Applications: Medical Implants

Total Hip Prosthesis Neck Segment

Low Plasticity Burnishing (LPB™) improves high cycle fatigue performance and eliminates the occurence of fretting-induced fracture in modular hip prosthesis.

Total hip replacement (THR) surgery is often necessary to relieve pain and increase mobility of hips damaged from disease or fracture. The number of THR surgeries has increased substantially and is predicted to increase in the future. Improved technologies and surgical techniques have revolutionized hip replacement offering patients more options.

Modular total hip prosthesis (THP) construction has become more widely used because it gives the surgeon the opportunity to interoperatively choose the proper size prosthesis and offers the surgeon flexibility in treating an array of hip defects and patient anatomies. Fretting damage, caused by small displacements between mating parts, can occur in the tapered interlock that hold the modular THP together. Fretting damage can greatly reduce the overall fatigue strength and life of the construct.

A customized compressive residual stress distribution was designed for the Exactech M-Series neck segment taper, shown in Figure 1. Compression was designed using finite element modeling, Lambda’s patented Fatigue Design Diagram (FDD) and fatigue life prediction software. Low plasticity burnishing (LPB™) was used to place the custom designed compressive residual stress in the taper region, shown in Figure 2. LPB™ produced the desired depth and magnitude of compression required with superior surface finish and distortion within the dimensional tolerances required by the customer.

High cycle fatigue tests were conducted to characterize the overall improvement in fatigue strength resulting from the LPB™ treatment. Fatigue tests were conducted in accordance with ISO standards for hip implants. Fatigue test results, shown in Figure 3, revealed a 40% improvement in fatigue strength and a remarkable 100X improvement in life. LPB™ eliminated fatigue failure from the fretting damage. In fact, the full fatigue benefit from LPB™ was not determined because other non-treated sub-components of the modular construct were failing before the LPB™ treated neck stem.

A turn-key LPB™ system was developed for production LPB™ treatment of the neck stem at Exactech’s facility. LPB™ tooling and software were integrated with a CNC lathe, shown in Figure 4. Exactech employees were provided with formal training through classroom instruction, hands-on demonstrations, and manuals. Production cycle times are on the order of 1 minute per sample including sample mounting, sample removal, and LPB™ treatment. Lambda Technologies provides complete maintenance and quality assurance support for the remote LPB™ system.

For further information, see Lambda Technologies Technical Paper, "Application of Low Plasticity Burnishing (LPB™) to Improve the Fatigue Performance of Ti-6Al-4V Femoral Hip Systems". Also, download the Lambda Technologies LPB™ Application Note, "Medical Implants: Total Hip Prosthesis".