Sliding Direction Dependence of Polyethylene Wear for Metal Counterface Traverse of Severe Scratches

Abstract

Third body effects appear to be responsible for an appreciable portion of the wear rate variability within cohorts of patients with metal-on-polyethylene joint replacements. The parameters dominating the rate of polyethylene debris liberation by counterface scratches are not fully understood, but one seemingly contributory factor is the scratch's orientation relative to the direction of instantaneous local surface sliding. To study this influence, arrays of 550 straight parallel scratches each representative of the severe end of the clinical range were diamond stylus-ruled onto the surface of polished stainless steel plates. These ruled plates were then worn reciprocally against polyethylene pins (both conventional and highly cross-linked) at traverse angles varied parametrically relative to the scratch direction. Wear was measured gravimetrically, and particulate debris was harvested and morphologically characterized. Both of the polyethylene variants tested showed pronounced wear rate peaks at acute scratch traverse angles (15º for conventional, 5º for cross-linked), and had nominally comparable absolute wear rate magnitudes. The particulate debris from this very aggressive test regime primarily consisted of extremely large and elongated strands, often tens or even hundreds of microns in length. These data suggest that counterface damage regions with preferential scratch directionality can liberate large amounts of polyethylene debris, apparently by a slicing/shearing mechanism, at critical (acute) attack angles. However, the predominant manifestation of this wear volume was in the form of particles far beyond the most osteolytically potent size range.

Scratching Vulnerability of Conventional vs Highly Cross-Linked Polyethylene Liners Because of Large Embedded Third-Body Particles

“…Analogous to cross-shearing effects for wear of non-damaged surfaces, the counterface scraping transversely across a scratch lip would tend to break off this piled-up material. While the particle size of broken-off scratch lip material would tend to be outside the most osteolytic range [15], its presence in the joint space would nevertheless be deleterious [16]. …”

A novel formulation for scratch-based wear modelling in total hip arthroplasty

“…The specific local wear coefficient elevations were based on the scratch segment’s lip height and orientation (Glennon et al 2008). Physical wear test results from the literature (Dowson et al 1987) were used to determine the relationship between scratch severity and wear acceleration.…”

“…Similarly, R p fails to differentiate between a single asperity versus multiple asperities of similar height (Hall et al 1997). Additionally, for actual scratch damage, the angle at which a given scratch instantaneously traverses the apposing polyethylene is known to play a critical role in wear rate (Glennon et al 2008). …”

Problematic sites of third body embedment in polyethylene for total hip wear acceleration

“…Parallel scratches may be seen in some particular directions at some sites. Studies have shown that scratches oriented in specific directions result in different wear volumes, depending on the angle of scratch-traverse (Dowson et al, 1987;Glennon et al, 2005); therefore scratch directionality may have a significant impact on the amount of wear acceleration from roughened femoral heads. Wear from embedded debris resulting in scratches in specific directions could potentially be represented with this model if the wear coefficient in the original sliding-distance-coupled finite element wear model was modified, for example, by using a wear coefficient that varied depending on the instantaneous direction of sliding along scratches, as reflected in continuum surrogates such as that of Paul et al (2005).…”