Failure Modes In Modular Total Joint Implants

Despite the success achieved with most total-joint procedures, there is a steady increase in the number of failed implantations leading to revisions. Research has demonstrated that approximately 10% of the total number of total hip arthroplasties performed in the US are related to revision procedures. Several clinical reasons lead to malfunctioning of an implant, therefore it is crucial to understand these reasons to better design or redesign an implant. Typically, failure is associated to aseptic or mechanical loosening caused by severe corrosion or fatigue processes, dislocation, osteolysis, infection, pain, instability and periprostetic failure. One way to better understand these failure modes is the evaluation of retrieval collections and patient history.

We are interested in actively collaborating with orthopedic surgeons to perform analysis of retrieved total-joint implants upon revision surgery.

Cases of severe corrosion have been reported at the interface between cobalt chromium (CoCr) heads and titanium alloy (Ti) necks, and at the modular taper connections on the stem/sleeve region of cementless hip-replacements. There is an increasing trend for modular designs of titanium hip systems since these designs provide the surgeon with a wide variety of options for intraoperative adjustments (Figure 1). The conjoint effects of cyclic loading and fretting motion between modular junctions is thought to result in abrasion of the oxide film covering the titanium surface. This breakdown of the oxide film will exposure the bulk metal to active dissolution and attack.

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Figure 1. Example of a modular hip system revised due to pain. Modular components are located in the head-neck couple and stem-sleeve region. Modular implants are typically used in complex revision surgeries giving the surgeons with the flexibility to combine sizes and materials that best fit the patient anatomy. However, modularity results in large crevice geometries with differential aeration that will be subjected to micromotion during loading. Micromotion between the mating interfaces will result in abrasion of the titanium oxide film and trigger a series of reactions in the crevice that will lead to events such as cracks, pitting and cracks. Corrosion debris (black deposits) is shown in the stem-sleeve mating interface.

Therefore, the combination of a large crevice-like environment and fretting can lead to a series of corrosion events or mechanically assisted corrosion (MAC) processes (Figure 2). These corrosion processes may ultimately lead to release of ionic species and particulate debris that may induce adverse reactions in local tissues and accelerate bearing surface wear by a third body mechanism. Therefore, the characterization and understanding of in vivo mechanically and electrochemically induced degradation occurring in these modular tapers are of great importance. We are interested in studying the mechanisms of fretting crevice corrosion to ultimately mitigate the potential for this phenomenon in modular devices, particularly focusing on the study of:

  • microenvironments within counter-interfaces
  • role of materials combination and surface finish in these areas
  • techniques to improve lubrication in crevice areas
  • surface protection of taper interfaces
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Figure 2. Example of a component of a retrieved total hip implant revised due to adverse local tissue reaction (ALTR). The use of metal-on-metal (MoM) total hip arthroplasty (THA) implants has decreased in recent years due to concerns of high failure rates and adverse local tissue reaction (ALTR) including periprosthetic fluid collections and pseudotumors. It has been hypothesized that cobalt (Co), chromium (Cr) and Nickel (Ni) ions and wear debris released from CoCr heads articulating against CoCr cups may provoke delayed hypersensitivity reactions. High concentrations of metallic ions and oxide wear can lead to soft tissue metallosis and ALTR.