Table 3.

Chromium 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 chromium
MOP†	1.00 (0.75–1.45)	1.20 (0.80–1.58)	1.20 (0.70–1.60)	1.10 (0.70–1.73)	0.609
MOM28‡	1.20 (1.10–1.55)	1.20 (0.80–1.60)	1.10 (0.87–1.42)	1.10 (0.60–1.70)	0.198	0.864
MOM36§	0.90 (0.70–1.47)	1.10 (0.80–1.68)	1.20 (1.00–1.85)	1.20 (0.78–1.63)	0.525	0.527	0.427
Serum chromium
MOP	0.20 (0.13–0.29)	0.20 (0.14–0.32)	0.24 (0.13–0.37)	0.17 (0.12–0.29)	0.904
MOM28	0.22 (0.10–0.28)	0.99 (0.49–1.30)	1.07 (0.65–1.46)	1.29 (0.77–1.72)	< 0.001	< 0.001
MOM36	0.21 (0.12–0.34)	0.97 (0.55–1.39)	1.06 (0.61–1.63)	0.91 (0.64–1.78)	0.001	< 0.001	0.600
Urine chromium
MOP	0.30 (0.15–0.49)	0.30 (0.18–0.50)	0.31 (0.22–0.37)	0.27 (0.19–0.53)	0.819
MOM28	0.24 (0.18–0.46)	1.64 (0.74–2.19)	1.88 (1.20–2.40)	1.92 (1.44–2.90)	< 0.001	< 0.001
MOM36	0.36 (0.18–0.58)	1.65 (1.19–2.17)	1.80 (1.33–3.04)	2.02 (1.08–3.74)	< 0.001	< 0.001	0.788

*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 chromium ion concentrations within each group using the Wilcoxon test; ^¶intergroup p value = difference in chromium 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.