<qa>  What are metal prostheses?Pacemakers, brain stimulators, dental implants or metallic braces, aneurysm clips metal clips on the wall of a large artery, metallic prostheses including metal pins and rods, heart valves, and cochlear implants, permanent eyeliner, insulin pump, or shrapnel fragments. Welders and metal workers are also at risk for injury because of possible small metal fragments in the eye which they may be unaware. Subjects will be screened for these contraindications prior to the study. 