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Short-Term Metal Ion Trends Following Removal of Recalled Modular Neck Femoral Stems
The Journal of Arthroplasty 30 (2015) 1191–1196
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The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Short-Term Metal Ion Trends Following Removal of Recalled Modular Neck Femoral Stems Brian T. Barlow, MD a, Joseph Assini, MD a, John Boles, BS a, Yuo-yu Lee, MS b, Geoffrey H. Westrich, MD a a Division of Adult Reconstruction and Joint Replacement Surgery, Department of Orthopaedic Surgery, Hospital for Special Surgery, Weill Cornell Medical College of Cornell University, New York, New York b Epidemiology and Biostatistics Core, Hospital for Special Surgery, New York, New York
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Article history: Received 7 November 2014 Accepted 21 February 2015 Keywords: metal ion taper corrosion dual taper modular neck Rejuvenate stem failure
a b s t r a c t Elevated serum metal ions have been well documented with the Rejuvenate modular neck femoral stem (Stryker, Mahwah, NJ); however, the rate at which ion levels decline following revision is less clear. This study included fifty-nine consecutive revisions of Rejuvenate stems for symptomatic ALTR. Blood tests prior to revision and postoperatively at 6 weeks, 3 months, 6 months, and 1 year measured serum cobalt and chromium concentrations, ESR, and CRP. At six weeks following revision of a unilateral Rejuvenate, cobalt and chromium levels dropped from preoperative levels by 67% and 42%, respectively. At three months, cobalt levels declined to 19% of preoperative values, but chromium levels remained stable. With this information, surgeons can set realistic expectations for serum metal ion levels following Rejuvenate stem revision. Published by Elsevier Inc.
The Rejuvenate Modular Hip Stem (Stryker Orthopedics, Mahwah, NJ) was voluntarily recalled after an elevated failure rate was noted during routine post-market analysis [1,2]. Symptomatic patients were found to have pain, elevated chromium (Cr) and cobalt (Co) ion levels, and adverse local tissue reactions (ALTRs). Certain series have shown that up to thirty percent of patients with Rejuvenate stems are symptomatic [3]. The Rejuvenate modular stem was designed to allow the surgeon intra-operative flexibility to optimize stability, range of motion and leg length by adjusting the neck length. The taper junction of the neck stem interface has fallen under scrutiny after several studies reported that mechanically assisted crevice corrosion (MACC) at the dual taper junction leads to elevated Co and Cr ion resulting in ALTR [2,4–7]. The diagnosis of a taper junction corrosion and metallosis usually relies on the patient narrative, physical exam, laboratory tests, and advanced imaging, specifically MRI [8]. The natural history of metal ion levels after removal of poorly performing total hip arthroplasty (THA) implants is not well characterized in the literature. The senior author (GW) has found that, although revision surgery improves pain symptoms, patients remain concerned about metal ion levels after revision. However, currently there are no data examining the trends of serum metal ions levels following revision of the Rejuvenate stem. Pivec et al recently published
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.02.033. Reprint requests: Brian T. Barlow, MD, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. http://dx.doi.org/10.1016/j.arth.2015.02.033 0883-5403/Published by Elsevier Inc.
an expert's opinion of how to deal with a recalled implant, including surveillance of serum metal ions. The authors suggest that erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and serum metal ions should be ordered at presentation and followed serially [8]. We prospectively examined 54 patients, including five patients with bilateral Rejuvenate stems, to determine the trends of metal ion levels and ESR/CRP both pre and post-revision. The goal of the study was to determine the change in metal ion levels and inflammatory markers after Rejuvenate stem revision. We hope that our data will assist surgeons as they counsel patients before and after revision of a femoral stem with a dual taper modular neck. Materials and Methods Study Design, Patient Cohort The senior author implanted 199 Rejuvenate modular stems between April 2010 and March 2012 and the revision rate is approximately 30%, consistent with previous clinical series [3]. From this group of patients, we have collected a consecutive series of 59 Rejuvenate Modular hip implants from 54 patients. Patients included in the study underwent revision surgery of a modular Rejuvenate stem at our institution and all patients had greater than six months of follow up. Cohort 1 included three unilateral patients who had the index surgery elsewhere. We excluded patients who were revised for reasons other than pain and suspicion of ALTR. The Rejuvenate stem is made of a titanium-alloy (Ti12-Mo-6Zr-2Fe or TMZF), and the modular neck is made of cobalt– chromium alloy (Co-Cr-Mo). Cohort 1 included patients with unilateral Rejuvenate stems and consisted of 27 female and 22 male patients.
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Table 1 Cohort 1 Patient Demographics. Characteristic (n = 49)
Number of Patients
Sex Male Female Age b50 50–59 60–69 70–79 N80 Side Left Right Has another artificial joint Peri-prosthetic fracture intra-operatively or within 6 weeks
Table 3 Histologic Grades of Intraoperative ALTR Tissue Samples According to the ALVAL (Aseptic Lymphocytic Vasculitis-Associated Lesion) Scoring Criteria.a Cohort 1 (Unilateral)
22 (44.9%) 27 (55.1%) 2 (4.1%) 7 (14.3%) 22 (44.9%) 17 (34.7%) 1 (2.0%) 22 (44.9%) 27 (55.1%) 9 (17.7%) 5 (10.2%)
Cohort 2 included 5 patients (4 females and 1 male) with bilateral Rejuvenate stems. The average patient age at revision was 65.2 (±8.1) years in cohort 1 and 60.0 (±5.8) years in cohort 2 [Table 1]. Prior to revision, patient demographics, revision indication, length of implantation, MRI/ pathology results, metal ion levels (Co/Cr), and ESR/CRP were recorded into our prospective database. All MRI imaging was completed using our institution's metal suppression protocol. We obtained a preoperative MRI using a standardized protocol (multiacquisition with variable resonances image combination (MAVRIC)) optimized to reduce metallic susceptibility artifact on all patients in the unilateral cohort (49 patients, 49 hips) [9]. The unilateral cohort's MRI findings were graded as 12 mild, 18 moderate, and 14 severe cases [Table 2]. Using the histologic description of pseudotumor-like tissues for metal on metal hips, nearly 90% of the unilateral hips had an ALVAL score N5 and therefore were considered positive ALVAL cases [10]. The ALVAL scoring system described by Campbell et al, the mean synovial lining score was 2.54 (SD 0.65, range 1–3), the mean inflammatory infiltrate score was 3.18 (SD 0.89, range 0–4), and the mean tissue organization score was 2.25 (SD 0.86, range 1–3). The mean total histological ALVAL score was 8.02 (SD 2.07, range 3–10). Additionally, 71.4% of unilateral patients had histologic evidence of corrosion products [Table 3]. Using the preoperative MRI and the previously mentioned criteria for ALVAL for the bilateral Rejuvenate patients, 2 were mild, 5 were moderate, and 3 were severe grades. 100% of the bilateral patients had a histological diagnosis of ALVAL based on the score N5. The mean synovial lining score was 2.6 (SD 0.52, range 2–3), mean inflammatory infiltrate score of 3.2 (SD 0.42, range 3–4), and mean tissue organization score of 2.4 (SD 0.84, range 1–3). The mean total histological ALVAL score was 8.2 (SD 1.3, range 6–10). Additionally, 70% of the bilateral patients had histological evidence of corrosion products [Table 3]. Nine patients (18.36%) of our unilateral patient cohort had other metallic orthopedic implants within their bodies [Table 1]. Obviously, 100% of our bilateral patients had multiple orthopedic implants. Blood collection was performed by trained phlebotomists in our clinic. All samples were sent to one of two laboratories (Quest Laboratories, Chantilly, VA or ARUP Laboratories, Salt Lake, UT) depending on each patient's insurance providers. Patients were encouraged to avoid diet and nutritional supplementation prior to blood draws. Whole
ALVAL Grading Category Synovial lining Inflammatory infiltrate Tissue organization Total Percentage of samples with ALVAL score ≥5 Percentage of samples with evidence of corrosion products
Average and SD
Range
Cohort 2 (Bilateral) Average and SD
Range
2.54 ± 0.65 1–3 3.18 ± 0.89 0–4 2.25 ± 0.86 1–3 8.02 ± 2.07 3–10 87.75% (43/49)
2.6 ± 0.52 2–3 3.2 ± 0.42 3–4 2.4 ± 0.84 1–3 8.2 ± 1.3 6–10 100% (10/10)
71.43% (35/49)
70% (7/10)
a Campbell P, Ebramzadeh E, Nelson S, Takamura K, De Smet K, Amstutz HC. Histological features of pseudotumor-like tissues from metal-on-metal hips. Clinical Orthopaedics and Related Research® 2010;468:2321-7.
