Abstract
Having previously been a proponent of the advantages of the modular neck in total hip arthroplasty, I now take the opposite argument because of corrosion that happens with all taper junctions. The advantage of the modular neck is the “uncoupling” of femoral stem position from the final position of the femoral head. Surgical priorities frequently compete, whether positioning the stem for the best press-fit (for cementless fixation) or the best cement mantle (for cemented fixation), and positioning of the stem for preventing dislocation and improving function. My personal use of the modular neck spanned approximately 4 years from 2003–2008 and encompassed a total of 390 primary and revision cases. Excellent functional results were obtained, but some problems occurred that were associated with the modular neck and with large diameter head metal-metal articulations.
The modular neck was designed and studied at the Rizzoli Institute in Bologna, Italy with the conclusion that the strength of construct (titanium alloy neck in the titanium alloy stem) was sufficient and the potential for fretting at the modular junction was small and acceptable. Pre-market testing of the device met and exceeded all FDA suggested benchmarks.
The first modular neck fracture in my personal series occurred more than 3 years after implantation, in a large man with a long, varus modular neck. Within a year another fracture of a long, varus modular neck occurred in a heavy man. I now know of 6 modular neck fractures among the 390 cases. We have found evidence of corrosion, some very severe, in modular necks that we have revised (both fractured and intact modular necks). This corrosion is caused by Mechanically Assisted Crevice Corrosion associated with fretting at the modular junction which leads to removal of the titanium oxide “passivation” layer that generally forms on a titanium implant. This exposes more of the substrate metal to oxidation and can create pits that, in the notch-sensitive titanium alloy, can lead to the initiation of fracture. The hydrogen that is created from the corrosion reaction and diffused into the metal can cause “embrittlement” which predisposes it to fracture. We also have seen “hydrogen pneumarthrosis” associated with corrosion of the titanium modular neck in which the corrosion concentrated the hydrogen gas in the femoral stem below the modular neck and suddenly was released into the joint with significant pain. The hydrogen gas is irritating to the joint capsule and the patient presents with intense pain and gas in the joint, a clinical picture that can be confused with infection in the joint with a gas-forming organism. We now know that the condition is self-limiting, but suggests that revision of the modular neck construct would be a reasonable course of action.
Recently cobalt chromium modular necks have replaced those made of titanium alloy. Since cobalt-chromium is harder and stiffer, the milieu of the taper junction will be different than that of the titanium-titanium junction, and it has been suggested that this will allow safe and long-term use of the modular neck. The first titanium alloy necks were introduced in the early 1990s and it took until the mid-2000s to recognise problems. Last year the Stryker modular neck used with the Rejuvenate stem was recalled because of significant reaction associated with corrosion at the neck-to-stem junction.
Corrosion is inevitable at modular junctions exposed to cyclic loading, especially in the milieu of body fluids. We now know that ALTR occurs in response to taper junction corrosion as well, and the more modular junctions there are in a total hip construct, the more debris and potential reaction likely. Fixed neck stems provide satisfactory long-term fixation and function for patients, so despite a functional advantage to the modular neck, it is “a bridge too far”.
