A Prospective Metal Ion Study of Large-Head Metal-on-Metal Bearing: A Matched-Pair Analysis of Hip Resurfacing Versus Total Hip Replacement

The current study measured ion release among large-head metal-on-metal hip bearings. Twenty-six patients with a modular, Profemur® TL with A-Class® big femoral head total hip replacement were matched (gender, femoral size, BMI) with a group of 26 patients with the Conserve® Plus implant hip resurfacing. Compared with HR patients, THR patients had higher median serum cobalt ion levels at 6 months (3.26 vs 1.12 μg/L, P = .002) 1 year (4.51 vs 1.02, P = .002), and 2 years (3.77 vs 1.22, P <.001) following surgery. No differences in chromium ions were observed. Further research is required to determine the clinical significance of elevated serum cobalt ions.

Recent improvements in metal-on-metal bearing technology have further led to the development of a total hip arthroplasty (THA) system that makes use of large-diameter femoral heads in both stem type and resurfacing hip arthroplasties. Metal ion release remains a concern with using these bearings because of possible adverse tissue reactions and subsequent prosthetic loosening. Recent reports have shown that metal ion release can be affected by bearing design, acetabular component orientation, as well as modularity due to fretting corrosion. Although several clinical trials have shown relatively low metal ion levels after metal-on-metal hip resurfacing (HR) as well as 28-mm diameter Metasul (Zimmer, Warsaw, IN, USA) and 36-mm metal-on-metal total hip replacements, it is unclear if these studies can be applied to other implant designs as well as even larger-diameter metal-on-metal total hip replacements. Initial in vitro laboratory tribological wear testing have shown lower wear rates with the larger-diameter metal-on-metal bearings, with further reduction in bearing wear being proposed using differential hardness bearings, which are intended to limit abrasive, adhesive, and surface fatigue damages.

The primary purpose of the current study in this article is to measure metal ion release due to large-head metal-on-metal bearing by comparing HR with total hip replacement.

Methods

The study was approved by and performed in accordance with the guidelines of the institutional review board. Informed consent was obtained from all patients included in this analysis. Twenty-six patients who received a modular stem type total hip replacement (Profemur® TL, Wright Medical Technology, Memphis, TN, USA) with a A-Class® differential hardness big femoral head (BFH) were matched on the basis of gender, femoral head size, and body mass index (BMI) with a group of patients who received the Conserve® Plus (Wright Medical Technology, Memphis, TN, USA) metal-on-metal HR implant ( Fig. 1 ). The BFH and metal-on-metal HR groups did not differ on the matching variables: head size ( P = .083), BMI ( P = .313), or gender ( P = .337). However, patients in the BFH group were older than those in the Conserve® Plus group ( P <.001) ( Table 1 ).

Table 1

Means, medians, and standard deviations of demographic variables

Variables	A-Class® THR	HR	P Value
Gender (Men)	69.2%	80.8%	.337
	Mean, Median (SD)
BMI	28.46, 28.00 (5.21)	27.24, 26.60 (3.81)	.313
Head Size	48.08, 48.00 (2.74)	48.31, 48.00 (2.51)	.083
Age	60.15, 59.00 (9.18)	54.38, 54.00 (5.96)	<.001

Abbreviation: THR, total hip replacement.

The acetabular component was identical in both groups, which was a monoblock cobalt-chrome shell with a cobalt-chrome porous beaded surface. The acetabular components and the femoral components for the HR are made of a high-carbon cast alloy conforming to the ASTM F-75 standard. The castings undergo 2 heat treatment regimes before final machining and polishing. Hot isostatic pressing is done to eliminate tiny voids left in the castings during the cooling process. Solution annealing is done for the dissolution of large blocky carbides into the matrix. The A-Class® femoral head is made of a wrought alloy and has an open design with no modular sleeve and 3 different neck lengths. The stem used has a modular neck made of a titanium-vanadium alloy.

