Durham, N.C. — Eighteen more patients who were exposed to hydraulic fluid on surgical instruments in 2004 have filed suite against Duke University Health System.

About 3,800 surgical patients at Durham Regional Hospital and Duke Health Raleigh Hospital were exposed to instruments that had been washed with used hydraulic fluid. The fluid had been drained from hospital elevators and put in empty detergent drums, then was mistakenly shipped back to the hospitals.

Dozens of patients said they suffered health problems ranging from infections to immune system reactions after the exposure.

Duke Health officials have maintained the instruments were safe because they were sterilized after being washed in the hydraulic fluid. The hospital system recently settled the claims of dozens of other patients in the case for an undisclosed sum.