The use of multiple-component systems in orthopedic surgery gives the surgeon increased flexibility in choosing the optimal implant, but introduces the possibility of interfacial corrosion. Such corrosion could limit the longevity of prostheses due either to tissue reactions to corrosion products, or to device failure. The incidence and nature of corrosion of modular total hips was evaluated in a consecutive series of 79 retrieved implants from University Hospitals of Cleveland. Surfaces were examined with stereo- and scanning electron microscopy. Several laboratory studies were undertaken to examine mechanisms that might contribute to the initiation of corrosion. The first set of experiments investigated the effect of head neck extension; the second study looked at the effect of material combinations on fretting corrosion and crevice corrosion. Analysis of retrieved implants demonstrated that fretting corrosion played a major role in the initiation of interface corrosion, and that a correlation existed between corrosion and length of neck extensions. Laboratory studies showed that longer head neck extensions may be more susceptible to fretting corrosion because of an instability at the interface. Short-term mixed-metal corrosion studies demonstrated that the coupling of cobalt and titanium alloys did not render the interface more susceptible to corrosion. It is hypothesized that fretting corrosion contributes to the initiation of modular interface corrosion, and that the problem can be reduced by design changes that increase the stability of the interface.

Examination of retrieved modular total hip replacements (THR's) has identified fretting corrosion as one of the principal mechanisms of implant corrosion at the bore and cone interface. Increased instability due to increased neck extension has been attributed to one of the design factors affecting corrosion rates. A series of experiments was conducted on THR's with cobalt chromium molybdenum alloy heads and Ti 6Al 4V stems, which demonstrated higher fretting corrosion currents with longer neck extensions. Shortening the skirt reduced corrosion rates. In a second series, the effects of wave form and cycling frequency demonstrated higher currents with a ramp versus a sine wave, and higher currents with higher frequencies. Examination of the frequency components of the Paul curve for loads on the hip during gait demonstrated that higher frequencies may be appropriate for device testing.

A recent advancement in the design of the total artificial hip is the introduction of modularity. Within the last year clinical retrievals have shown that significant corrosion can occur at the cone-taper interface in some variations of the modular hip. The hypothesis of this study was that the stability of the cone-taper interface affects the amount of fretting corrosion that occurs. The development of a method that can study the taper corrosion and determine the most stable design is therefore necessary. Design characteristics that may affect corrosion include the use of dissimilar metals, taper angle, taper diameter, percent coverage, machining tolerance, and head neck extension. The fretting that occurred at the interface was initially studied by measuring potential changes while cyclically loading hips in a universal testing machine. This method of measurement proved ineffective at isolating the fretting at the taper interface so current changes were monitored instead. The current was measured when the saline was below the crevice and when it was just above the crevice, and the two measurements were subtracted from each other to produce the taper fretting current. When comparing two different designs, one showed a significant taper fretting current while the other's was minimal. In a second study, when comparing two different head neck extensions placed on identical stem designs, a significant taper fretting current was seen with the + 10-mm head as compared to a +0-mm head. It is concluded that this method can be used in the evaluation of designs to optimize stability and reduce the resulting fretting corrosion.