To investigate the mid-and long-term trend of metal ion levels after metal-on-metal hip resurfacing arthroplasty.

A retrospective analysis was performed on 72 patients with 81 hips, including 30 males (31 hips)and 42 females(50 hips)who underwent metal-on-metal hip resurfacing arthroplasty in Tianjin First Central Hospital from September 2006 to August 2016. The age range was 24-66 years. Twenty-nine hips(35.8%)were diagnosed as acetabular dysplasia before the operation, 34 hips (42.0%) were diagnosed femoral head necrosis, three hips were diagnosed osteoarthritis (3.7%), five hips were diagnosed rheumatoid arthritis (6.2%), 10 hips were diagnosed ankylosing spondylitis (12.3%). The patients who were allergic to metal ions and using metal-containing drugs were excluded. Twenty-two hips used Conserve Plus prosthesis from Wright Company(USA), and 59 hips used Birmingham Hip Resurfacing or Birmingham Mid Head Resection System (BHR or BMHR) from Smith & Nephew Company (United Kingdom). Serum samples from 20 patients were randomly selected before surgery to detect metal ion levels, and served as control groups. Inductively coupled plasma mass spectrometer was used to detect the levels of Co, Cr and Mo ions in serum samples before and after surgery, three months, six months, and every year after surgery, and analyze their changes. The t-test was used to compare the differences in metal ion levels before and after surgery, and the Co, Cr, and Mo ion levels at the same time point were analyzed by single-factor repeated measurement data analysis of variance.

The mean follow-up time was (7.85±0.34)years. The preoperative levels of Co, Cr and Mo were (1.13±0.24)μg/L, (2.22±0.54)μg/L, and(2.62±1.00)μg/L. The levels of the three metal ions increased after the operation, among which the differences of Co and Cr ion levels were statistically significant (F=51.93, 61.00, both P<0.05). Co ions reached a peak six months after the operation, 3.2 times to that before the operation, and then gradually decreased. Seven years after the operation, the levels of Co ions returned to the levels before the operation. The levels of metal ions increased again at eight years and 10 years after surgery, 2.0 times and 2.5 times to that before surgery respectively, and the differences were statistically significant (both P<0.05). After 10 years, the levels of metal ions gradually decreased to 13 years after surgery. Co ion level basically recovered to the preoperative level, (1.28±0.79)μg/L. One years after Cr ion, the metal ion level reached a peak, which was 2.2 times to that before the operation, and the difference was statistically significant (P<0.05) At seven years after operation, the metal ion level increased significantly, compared with the metal ion level at the peak. Similarly, it was 2.0 times to the metal ion level before surgery, and the difference was statistically significant (P<0.05). The level of metal ions was in a downward trend. By 13 years after operation, the level of metal ions was lower than before surgery, and the difference was not statistically significant (P>0.05). Although Mo ion level increased following the surgery, the difference was not statistically significant(F=35.93, P>0.05). Mo ion level reached the peak of the follow-up period at three months after the operation, which was 2.2 times to the preoperative level. Four years after the operation, Mo ion level showed a significant decline process, and Mo ion was the lowest eight years after the operation. The lowest level was (2.62±1.43) μg/L, which was not statistically significant compared with preoperative difference (P>0.05). From nine to 10 years after operation, the ion level increased again, which was (5.29±2.07) μg/L, which was 2.02 times preoperatively, and reached a plateau, dropped to the preoperative level 13 years after operation, yet the difference was not statistically significant (P>0.05).

After metal-on-metal hip resurfacing arthroplasty, in the early stage increased and reached a peak, and the metal ion level in the medium and long term shows a downward trend. The fluctuation of metal ion level needs to be continuously monitored and closely observed.