Long-term follow-up and metal ion trend of patients with metal-on-metal total hip arthroplasty

Abstract

Purpose

Long-term studies are required to support the use of metal-on-metal (MoM) bearings in total hip arthroplasty (THA) given the concern about systemic metal ion release and reports of adverse local soft tissue reactions. The purpose of this study was to report the seven to 13-year clinical, radiographic, and metal ion results in patients following MoM THA.

Methods

We studied 163 prostheses after second-generation MoM THA between July 1997 and November 2003. Cobalt and chromium metal ions were collected using whole and analysed by inductively-coupled plasma-mass spectrometry.

Results

The mean follow-up was 8.87 years (range, 7–13 years). Four hips (2.5 %) were revised. The Kaplan-Meier survivorship was 91.3 % for revision for all causes, and 97.5 % when excluding the hips revised for a manufacturer’s defect. Median whole blood cobalt levels peaked at a value of 2.87 μg/L at four years (p < 0.0001 vs. pre-operative) and subsequently decreased to 2.0 μg/L after nine years (p = 0.002 vs. four years). Median chromium levels maximally increased up to 0.75 μg/L after five years (p < 0.0001 vs. pre-operative) and tended to decrease thereafter to values of 0.56 μg/L after seven years.

Conclusions

This seven to 13-year follow-up study indicates that the clinical and radiological results following MoM THA are satisfactory with low revision rates. Cobalt and chromium ion levels peaked at four and five years, respectively, and gradually decreased thereafter.

Introduction

Metal-on-metal (MoM) articulations in total hip arthroplasty (THA) have become an attractive option for young, active patients. This is partly due to decreased volumetric wear rates when compared to conventional THA with metal-on-polyethylene bearings [1, 2]. Our increased understanding of tribology from hip simulator studies has led to improved manufacturing allowing for production of components with higher carbon contents, smoother surfaces and increased sphericity [3]. Despite the technological advances, recent recalls of MoM implants due to biological intolerance amongst certain patients has tempered their use amongst arthroplasty surgeons [4]. The ideal patient for MoM bearings is still unknown, although accepted contraindications include women of childbearing age, renal insufficiency and known metal allergy.

It is evident from many studies that MoM bearings made from cobalt and chromium alloys have the potential to release metal debris in the surrounding periprosthetic tissues [5–8]. Short-term reports have already demonstrated elevated systemic metal ion levels in the blood and urine [9]. Disseminated concentrations of cobalt and chromium have raised concern regarding cellular toxicity, chromosomal damage and adverse local soft tissue reactions [10, 11].

Published Kaplan-Meier survivorships have been reported as excellent, between 91 and 98 % at long-term follow-up periods [12–19]. Long-term studies are required to support the increased use of MoM bearings in younger patients given their potential deleterious effects. Previous long-term studies have not uniformly reported on clinical outcome measures, digital radiographic evaluations, and whole blood metal ion trends. The purpose of this study was to describe the seven to 13-year clinical, radiographic, and metal ion results in patients following MoM THA.

Patients and methods

We prospectively followed 165 patients (196 hips) after second-generation MoM THA between July 1997 and November 2003. These patients are maintained in a database that collects clinical and radiological data. The data was analysed retrospectively. Prior to the onset of the study, institutional review board approval was obtained and each patient provided informed consent.

Twenty hips were lost to follow-up, and two died of causes unrelated to their surgery. Eleven patients failed between 0.4 and 2.8 years (mean, 1.1 years) due to a manufacturer’s recall, which allegedly occurred from an oil residue that remained on the acetabular cups preventing osteointegration leading to early failure [20]. This provided 163 prostheses to analyse for Kaplan-Meier survivorship using clinical and radiographic outcome measures. Eighty-two patients (87 hips) had complete metal ion data for serial outcome measurements. This subset had concominant clinical and radiographic parameters to substantiate our metal ion data.

The aetiology of hip arthritis in the cohort was: osteoarthritis (80 %), septic arthritis (6 %), acetabular protrusio (3 %), developmental dysplasia of the hip (3 %), osteonecrosis (3 %), inflammatory arthritis (3 %), and arthrodesis (2 %). Two arthroplasty surgeons performed all procedures through a direct lateral approach. The Natural-Hip (Zimmer, Warsaw, IN) was used in all patients. It is a titanium femoral implant engineered to mimic human cancellous bone. It is coated with hydroxyapetite and can be porous-coated or non-porous coated (Table 1).

