Table 2.

Cobalt ion concentration in erythrocytes, serum, and urine

Treatment group	Median preoperative concentration* (interquartile range)	Median 6-month concentration (interquartile range)	Median year 1 concentration (interquartile range)	Median year 2 concentration (interquartile range)	Intragroup p value||	Intergroup p value¶ (MOP and MOM)	Intergroup p value (MOM28 and MOM 36)
Erythrocyte cobalt
MOP†	0.13 (0.11–0.16)	0.14 (0.11–0.18)	0.11 (0.10–0.14)	0.11 (0.09–0.15)	0.249
MOM28‡	0.13 (0.11–0.19)	0.41 (0.35–0.73)	0.42 (0.27–0.59)	0.42 (0.30–0.60)	< 0.001	< 0.001
MOM36§	0.11 (0.09–0.16)	0.32 (0.21–0.51)	0.45 (0.30–0.65)	0.42 (0.27–0.71)	< 0.001	< 0.001	0.915
Serum cobalt
MOP	0.15 (0.10–0.22)	0.15 (0.11–0.20)	0.12 (0.10–0.17)	0.14 (0.09–0.19)	0.252
MOM28	0.16 (0.09–0.29)	0.72 (0.49–0.98)	0.72 (0.48–1.02)	0.77 (0.48–1.19)	< 0.001	< 0.001
MOM36	0.14 (0.08–0.22)	0.66 (0.37–0.85)	0.70 (0.49–1.10)	0.73 (0.47–1.99)	< 0.001	< 0.001	0.831
Urine cobalt
MOP	0.41 (0.26–0.75)	0.50 (0.27–0.80)	0.42 (0.29–0.78)	0.44 (0.35–0.60)	0.809
MOM28	0.48 (0.29–1.03)	3.52 (1.83–5.01)	4.13 (2.79–7.51)	4.55 (2.74–6.86)	< 0.001	< 0.001
MOM36	0.48 (0.32–0.78)	4.16 (2.29–6.50)	4.20 (2.56–8.48)	5.42 (2.07–14.22)	0.001	< 0.001	0.612

*All concentrations are given in μg/L; ^†MOP = patients randomized to metal-on-polyethylene THA; ^‡MOM28 = patients randomized to metal-on-metal, 28-mm THA; ^§MOM36 = patients randomized to metal-on-metal, 36-mm THA; ^||intragroup p value = difference in preoperative and 2-year postoperative cobalt ion concentrations within each group using the Wilcoxon test; ^¶intergroup p value = difference in cobalt ion concentrations at the 2-year postoperative interval between the metal-on-polyethylene group and each of the metal-on-metal groups and between the two metal-on-metal groups using the Mann-Whitney test.