Allergies May Explain Post-Op Knee Pain December 3, 2014 NEW ORLEANS -- Pain that persists after a successful joint replacement operation might be due to allergic reaction to components of the implant, researchers suggested here. When a revision implant was chosen based on allergy testing, 14 of 15 patients said their condition was improved moderately or "a lot" compared with three of 20 patients whose allergy testing was not used to select the new implant (P<0.001), said Karin Pacheco, MD, of the University of Colorado Denver School of Medicine/National Jewish Health. "We had small numbers of patients in this study, but the difference was still highly significant," Pacheco told MedPage Today at her poster presentation and discussion session during the annual meeting of the American Academy of Orthopaedic Surgeons. "What we show is that if you use the information regarding allergic sensitivity in revision surgery, you did well," she said. "If you didn't use that information, the patient did poorly." She included patients who underwent hip, knee, and shoulder replacement surgery. Overall, she said, "58% of the people we tested had a positive test to something relative to their joint replacement. If you were tested pre-operation, the reactions were mainly to metals. If the problem was post-operation, about half had a reaction, and that was where cement emerged as a problem. "We also looked at clinical relevance because you can have a positive patch test and it may not be related to your implant. What we found was that the majority of people who had a positive test actually had the implant in place. If they were sensitized to bone cement most of them had bone cement. So we think this is clinically relevant." She said that people who have known allergies to metal jewelry should consider patch testing prior to undergoing implant surgery. Of the 179 patients in the study, the researchers were able to perform follow-up investigation in 106 of them. About 33% were men; the average age was 61. About 38% of the group had positive patch tests to metal; 27% were positive to any bone cement. "These were patients who were referred by an orthopedic surgeon because they had implant failure that was not due to infection and was not due to biomechanical issues. The x-rays look good, it may or may not look loose, they tapped it and it was culture negative," she said. "We are all faced with this problem," commented session moderator William Maloney, MD, of Stanford University Medical Center. "What we need to know as surgeons is that if you have a patient who comes in and has a knee that looks okay, but they hurt and you do a patch test that is positive, will you do a revision based on that information alone? "I still would personally be cautious about revising solely on the patch testing. We certainly have this small group of patients that hurts for no clear reason. Maybe this allergy thing is real. There aren't a lot of data out there. It could also be something else going on in their life such as work, depression. We need to be careful about over-interpreting it." Co-moderator David Blaha, MD, of the University of Michigan, Ann Arbor, noted that, "You stated that there was nothing going wrong biomechanically for the implant. We can't really say that. There are a lot of things that can go wrong with these implants that we can't understand and maybe what was fixed was some sort of subtle instability that made the patient better and it had nothing to do with allergy." "The people are here because they had a problem," Pacheco said. "There are going to be people who have allergies and do not have a problem joint. We need more outcome studies." Primary Source American Academy of Orthopaedic Surgeons Source Reference: Pacheco K, et al "Allergy assessment provides clinically relevant results in joint replacement patients" AAOS 2014; Abstract P120.