Davim J.P. Tribology for Engineers: A Practical Guide

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materials studied, without reproducing either the features of

the shape of the implant, or the environment with which it

will have to interact, and they are short. Figure 6.8 shows a

representation of some quick-tests.

They are quick, do not accurately represent the specimen

geometry of the biomaterials used that can inﬂ uence the

lubrication or the contact stress, and do not reproduce

accurately the wear mechanism operating in vivo. They are

based on physical mechanisms of material removal or

displacement and operational mechanisms such as mechanical

action: rolling wear, sliding wear, material interaction, etc.

They are less useful in predicting wear rates in the implanted

joint, unless care is taken to simulate the loading cycle, all

aspects of the motion between head and cup, body environment

and the effective mode of lubrication (Dowson, 2001 and

standards ASTM-F732-00, 2006 and ASTM-G133, 1995).

Standard wear screening devices used in order

to give information exclusively on the intrinsic

features of the materials studied

Figure 6.8

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These devices have some disadvantages, such as the inability

to extrapolate the results obtained and to predict wear of

the specimens tested, and the wear behaviour may fail to

predict in vivo results. In these machines, the wear mechanism

may also be inﬂ uenced by the great heat, generated by friction,

which is transferred to the surfaces. In metal against

polyethylene simulations, Davidson and co-workers (Davidson

and Schwartz, 1987; Davidson et al., 1988) found that the

temperature at the interface increased by about 10°C.

For orthopaedic applications, international guidelines

(ASTM F & G) give some recommendations in order to

conduct a wear test, but some of the most important quick-

tests will be brieﬂ y described here.

■ Pin-on-disk: This test consists of a laboratory procedure

for determining the wear of materials during sliding using

a pin-on-disk apparatus. Materials are tested in pairs

under nominally non-abrasive conditions. This practice

follows the guidelines contained in ASTM G99-05.

■ Pin-on-ﬂ at: This test consists of a laboratory method for

evaluating the friction and wear properties of combinations

of materials (that are being considered for use as the

bearing surfaces of human total joint replacement

prostheses). This practice follows the guidelines contained

in ASTM F732-06. It is used to rank the materials with

regard to friction levels and polymer wear rate under

simulated physiological conditions.

■ Block-on-ring: This test consists of a laboratory

procedure for determining the resistance of materials to

sliding wear. The test utilizes a block-on-ring friction and

wear testing machine to rank pairs of materials according

to their sliding wear characteristics under various

conditions. This practice follows the guidelines contained

in ASTM G77-05.

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■ Crossed-cylinder: This test consists of a laboratory test

for ranking metallic couples in their resistance to sliding

wear using the crossed-cylinder apparatus. This practice

follows the guidelines contained in ASTM G83-96.

6.6.2 Wear joint simulators

These machines represent a more complex dynamic situation

of analysis than those above and typical hip joint and knee

joint wear simulators are shown in Fig. 6.9 and Fig. 6.10. In

these simulators, real prostheses are tested in an environment

that simulates physiological conditions in order to predict

some aspect of clinical performance of the materials tested

in vivo wear patterns (Bragdon et al., 1996; Barbour et al.,

2000; Goldsmith et al., 2000; Clarke et al., 2001; Saikko,

2005; Affatato et al., 2006).

The development of improved materials to extend the

lifetime of orthopaedic implants, such as knees and hips, up

Schematic view of a typical hip joint

wear simulator

Figure 6.9

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to a minimum of thirty years is a critical social objective.

Considering that all new materials have to be tested before

clinical trials, joint simulators play an important role in this

pre-clinical validation. These machines can also be used as

research tools allowing experiments to be conducted in a

relatively controlled environment where variables such

as surface roughness and scratching can vary and the

effects measured. To replicate/simulate particularly extreme

conditions, a wear joint simulator could be of great help. In

particular, simulator wear tests can be used to conduct

accelerated protocols so that it is possible to establish the

limits of performance for the material in short time frames.

Schematic view of a typical knee joint

wear simulator

Figure 6.10

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Hip and knee wear simulators are widely used and have been

successful in evaluating the wear properties of total hip/knee

arthroplasty articulating surfaces. These test results have

been shown to correlate well to clinical experience. Since this

type of test machine is widely used, it is important that

parameters can be standardized to make results more uniform

and comparable over all laboratories. The simulators

currently in use differ from each other in many parameters:

number of stations, loading (physiological or simpliﬁ ed),

degrees of freedom, anatomical or inverted conﬁ guration,

and temperature-controlled test ﬂ uid baths for each hip joint

assembly. Finally, yet important, is the simulator that allows

independence among the stations (for example if one specimen

needs to be removed for examination or replacement, the

testing could be disabled while the remaining stations keep

running).