Table 1.

Demographic data

Characteristic	Finding
Agea (range)	50.9 (17–65)
Gender (number of hips)	47 male / 40 female
Side of surgery	56 right / 31 left
Etiology
Idiopathic osteoarthritis	80 %
Septic arthritis	6 %
Protrusio	3 %
Developmental dysplasia	3 %
Osteonecrosis	3 %
Inflammatory arthritis	3 %
Arthrodesis	2 %
Femoral stem
Natural hip	100 %
Acetabular cup
Inter-op	78 %
Converge	13 %
Fitek	9 %
Femoral head
Metasul	100 %
Femoral head diameter
28 mm	100 %

^a This value is reported as the mean in years

The acetabular components were Interop (Sulzer, Winterthur, Switzerland), Converge (Zimmer, Warsaw, IN), and Fitek (Centerpulse Orthopaedics, Winterthur, Switzerland). Interop is a hemispherical modular cup with a titanium-roughened backside to promote bony ingrowth and derotational fins to aid in stability. The Converge porous-coated acetabular shell is made of commercially available titanium and offers six different types: hemispherical, cluster hole, rimflare, rimflare with screwholes, multihole and protrusio. The Fitek shell is hemispherical with a sandblasted titanium mesh with a porosity that mimics cancellous bone.

All implanted femoral heads were Metasul (Zimmer, Warsaw, IN), 28 mm in diameter. Metasul is a forged, wrought, high-carbon, cobalt-chrome alloy component. The surface roughness is less than 0.005 micrometers, and has a radial clearance of 75 micrometers (manufacturer’s data; Zimmer).

The preoperative renal function was normal in all patients, which was determined from preoperative screening serum creatinine (range 55–110 micromol/liter). None of the patients had previous metal components elsewhere in the body or chronic medical illness that could account for alternate sources of increased metal ions.

Patients were evaluated at six weeks, three months, and annually thereafter. We obtained clinical, radiographic and metal ion data for all patients at each follow-up visit. Laboratory personnel who collected the data and analysed the metal ion concentrations were blinded to the study.

Functional outcome was measured using the Harris hip score (HHS) and the University of California Los Angeles (UCLA) activity score. Radiographic analysis was performed using Einzel-Bild-Roentgen-Analyse by two of the authors blinded to the study [21].

Cobalt and chromium metal ion levels were analysed from whole blood. This was performed in order to minimise metal ion contamination during the blood collection process [6, 22]. Blood was drawn using Sarstedt Monovette tubes manufactured with 21-gauge needles for trace metal analysis (Starstedt, Montreal, QC) and stored at −80°C until ready for dilution and analysis. Metal ion concentration was analysed using inductively coupled plasma-mass spectrometry (SCIEX Elan 6100 DRC ICP-MS system, PerkinElmer Instruments, Norwalk, CT). A quality-control sample was analysed using the biological reference standard SeroNorm Trace Elements Whole Blood, Level 2 (Sero AS, Billingstad, Norway).

Symmetrically distributed data, the HHS and UCLA Activity Score, were calculated with analysis of variance (ANOVA) followed by Fisher's protected least significant difference (PLSD). Metal ion data was not symmetrically distributed. Thus, the Kruskal-Wallis test, a nonparametric equivalent of one-way analysis of variance was used. A p-value of <0.05 was considered statistically significant.

Results

The study demonstrated a Kaplan-Meier survivorship of 91.3 % for revision for all causes, and 97.5 % when excluding the hips revised for a manufacturer’s defect at a mean of 8.87 years (range, 7–13 years) (Fig. 1). Four hips (2.5 %) were revised: two for infection at 0.2 and seven years; one for a loose stem at 1.3 years; and one for a loose cup at nine years. One patient required wound debridement for a superficial infection and did not have any components revised.

Fig. 1.

Kaplan-Meier survivorship curve at a mean of 8.87 years (range, seven to 13 years) was 91.3 % for revision for all causes. When excluding the hips revised for a manufacturer’s defect, the survivorship rate was 97.5 % (graph not shown)

Of the subset of patients with full metal ion data, the mean age at surgery was 50.9 years (range, 17–65 years). Of the total hips, 40 were female and 47 were male. The mean HHS and UCLA scores at the last follow-up were 91 and 6.8, respectively, from pre-operative values of 38 and 4.2. During the follow-up, the mean HHS varied from 90 to 94 while the mean UCLA score varied from 6.7 to 6.9. The mean acetabular inclination and anteversion was 40 degrees (range, 24–57 degrees), and 19 degrees (range, 3–39 degrees), respectively (Table 2).

