The univariate linear regression models showed significant and positive associations between the summed area of the 10 traces taken along the neck male taper and time to revision (coefficient, 0.040 [95% confidence interval (CI), 0.028 to 0.051]; p < 0.0001) (Fig. 7). For every 1-unit increase in time, there was a 0.04-unit increase in the sum of the 10 traces taken along the taper.
No significant relationship was detected between the summed area of the 10 traces and any of the following variables: age, sex, BMI, head diameter, implant horizontal offset, neck length, and outer diameter of polyethylene liners of dual-mobility cups (Table II). There was no significant difference in area of damage between bearing group types (dual-mobility or standard) or head materials (ceramic and/or metal) (t test, p > 0.05).
The ICC showed excellent reproducibility between the 2 examiners for the measurement of the area of damage (ICC, 0.95 [95% CI, 0.87 to 0.98]; p < 0.0001).
The multivariate analysis showed an association between the severity of taper damage and time to revision (p < 0.0001).
The medial parts of the necks exhibited deeper depth of wear compared with their lateral counterparts (Fig. 9). The maximum linear wear depth on the proximal medial aspect of the necks ranged from 20 to 200 µm, with a mean of 68.25 µm. On the lateral side, the mean maximum wear depth was 41.03 µm (range, 8 to 130 µm). The difference between the medial and lateral aspects was found to be significant (t test, p < 0.0001).
The median taper material loss rates (both head bore and trunnion surfaces) were 0.210 mm3/yr (range, 0.030 to 0.448 mm3/yr) for the metal head group and 0.084 mm3/yr (range, 0.059 to 0.108 mm3/yr) for the ceramic head group. The difference was not significant (p = 0.58). In all cases, the losses were very low, <1 mm3/yr30.
We aimed to better understand the impact of clinical and implant factors on the variability of the in vivo performance of recalled dual-taper implants to help surgeons to identify patients at risk of a revision surgical procedure. The analyzed devices were explanted because of an adverse reaction to metal debris and had been sent to our center for forensic examination together with the patient’s clinical data. We applied engineering methods to evaluate and quantify taper damage and correlated this with the available clinical data. Multivariate analysis using 8 variables from 116 components revealed a strongly significant correlation between neck-stem taper wear and time to revision. Moreover, univariate linear regression analysis showed that the wear rate was significantly associated with increased cobalt and chromium concentrations in the blood.
None of the available implant or patient-specific data correlated with the measured value of material loss observed. The bearing type and material of the heads made no difference in the outcomes measured. The ranges of maximum linear wear depth found were twofold higher than those previously reported for studies looking at head-neck taper junctions with metal-on-metal bearings (cobalt-chromium and titanium combination) that had failed because of an adverse reaction to metal debris32.
To our knowledge, this current study constitutes the largest retrieval study investigating neck-stem taper damage in dual-taper hip implants. An elevated blood ion ratio of cobalt relative to chromium has been associated with taper damage33,34, and the present findings support this. Despite their unsatisfactory clinical performance, dual-taper hip implants provide information helpful for understanding the clinical importance of taper corrosion, because there are no confounding effects from metal-on-metal bearings and negligible corrosion or material loss has been reported at the head-neck taper junction25,30,35.
For the first time for this specific design of implant, the cobalt and chromium concentrations in patients (in other words, the in vivo biomarkers of the wear rate) were available, thus giving a clinical importance to the retrieval analysis. Patients reported significantly elevated median levels of cobalt (7.4 μg/L) and chromium (1.7 μg/L).
Gross evidence of black deposits on all surfaces of the neck-stem taper junctions was found, more extensively on the medial conical aspect (Fig. 1). A number of studies have documented substantial corrosion on the removed components6,11,13,24,36-40 of this design. Compositional analysis confirmed chromium-rich deposits, phosphorus, and depletion of cobalt, indicative of corrosion processes taking place.
A characteristic wear pattern was observed in areas of maximum stresses. In many cases, the patches showed a retained topography of the original manufactured metal surface with a superficial TMZF transfer (Figs. 4 and 5). These findings are in agreement with recent research25,27. In addition, in this current study, the small size of the probe of the contact profilometer allowed us to capture complementary patches on the surfaces of the female taper, the localized removal of the TMZF alloy that had been transferred from the surfaces of the female taper of the stem onto the surface of the necks. The patches were complementary in size and shape to those seen on the surfaces of the neck male tapers (Fig. 4). It appears that, during in vivo function, the high stresses transmitted to the taper junction have led to material transfer of the titanium alloy on the cobalt-chromium alloy in areas of zero or partial slip fretting regime27. Although there were extensive areas of metal removal all around the patches on the neck male parts, this was not seen on the female counterparts. This could be partially explained by the increase in interfacial hardness of the TMZF alloy when coupled with the cobalt-chromium alloy27. Distribution of taper damage was random and more superficial on the surfaces of the female stem tapers, confirming the process of the hardening of the titanium alloy.
These findings can contribute to the understanding of the complex TMZF and cobalt-chromium interaction in the in vivo environment, considering the recent reports on the gross failure of TMZF trunnions mating with cobalt-chromium heads41-43.
The recall of this prosthesis, in addition to the disappointing performance of other dual-taper designs, whose approval had been based on the principle of substantial equivalence, once again highlights the importance of long-term clinical trials.
There were limitations in our study. In this cohort of failed implants, none of the patient or implant variables investigated was found to be predictive of failure. This may be because the retrieval analysis could not account for factors that could have had an influence on the severity of taper damage, such as patient activity levels44 and weight45 or surgical factors such as taper assembly condition46 and assembly force47. Our findings, although reproducible, cannot be used to directly compare volumetric metal losses; however, we reported maximum wear depths that constitute valuable data for comparison in future work. Moreover, our analysis was restricted by the limitations of the measurement tool, which could only analyze conical-shaped surfaces, thus excluding the contribution of the flat surfaces that account for one-fourth of the whole male tapers.
In conclusion, forensic examination of the retrieved components that failed secondary to adverse reaction to metal debris showed, in all cases, visible corrosion. Maximum linear wear depth recorded was twofold higher than that reported for head bore tapers in cobalt-chromium and titanium couples that failed because of an adverse reaction to metal debris32. The severity of damage was found to increase with time in situ and to be associated with an increase of cobalt and chromium concentrations in the blood, thus increasing the risk of adverse tissue reactions. There is evidence in the literature that an increase in the level of cobalt relative to chromium can represent as a biomarker for taper tribocorrosion8,33,34. Surgeons should scrupulously follow up and consider revision for these patients to avoid extensive tissue excision.
A description of the method of measurement of material loss profile and figures showing the correlation of the maximum linear penetration depth on the medial and lateral aspects of the necks with the summed area of the 5 traces are available with the online version of this article as a data supplement at jbjs.org (http://links.lww.com/JBJS/E915).
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