Fig. 1. Left primary THA using the Rejuvenate modular neck femoral stem (Stryker, Mahwah NJ).
Table 2 Pre-Operative MRI ALTR Grades. MRI Grade Mild Moderate Severe MRI score not available Total
Cohort 1
Cohort 2
12 (24.5%) 18 (36.7%) 14 (28.6%) 5 (10.2%) 49
2 5 3 0 10
Fig. 2. Left revision THA using the Restoration Modular Stem (Stryker Orthopedics, Mahwah, NJ).
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blood samples were spun in centrifuge and serum layer was placed into the appropriate Trace Element-Free Transport Tube. The serum Co and Cr levels were measured using a quantitative inductively coupled plasma-mass spectrometer. Implant characteristics such as stem size, offset, neck/shaft angle and neck version were also recorded. All implants were transferred into our institutional review board-approved implant retrieval system. All implants were revised at our institution by the senior author (GW) [Figs. 1 and 2]. Revisions were done for pain or adverse local tissue reaction (ALTR), which was diagnosed by increased metal ion levels and advanced axial imaging using the MAVRIC MRI protocol [Fig. 3]. Mean length of implantation (LOI) was 21.9 months (8–37). All primary and revision surgeries were completed via a posterolateral approach. On visual inspection, all implants were noted to have corrosion at the modular neck-stem junction that was most severe on the medial aspect of both the male and female tapers [Fig. 4]. No implants demonstrated corrosion or damage at the head-neck taper or articulating surface. All Rejuvenate implants were removed proximally an identical fashion. First, the pathologic soft tissues affected by the ALTR were resected and sent for culture and pathology [Fig. 5]. Next, the bone ingrowth surfaces were separated from the implant using a Midas Rex burr and flexible osteotomes [Figs. 6-7]. Lastly, the implant was removed using a screwin explantation tool [Figs. 8 and 9]. The Restoration Modular Stem (Stryker Orthopedics, Mahwah, NJ) was used for the revision femoral component in all cases. At the primary THA procedure, 52 patients (57 hips) received the Restoration Anatomic Dual Mobility (ADM) Acetabular Shell (Stryker Orthopedics, Mahwah, NJ), and 2 patients (2 hips) received a Trident PSL Acetabular Shell (Stryker Orthopedics, Mahwah, NJ). Both PSL cups were well positioned and well-fixed, and thus left in situ since the PSL cup is composed of titanium alloy. Patients with the ADM cup underwent revision of both the Rejuvenate stem and ADM cup as the senior surgeon preferred to remove all sources of Co and Cr during the time of this series. None of the ADM cups had visible scratching or evidence of damage. Four patients received a Trident Hemispherical HA cup, 11 received a Trident PSL cup, and 42 received a Trident Tritanium cup. All were mated to a polyethylene liner. A constrained polyethylene liner was used in a second revision procedure in the one patient in whom severe instability was encountered following the initial revision THA. Primary femoral head distribution consisted of 55 cobalt chrome femoral heads and 4 ceramic femoral heads. The senior author used exclusively ceramic heads at the time of the revision procedure again in an effort to minimize the Co
Fig. 3. Severe adverse local tissue reaction (ALTR) as seen on an MRI using our institution's metal suppression protocol (MAVRIC).
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Fig. 4. Modular neck with black corrosion material just after removal.
and Cr exposure. All Rejuvenate stems in both cohorts were removed proximally without the need for extended trochanteric osteotomy (ETO) or cables. Overall, five patients (10.2%) within the unilateral Rejuvenate cohort (n = 49) sustained a periprosthetic fracture of the greater trochanter [Table 1]. No patients in the bilateral Rejuvenate cohort sustained a periprosthetic fracture. Two (4.0%) of these fractures occurred intraoperatively, both were non-displaced calcar fractures that were treated with open reduction internal fixation using a trochanteric plate and cables. Three patients (6.1%) fractured within 6 weeks post-revision surgery following a traumatic event. In these three patients, radiographs at 6 weeks demonstrated displaced fractures of the great trochanter and all were treated with open reduction internal fixation using a trochanteric plate and cables. Following all Rejuvenate revision surgeries, all patients were partial weight bearing for 6 weeks, so the fracture treatment did not alter their weight bearing status. However, trochanteric precautions (no active abduction or passive adduction) were added to the physical therapy protocol following all of the above fractures. Metal Ion Analysis All patients scheduled for Rejuvenate modular hip revision undergo pre-operative blood work for serum cobalt, chromium, and ESR/CRP. If serum metal ion levels are elevated, further imaging was obtained, including MARS/MAVRIC MRI or ultrasound is obtained. Asymptomatic patients with normal metal ion levels are followed annually. Patients that undergo revision surgery have serum labs taken preoperatively and at 6 weeks, 3 months, 6 months, and 1 year postoperatively. We isolated all patients with a minimum of 6 month metal ion follow-up and included them in the study. As mentioned previously,
Fig. 5. Removal of ALTR tissue using electrocautery from over the trochanteric region.
