Table 2.

Cobalt and chromium ion concentration in erythrocytes and serum

Treatment group	Median preoperative concentration	Median Year 1 concentration	Median Year 5 concentration	Intergroup p value (COM, MOM at Year 5)	Intragroup p value (preoperative and Year 5)	Intragroup p value (Year 1 and Year 5)	Intragroup variance comparison (Year 1 and Year 5)
Erythrocyte cobalt
CoM	0.08 (0.07, 0.13)	0.45 (0.28, 0.83)	0.55 (0.38, 0.69)	0.72	< 0.001	0.11	0.39
MoM	0.09 (0.06, 0.16)	0.32 (0.26, 0.44)	0.51 (0.31, 1.21)		< 0.001	< 0.01	< 0.05
Serum cobalt
CoM	0.12 (0.07, 0.23)	0.88 (0.54, 1.49)	0.85 (0.73, 1.00)	0.54	< 0.001	0.55	0.60
MoM	0.12 (0.09, 0.16)	0.65 (0.42, 0.80)	1.01 (0.64, 2.20)		< 0.001	< 0.01	< 0.01
Erythrocyte chromium
CoM	0.98 (0.55, 1.25)	1.25 (0.75, 1.70)	0.92 (0.77, 1.36)	< 0.05	0.39	0.08	0.07
MoM	0.85 (0.43, 1.25)	1.43 (0.75, 2.60)	0.56 (0.34, 0.99)		0.17	< 0.01	< 0.01
Serum chromium
CoM	0.16 (0.14, 0.20)	1.01 (0.65, 1.99)	1.13 (0.73, 1.35)	0.35	< 0.001	0.67	0.81
MoM	0.14 (0.11, 0.22)	0.84 (0.61, 1.24)	0.95 (0.60, 1.41)		< 0.001	0.70	0.26

Medians are reported with the interquartile range. The Mann-Whitney test was used to test for differences between groups. The Wilcoxon signed-rank test was used for related variables. Levene’s test for homogeneity of variance was used to check for differences in variance. The reference ranges for erythrocyte cobalt and serum cobalt are 0.02–0.29 ppb and 0.03–0.40 ppb, respectively. The reference ranges for erythrocyte chromium and serum chromium are 0.04–0.64 ppb and 0.10–0.20 ppb, respectively; CoM = ceramic-on-metal; MoM = metal-on-metal.