Is there material loss at the backside taper in modular CoCr acetabular liners?

Abstract

Background: Metal wear and corrosion products generated by hip replacements have been linked to adverse local tissue reactions. Recent investigations of the stem/head taper junction have identified this modular interface as another possible source of metal debris; however, little is known regarding other modular metallic interfaces, their ability to produce metal debris, and possibly to provide insight in the mechanisms that produce metal debris.

Questions/purposes: We asked three questions: (1) can we develop a reliable method to estimate volumetric material loss from the backside taper of modular metal-on-metal liners, (2) do backside tapers of modular metal-on-metal liners show a quantifiable volumetric material loss, and, if so, (3) how do regions of quantitatively identified material loss correspond to visual and microscopic investigations of surface damage?

Methods: Twenty-one cobalt-chromium (CoCr) liners of one design and manufacturer were collected through an institutional review board-approved retrieval program. All liners were collected during revision surgeries, where the primary revision reason was loosening (n=11). A roundness machine measured 144 axial profiles equally spaced about the circumference of the taper region near the rim to estimate volume and depth of material loss. Sensitivity and repeatability analyses were performed. Additionally, visual and scanning electron microscopy investigations were done for three liners.

Results: Our measurement method was found to be reproducible. The sensitivity (how dependent measurement results are on experimental parameters) and repeatability (how consistent results are between measurements) analyses confirmed that component alignment had no apparent effect (weak correlation, R2=0.04) on estimated volumetric material loss calculations. Liners were shown to have a quantifiable material loss (maximum=1.7 mm3). Visual investigations of the liner surface could identify pristine surfaces as as-manufactured regions, but could misidentify discoloration as a possible region of material loss. Scanning electron microscopy more accurately distinguished between as-manufactured and damaged regions of the taper.

Conclusions: The roundness machine has been used to develop a repeatable method for characterizing material loss; future work comparing a gravimetric standard with estimations of material loss determined from the roundness machine may show the accuracy and effectiveness of this method. Liners show rates of material loss that compare with those reported for other taper junctions. Visual inspection alone may misidentify as-manufactured regions as regions of material loss.

Clinical relevance: This study identifies the acetabular liner/shell interface in modular metal-on-metal devices as a potential source of metal wear or corrosion products. The relation between metal debris and clinical performance, regardless of the type of bearing couple, is a concern for clinicians. Therefore, it is important to characterize every type of modular junction to understand the quantity, location, and mechanism(s) of material loss.

Fig. 1A–D

The alignment procedure used in this study progressed from: (A) manual alignment to the (B) auto center and level of a ring gauge to (C) creation of a part datum from circumferential profiles. (D) This is a representation of the direction of the axial traces and their spacing about the circumference of the taper (not to scale).

Fig. 2A–B

Knowledge of the component geometry was used to identify likely locations for the as-manufactured regions. (A) A cross-sectional schematic view of the acetabular shell shows the mating surface for the modular liners. (B) A photograph of a liner taper where damaged regions, corresponding to the points of contact with the acetabular shell, may be clearly identified.

Fig. 3

The effect of length and placement of the selected fit region on estimated volumetric material loss is shown. Variability decreased with each sequential increase in length.

Fig. 4

The sensitivity analysis of volumetric calculations to component alignment showed a weak correlation (R² = 0.04).

Fig. 5

Normalized volumetric material loss was calculated using 15 to 144 axial profiles. Three trials of 144 measurements were made on one component. The other data were simulated by excluding various numbers of axial traces from the data set; as there are permutations of which traces can be excluded, some simulations have more data points than others.

Fig. 6A–B

(A) Average volumetric material loss and (B) maximum depth of material loss are shown. The horizontal line indicates the mean for the entire cohort.

Fig. 7A–D

Visual indications of liner damage, which was shown to have quantifiable material loss, can be seen. (A) A visual overview of the taper surface is shown. (B) Scanning electron microscopy, 10 kV, shows topographic changes at low magnification (×25). (C) Machine marks from manufacturing can be clearly seen (×100). (D) There is a distinct change in surface topography in a damaged region (×100). The axial profile (overlay) shows quantitative surface topography along the taper. The fit region was −3.3 to −1.3 mm on the distance axis.

Fig. 8A–D

Visual indications of liner damage, which did not show quantifiable material loss, are shown. (A) This visual overview of the taper surface shows discoloration and resulted in a visual damage score of three. (B) Scanning electron microscopy, 10 kV, shows relatively consistent topography at low magnification (×25). (C) Machine marks from manufacturing can be seen inside and outside the bands of discoloration (×100). (D) There was no apparent change in surface topography in the discolored region; the vertical lines are from the roundness machine measurements (×100). The axial profile (overlay) shows quantitative surface topography along the taper. The fit region was −3.3 to −1.3 mm on the distance axis.

Fig. 9A–D

A severely damaged component shows the contact geometry. (A) The acetabular liner and shell indicate the presence of a noncontact region. (B) Liner taper damage could be identified visually by discoloration and texture. (C) A change in the surface topography could be seen at higher magnification (×15) and (D) evidence of material loss could be identified at ×200 magnification. The axial profile (overlay) shows surface measurement along the taper and agrees with scanning electron microscopy analysis. The fit region was −3.2 to −1.6 mm on the distance axis.