Fretting corrosion studies of universal femoral head prostheses and cone taper spigots.

Short-duration, cyclically loaded, axial, fretting corrosion tests were designed and performed to compare the fretting behaviour of different metal Howmedica universal heads connected to coated and uncoated metal cone taper spigots. Concurrent axial push-on and pull-off taper friction tests were also performed. There was no measurable fretting debris found in any test and SEM studies of the contact surfaces showed no evidence of fretting. It is concluded that no significant, long-term fretting corrosion of the Vitalium Co-Cr-Mo heads on the Vitallium or uncoated Ti-6AI-4V spigots, nor of the Orthinox stainless steel heads on the Orthinox spigots, is likely to occur in vivo.

Friction in femoral prosthesis and photoelastic model cone taper joints.

Static axial push-on and lift-off, and push-on and twist-off experiments were designed and performed to measure the effective, room-temperature coefficient of friction mu for different design femoral prosthesis cone taper joints comprising a universal head on a stem spigot. Alumina and metal heads were tested on metal spigots using either distilled water, Ringer's solution, blood or no lubricant. Complementary push-on and lift-off friction tests of Araldite model joints for subsequent frozen-stress, photoelastic stress analysis were performed at 130 degrees C. It was found that lubricant caused little decrease in the values of the coefficient of friction in prosthesis tapers. The values measured were typically mu = 0.2 for an alumina head on a Co-Cr-Mo or Ti-6Al-4V spigot, mu = 0.15 for a Co-Cr-Mo head on a Co-Cr-Mo or Ti-6Al-4V spigot and mu = 0.13 for a stainless steel head on a stainless steel spigot. For Araldite photoelastic models of an alumina head on a Vitallium spigot, as-cast taper surfaces lubricated with silicone grease gave consistent friction of typically mu = 0.14. The axial displacement of model heads on their spigots were compared with predicted values and previously measured values for prosthesis heads.

A study of stresses in alumina universal heads of femoral prostheses.

Three-dimensional, frozen-stress photoelasticity was used to study the best shape for a proposed alumina universal head loaded on to a Vitallium cone taper spigot with a 30 degrees inclined force, as in vivo. Typical cone taper friction values were reproduced in the photoelastic models. The location of the highest tensile stresses in the Mark I shape with a flat crown was found to be on the inner surface of the crown. Changing to a torispherical surface in the Mark II shape reduced this magnitude. However, the Mark III shape with a hemispherical inner crown surface gave even lower stress there, equal to the maximum value of the hoop stress at the taper, which was measured to be fairly uniform both around and along the taper except at the ends where contact pressure concentrations were found to occur and it became reduced. Lamé axisymmetric cylinder stress predictions were found to be useful approximations to measured values and were generally overestimates of the tensile hoop stress at the head taper surface.