Table 2.

Clinical, radiographic and metal ion results

Clinical parametersa	Preoperative	Final follow-up
Harris hip score	38	91 (90–94)
UCLA activity score	4.2	6.8 (6.7–6.9)
Radiographic parametersa
Acetabular anteversion	N/A	19° (3–39°)
Acetabular inclination	N/A	40° (24–57°)
Metal ionb	Mid-term	Final follow-up
Cobalt (μg/L)	2.87	2.00
Chromium (μg/L)	0.75	0.56

N/A  not applicable

^aThe values are reported as the mean

^bThe values are reported as the median

Median cobalt levels peaked at a value of 2.87 μg/L at four years (p < 0.0001 vs. pre-operative) and subsequently decreased to 2.0 μg/L after nine years (p = 0.002 vs. four years) (Fig. 2). Median chromium levels maximally increased up to 0.75 μg/L after five years (p < 0.0001 vs. pre-operative) and tended to decrease thereafter to values of 0.56 μg/L after seven years (Fig. 3).

Fig. 2.

Annual whole blood cobalt (Co) ion concentrations in patients after metal-on-metal total hip arthroplasty. The results are presented as a box plot in which outliers are represented by the dots (•) while the box represents the middle 50 % (25–75 % percentiles) of the data. Cobalt peaked at four years

Fig. 3.

Annual whole blood chromium (Cr) ion concentrations in patients after metal-on-metal total hip arthroplasty. The results are presented as box plot in which outliers are represented by the dots (•) while the box represents the middle 50 % (25–75 % percentiles) of the data. Chromium peaked at five years

Discussion

Metal-on-metal bearing use in THA has undergone significant manufacturing improvements; however, ongoing concerns exist with regards to biological compatibility in certain patients. At present, it remains unclear as to which patients are predisposed to developing adverse local soft tissue reactions as most of the recent evidence has been focused on measuring systemic concentrations of metal ions and using them as surrogates to wear, abnormal component position, and early failure [23–29]. Therefore long-term studies are important to substantiate the use of this bearing in a given cohort of patients. Our study includes 163 prostheses with a mean follow-up of 8.87 years (range, seven to 13 years). We report a Kaplan-Meier survivorship of 91.3 %, which includes revision for all causes. When excluding revisions due to a manufacturer’s defect the survivorship is 97.5 %. Metal ion trend revealed the cobalt and chromium values peaked at four and five years, respectively, and decreased thereafter. We did not revise any patient for adverse local soft tissue reactions, including pseudotumours.

There is limited published data to compare as most studies do not include annual whole blood metal ion values and clinical outcome scores (Table 3). Grubl et al. reported a 98.6 % survivorship at a minimum ten-year follow-up in 73 patients [13]. The median cobalt and chromium ions values were 0.75 μg/L and 0.95 μg/L, respectively, but were analysed in only 22 patients at the latest follow-up. Maezawa et al. analysed serum chromium concentrations annually in a seven-year study of 44 patients [14]. They reported that chromium peaked at three years post-operatively; mean 1.76 μg/L (range, 0.1–9.9) and remained constant until final follow-up. Limitations to their study included lack of cobalt values and the use of serum rather than whole blood for metal ion analysis, which has been shown to be inaccurate [30]. Metal ion data is not asymmetrically distributed; thus, metal ion data should be reported as the median for a more accurate statistical representation of the data [9]. Despite our study demonstrating median whole blood cobalt and chromium values peaking at approximately four and five years, 2.87 μg/L and 0.75 μg/L, respectively, comparison to current literature is difficult because of varied sampling methods and statistical reporting.

Table 3.