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Fig. 6. Circumferential use of a Midas-Rex burr to remove the bony ongrowth from the proximal stem.
patients with bilateral modular hips (cohort 2) were analyzed separately. Blood samples were collected and sent to one of two labs (Quest and ARUP) for analysis. At the Quest lab, the normal reference range for cobalt was 0.1–0.4 μg/L. The limit of detection for cobalt was 0.1 μg/L. The normal reference range for chromium was b1.4 μg/L and the limit of detection was 0.1 μg/L. At the ARUP lab, the normal reference range for cobalt was 0.5–3.9 μg/L. The limit of detection for cobalt was 0.1 μg/L. The normal reference range for chromium was b 5.0 μg/L and the limit of detection was 1.0 μg/L. The normal reference range for ESR levels in both labs was 0–20 mm/h. The normal reference range for CRP was b 0.7 mg/dL for both labs. Statistical Analysis Descriptive statistics were used to summarize patient demographics and MRI/ pathology results. Mean and standard deviation were calculated for continuous variables. Frequency and percentage were calculated for categorical variables. Metal ion levels (Co/Cr) at 6 weeks, 3 months, 6 months, and 1 year postoperatively were summarized in line graphs and compared between cohort 1 and cohort 2 using Wilcoxon ranksum test, a non-parametric alternative to two-sample t-test. Comparisons were also made within each cohort between subsequent time points using Wilcoxan signed rank test. All tests were two-sided, with a significance level of b 0.05. All analyses were conducted using SAS for Windows 9.3 (SAS Institute Inc, Cary, NC).
Fig. 8. Removal of the femoral stem using the screw-in explantation device.
mean pre-operative Cr levels were found to be 1.72 (±1.04) μg/L. Six weeks following revision and removal of the modular component, both the Co and Cr levels had significantly dropped in all patients. The mean serum cobalt ion levels dropped nearly 3-fold to 2.68 (± 2.67) μg/L (P b 0.0001) and the serum chromium ion levels dropped nearly in half to 1.0 (±.041) μg/L (P b 0.0001) [Fig. 10]. At three months, cobalt levels continued to decrease significantly to a mean of 1.58 (±1.57) μg/L (P b 0.0001). The cobalt level continued gradually drop to 1.27 (±1.48) μg/L at 1 year [Fig. 10]. Chromium levels demonstrated a similar pattern in serum ion level decrease. After the initial 6 week drop, serial follow-up to 1 year demonstrated no significant changes in the mean concentration [Fig. 10]. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) both demonstrated downward trends similar to those of the metal ions after revision surgery. The pre-operative ESR was 21.57 mm/h. The ESR dropped to 19.86 mm/h at six weeks and continued to decrease fairly uniformly to 12.4 mm/h at 12 months. The mean preoperative CRP concentration was 2.4 mg/dL. Postoperatively, the mean CRP had dropped to 1.9 mg/dL at 6 weeks and returned to a normal concentration of 0.9 mg/dL at 1 year [Fig. 11].
Cohort 2
Cohort 1 included all patients with unilateral modular Rejuvenate hip implants. Mean pre-operative Co levels were 8.19 (±5.54) μg/L and the
Cohort 2 included all patients with bilateral modular Rejuvenate hip implants. Cohort 2 preoperative cobalt levels were significantly higher than the preoperative values in cohort 1 (P = 0.013). Similarly for chromium, the preoperative levels in cohort 2 were significantly higher than the preoperative values in cohort 1 (P = 0.0157). Cohort 2 also demonstrated significant decreases in serum cobalt levels from 13.33 (±6.45) μg/L preoperatively to 3.73 (±2.19) μg/L at six weeks postoperatively (P = 0.0009). Correspondingly, serum chromium levels also
Fig. 7. Use of osteotomes to further remove bony ongrowth from the proximal stem.
Fig. 9. Femoral stem immediately following removal, note the minimal bone loss with careful removal.
Results Cohort 1
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Fig. 10. Graph comparing serum cobalt and chromium levels in both unilateral and bilateral patients spanning from pre-revision levels to one year follow up levels.
dropped in half from 2.56 (±1.37) μg/L preoperatively to 1.34 (±0.41) μg/L (p = 0.01) at six weeks [Fig. 10]. The cobalt and chromium levels remained relatively stable at 3 months. It should be noted that three patients had revision of the 2nd stem between 6 and 12 weeks, whereas the other two patients had the 2nd revision between 12 weeks and 6 months. This may be a significant confounding factor in the determination of average changes in serum cobalt and chromium levels between 6 weeks and 3 months. At six months, the mean serum cobalt level fell to 1.62 (± 0.74) μg/L and decreased to 1.12 (±0.32) μg/L at one year. The chromium levels in cohort 2 at 6 months had fallen to 0.92 (± .06) μg/L and remained stable at 0.98 (±0.11) μg/L at one year [Fig. 10]. The mean ESR level in cohort 2 began at 22.0 mm/h preoperatively and fell to 12.75 mm/h at 6 weeks. The ESR then slowly decreased over to 12.6 mm/h at 12 months. The CRP dropped from a preoperative value of 1.45 mg/dL to 0.98 mg/dL at six weeks. The CRP then continued to slowly decrease to 0.76 mg/dL at 12 months. Again, the fact that three patients had revision of the 2nd stem between 6 and 12 weeks, whereas two patients had the 2nd revision between 12 weeks and 6 months
Fig. 11. Graph of the serum ESR and CRP trend in patients with unilateral Rejuvenate modular femoral stems spanning from pre-revision levels to one year follow up levels.
confounds the pattern of ESR and CRP decline between 6 weeks and 3 months after Rejuvenate stem removal.
