BACKGROUND AND PURPOSE
Hip prostheses that use a metal-on-metal articulation expose the brain to elevated metal concentrations that, in acute excess due to prosthesis malfunction, is associated with neurologic damage, including visual and hearing loss and motor deficits. Here, we examined whether chronic exposure to lower elevated metal levels, typical of well-functioning prostheses, also affects brain structure and function.

MATERIALS AND METHODS
We compared brain volumes, metal deposition, and gray matter attenuation by MR imaging and clinical neurologic function in patients 8 years after receiving a metal-on-metal hip resurfacing versus a matched group of patients with the same duration exposure to a conventional hip prosthesis.

RESULTS
Twenty-nine patients (25 men; mean, age 59 ± 7 years) after metal-on-metal hip resurfacing and 29 patients (25 men; 59 ± 8 years) after total hip arthroplasty were compared. Whole blood cobalt and chromium concentrations were 5–10 times higher in the metal-on-metal hip resurfacing group (P < .0001). Occipital cortex gray matter attenuation tended to be lower (P < .005 uncorrected, P > .05 corrected), and the optic chiasm area tended to be lower (mean difference, −2.7 mm²; P = .076) in the metal-on-metal hip resurfacing group. Subgroup analyses in 34 patients (17 per group), after exclusion of primary ocular pathology, showed the same trend in gray matter attenuation in the occipital cortex and basal ganglia and a smaller optic chiasm in the metal-on-metal hip resurfacing group (mean difference, −3.9 mm²; P = .048). No other structural or functional differences were found between the groups.

CONCLUSIONS
Chronic exposure to metal-on-metal hip resurfacing is associated with subtle structural change in the visual pathways and the basal ganglia in asymptomatic patients.