Recent published studies reporting on long-term survivorships in patients with metal-on-metal total hip arthroplasty

Authors	Patients / hips	Stem	Cup	Articulation (bearing diameter)	Metal ion analysis	Follow-up (range)	Survivorship
Neumann et al. [15]	94 hips	Uni stem, Alloclassic stem (rectangular, cementless)	HI cup	Lubrimet (32 mm)	None	Mean 10.5 years (10–11.9)	93 %
Saito et al. [16]	90 hips	Not reported	Not reported	Not reported (28 mm)	None	Mean 12.3 years (10–14)	94.40 %
Grubl et al. [13]	73 patients	Alloclassic (rectangular cementless)	Not reported	Not reported (28 mm)	Serum Co and Cr in 22 patients at final follow-up	Minimum 10 years	98.60 %
Maezawa et al. [14]	44 patients	Natural hip	Wagner, APR cups	Metasul (28 mm)	Serum Cr	7 years	None reported
Eswaramoorthy et al. [12]	85 hips	Allopro CF30, cemented	Stuehmer-Weber cemented polyethylene liner; Allofit cementless	Metasul (28 mm)	None	Mean 10.8 years (10.2–12.2)	94 %
Dastane et al. [17]	69 hips	Not reported	Weber, cemented; APR cup cementless	Metasul (28 mm)	None	Mean 13 years	92.20 %
Milosev et al. [18]	611 hips	423-Plus, Ribbed hip, Thrust plate prostheses, Anca, Versys, Copf/Holz, SLR-Plus	Bicon-Plus	Sikomet SM21 (28 mm)	None	Mean 7.1 years	91 %
Randelli et al. [19]	138 hips	SL stem, custom-made stem, Wagner cone prosthesis	Alloclassic CSF Cup	Metasul (28 mm)	None	Mean 13 years	94 %
Current study	163 hips	Natural hip	Interop, Converge, Fitek	Metasul (28 mm)	Annual whole blood Co and Cr	Mean 8.87 years (7–13)	a97.5 %

^aKaplan-Meier survivorship excluding hips revised for a manufacturer’s defect. Revision for all causes was 91.3 %

The articulation used in our study was Metasul (Zimmer, Warsaw, IN). It is a forged, wrought, high-carbon cobalt-chrome alloy. The manufacturing is comparable to other so-called second-generation components and maintains excellent published survivorships. Eswaramoorthy et al. reported a Kaplan-Meier survivorship of 94 % on 85 hips at a mean of 10.8 years using the Metasul bearing [12]. They used hybrid fixation with a variety of acetabular cups. Sixty-four patients (62 %) were women and had a mean age of 61.6 years (range, 44–84 years). Of concern, two of their six failures had evidence of aseptic lymphocytic vasculitis associated lesions (ALVAL). They speculated that an additional three had a similar diagnosis, however, they lacked the histological evidence for the latter. It would have been interesting to note their acetabular cup position, renal function as well as potential other sources of metal ions in their cohort, as these can increase metal ion load [23, 24, 31]. Furthermore, it lacked metal ion data to help substantiate their findings.

Another study with the Metasul bearing that included 69 hips used a mixture of cemented and uncemented acetabular cups and reported a Kaplan-Meier survivorship of 92.2 % at a mean 13 years follow-up [17]. Limitations to the study included lack of serial metal-ion data as well as an unknown femoral stem.

The drive to generate bearings with minimal wear and increase understanding of tribology has led to multiple in vitro and clinical studies supporting the use of alternate bearings [3, 7, 29]. Notably, while maintaining all other manufacturing parameters, increasing femoral head diameters allows for improved lubrication and lamda ratios [3]. Our study, in line with recent literature on 28-mm metal-on-metal femoral heads has reported excellent survivorship rates between approximately 91 and 98 % [12–19]. With regards to implant stability, the selected studies have demonstrated six recurrent dislocations requiring revisions in greater than 1,000 hips [12–19].

There are several limitations to this study. Although the data was collected prospectively, data analysis was performed retrospectively and therefore has inherent bias. Two different surgeons performed the procedures; however, this should not introduce any bias since both are well-experienced fellowship-trained arthroplasty surgeons. The results of our study can also be generalised given that we are reporting on more than one surgeon’s experience.

Conclusions

The Kaplan-Meier survivorship is comparable to other studies with similar follow-ups. Our study demonstrates that MoM implants are durable. There were no pseudotumours or adverse local soft tissue reactions in our patient cohort. Metal ions peaked at approximately four years, and decreased thereafter. Clinical scores have demonstrated that patients maintain excellent outcomes. We aim to continue to follow this cohort and update the long-term data to ensure that the outcomes maintain an acceptable level.