The Conserve® Plus (Wright Medical Technology Inc., Arlington, TN) was introduced clinically in the United States in 1996. A study of the serum cobalt and chromium ion levels was started in 2000 in our center to monitor the metal ion levels over time as part of an FDA clinical trial.

Thirteen male and five female patients received this resurfacing for idiopathic osteoarthritis (14), post-traumatic degenerative changes (3) or developmental dysplasia (1). Fourteen received a unilateral implant but four subsequently received a contralateral device from 52 to 86 months post-op. Four patients had bilateral resurfacings done in a one-stage procedure. All surgeries were performed by the senior author. None of these patients had known exposure to cobalt or chromium, kidney disease or other metal implants elsewhere in their bodies. Each prospectively provided blood samples and then yearly thereafter to measure cobalt and chromium levels for up to 11 years. Metal levels were measured using atomic absorption spectrophotometry and inductively coupled plasma mass spectrometry by a specialized trace element analysis laboratory. Acetabular component position was evaluated using Einzel-Bild-Röentgen-Analysis (EBRA) software. Contact patch to rim (CPR) distance was computed as described by Langton et al JBJS Br 91: 2009. A mixed model linear regression analysis was performed to evaluate long term trends, and multivariate analysis was performed to examine effects of implant and patient covariates on the metal ion levels.

One bilateral patient underwent revision for femoral loosening, all other patients were clinically well-functioning at the time of last follow-up (ave 89 mos). The median pre-operative Co was below the detection limit (d.l) of 0.3μg/L and the median pre-operative Cr was 0.069μg/L (d.l. 03μg/L). Metal levels increased within the first year then decreased and stabilized (fig 1). For unilaterals over all time intervals, the median Co was 1.06 μg/L, while the median Cr was 1.58 μg/L. For bilaterals, the mean post-operative Co was 2.80 μg/L, while the mean Cr was 5.80 μg/L. Generally, Cr levels were higher at all time points than Co. Bilateral patients had Co values 1.96 times greater on average than the unilateral patients (p<0.001). None of the possible covariates studied (femoral size, cup abduction angle, cup anteversion, CPR distance, activity, BMI and testing method) were related to the assay values.

The results of this study have shown that serum metal levels in well functioning implants can be low and do not increase over time. These are among the lowest levels reported for resurfacing devices and comparable to levels reported for well functioning small diameter metal-on-metal total hips. The study is limited due to the relatively small sample size and limited range of values for the covariates studied. However, it included patients who were active, female or bilateral and we collected ion levels up to 11 years. We now recommend that patients who have well-oriented Conserve Plus components with stable radiographic interfaces and no incidences of unexplained pain or hip noises be scheduled for follow-up every 2–3 years, rather than annually.

Ion levels in the serum and urine of patients with metal-on-metal hip resurfacing implants can provide a means to monitor bearing wear. This presentation will discuss the current results, now out to 5 years for the Conserve Plus resurfacing. In particular, the effect of bilateral implantation on ion levels was examined

Forty-eight patients were studied. Forty-three of these cases were initially implanted with a unilateral resurfacing. Nine of these cases subsequently were implanted with a resurfacing implant on the contra-lateral side 4 to 48 months following the first implantation (staged implantations). Five cases had bilateral resurfacings done simultaneously. All surgeries were done in one institution by a single surgeon. Serum and urine samples were collected pre-operatively, and at 4 months, 12 months and annually thereafter. The samples were analysed for cobalt and chromium using atomic absorption spectrometry with a detection limit of 0.3 to 0.03ng/ml respectively. The data were compared between the groups and also correlated with UCLA activity scores, cup angle, BMI and component size.

All patients showed a rise in ions following implantation. The simultaneous bilateral levels were higher at all time periods compared with the staged bilaterals monitored at the same time point for the second hip, for example cobalt serum at 12 month uni = 2.24, simultaneous bilat = 2.53, staged bilat = 2.05ng/ml, and at 4 years uni = 1.20, simultaneous bilat = 2.93, staged bilat = 2.27ng/ml. There was no correlation between ion levels and UCLA activity score, gender, component size or cup angle (but only 4 hips had cups > 55 degrees).

Bilateral metal-on-metal hip resurfacings performed simultaneously resulted in higher levels of metal ions, particularly chromium, compared to staged implantations monitored at the same time periods. With the exception of a small number of outliers, the levels in this group of hip resurfacings were within the range of metal levels reported for other metal-on-metal total hips.

Background: Metal-metal bearings are frequently implanted in young, active patients. The relationship between patient activity and Co and Cr ion levels has not been scientifically investigated.

Methods: Seven patient subjects with well-functioning metal-metal bearing hip prostheses and one control subject (no implants), all with normal renal function, were monitored during a two-week long activity protocol. Lower extremity activity was continuously assessed by a computerized, two-dimensional accelerometer (Step Activity Monitor; SAM). During the first week, subjects were requested to limit physical activity. Subjects then completed an hour-long treadmill test followed by a week where they were encouraged to be as physically active as possible. Serum Co and Cr ion levels and urine Cr levels were assessed at 10 different time points during these two weeks.

Results: Regardless of activity, the serum ion levels for a given patient were essentially constant and there was no correlation between patient activity and serum or urine ion levels. A mean increase in activity of 28% (95% CI, 13 to 43%; SE, 6%) during the high-intensity activity week resulted in a mean decrease of 2.6% (95% CI, −14.2 to 8.9%; SE, 4.7%) in serum Co and a mean increase of 2.0% (95% CI, −5.3 to 9.3%; SE, 3.0%) in serum Cr. During the treadmill test, a mean activity increase of 1,621% (16-fold) (95% CI, 972 to 2,271%; SE, 265%) resulted in a mean increase of 3.0% (95% CI, −5.7 to 11.7%; SE, 3.6%) in serum Co and a mean increase of 0.8% (95% CI, −3.5 to 5.0%; SE, 1.7%) in serum Cr. This effectively constitutes no change in serum ion levels for these changes in activity because the differences are within the variability for the measurement accuracy of these tests.

Conclusions: In patients with normal renal function and a well-functioning metal-metal bearing, ion levels are not affected by patient activity. Periodic measurements of serum ion levels can be used to monitor the tribologic performance of prosthesis with a metal-metal bearing without adjusting for patient activity.

Aims: Measure serum chromium (SrCr) and cobalt (SrCo) and urine chromium (UCr) levels in patients with metal on metal surface arthroplasty of the hip. Methods: Ion levels were measured prospectively in 22 patients implanted with the Conserve Plusª (Wright Medical, TN) CoCr hip resurfacing. There were 15 males and 7 females with an average age of 49 years (range 28 Ð 62 yr). Serum and urine samples were collected using strict anti-contamination techniques pre-operatively and at 3, 6 and 12 months using graphite furnace atomic absorption spectrophotometry. Results: All postoperative metal levels were increased compared to their pre-operative levels. SrCr and SrCo values are at their highest at 3 months post operative and then begin to decrease. UrCr although elevated at 3 and 6 months postoperatively compared to the preop values, the levels continued to increase after the 6-month interval. These values are approximately 4-fold, 7-fold and 3-fold higher in SrCr, UrCr and SrCo, respectively, compared to the values seen in a group of patients with well functioning conventional metal (CoCr) on polyethylene total hips at 84 months postoperative measured by our group. Conclusions: The present levels are 2-fold lower in both SrCr and UrCr and 3-fold lower in SrCo than a group of patients with older generation surface arthroplasties reported previously by our group, suggesting improved manufacturing techniques and material properties have resulted in reduced component wear and generation of wear particles. 12 and 24 month data are currently being collected and analyzed.