Abstract
Introduction
Little is known about variations in blood metal ions following bearing exchange for poorly functioning metal-on-metal hip replacements (MoMHRs). This study aimed to determine the changes that occur in blood and urine metal ion concentrations following MoMHR revision.
Methods
Between 2005–2012, a single-centre prospective cohort study was undertaken of patients with large-diameter MoMHRs and high blood metal ions requiring revision. Whole blood and urine were collected both pre-revision and post-revision for metal ion analysis.
Results
Twenty-three MoMHRs (21 hip resurfacings and 2 total hip replacements) were revised at a mean time of 7.9 years from primary surgery (mean age 56.0 years; 65% female). All revisions were performed using primary total hip implants and non-metal-on-metal bearings. Mean (range) metal ion concentrations pre-revision were: blood cobalt 13.9µg/l (1.32–74.7µg/l), blood chromium 8.9µg/l (1.29–57.3µg/l), urine cobalt 104.6µg/24 hours (4.35–747.3µg/24 hours), urine chromium 33.2µg/24 hours (4.39–235.4µg/24 hours). After revision the mean metal ion concentrations (percentage of pre-revision values) were: blood cobalt at 2 days=10.7µg/l (77%), 4 days=8.4µg/l (61%), 6 days=7.7µg/l (55%), 2 months=3.4µg/l (24%), 1 year=1.0µg/l (7%), 2 years=0.42µg/l (3%); blood chromium at 2 days=8.7µg/l (98%), 4 days=6.5µg/l (73%), 6 days=5.5µg/l (62%), 2 months=2.2µg/l (25%), 1 year=1.5µg/l (16%), 2 years=0.97µg/l (11%); urine cobalt at 2 days=31.9µg/24 hours (30%), 4 days=22.5µg/24 hours (22%), 6 days=21.5µg/24 hours (21%), 2 months=6.1µg/24 hours (6%), 1 year=0.99µg/24 hours (1%), 2 years=0.61µg/24 hours (1%); urine chromium at 2 days=34.4µg/24 hours (103%), 4 days=19.2µg/24 hours (58%), 6 days=15.8µg/24 hours (48%), 2 months=9.3µg/24 hours (28%), 1 year=2.8µg/24 hours (8%), 2 years=1.9µg/24 hours (6%).
Discussion
The most dramatic reductions in systemic metal ion concentrations in MoMHR patients occur early following bearing exchange, with little variation after one-year.
Conclusions
Blood and urine metal ion sampling performed as early as two-months following MoMHR revision is sufficient to determine the success of bearing exchange.
