and that would be the critical factor in determining the final solution."
Low Plasticity Burnishing
impart ???compressive residual stresses in metal components. By rolling a high modulus ball over the surface of a metal component under high pressure, a deep stable layer of compressive residual stress is introduced. The compressive surface layer becomes resistant to damage mechanisms including foreign object damage, stress corrosion cracking, fretting fatigue or HCF. The low cold working of the component surface provides for thermal and mechanical stability of the residual stresses at elevated temperatures or extreme loading.
The Way Forward
Residual stress measurements were made on the tapered region of the neck segment in the as-machined condition by Lambda Technologies - Laboratory Services using x-ray diffraction (XRD). The residual stresses were mapped along the specimen using a specialized translation table designed and built at Lambda to enable automated measurements. Following that, a 3-dimensional finite element model (FEM) of the neck segment was used to determine the in-service applied stresses. Figure 3 shows the FEM used for this analysis. The model consists of the neck segment and femoral head assembly. Loads and boundary conditions were applied to simulate those experienced in-service. Applied stress data from the FEM and fractography results (derived from fatigue tested components) were used to determine where to place the beneficial compressive residual stresses.
A 3-dimensional elastic FEM was built for the residual stress field design and development effort that was specific to this application. Examples of design considerations include re-allocation of the residual tensile stresses, component distortion, and ease of processing. For example, the medial portion of the neck taper was not an area of concern. However, LPB processing the medial portion of the neck taper would permit a lathe to be used. A view of the FEM showing the designed compressive residual stress distribution for the tapered region is depicted in Figure 4.
At this point, preliminary LPB processing runs of the neck segment were conducted. The LPB processed neck specimens were measured by x-ray diffraction to determine the compressive residual stresses after LPB processing. Three identically LPB processed neck segments were measured and compared to earlier measurements of both as-machined and roller burnished specimens. Compressive residual stresses from LPB processing were consistent across all the three neck segments. Moreover, LPB produced compression that was significantly deeper and higher in magnitude than either the roller burnished or as-machined neck segments. This deep layer of compressive residual stress was sufficient to protect the tapered region against fretting-related micro-cracking damage.
Validating the Design
While the XRD measurements showed the LPB process had imparted the designed compressive residual stress distribution, fatigue testing was essential to validate that fretting failure mitigation and the HCF goals were met. M-Series hip stem assemblies were tested in accordance with ISO 7206-6, "Determination of endurance properties of head and neck region of stemmed femoral components". This testing method isolates the LPB-processed taper that allows the fatigue strength of the taper junction to be characterized. The fatigue testing setup is shown in Figure 5.
The results of the validation were impressive. Prior to LPB application, the fatigue strength of the M-Series prosthesis was 1050 pounds at 10 million cycles, and the failure mode was fretting-induced fracture of the neck taper. LPB processing of the neck taper increased the fatigue strength of the worst-case construct by 33% to 1400 pounds at 10 million cycles, and eliminated the occurrence of fretting-induced fracture. The smallest size components were tested to maximize the applied stresses and thereby simulate the worst-case implant construct. In addition, preliminary testing of M-Series assemblies using larger metaphyseals with LPB-processed necks indicates that the fatigue strength may actually increase to 1600 pounds for larger constructs. However, further testing is required to confirm this trend. With these results in-hand, the development phase of the program concluded successfully.
The Next Hurdle – Seamless Integration with Manufacturing Operations
The M-Series neck segments are machined from near net-shaped Ti-6Al-4V forgings and the machining operations primarily consist of turning the neck tapers. Although LPB can be applied with either a mill or a lathe, the axisymmetric taper geometry and potential for shorter cycle times favored a lathe application. Based upon machine availability and anticipation of future development efforts, Exactech decided that a dedicated LPB machining center (a 3-axis CNC lathe platform) would provide the best fit with their manufacturing operation. Another consideration was that Exactech had a suitable lathe, a Mazak Quick Turn 6T, at their facility that could be re-allocated for use as the dedicated machine.