Discussion In order for a cementless proximal fit femoral implant to achieve stable fixation, it must reasonably conform to the version of the native femur. Proximal femoral deformities such as increased anteversion, femoral retroversion, coxa vara, cox breva, or coxa valga can arise idiopathically or be associated with other syndromes (i.e. hip dysplasia, SCFE, Legg-Calve-Perthes, Down's syndrome, etc.). Proximal femoral deformity may occasionally compromise the surgeon's ability to place a proximal fit cementless stem with appropriate anteversion. Modular neck or dual taper implants, such as the Rejuvenate stem, were designed to increase the surgeon's ability to match the patient's femoral offset, length, and version. Unfortunately, data from the 2012 Australian Registry Annual Report and UK National Joint Registry noted the Rejuvenate stem to have increased risk for early revision. Subsequently, a type II voluntary recall was initiated by the manufacturer, Stryker Orthopedics in July 2012 [2]. A similar modular dual taper stem, the ABG II was also recalled at the same time for similar increased risk of revision [11]. Since the initial reports, numerous articles have reaffirmed the poor performance of these implants and have also studied the elevated metal ion levels, abnormal soft tissue masses on MRI, and early onset pain experienced by patients [1,3,8]. While some authors have observed a rapid decline in metal ion levels in large series of Rejuvenate stems, the rate of decline has not been well characterized [3]. Increased metal ions are not only attributed to modular neck designs. In fact elevated metal ions have been seen in patients with the conventional THA designs with modular heads and polyethylene liners. Previous knowledge from the metal on metal hip arthroplasty experience suggests that measuring serum metal ions is a way of tracking a patient's progress after revision. Ball et al performed a similar study of serum metal ion decline after revision of metal on metal hip arthroplasty in 25 patients. The authors reported a drop in serum cobalt and chromium levels of 80% by 6 weeks and of 90% by 12 weeks [12]. Their study did not include any bilateral patients, though they included a single patient that had undergone a prior initial stage of a bilateral metal on metal THA revision. Additionally, there is concern that elevated metal ion levels associated
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with an adverse soft tissue reaction will remain elevated even after the offending implant is removed. Patients want to know how much time to expect to pass before their metal ion levels return to normal, if ever. The goal of this study was to answer these questions. To address this gap in the literature, our group decided to investigate the trend of metal ions and inflammatory markers following the removal of the Rejuvenate stem in a single surgeon cohort. It should be noted that taper corrosion of modular neck implants like the Rejuvenate and ABG II modular stem may be due to a different mechanism than the metal on metal hip metallosis problem. Authors have suggested that the chromium released from MACC precipitates as chromium orthophosphate whereas cobalt is more soluble and distributed into the serum [7]. This is seen clinically as there is often marked mismatch between serum cobalt and chromium ion levels with mechanically assisted crevice corrosion in contrast to metal on metal THA where cobalt and chromium ion levels are often similar [13]. MACC also seems to produce significant pain and symptomatic ALTR at lower metal ion levels than seen metal on metal total hip arthroplasty [13]. Our study demonstrated a statistically significant and clinically relevant drop in serum cobalt and chromium ion levels within 6 weeks after surgery in both the unilateral and bilateral Rejuvenate groups. Cohort 1, the unilateral Rejuvenate stem group, also demonstrated a significant decrease in both cobalt and chromium levels between 6 weeks and 3 months. Cohort 2, however, demonstrated a relative plateau in cobalt and chromium levels between 6 weeks and 3 months but this plateau effect may simply reflect the small sample size and the fact that three of the bilateral patients completed both Rejuvenate implant revisions within 3 months of each other while the other two patients had a residual Rejuvenate implant at 3 months. This certainly may confound the pattern of serum metal ion decrease in our bilateral cohort group. This plateau effect may also be caused by a nonlinear relationship between the renal clearance and serum metal ion levels. Limitations of this series include lack of a control group, limited follow up, single implant, wide variation in metal ion levels, and small numbers. The current series does not have a control group against which to compare metal levels. Metal levels from a well functioning total hip arthroplasty would have been an appropriate comparison group, but were not available as this is not our usual practice or approved by our institutional review board. With respect to follow up, only 52% of patients were eligible for one year serum metal ion testing at the time of the study. However, our study had 98% follow up at six months whereupon the majority of patients' serum cobalt and chromium ion levels had returned to near-normal levels. The added expense of continued testing was not warranted following the return of cobalt and chromium ion levels to near normal after Rejuvenate stem revision. While we acknowledge that a single senior surgeon is a limitation of the study, the failure of the Rejuvenate stem is likely a flaw inherent to the modular CoCr neck design in a susceptible host, rather than a failure of surgical technique. Ultimately, this study examines only the Rejuvenate stem and cannot make any conclusions about other modular neck dual taper stems. This study cannot determine if the high failure rate of the Rejuvenate applies to the modular neck stem design as a group. Unique to Stryker, both the Rejuvenate and ABG II stems are made from a proprietary TMZF alloy. It has been suggested that the alloyed metals may be more susceptible to corrosion when mixed with a cobalt–chrome implant compared to single element metals [1,14]. Other modular stems with titanium stems and cobalt–chrome modular necks may be at less risk for MACC on account of their metallurgy. There is concern using a modular implant (Restoration Modular, Stryker) for revision of a failed modular implant. Unlike the Co-Cr and
TMZF stem-neck taper in the Rejuvenate system, the tapers in the Restoration Modular system are titanium–titanium interfaces. The Rejuvenate taper was between 2 different metals (TMZF and CoCr) which has been shown to be more susceptible to corrosion than identical metals [1]. Additionally, the taper between the stem-body of the Restoration Modular is axially loaded rather than the shear loading of the neckstem taper of the Rejuvenate taper, a mechanically unfavorable loading mechanism. Finally, the Restoration Modular system was released in the United States in 2003 and has more than 10 years of clinical follow up with no reports of failure secondary to taper corrosion. We have published on the satisfactory results of modular femoral stems in revision total hip arthroplasty in the past [15]. In conclusion, this study demonstrated that cobalt and chromium ion levels significantly decrease after removal of a unilateral Rejuvenate femoral stem with a modular neck. The largest drops serum ion levels were noted from pre-revision to six weeks postoperatively. A patient with a unilateral Rejuvenate stem can expect the serum cobalt and chromium levels to drop by 67% and 42%, respectively, from preoperative levels at six weeks following revision. Furthermore, at three months, serum cobalt levels can be expected to decline by 81% compared to preoperative values and then gradually decrease thereafter. Chromium levels remained stable after 6 weeks at a level of about 1.0 μg/L, which is within the normal reference range in the unilateral cohort. Inflammatory markers like ESR and CRP gradually decreased and remained elevated even at 12 months following revision. Statistical analysis of the bilateral Rejuvenate stem group demonstrated significant declines in serum cobalt and chromium ions at six weeks, but a plateau between six weeks and three months although this effect may be due to small sample size and inconsistent timing between the first and second revisions. Surgeons can use this study when counseling patients that they can expect serum cobalt and chromium ion levels to fall to nearnormal levels within about 3 months following removal of a unilateral Rejuvenate femoral stem. References 1. Krishnan H, Krishnan S, Blunn G, et al. Modular neck femoral stems. Bone Joint J 2013; 95:1011. 2. Stryker initiates voluntary product recall of modular-neck stems. Action Specific to Rejuvunate and ABG II Modular-Neck Stems: Food and Drug Administration; 2012. 3. Meftah M, Haleem AM, Burn MB, et al. Early corrosion-related failure of the rejuvenate modular total hip replacement. J Bone Joint Surg 2014;96:481. 4. Jacobs JJ, Gilbert JL, Urban RM. Current concepts review—corrosion of metal orthopaedic implants*. J Bone Joint Surg 1998;80:268. 5. Kretzer JP, Jakubowitz E, Krachler M, et al. Metal release and corrosion effects of modular neck total hip arthroplasty. Int Orthop 2009;33:1531. 6. Kop AM, Swarts E. Corrosion of a hip stem with a modular neck taper junction: a retrieval study of 16 cases. J Arthroplast 2009;24:1019. 7. Cooper HJ, Della Valle CJ, Berger RA, et al. Corrosion at the head-neck taper as a cause for adverse local tissue reactions after total hip arthroplasty. J Bone Joint Surg 2012;94:1655. 8. Pivec R, Meneghini RM, Hozack WJ, et al. Modular taper junction corrosion and failure: how to approach a recalled total hip arthroplasty implant. J Arthroplast 2014;29:1. 9. Hayter CL, Koff MF, Potter HG. Magnetic resonance imaging of the postoperative hip. J Magn Reson Imaging 2012;35:1013. 10. Campbell P, Ebramzadeh E, Nelson S, et al. Histological features of pseudotumor-like tissues from metal-on-metal hips. Clin Orthop Relat Res 2010;468:2321. 11. Restrepo C, Ross D, Restrepo S, et al. Adverse clinical outcomes in a primary modular neck/stem system. J Arthroplast 2014;29:173. 12. Ball ST, Severns D, Linn M, et al. What happens to serum metal ion levels after a metal-on-metal bearing is removed? J Arthroplast 2013;28:53. 13. Gill I, Webb J, Sloan K, et al. Corrosion at the neck-stem junction as a cause of metal ion release and pseudotumour formation. J Bone Joint Surg Br 2012;94:895. 14. Molloy DO, Munir S, Jack CM, et al. Fretting and corrosion in modular-neck total hip arthroplasty femoral stems. J Bone Joint Surg 2014;96:488. 15. Stimac JD, Boles J, Parkes N, et al. Revision total hip arthroplasty with modular femoral stems. J Arthroplast 2014;29:2167.