One 10-mm syringe of blood was collected from each individual at each assessment interval. Each syringe was labeled to indicate the sequence of collection. Before sample collection, all collection containers, syringes, and apparatus were triple acid-washed with Ultrex-grade chemicals and verified for absence of contamination by flushing with double deionized water, followed by analysis for residual trace metals. The samples of blood were maintained at room temperature until they had fully clotted (approximately 20 minutes) and then centrifuged at 1850 g for 30 minutes. Serum and clot fractions were then separated, and the fractions were frozen and stored at −80°C in polypropylene tubes until analysis. All manipulations of the specimens were performed in a class 100 environment with a SterilGARD Hood (The Baker Company, Sanford, ME, USA) and class 100 gloves. All specimens were shipped to the Trace Elements Laboratory at the University of Western Ontario for elemental analysis using a high-resolution inductively coupled mass spectrophotometer.

The 2 groups were compared in terms of serum ion levels of cobalt and chromium at baseline and 6, 12, and 24 months postoperatively. The functional outcome at 2 years postoperation was assessed using the Harris hip score, the Western Ontario and McMaster Universities Osteoarthritis index (WOMAC), and the University of California, Los Angeles (UCLA) activity score. Nonparametric Wilcoxon signed ranks tests were performed to determine differences in ion levels and functional outcome scores between the groups. Spearman correlation was used to measure the association between ion levels and BMI, age, femoral head diameter, and inclination, whereas Mann-Whitney U test was used to examine the relationship between gender and ion levels in both study groups ( Table 2 ).

Table 2

Analysis of demographic variables and head size versus cobalt and chromium ion levels

Variables	HR (n = 26)				Big-Head A-Class® (n = 26)
	Chromium		Cobalt		Chromium		Cobalt
	r ^a	P Value	r ^a	P Value	r ^a	P Value	r ^a	P Value
BMI
6 mo	−.21	.29	−.25	.23	.11	.64	.01	.99
12 mo	−.34	.11	−.34	.12	−.43	.08	−.13	.60
24 mo	−.49	.01 ^∗	−.45	.02 ^∗	−.12	.59	−.02	.94
Age
6 mo	.17	.42	.12	.55	.16	.47	.41	.06
12 mo	−.09	.68	−.07	.75	.01	.99	.36	.14
24 mo	.07	.72	.11	.58	−.04	.87	.30	.18
Femoral Head Diameter
6 mo	.03	.87	−.11	.59	.25	.25	.16	.48
12 mo	−.31	.15	−.30	.17	.06	.81	.40	.10
24 mo	−.25	.22	−.19	.35	−.03	.88	.16	.47
Inclination
6 mo	−.17	.50	−.30	.23	.27	.42	.38	.26
12 mo	−.12	.65	−.31	.24	.63	.10	.03	.95
24 mo	−.01	.99	−.24	.35	.13	.69	.37	.24

Mann-Whitney U ( P Value)
Variable	HR (n = 26)		Big-Head A-Class® (n = 26)
Variable	Chromium	Cobalt	Chromium	Cobalt
Gender
6 mo	.50	.20	.48	.42
12 mo	.37	.37	.56	.14
24 mo	.24	.31	.77	.42

Significant results in bold; * P <.05.

a r, Spearman correlation.

Results

At the 2-year follow-up, the Harris hip score ( P = .03) and the pain ( P = .049) and stiffness ( P = .002) subscale scores of the WOMAC were higher for the HR group than for the BFH group. However, no significant differences were found in terms of the WOMAC function, the UCLA activity score, or the cup inclination ( Table 3 ).

Table 3

Means, medians, and standard deviations of outcome scores at 2 years

Outcome Measure	A-Class® THR; Mean, Median (SD)	HR; Mean, Median (SD)	P Value
Harris Hip Score	86.04, 92.95 (15.35)	95.26, 99.80 (7.92)	.030
WOMAC
Pain Score	82.25, 87.50 (18.53)	95.00, 100.00 (9.24)	.049
Stiffness Score	66.88, 68.75 (21.94)	87.50, 87.50 (13.50)	.002
Function Score	81.54, 86.76 (18.28)	90.71, 97.06 (13.08)	.218
UCLA Score	6.83, 7.00 (2.23)	7.83, 8.00 (1.69)	.263
Cup Inclination	46.00, 45.00 (3.40)	46.94, 47.50 (4.28)	.108