**2.3.1.3 Corrosive wear**

This type of wear occurs by the combination of mechanical wear and chemical reaction. (Howcroft 2008)

### **2.3.1.4 Surface fatigue wear**

Fatigue acts during repeated sliding or rolling over the same wear track. The repeated loading and unloading can induce the initiation and propagation of microcracks parallel and orthogonal to the surfaces for mechanical or material-related reasons. As a result, shallow pits and filaments (delaminations) are generated.

### **2.3.2 Wear mode**

The wear mode defines the general mechanical conditions under which the bearing is functioning when wear occurs. Wear modes are defined by two sets of criteria: first by the macroscopic structure of the tribosystem and the kinematic interaction of its elements and second by the combination of acting wear mechanisms. It should be noted that the wear mode is not a steady-state condition and can change from one form to another. For example, particulate debris generated by two-body abrasion may function as an interfacial medium and turn the problem into a particle-related (third-body) phenomenon. Depending on the circumstances, this may reduce or increase the wear rate. Mode 1 wear results from the motion of two primary bearing surfaces one against each other, as intended. Mode 2 refers to the condition of a primary bearing surface moving against a secondary surface, which is not intended. Usually, this mode of wear occurs after excessive wear in mode 1. Mode 3 refers to the condition of the primary surfaces moving against each other, but with thirdbody particles interposed. In mode 3, the contaminant particles directly abrade one or both of the primary bearing surfaces. This is known as three-body abrasion or three-body wear. The primary bearing surfaces may be transiently or permanently roughened by this interaction, leading to a higher mode 1 wear rate. Mode 4 wear refers to two secondary (nonprimary) surfaces rubbing together. Examples of mode 4 wear include wear due to metal-cement or bone-cement interface motion or from relative motion of a porous coating, or other metallic surface, against bone; relative motion of the superior surface of a modular PE component against the metal support, so-called back-side wear; and fretting and corrosion of modular taper connections and extra-articular sources.(Jacobs 2006) (Fig.4)

Fig. 4. The modes of wear for a total hip arthroplasty: **A:** Mode 1 or normal wear, **B:** Mode 2 or subluxation wear, **C:** Mode 3 or third body abrasive wear, **D and E:** Mode 4

Particles produced by mode 4 wear can migrate to the primary bearing surfaces, inducing third-body wear (mode 3). Wear particles are a function of the type of wear. A smooth, highly polished femoral head wearing against PE in the absence of third bodies generates very small wear particles with comparatively little variation in size and shape.