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Short-Term Metal Ion Trends Following Removal of Recalled Modular Neck Femoral Stems Brian T. Barlow, MD a, Joseph Assini, MD a, John Boles, BS a, Yuo-yu Lee, MS b, Geoffrey H. Westrich, MD a a Division of Adult Reconstruction and Joint Replacement Surgery, Department of Orthopaedic Surgery, Hospital for Special Surgery, Weill Cornell Medical College of Cornell University, New York, New York b Epidemiology and Biostatistics Core, Hospital for Special Surgery, New York, New York
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Article history: Received 7 November 2014 Accepted 21 February 2015 Keywords: metal ion taper corrosion dual taper modular neck Rejuvenate stem failure
a b s t r a c t Elevated serum metal ions have been well documented with the Rejuvenate modular neck femoral stem (Stryker, Mahwah, NJ); however, the rate at which ion levels decline following revision is less clear. This study included fifty-nine consecutive revisions of Rejuvenate stems for symptomatic ALTR. Blood tests prior to revision and postoperatively at 6 weeks, 3 months, 6 months, and 1 year measured serum cobalt and chromium concentrations, ESR, and CRP. At six weeks following revision of a unilateral Rejuvenate, cobalt and chromium levels dropped from preoperative levels by 67% and 42%, respectively. At three months, cobalt levels declined to 19% of preoperative values, but chromium levels remained stable. With this information, surgeons can set realistic expectations for serum metal ion levels following Rejuvenate stem revision. Published by Elsevier Inc.
The Rejuvenate Modular Hip Stem (Stryker Orthopedics, Mahwah, NJ) was voluntarily recalled after an elevated failure rate was noted during routine post-market analysis [1,2]. Symptomatic patients were found to have pain, elevated chromium (Cr) and cobalt (Co) ion levels, and adverse local tissue reactions (ALTRs). Certain series have shown that up to thirty percent of patients with Rejuvenate stems are symptomatic [3]. The Rejuvenate modular stem was designed to allow the surgeon intra-operative flexibility to optimize stability, range of motion and leg length by adjusting the neck length. The taper junction of the neck stem interface has fallen under scrutiny after several studies reported that mechanically assisted crevice corrosion (MACC) at the dual taper junction leads to elevated Co and Cr ion resulting in ALTR [2,4–7]. The diagnosis of a taper junction corrosion and metallosis usually relies on the patient narrative, physical exam, laboratory tests, and advanced imaging, specifically MRI [8]. The natural history of metal ion levels after removal of poorly performing total hip arthroplasty (THA) implants is not well characterized in the literature. The senior author (GW) has found that, although revision surgery improves pain symptoms, patients remain concerned about metal ion levels after revision. However, currently there are no data examining the trends of serum metal ions levels following revision of the Rejuvenate stem. Pivec et al recently published
One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.02.033. Reprint requests: Brian T. Barlow, MD, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021. http://dx.doi.org/10.1016/j.arth.2015.02.033 0883-5403/Published by Elsevier Inc.
an expert's opinion of how to deal with a recalled implant, including surveillance of serum metal ions. The authors suggest that erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and serum metal ions should be ordered at presentation and followed serially [8]. We prospectively examined 54 patients, including five patients with bilateral Rejuvenate stems, to determine the trends of metal ion levels and ESR/CRP both pre and post-revision. The goal of the study was to determine the change in metal ion levels and inflammatory markers after Rejuvenate stem revision. We hope that our data will assist surgeons as they counsel patients before and after revision of a femoral stem with a dual taper modular neck. Materials and Methods Study Design, Patient Cohort The senior author implanted 199 Rejuvenate modular stems between April 2010 and March 2012 and the revision rate is approximately 30%, consistent with previous clinical series [3]. From this group of patients, we have collected a consecutive series of 59 Rejuvenate Modular hip implants from 54 patients. Patients included in the study underwent revision surgery of a modular Rejuvenate stem at our institution and all patients had greater than six months of follow up. Cohort 1 included three unilateral patients who had the index surgery elsewhere. We excluded patients who were revised for reasons other than pain and suspicion of ALTR. The Rejuvenate stem is made of a titanium-alloy (Ti12-Mo-6Zr-2Fe or TMZF), and the modular neck is made of cobalt– chromium alloy (Co-Cr-Mo). Cohort 1 included patients with unilateral Rejuvenate stems and consisted of 27 female and 22 male patients.
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Table 1 Cohort 1 Patient Demographics. Characteristic (n = 49)
Number of Patients
Sex Male Female Age b50 50–59 60–69 70–79 N80 Side Left Right Has another artificial joint Peri-prosthetic fracture intra-operatively or within 6 weeks
Table 3 Histologic Grades of Intraoperative ALTR Tissue Samples According to the ALVAL (Aseptic Lymphocytic Vasculitis-Associated Lesion) Scoring Criteria.a Cohort 1 (Unilateral)
22 (44.9%) 27 (55.1%) 2 (4.1%) 7 (14.3%) 22 (44.9%) 17 (34.7%) 1 (2.0%) 22 (44.9%) 27 (55.1%) 9 (17.7%) 5 (10.2%)
Cohort 2 included 5 patients (4 females and 1 male) with bilateral Rejuvenate stems. The average patient age at revision was 65.2 (±8.1) years in cohort 1 and 60.0 (±5.8) years in cohort 2 [Table 1]. Prior to revision, patient demographics, revision indication, length of implantation, MRI/ pathology results, metal ion levels (Co/Cr), and ESR/CRP were recorded into our prospective database. All MRI imaging was completed using our institution's metal suppression protocol. We obtained a preoperative MRI using a standardized protocol (multiacquisition with variable resonances image combination (MAVRIC)) optimized to reduce metallic susceptibility artifact on all patients in the unilateral cohort (49 patients, 49 hips) [9]. The unilateral cohort's MRI findings were graded as 12 mild, 18 moderate, and 14 severe cases [Table 2]. Using the histologic description of pseudotumor-like tissues for metal on metal hips, nearly 90% of the unilateral hips had an ALVAL score N5 and therefore were considered positive ALVAL cases [10]. The ALVAL scoring system described by Campbell et al, the mean synovial lining score was 2.54 (SD 0.65, range 1–3), the mean inflammatory infiltrate score was 3.18 (SD 0.89, range 0–4), and the mean tissue organization score was 2.25 (SD 0.86, range 1–3). The mean total histological ALVAL score was 8.02 (SD 2.07, range 3–10). Additionally, 71.4% of unilateral patients had histologic evidence of corrosion products [Table 3]. Using the preoperative MRI and the previously mentioned criteria for ALVAL for the bilateral Rejuvenate patients, 2 were mild, 5 were moderate, and 3 were severe grades. 100% of the bilateral patients had a histological diagnosis of ALVAL based on the score N5. The mean synovial lining score was 2.6 (SD 0.52, range 2–3), mean inflammatory infiltrate score of 3.2 (SD 0.42, range 3–4), and mean tissue organization score of 2.4 (SD 0.84, range 1–3). The mean total histological ALVAL score was 8.2 (SD 1.3, range 6–10). Additionally, 70% of the bilateral patients had histological evidence of corrosion products [Table 3]. Nine patients (18.36%) of our unilateral patient cohort had other metallic orthopedic implants within their bodies [Table 1]. Obviously, 100% of our bilateral patients had multiple orthopedic implants. Blood collection was performed by trained phlebotomists in our clinic. All samples were sent to one of two laboratories (Quest Laboratories, Chantilly, VA or ARUP Laboratories, Salt Lake, UT) depending on each patient's insurance providers. Patients were encouraged to avoid diet and nutritional supplementation prior to blood draws. Whole
ALVAL Grading Category Synovial lining Inflammatory infiltrate Tissue organization Total Percentage of samples with ALVAL score ≥5 Percentage of samples with evidence of corrosion products
Average and SD
Range
Cohort 2 (Bilateral) Average and SD
Range
2.54 ± 0.65 1–3 3.18 ± 0.89 0–4 2.25 ± 0.86 1–3 8.02 ± 2.07 3–10 87.75% (43/49)
2.6 ± 0.52 2–3 3.2 ± 0.42 3–4 2.4 ± 0.84 1–3 8.2 ± 1.3 6–10 100% (10/10)
71.43% (35/49)
70% (7/10)
a Campbell P, Ebramzadeh E, Nelson S, Takamura K, De Smet K, Amstutz HC. Histological features of pseudotumor-like tissues from metal-on-metal hips. Clinical Orthopaedics and Related Research® 2010;468:2321-7.
