The demand for total hip replacement surgery has risen dramatically for the last decade. If rates continue to grow, the annual number of surgeries will top 600,000 by 2015.1 The use of titanium modular implant systems is popular with surgeons because it allows for precise fit to the patient with a less invasive surgical procedure. However, modular prosthetics can be prone to fretting damage at the contacting surfaces of the tapered connections between subcomponents. Surface micro-cracks, caused by shear stresses at the contact surface during fretting, can lead to a reduction in fatigue strength that significantly reduces the functional life of the prosthesis. Implanted hip systems experience a spectrum of cyclic loading from normal day-to-day patient activities that can cause the micro-cracks to propagate. Further, risk of failure is affected by a patient’s size and level of activity. Implant failure leads to revision for the patient, and the possibility of litigation for the prosthetic providers.
Manufacturers have tried to offset these issues by creating hips out of nickel, cobalt and chrome alloys, intending to offer a stronger implant that is less prone to failure. Recent research has demonstrated that these materials have severe problems with biocompatibility. For example, 30% of patients with hips made from these alloys suffer from hypersensitive metal allergy. If the hip begins to fail, this number jumps to 66%.2
Adding a layer of residual compression to a metallic part has been shown to slow or even terminate fatigue crack initiation and growth.3-6 Traditional methods such as shot peening have been used across industries to add a protective layer of residual stress to components, but some of these carry serious issues for use on medical devices. They may not provide the depth of compression necessary to offset fretting damage in modular hip implants. These surface treatments can also produce a rough surface that prevents their use on precision tapered joints, and can leave behind residue which can lead to contamination of processed pieces.
Low Plasticity Burnishing (LPB®) has been developed by Lambda Technologies to impart controlled residual compressive stresses in metal components to increase damage tolerance and improve fatigue life. LPB creates a deep, stable layer of beneficial compressive residual stress in the component surface without introducing damage. This protective layer of compression suppresses fatigue cracking from flaws shallower than the depth of compression making the piece resistant to a variety of damage, such as machining marks, handling damage, stress corrosion cracking, corrosion pitting and fretting.7
LPB was initially applied and proven to eliminate fretting fatigue failures at the dovetail joints of titanium alloy (Ti-6Al-4V) jet engine compressor blades, the same alloy that is used in hip prostheses. Review and acceptance by the FAA for use in commercial aircraft supported extending the application of LPB to solve the similar medical prosthesis fretting problem. The deep compressive surface layer suppresses fatigue initiation and crack propagation from the fretted surface, while producing the high quality surface finish needed for the tapered joints.
LPB and Replacement Hips
In 2006, Exactech began searching for a new way to mitigate fretting-initiated fatigue. During a continuous improvement assessment for the Exactech M-Series modular hip prosthesis, it was hypothesized that the controlled application of compressive residual stresses to the taper joint surface could improve fatigue strength. The possibility of using residual compression to extend life offered important advantages, including maintenance of the existing material and design that already had FDA clearance.
After experimentation with conventional roller burnishing and research into laser and shot peening, the LPB process was identified as the most promising surface treatment method for fatigue strength improvement. An LPB process and tooling were designed to improve the fatigue strength and fretting damage tolerance of the M-Series modular hip prosthesis.