**2.3.1 Types of wear**

166 Recent Advances in Arthroplasty

Sommerfield number is the determiner of the thickness of the lubrication fluid and depends

Sommerfield number ∝ Fluid viscosity x Sliding velocity/Applied pressure (2)

The wettability of the materials also plays a part. This is essentially describes how hydrophobic or hydrophilic they are. The ceramics are the most wettable of the currently

The lambda ratio (λ) refers to the ratio of fluid-film thickness to the surfaces roughness. Lambda values greater than 3 imply that the fluid-film thickness is greater than the height of asperities on the articular surface and represent fluid-film lubrication. Lambda values between 1 and 3 represent mixed film lubrication, and values less than 1 represent boundary

Lubrication between the bearing surfaces of hip implants and its effect on friction generated

The Stribeck curve is traditionally depicted in three phases. When the thickness of the fluid film is less than or equal to the average surface roughness of the articulating surfaces, boundary lubrication (BL) is achieved. In this phase, the asperities of the articulating surfaces are in contact at all times. It is not ideal and is more likely to occur in rough bearing surfaces, or as a result of third body formation or protein deposition. It is improved with better manufacture tolerances of the bearing surfaces. The longer implants remain in situ the more likely they are to develop this type of lubrication. As the thickness of the fluid film increases, the articulating surfaces become separated from each other. There is a transition stage called mixed lubrication (ML), where there is a combination of

Sliding velocity (velocity at which the fluid is forced into gaps)

The higher the value, the thicker the lubrication film.(Howcroft 2008)

during articulation is commonly illustrated by a Stribeck diagram. (Fig.2)

Fig. 2. The Stribeck diagram of different bearing combination materials

on a number of factors:

Applied pressure

Viscosity

used bearings.

lubrication.

It is important, especially when describing wear, to distinguish clearly between the nature of the relative motion responsible for the wear and the physical mechanisms by which the material is removed or displaced in wear. The wear mechanisms in bearing surfaces are as follows:

### **2.3.1.1 Adhesive wear**

The bonds that form between to surfaces need to be broken to allow movement. If the bonds are the weakest point then they will break. But sometimes one of the materials is weaker than the bonds so it breaks preferentially. Thus a layer of the weaker material lines the stronger material, changing the interface at which movement takes place. During mechanical action, these microjunctions are torn off, and fragments may become particles or be transferred from body to counterbody and vice versa, bringing about surface damage in the form of flakes and pitting. If the generated flakes and particles are bigger than the clearance of the bearing, they may act as abrasive particles or even block the joint.(Howcroft 2008)

### **2.3.1.2 Abrasive wear**

When material is removed from a surface by hard asperities on the counterface or hard particles (third body) trapped between the two contact surfaces, abrasive wear occurs.(Howcroft 2008) (Fig.3)

The Bearing Surfaces in Total Hip Arthroplasty – Options, Material Characteristics and Selection 169

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

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

Materials used in the manufacturing of femoral heads for metal-on-polyethylene (MOP) total hip replacement (THR) include the metal alloys, stainless steel, cobalt-chromium, and titanium alloy as well as ceramic materials, aluminum oxide, and zirconium oxide. Properties to consider when evaluating materials for bearings in THR include corrosion resistance, strength, ductility, hardness, and frictional characteristics. Frictional characteristics are a result of material properties such as wettability (related to surface energy), manufacturing variables such as surface finish, and operating conditions such as lubrication. The degree of resistance is proportional to the load. Because both chemical and mechanical interactions may occur, frictional forces depend on both the material composition and the roughness of the opposed surfaces. Lubricating conditions can change the nature of the interface between the moving surfaces and decrease friction. As described earlier, the coefficients of friction depend upon the nature and amount of lubricant present,

A thorough surface roughness evaluation should include a visual comparison of the actual tracings and representative photomicrographs (scanning electron microscope) of the surface. The surface roughness of currently available femoral heads ranges from an average height (Ra) of less than 0.03 µm to about 0.10 µm and maximum height ranging from less than 0.10 to about 0.40 µm. The surface roughness of a femoral bearing can change over time in vivo. In the presence of hard third bodies, as can occur in vivo, surface abrasions (scratches) result

or subluxation wear, **C:** Mode 3 or third body abrasive wear, **D and E:** Mode 4

very small wear particles with comparatively little variation in size and shape.

as well as the speed of relative motion and the applied load.

**2.4 Material properties** 

**2.5 Surface roughness** 

### Fig. 3. Examples of abrasive wear
