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Metal on Metal Are Metal Ion Levels a Useful Trigger for Surgical Intervention?

Josephine

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Are Metal Ion Levels a Useful Trigger for Surgical Intervention? PAPER NO. 558
William L. Griffin, MD, Charlotte, NC
Thomas K. Fehring, MD, Charlotte, NC
James C. Kudrna, MD, Glenview, IL
Robert H. Schmidt, MD, Fort Worth, TX
Michael J. Christie, MD, Nashville, TN
Susan M. Odum, Charlotte, NC
Anne C. Dennos, BS, Charlotte, NC


INTRODUCTION: Adverse local tissue reactions have been reported with a variety of metal on metal (MOM) implants. These reactions range from metallic staining of tissues to pseudotumors with peri-articular necrosis. It has been suggested that metal ion levels may have prognostic value in determining timing or need for surgical intervention. The British Orthopedic Association’s alert concerning MOM implants chose ion levels of 7ppb as a threshold for concern. The purpose of this study was to determine if cobalt and chromium ion levels can predict soft tissue damage at the time of revision.

METHODS: This study included patients undergoing revision of a MOM hip and who had pre-operative Co and Cr ion levels. Tissue damage noted at the time of revision surgery was graded on a four-point scale. Damage scores were compared to ion levels and time in situ independently and in combination to determine a time/dose correlation. The data were also analyzed to determine an ion level threshold that could serve as a trigger for surgical intervention.

RESULTS: Eighty-four patients had Co and Cr ion levels obtained prior to revision of a symptomatic MOM hip. Ion levels ranged from 0.8- 236ppb for Co, and 0-112ppb for Cr. There was a trend toward higher ion levels in cases with more severe tissue damage (Co p=0.50, Cr p=0.99). Both Co and Cr ion levels had poor sensitivity and specificity values as predictors of soft tissue damage (Co- 60%, 62%; Cr- 36%, 80%). The positive predictive values for Co and Cr were only 46%, and 26% respectively. The length of time to revision significantly correlated with tissue damage (p=.05).

DISCUSSION AND CONCLUSION: Co and Cr ion levels are not reliable predictors of peri-articular soft tissue damage and should not be used in isolation as surgical intervention triggers. Furthermore, the 7ppb threshold was not a useful predictor of damage.
 

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