4. Discussion
Eight distinct zones of the stem tapers including anterior-distal, anterior-proximal, medial-distal, medial-proximal, posterior-distal, posterior-proximal, lateral-distal, and lateral-proximal were scored and statistically compared to identify the zone(s) with the most severe corrosion damage in the retrieved implants studied in this work. It is noted that there are several studies in the literature that chose to score stem tapers holistically, not locally [
9,
10,
24,
25,
26].
Within the studies [
11,
12,
15,
16,
17,
18,
19] that scored stem tapers locally, the pools of implants had a limited diversity in terms of implant properties (e.g., head diameter, articulation type, and stem design). Therefore, it was deemed necessary to explore whether a similar distribution of corrosion damage can be seen in a more heterogeneous pool of implants.
To the best of our knowledge, there are only two studies [
18,
19] in the literature that, similar to this work, have assigned eight local scores to the stems with the rest using lower numbers of zones. In those two studies, one did not compare the scores between the zones [
18]. The other compared the four quadrants, and the two distal and proximal regions separately in terms of corrosion severity and did not determine which zone(s) had the most severe damage [
19].
Routine causal-explanatory statistical analyses require only one score as the descriptor of damage for each implant. The majority of these studies have chosen to combine the local scores by calculating an overall value [
8,
11,
12,
13,
14]. This approach has led to the presumption that this global score is a continuous variable; and, thus, the statistical analyses for continuous variables have been utilised. Analysing a continuous variable with an interval or ratio level of measurement is generally less complex in nature. However, an increased number of levels in the global score does not necessarily imply a known “distance” between the score levels. Therefore, this approach was treated with suspicion in this study and was not adopted.
Here, the corrosion scores were analysed using a univariate OLR model, and the odds ratios along with their
p-values were reported. Since there was no particular hypothesis about the relative level of corrosion at the eight zones, 28 pairwise comparisons were carried out to exhaust the entire pairwise comparison of the zones. The distal region of the medial quadrant was found to have the highest odds of receiving a higher corrosion score which is aligned with the previous findings in the literature that identified the distal region [
19,
20,
27] and the medial quadrant [
7,
10,
16,
28] having the highest corrosion scores. Also, this study shows that the distal region of all the four quadrants had more corrosion damage in comparison with the proximal region of those quadrants. Therefore, it was found that, regardless of the quadrant, corrosion damage is more present distally than proximally.
Generally, the higher severity of wear or corrosion at a specific zone has been attributed to several factors such as increased micro-motions at the interface, head or stem materials, head diameter, high friction moments, and poor lubrication of the bearing articulation. While some act as root causes, the others play the role of causal factors. Also, damage at the head-neck taper junction usually appears as a combination of wear and corrosion mechanisms. Some of these factors may only contribute to a specific mode of damage, while others may contribute towards a set of damage mechanisms.
In a retrieval study of 231 implants [
7] the stem tapers received four fretting and corrosion scores corresponding to the four quadrants. The medial and lateral scores were observed to be significantly higher than the scores at the other two quadrants (posterior and anterior). This was explained to be due to a higher likelihood of micro-motions between the head and neck about an axis in the sagittal plane. Similar to the present study, the pool of implants in this work had a wide diversity, and higher corrosion scores at the medial quadrant suggest that it could be a phenomenon independent of the included patient and implant factors.
Wilson et al. [
29] explained how at the double-tapered cone design of Profemur Z, the proximal end of the neck experiences an almost pure compression and shear loading. High frictional moments at taper junctions were related to poor lubrication of the articulation interfaces by another study [
30].
The medial quadrant was identified to have higher corrosion scores in a retrieval study of 52 S-ROM components [
16]. It was hypothesised that greater micro-motions at this quadrant could result in a more frequent disruption of the passive oxide layer; and consequently, more severe corrosion damage. Similar to the conclusion of the Wilson et al. [
29] study, they reported that this region is generally under a compression-loading regime. A computational modelling of the stem taper stresses paired with large diameter heads confirmed this hypothesis after witnessing maximum levels of principal stresses at the medial quadrant [
31]. In that work, a 3D model of a 12/14 titanium taper was paired with cobalt-chromium and alumina heads. Increasing the head diameter increased this quadrant’s stresses distal to the junction significantly. It was highlighted that the pairing of a small taper and a large head leads to a larger moment arm transmitting a higher force to a small surface area which facilitates tribo-corrosion.
A relatively higher amount of load and stress at the medial quadrant causes elastic strains which appear as surface compression. This condition may lead to micro-motions of approximately 5 to 40 µm [
32] which in turn may result in abrasion or fracture of the oxide layer. The subsequent changes in the metal surface potential and the continuous re-passivation of the oxide layer change the chemistry of the crevice solution. Ultimately, the deaeration and pH decrease of the solution initiate crevice corrosive attacks [
33,
34]. Crevice corrosion has been reported to occur near the bore opening which may explain observing more severe corrosion at the distal region [
35].
Besides micro-motions, galvanic corrosion at this interface due to using mixed metal components is a potential source of material loss. In this study, 18 (13.1%) implants had mixed head and stem materials, whereas 45 (32.8%) had similar materials. Therefore, galvanic corrosion cannot be nominated as the sole mechanism of corrosion.
These studies have used relatively homogenous pools of implants, yet they observed higher levels of corrosion at the medial quadrant or distal zones of stem tapers. Based on the findings of the present study which shows that the distal region of the medial quadrant sustains the most severe corrosion damage, it is understood that this particular zone is most severely damaged versus all the other zones regardless of the properties and patient characteristics of the investigated pool of implants.