For orthopaedic applications, international guidelines

(ISO 14242 for hip and ISO 14243 for knee) give some

recommendations in order to conduct a wear test. In

particular, these international guidelines specify the relative

angular movement between articulating components, the

pattern of the applied force, speed and duration of testing,

sample conﬁ guration and test environment to be used for the

wear testing of total hip/knee joint prostheses. These wear

tests need a complex set up because the test specimens have

to be placed onto the simulator in a particular conﬁ guration.

The test apparatus transmits a speciﬁ ed time-varying force

between the components, together with speciﬁ ed relative

angular displacements, and the test takes place in a controlled

environment simulating physiological conditions. Normally

the ﬂ uid test medium is ﬁ ltered through a 2-μm ﬁ lter, and has

a protein mass concentration of not less than 17g/l as

recommended by the aforementioned guidelines. To minimize

microbial contamination, the ﬂ uid test medium should be

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stored frozen until required for testing. An anti-microbial

reagent (such as sodium azide) may be added, but such

reagents can be potentially hazardous.

The material loss (the wear) from components of the

prosthetic joint, due to combined movement and loading, is

evaluated using different methodologies, but the gravimetric

way is considered the gold standard. As recommended by the

aforementioned international guidelines, the test specimen is

soaked in a lubricant (usually Bovine calf serum) and is

repeatedly removed from the lubricant, cleaned, dried and

weighed until a steady rate of ﬂ uid sorption is established.

After recording the mass of the specimens and mounting

them in the testing machine, the wear test is conducted in

accordance with ISO 14242-1/2 (for hip specimens) and

ISO 14243-1/2 (for knee specimens). The test specimen is

assessed subsequently for wear by testing for loss in mass in

a hip/knee simulator. A loaded, non-articulating control

specimen is intended to allow for ﬂ uid sorption and undergoes

the same procedure for reference purposes.

On each occasion when the test specimen and control

specimen are removed from the wear-testing machine, clean

and dry the components and calculate the gravimetric wear

as follows:

Wn = Wan + Sn [6.1]

where Wn is the net mass loss after n cycles of loading, Wan

is the average uncorrected mass loss, and Sn is the average

increase in mass of the control specimen over the same period.

6.7 Biological effects of wear

Willert and Semlitsch in the 1970s (Willert et al., 1974; Willert

and Semlitsch, 1977) proposed that aseptic loosening of joint

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prostheses resulted from the abundant wear debris seen

microscopically around and within reactive blood cells

(macrophages), which comprised the majority of the

periprosthetic ﬁ brous tissue membrane. They suggested that

wear debris was biologically active and induced a macrophage

response to the surrounding tissues. Since than, numerous

investigators have shown that the cellular activity in the

membrane is capable of producing a variety of enzymes,

prostaglandins, and cytokines, which are capable of stimulating

osteoclastic bone resorption and ﬁ brous tissue formation

(Goldring et al., 1983; Kim et al., 1993; Shanbhag et al., 1995;

Tuan et al., 2008).

The factors that contribute to osteolysis (bone resorption

around or at the implant–bone interface) are related to the

number, size, shape, rate of generation, time of exposure,

and antigenic properties of the wear debris particles. The

macrophage is the predominant cell type with respect to

biomaterial particles in inciting periprosthetic inﬂ ammatory

bone loss. Other cells recognized as taking part in the process

of osteolysis are ﬁ broblasts, osteoblasts, osteoprogenitor cells

(adult mesenchymal stem cells), synovial cells, and osteoclasts

(Tuan et al., 2008). The activation process by which wear

debris activates macrophages and other cells within the

interfacial membrane is similar. During the life of an implant,

and especially in the case of poor primary stability with a

constant accumulation of wear particles over time, marrow

cells are exposed to high concentrations of debris and also

when implant ﬁ xation is at the beginning relatively weak. As

a result of the prolonged exposure to particles, the normal

osteogenic differentiation process may disrupt, diminishing

the population of functional osteoblasts, thus compromising

osseointegration at the bone–implant interface. In vitro

evidence suggests that wear debris can alter osteoblast and

osteoprogenitor cell function and increase osteoclast activity,

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resulting in decreased bone matrix production and osteolysis.

Tuan et al. (2008) suggest a possible genetic predisposition to

osteolysis on the basis of the clinical variation seen in the

osteolytic response to implant wear. The heritable component

should depend on multiple minor DNA sequence variations

occurring with a stable frequency within the population,

leading to subtle changes in gene function, giving rise to

altered susceptibility of severity for osteolysis.

Clinically the bone loss appears as a linear, diffuse,

dissecting phenomenon compromising the bone–implant

interface, resulting in a generalized enlargement of the bone

canal and endosteal bone lysis; alternatively, the bone loss

can appear to be focal, manifesting as an endosteal lytic

lesion. The end result is typically a loose prosthesis requiring

revision surgery, or possibly a periprosthetic fracture.

6.8 Acknowledgements

The authors would like to thank Luigi Lena for the

illustrations (Laboratorio di Tecnologia Medica, Rizzoli

Orthopaedic Institute, Bologna) and Stefania Affatato for

her help with the English language.

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