Retrieval analysis of metal and ceramic femoral heads on a single CoCr stem design

Implant and patient selection

This study involved a series of 48 retrieved THAs that were revised at a single institution and subsequently sent to our centre for analysis. All hips were of a dual-taper design, with two junctions: at the neck-stem and neck-head. This study focused on the retrieval findings at the neck-head surfaces.

The hips were all of a single Rejuvenate (Stryker Orthopaedics, Mahwah, New Jersey) design and consisted of CoCr alloy V40 trunnions that were paired with either 21 CoCr alloy heads (Stryker) or 27 zirconia-toughened alumina (BIOLOX delta) ceramic heads (CeramTec GmbH, Plochingen, Germany). All heads were articulated against an ultra-high-molecular-weight polyethylene (UHMWPE) bearing. The CoCr heads had a median diameter of 40 mm (32 to 40) and the ceramic heads were all 36 mm in diameter except for one 32 mm head. Reasons for revision were pain, elevated blood metal ion levels and bony erosion due to severe corrosion at the neck-stem interface.

Patients with metal heads (11 female and ten male) had a median age of 66 years (42 to 81), a median implantation time of 23 months (19 to 59) and a median BMI of 28.7 kg/m² (25 to 42).

Patients with ceramic heads (11 female and 16 male) had a median age of 61 years (36 to 78), a median implantation time of 37.5 months (5 to 71) and a median BMI of 29 kg/m² (24 to 46.4).

The two groups were not found to vary in terms of their gender or body mass index (BMI) but there was some evidence that the age of revision varied between them, with the ceramic group younger, on average, by around seven years. However, this difference was only of borderline statistical significance. There was no difference in the length of implantation.

The two groups did not vary in terms of their head offset or lateral offset. The diameter of the metal heads was greater than that of the ceramic heads (Table I).

Sample preparation

All retrieved implants were decontaminated upon receipt by following an established protocol developed at our centre. The components were then cleaned in an ultrasonic cleaning bath (Pulsatron MKC6, Kerry, Guyson, UK) with a 10% mild detergent solution for 30 minutes, followed by rinsing under water and then being allowed to dry in air; this was performed in order to eliminate obscuring factors such as proteinaceous films on the surfaces.

Taper corrosion assessment

The CoCr male trunnions were inspected visually and with the aid of a Leica M50 light microscope (Leica Microsystems, Mannheim, Germany), by two examiners (AD and HH), experienced in retrieval analysis, to score for evidence of corrosion damage using a previously published method.²⁰ A score of 1 indicated minimal fretting or corrosion (no corrosion damage); 2 indicated mild damage (corrosion attack confined to one or more small areas); 3 indicated moderate damage (aggressive local corrosion attack with corrosion debris); and 4 indicated severe damage (severe corrosion attack and abundant corrosion debris). The two examiners were blinded for head material and in the case of disagreement between them, a consensus score was achieved after consultation. The bore tapers of the metal heads were also assessed for severity of corrosion. We did not compare the scores for the head tapers between the two groups, given that the corrosion score is applicable to metal heads only and comparison between corrosion and metal transfer on ceramic would not have been of significance.

Each neck-stem interface was also assessed for corrosion using the method described above.²⁰

Material loss measurement

The volume of material loss from the head bore taper surfaces was measured using a Talyrond 365 (Taylor Hobson, Leicester, United Kingdom) roundness-measuring machine. A total of 180 vertical measurement traces were taken along the taper axis using a 5-micron diamond stylus, with the femoral head positioned on a rotating air spindle, levelled and centred with respect to the spindle axis. The vertical traces from each head taper were combined into a rectangular surface map and the unworn surface on the taper was manually selected in order to level the data obtained.^11,21 The same method was used to analyse the male trunnion surfaces, and the annual rate of material loss was calculated by dividing the total volume by implantation time.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

A scanning electron microscope (SEM) (Jeol JSM5500; Jeol Ltd, Tokyo, Japan) was used to characterise the fretting/corrosion damage occurring at the male trunnion surfaces mating either ceramic or metal heads in both secondary and backscattered modalities and, when appropriate, EDS for elemental analysis was performed.

Statistical analysis

Statistical analyses were performed using SPSS Statistics Version 23 (IBM, Armonk, New York). All continuous variables were found to be approximately normally distributed, and were compared between groups using the unpaired t-test. Categorical variables were analysed using the Fisher’s exact test.

There were three outcome variables relating to the amount of material loss: trunnion rate; head rate, and; total (trunnion and head) rate.