Fig. 1. Left primary THA using the Rejuvenate modular neck femoral stem (Stryker, Mahwah NJ).
Table 2 Pre-Operative MRI ALTR Grades. MRI Grade Mild Moderate Severe MRI score not available Total
Cohort 1
Cohort 2
12 (24.5%) 18 (36.7%) 14 (28.6%) 5 (10.2%) 49
2 5 3 0 10
Fig. 2. Left revision THA using the Restoration Modular Stem (Stryker Orthopedics, Mahwah, NJ).
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blood samples were spun in centrifuge and serum layer was placed into the appropriate Trace Element-Free Transport Tube. The serum Co and Cr levels were measured using a quantitative inductively coupled plasma-mass spectrometer. Implant characteristics such as stem size, offset, neck/shaft angle and neck version were also recorded. All implants were transferred into our institutional review board-approved implant retrieval system. All implants were revised at our institution by the senior author (GW) [Figs. 1 and 2]. Revisions were done for pain or adverse local tissue reaction (ALTR), which was diagnosed by increased metal ion levels and advanced axial imaging using the MAVRIC MRI protocol [Fig. 3]. Mean length of implantation (LOI) was 21.9 months (8–37). All primary and revision surgeries were completed via a posterolateral approach. On visual inspection, all implants were noted to have corrosion at the modular neck-stem junction that was most severe on the medial aspect of both the male and female tapers [Fig. 4]. No implants demonstrated corrosion or damage at the head-neck taper or articulating surface. All Rejuvenate implants were removed proximally an identical fashion. First, the pathologic soft tissues affected by the ALTR were resected and sent for culture and pathology [Fig. 5]. Next, the bone ingrowth surfaces were separated from the implant using a Midas Rex burr and flexible osteotomes [Figs. 6-7]. Lastly, the implant was removed using a screwin explantation tool [Figs. 8 and 9]. The Restoration Modular Stem (Stryker Orthopedics, Mahwah, NJ) was used for the revision femoral component in all cases. At the primary THA procedure, 52 patients (57 hips) received the Restoration Anatomic Dual Mobility (ADM) Acetabular Shell (Stryker Orthopedics, Mahwah, NJ), and 2 patients (2 hips) received a Trident PSL Acetabular Shell (Stryker Orthopedics, Mahwah, NJ). Both PSL cups were well positioned and well-fixed, and thus left in situ since the PSL cup is composed of titanium alloy. Patients with the ADM cup underwent revision of both the Rejuvenate stem and ADM cup as the senior surgeon preferred to remove all sources of Co and Cr during the time of this series. None of the ADM cups had visible scratching or evidence of damage. Four patients received a Trident Hemispherical HA cup, 11 received a Trident PSL cup, and 42 received a Trident Tritanium cup. All were mated to a polyethylene liner. A constrained polyethylene liner was used in a second revision procedure in the one patient in whom severe instability was encountered following the initial revision THA. Primary femoral head distribution consisted of 55 cobalt chrome femoral heads and 4 ceramic femoral heads. The senior author used exclusively ceramic heads at the time of the revision procedure again in an effort to minimize the Co
Fig. 3. Severe adverse local tissue reaction (ALTR) as seen on an MRI using our institution's metal suppression protocol (MAVRIC).
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Fig. 4. Modular neck with black corrosion material just after removal.
and Cr exposure. All Rejuvenate stems in both cohorts were removed proximally without the need for extended trochanteric osteotomy (ETO) or cables. Overall, five patients (10.2%) within the unilateral Rejuvenate cohort (n = 49) sustained a periprosthetic fracture of the greater trochanter [Table 1]. No patients in the bilateral Rejuvenate cohort sustained a periprosthetic fracture. Two (4.0%) of these fractures occurred intraoperatively, both were non-displaced calcar fractures that were treated with open reduction internal fixation using a trochanteric plate and cables. Three patients (6.1%) fractured within 6 weeks post-revision surgery following a traumatic event. In these three patients, radiographs at 6 weeks demonstrated displaced fractures of the great trochanter and all were treated with open reduction internal fixation using a trochanteric plate and cables. Following all Rejuvenate revision surgeries, all patients were partial weight bearing for 6 weeks, so the fracture treatment did not alter their weight bearing status. However, trochanteric precautions (no active abduction or passive adduction) were added to the physical therapy protocol following all of the above fractures. Metal Ion Analysis All patients scheduled for Rejuvenate modular hip revision undergo pre-operative blood work for serum cobalt, chromium, and ESR/CRP. If serum metal ion levels are elevated, further imaging was obtained, including MARS/MAVRIC MRI or ultrasound is obtained. Asymptomatic patients with normal metal ion levels are followed annually. Patients that undergo revision surgery have serum labs taken preoperatively and at 6 weeks, 3 months, 6 months, and 1 year postoperatively. We isolated all patients with a minimum of 6 month metal ion follow-up and included them in the study. As mentioned previously,
Fig. 5. Removal of ALTR tissue using electrocautery from over the trochanteric region.
