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Today in the United States, 95% of all total hip implants contain at least one modular junction. The most common of these junctions connects the femoral head and neck taper. Because this connection utilizes a Morse taper, various materials and design variables can influence fretting and corrosion behavior at the mating surfaces, and ultimately the longevity of the device.
In order to isolate and study the behavior of the femoral head/neck taper connection, a simplified model of a neck taper was designed. Test specimens were manufactured of Ti-6Al-4V and Co-Cr, and were designed to accept a Co-Cr femoral head. These constructs provided control in the experiment by focusing on the femoral head/neck taper interface, and by facilitating debris collection. Each construct was encapsulated with Ringer's solution and subjected to a fatigue load of 490 N to 4900 N at a rate of 10 hertz for ten million cycles. Using direct current plasma-optical emission spectroscopy (DCP-OES), the Ringer's solution was analyzed for titanium and cobalt ions after the fatigue test. The titanium level measured for the Ti-6Al-4V/Co-Cr constructs was less than 0.05 mg/L. For the Ti-6Al-4V/Co-Cr and Co-Cr/Co-Cr constructs, the cobalt levels were 0.95 – 0.23 mg/L and 1.16 – 0.57 mg/L, respectively.
This test method was designed to determine the debris generated at the femoral head/neck taper interface by controlling for specific design/fixation variables. It can be used to predict how such factors as material, taper diameter, taper angle, taper engagement, and tolerances affect the generation of wear debris during fatigue. Although the results do not predict the debris generation of an actual hip prosthesis, they may indicate performance characteristics which can be generalized to in-vivo behavior of the device.
fretting, corrosion, modular hip, Morse taper, cobalt, titanium
Sr. Research Technician, Device Testing Technical Services, Smith & Nephew Orthopaedics, Memphis, Tennesses
Sr. Research Engineer, Device Testing Technical Services, Smith & Nephew Orthopaedics, Memphis, Tennessee