All three variables were measured on a continuous scale, and all were found to have positively skewed distributions. As a result of the distributions, all outcomes were given a log transformation before analysis. Both trunnion rate and head rate had some zero values, and thus to enable a log transformation to be applied, a small constant was added to all rates before transformation. Regression analyses were performed to compare the wear rate between the two groups. The first analysis examined the difference between groups without considering any further variables. The second analysis re-examined the group difference, adjusting for potential confounding factors found to show a difference (p < 0.2) between the groups in preliminary analysis. The analysis of the outcomes was performed using linear regression on the log-transformed values.

Spearman’s rank order correlation was used to identify correlations between corrosion scores, material loss rates and the clinical and implant variables considered in this study. The level of significance chosen for all statistical analyses was p < 0.05.

Taper corrosion assessment

The mean corrosion scores of the trunnions paired with metal and ceramic heads were 2.3 (1 to 4) and 2.5 (1 to 4), respectively; this difference was not significant (t-test, p = 0.48). The mean corrosion score for the metal head tapers was 2.7 (1 to 4).

Severe corrosion damage was seen for all implants at the neck-stem interface with a mean Goldberg corrosion score of 3.95 (3 to 4) for the neck male parts and 3.88 (3 to 4) for the stem bore tapers.

Material loss measurement

Table II presents the median (IQR) material loss rates at the trunnion head taper and the total volumetric losses (mm³/year) for the two groups. The total head-stem rate with metal and ceramic heads had a median of 0.210 mm³/year (0.030 to 0.448)and 0.084 mm³/year (0.059 to 0.108) respectively.

Scanning Electron Microscopy and Energy Dispersive Spectroscopy

We found evidence of mechanically assisted crevice corrosion at trunnions that had been paired with both head types. Dark deposits were found to be chromium-rich, indicative of corrosion processes.

Regression analysis

The results of our statistical analysis are summarised in Table III. The group differences are reported in the form of ratios, along with corresponding confidence intervals. These give the ratio of material loss in the ceramic group relative to metal group. P-values from our linear regression analysis are also reported.

Our results suggested no significant difference between metal and ceramic groups in either trunnion rate or cumulative material loss rate. The lack of significance for these outcomes was observed both in a simple comparison (unadjusted analysis), and after adjusting for potentially confounding factors.

There was a significant difference in head taper material loss rate between the groups in the unadjusted analysis (unpaired t-test, p = 0.007). After adjusting for potentially confounding factors, the head rate in the ceramic group was still lower (in terms of the estimated difference), but this difference was not statistically significant (p = 0.22).

Correlation analysis

We did not find a significant correlation between material loss or patient variables of gender (Spearman’s rho = 0.35, p = 0.33), age at surgery (Spearman’s rho = -0.42, p = 0.19), BMI (Spearman’s rho = 0.37, p = 0.50), implantation time (Spearman’s rho = -0.16, p = 0.63) or implant variables of head diameter (Spearman’s rho = 0.31, p = 0.36), head offset (Spearman’s rho = 0.42, p = 0.21) and lateral offset (Spearman’s rho = -0.11, p = 0.83) in the metal group. Similarly, taper material loss in the ceramic group was not affected by patient or implant factors (p > 0.05) (Table IV).

Stem trunnion corrosion for the metal group was not significantly correlated with gender (Spearman’s rho = 0.21, p = 0.41), age at surgery (Spearman’s rho = -0.17, p = 0.49), BMI (Spearman’s rho = 0.27, p = 0.67), implantation time (Spearman’s rho = -0.44, p = 0.06) or implant variables of head diameter (Spearman’s rho = -0.09, p = 0.70), head offset (Spearman’s rho = 0.33, p = 0.17) and lateral offset (Spearman’s rho = -0.29, p = 0.40). The head taper corrosion score was not correlated with any patient or implant variables, p > 0.05.

Stem trunnion score for the ceramic group was not correlated with gender (Spearman’s rho = 0.37, p = 0.06), BMI (Spearman’s rho = -0.48, p = 0.15), implantation time (Spearman’s rho = 0.11, p = 0.60) or implant variables of head diameter (Spearman’s rho = -0.09, p = 0.62), head offset (Spearman’s rho = 0.12, p = 0.54) and lateral offset (Spearman’s rho = -0.32, p = 0.34); however there was a negative correlation between trunnion corrosion and age at surgery (Spearman’s rho = -0.42, p = 0.03).

There was a positive correlation between visual scoring and volumetric material loss (mm³) in the ceramic group (Spearman’s rho = 0.69, p = 0.0002) for the stem trunnions. No other significant correlations were found.