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Fig. 6. Circumferential use of a Midas-Rex burr to remove the bony ongrowth from the proximal stem.
patients with bilateral modular hips (cohort 2) were analyzed separately. Blood samples were collected and sent to one of two labs (Quest and ARUP) for analysis. At the Quest lab, the normal reference range for cobalt was 0.1–0.4 μg/L. The limit of detection for cobalt was 0.1 μg/L. The normal reference range for chromium was b1.4 μg/L and the limit of detection was 0.1 μg/L. At the ARUP lab, the normal reference range for cobalt was 0.5–3.9 μg/L. The limit of detection for cobalt was 0.1 μg/L. The normal reference range for chromium was b 5.0 μg/L and the limit of detection was 1.0 μg/L. The normal reference range for ESR levels in both labs was 0–20 mm/h. The normal reference range for CRP was b 0.7 mg/dL for both labs. Statistical Analysis Descriptive statistics were used to summarize patient demographics and MRI/ pathology results. Mean and standard deviation were calculated for continuous variables. Frequency and percentage were calculated for categorical variables. Metal ion levels (Co/Cr) at 6 weeks, 3 months, 6 months, and 1 year postoperatively were summarized in line graphs and compared between cohort 1 and cohort 2 using Wilcoxon ranksum test, a non-parametric alternative to two-sample t-test. Comparisons were also made within each cohort between subsequent time points using Wilcoxan signed rank test. All tests were two-sided, with a significance level of b 0.05. All analyses were conducted using SAS for Windows 9.3 (SAS Institute Inc, Cary, NC).
Fig. 8. Removal of the femoral stem using the screw-in explantation device.
mean pre-operative Cr levels were found to be 1.72 (±1.04) μg/L. Six weeks following revision and removal of the modular component, both the Co and Cr levels had significantly dropped in all patients. The mean serum cobalt ion levels dropped nearly 3-fold to 2.68 (± 2.67) μg/L (P b 0.0001) and the serum chromium ion levels dropped nearly in half to 1.0 (±.041) μg/L (P b 0.0001) [Fig. 10]. At three months, cobalt levels continued to decrease significantly to a mean of 1.58 (±1.57) μg/L (P b 0.0001). The cobalt level continued gradually drop to 1.27 (±1.48) μg/L at 1 year [Fig. 10]. Chromium levels demonstrated a similar pattern in serum ion level decrease. After the initial 6 week drop, serial follow-up to 1 year demonstrated no significant changes in the mean concentration [Fig. 10]. Erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) both demonstrated downward trends similar to those of the metal ions after revision surgery. The pre-operative ESR was 21.57 mm/h. The ESR dropped to 19.86 mm/h at six weeks and continued to decrease fairly uniformly to 12.4 mm/h at 12 months. The mean preoperative CRP concentration was 2.4 mg/dL. Postoperatively, the mean CRP had dropped to 1.9 mg/dL at 6 weeks and returned to a normal concentration of 0.9 mg/dL at 1 year [Fig. 11].
Cohort 2
Cohort 1 included all patients with unilateral modular Rejuvenate hip implants. Mean pre-operative Co levels were 8.19 (±5.54) μg/L and the
Cohort 2 included all patients with bilateral modular Rejuvenate hip implants. Cohort 2 preoperative cobalt levels were significantly higher than the preoperative values in cohort 1 (P = 0.013). Similarly for chromium, the preoperative levels in cohort 2 were significantly higher than the preoperative values in cohort 1 (P = 0.0157). Cohort 2 also demonstrated significant decreases in serum cobalt levels from 13.33 (±6.45) μg/L preoperatively to 3.73 (±2.19) μg/L at six weeks postoperatively (P = 0.0009). Correspondingly, serum chromium levels also
Fig. 7. Use of osteotomes to further remove bony ongrowth from the proximal stem.
Fig. 9. Femoral stem immediately following removal, note the minimal bone loss with careful removal.
Results Cohort 1
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Fig. 10. Graph comparing serum cobalt and chromium levels in both unilateral and bilateral patients spanning from pre-revision levels to one year follow up levels.
dropped in half from 2.56 (±1.37) μg/L preoperatively to 1.34 (±0.41) μg/L (p = 0.01) at six weeks [Fig. 10]. The cobalt and chromium levels remained relatively stable at 3 months. It should be noted that three patients had revision of the 2nd stem between 6 and 12 weeks, whereas the other two patients had the 2nd revision between 12 weeks and 6 months. This may be a significant confounding factor in the determination of average changes in serum cobalt and chromium levels between 6 weeks and 3 months. At six months, the mean serum cobalt level fell to 1.62 (± 0.74) μg/L and decreased to 1.12 (±0.32) μg/L at one year. The chromium levels in cohort 2 at 6 months had fallen to 0.92 (± .06) μg/L and remained stable at 0.98 (±0.11) μg/L at one year [Fig. 10]. The mean ESR level in cohort 2 began at 22.0 mm/h preoperatively and fell to 12.75 mm/h at 6 weeks. The ESR then slowly decreased over to 12.6 mm/h at 12 months. The CRP dropped from a preoperative value of 1.45 mg/dL to 0.98 mg/dL at six weeks. The CRP then continued to slowly decrease to 0.76 mg/dL at 12 months. Again, the fact that three patients had revision of the 2nd stem between 6 and 12 weeks, whereas two patients had the 2nd revision between 12 weeks and 6 months
Fig. 11. Graph of the serum ESR and CRP trend in patients with unilateral Rejuvenate modular femoral stems spanning from pre-revision levels to one year follow up levels.
confounds the pattern of ESR and CRP decline between 6 weeks and 3 months after Rejuvenate stem removal.