While a dedicated machine may represent an optimum approach, a dedicated machine is not necessary for the LPB system. In fact, Lambda Technologies routinely performs machining operations on the same mill that is used to burnish components. In addition to the LPB tool, the LPB system consists of a digital controller that interfaces directly to the machine controller and a hydraulics cabinet. The total footprint of the LPB system is approximately eight square feet. LPB system hardware can be packaged to permit relocation from one machining center to another if warranted.
Upon reaching the decision to use a dedicated machine, Exactech and Lambda concurred that the integration would be streamlined if it took place at Lambda’s facility in Cincinnati, Ohio. According to Rick Woolf, Lead Tool Design Engineer at Lambda Technologies, "Having the lathe available to us in our facility was a big plus – but by no means essential. This project marked the first time that we interfaced an LPB system to Mazak equipment. Overall, I’d have to say that the integration went smoothly and, once started, was completed in less than a week."
After integration and initial testing, the entire system was shipped and installed at Exactech. Besides installation and check-out, Lambda conducted formal training to qualify machine operators and quality assurance inspectors. In addition to classroom instruction, Exactech employees were provided manuals and a hands-on demonstration. Lambda also provides complete maintenance and logistics support for the LPB system. A photograph of the lathe and LPB control system is shown in Figure 6.
Dan Szall, Exactech’s Shop Operations Coordinator, added this perspective, "Lambda Technologies promised a turn-key system and that’s what we got. Once the neck segment is secured in the fixture, the operator presses the start button and can begin preparing the next piece. The actual LPB processing takes about a minute."
Quality Assurance – The Critical Link
From the outset, Exactech was focused on integration of the LPB process with their rigorous quality assurance program. Early in the program, the correlation of LPB tool pressures (that are measured in real-time) to the level of compressive stress was established through XRD testing at Lambda Technologies. As an added level of assurance, specific trials were run at processing extremes to establish stringent processing limits. Overall, the LPB process proved to be highly robust. During LPB processing, the LPB system captures data on a number of LPB operating parameters. This data is displayed to the operator in the form of a point cloud. If all parameters are maintained within tolerance, the operator is given a “PART PASSED” message (See Figure 7), if not, the part is quarantined and the cause is investigated. In addition to the real-time feedback provided to the operator, the files for each component are up-loaded to the internet for Statistical Process Control (SPC) analysis by both Exactech and Lambda.
Ann Kelly, Senior Quality Engineer, states, "One of the most critical aspects of implementing LPB in our manufacturing operation was the need to ensure that the equipment and processes were properly qualified. Lambda Technologies’ ability to extract manufacturing performance data in real time helped to expedite the qualification process. We are certain, within a very tight tolerance, that when a part has been LPB processed that the compressive residual stress distribution is precisely what was specified by the engineer."
Dave Bogarde, Machinist, adds, "I’ve been working with this system for the last five months and I haven’t put a single neck segment in the scrap bin."
From the first meeting to final deployment, the program spanned about a year. That said, both companies believe that this duration can be markedly reduced, and LPB processing is under consideration for future Exactech prostheses. Both companies agree that this has been a highly rewarding effort.
Reflecting on the success of the program, Paul S. Prevey, President and Director of Research at Lambda Technologies, remarked, "Working with Exactech can be best characterized as a true partnership. They brought a problem and we, both Exactech and Lambda, worked to solve it. We have been extremely impressed with their drive for technical and market innovation and their commitment to LPB and its capability to enhance their product line. At Lambda Technologies, we look forward to a long-term relationship with an industry pacesetter like Exactech."
Dr. Gary Miller, Executive Vice President – Research and Development, summarizes the project best, "Exactech has always valued continuous quality improvement in its product design and development activities. Improvements continue to be important as patients impose ever-greater demands on their devices. Through our partnership with Lambda Technologies, we have now implemented an innovative and cost effective means of applying cutting edge technology in the continuing quest to improve our modular components."
Wm. David Butler
Director, Advance Application Development, Lambda Technologies
Tel. +.800.883 0851 or +1.513.561 0883
Edmund Loftus, Development Engineer, Exactech Inc
Tel. +1.800.392 2832 or +1.352.377 1140
Doug Hornbach, Vice-President, Laboratory Operations
Lambda Technologies – Laboratory Services