Discussion In order for a cementless proximal fit femoral implant to achieve stable fixation, it must reasonably conform to the version of the native femur. Proximal femoral deformities such as increased anteversion, femoral retroversion, coxa vara, cox breva, or coxa valga can arise idiopathically or be associated with other syndromes (i.e. hip dysplasia, SCFE, Legg-Calve-Perthes, Down's syndrome, etc.). Proximal femoral deformity may occasionally compromise the surgeon's ability to place a proximal fit cementless stem with appropriate anteversion. Modular neck or dual taper implants, such as the Rejuvenate stem, were designed to increase the surgeon's ability to match the patient's femoral offset, length, and version. Unfortunately, data from the 2012 Australian Registry Annual Report and UK National Joint Registry noted the Rejuvenate stem to have increased risk for early revision. Subsequently, a type II voluntary recall was initiated by the manufacturer, Stryker Orthopedics in July 2012 [2]. A similar modular dual taper stem, the ABG II was also recalled at the same time for similar increased risk of revision [11]. Since the initial reports, numerous articles have reaffirmed the poor performance of these implants and have also studied the elevated metal ion levels, abnormal soft tissue masses on MRI, and early onset pain experienced by patients [1,3,8]. While some authors have observed a rapid decline in metal ion levels in large series of Rejuvenate stems, the rate of decline has not been well characterized [3]. Increased metal ions are not only attributed to modular neck designs. In fact elevated metal ions have been seen in patients with the conventional THA designs with modular heads and polyethylene liners. Previous knowledge from the metal on metal hip arthroplasty experience suggests that measuring serum metal ions is a way of tracking a patient's progress after revision. Ball et al performed a similar study of serum metal ion decline after revision of metal on metal hip arthroplasty in 25 patients. The authors reported a drop in serum cobalt and chromium levels of 80% by 6 weeks and of 90% by 12 weeks [12]. Their study did not include any bilateral patients, though they included a single patient that had undergone a prior initial stage of a bilateral metal on metal THA revision. Additionally, there is concern that elevated metal ion levels associated
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with an adverse soft tissue reaction will remain elevated even after the offending implant is removed. Patients want to know how much time to expect to pass before their metal ion levels return to normal, if ever. The goal of this study was to answer these questions. To address this gap in the literature, our group decided to investigate the trend of metal ions and inflammatory markers following the removal of the Rejuvenate stem in a single surgeon cohort. It should be noted that taper corrosion of modular neck implants like the Rejuvenate and ABG II modular stem may be due to a different mechanism than the metal on metal hip metallosis problem. Authors have suggested that the chromium released from MACC precipitates as chromium orthophosphate whereas cobalt is more soluble and distributed into the serum [7]. This is seen clinically as there is often marked mismatch between serum cobalt and chromium ion levels with mechanically assisted crevice corrosion in contrast to metal on metal THA where cobalt and chromium ion levels are often similar [13]. MACC also seems to produce significant pain and symptomatic ALTR at lower metal ion levels than seen metal on metal total hip arthroplasty [13]. Our study demonstrated a statistically significant and clinically relevant drop in serum cobalt and chromium ion levels within 6 weeks after surgery in both the unilateral and bilateral Rejuvenate groups. Cohort 1, the unilateral Rejuvenate stem group, also demonstrated a significant decrease in both cobalt and chromium levels between 6 weeks and 3 months. Cohort 2, however, demonstrated a relative plateau in cobalt and chromium levels between 6 weeks and 3 months but this plateau effect may simply reflect the small sample size and the fact that three of the bilateral patients completed both Rejuvenate implant revisions within 3 months of each other while the other two patients had a residual Rejuvenate implant at 3 months. This certainly may confound the pattern of serum metal ion decrease in our bilateral cohort group. This plateau effect may also be caused by a nonlinear relationship between the renal clearance and serum metal ion levels. Limitations of this series include lack of a control group, limited follow up, single implant, wide variation in metal ion levels, and small numbers. The current series does not have a control group against which to compare metal levels. Metal levels from a well functioning total hip arthroplasty would have been an appropriate comparison group, but were not available as this is not our usual practice or approved by our institutional review board. With respect to follow up, only 52% of patients were eligible for one year serum metal ion testing at the time of the study. However, our study had 98% follow up at six months whereupon the majority of patients' serum cobalt and chromium ion levels had returned to near-normal levels. The added expense of continued testing was not warranted following the return of cobalt and chromium ion levels to near normal after Rejuvenate stem revision. While we acknowledge that a single senior surgeon is a limitation of the study, the failure of the Rejuvenate stem is likely a flaw inherent to the modular CoCr neck design in a susceptible host, rather than a failure of surgical technique. Ultimately, this study examines only the Rejuvenate stem and cannot make any conclusions about other modular neck dual taper stems. This study cannot determine if the high failure rate of the Rejuvenate applies to the modular neck stem design as a group. Unique to Stryker, both the Rejuvenate and ABG II stems are made from a proprietary TMZF alloy. It has been suggested that the alloyed metals may be more susceptible to corrosion when mixed with a cobalt–chrome implant compared to single element metals [1,14]. Other modular stems with titanium stems and cobalt–chrome modular necks may be at less risk for MACC on account of their metallurgy. There is concern using a modular implant (Restoration Modular, Stryker) for revision of a failed modular implant. Unlike the Co-Cr and
TMZF stem-neck taper in the Rejuvenate system, the tapers in the Restoration Modular system are titanium–titanium interfaces. The Rejuvenate taper was between 2 different metals (TMZF and CoCr) which has been shown to be more susceptible to corrosion than identical metals [1]. Additionally, the taper between the stem-body of the Restoration Modular is axially loaded rather than the shear loading of the neckstem taper of the Rejuvenate taper, a mechanically unfavorable loading mechanism. Finally, the Restoration Modular system was released in the United States in 2003 and has more than 10 years of clinical follow up with no reports of failure secondary to taper corrosion. We have published on the satisfactory results of modular femoral stems in revision total hip arthroplasty in the past [15]. In conclusion, this study demonstrated that cobalt and chromium ion levels significantly decrease after removal of a unilateral Rejuvenate femoral stem with a modular neck. The largest drops serum ion levels were noted from pre-revision to six weeks postoperatively. A patient with a unilateral Rejuvenate stem can expect the serum cobalt and chromium levels to drop by 67% and 42%, respectively, from preoperative levels at six weeks following revision. Furthermore, at three months, serum cobalt levels can be expected to decline by 81% compared to preoperative values and then gradually decrease thereafter. Chromium levels remained stable after 6 weeks at a level of about 1.0 μg/L, which is within the normal reference range in the unilateral cohort. Inflammatory markers like ESR and CRP gradually decreased and remained elevated even at 12 months following revision. Statistical analysis of the bilateral Rejuvenate stem group demonstrated significant declines in serum cobalt and chromium ions at six weeks, but a plateau between six weeks and three months although this effect may be due to small sample size and inconsistent timing between the first and second revisions. Surgeons can use this study when counseling patients that they can expect serum cobalt and chromium ion levels to fall to nearnormal levels within about 3 months following removal of a unilateral Rejuvenate femoral stem. References 1. Krishnan H, Krishnan S, Blunn G, et al. Modular neck femoral stems. Bone Joint J 2013; 95:1011. 2. Stryker initiates voluntary product recall of modular-neck stems. Action Specific to Rejuvunate and ABG II Modular-Neck Stems: Food and Drug Administration; 2012. 3. Meftah M, Haleem AM, Burn MB, et al. Early corrosion-related failure of the rejuvenate modular total hip replacement. J Bone Joint Surg 2014;96:481. 4. Jacobs JJ, Gilbert JL, Urban RM. Current concepts review—corrosion of metal orthopaedic implants*. J Bone Joint Surg 1998;80:268. 5. Kretzer JP, Jakubowitz E, Krachler M, et al. Metal release and corrosion effects of modular neck total hip arthroplasty. Int Orthop 2009;33:1531. 6. Kop AM, Swarts E. Corrosion of a hip stem with a modular neck taper junction: a retrieval study of 16 cases. J Arthroplast 2009;24:1019. 7. Cooper HJ, Della Valle CJ, Berger RA, et al. Corrosion at the head-neck taper as a cause for adverse local tissue reactions after total hip arthroplasty. J Bone Joint Surg 2012;94:1655. 8. Pivec R, Meneghini RM, Hozack WJ, et al. Modular taper junction corrosion and failure: how to approach a recalled total hip arthroplasty implant. J Arthroplast 2014;29:1. 9. Hayter CL, Koff MF, Potter HG. Magnetic resonance imaging of the postoperative hip. J Magn Reson Imaging 2012;35:1013. 10. Campbell P, Ebramzadeh E, Nelson S, et al. Histological features of pseudotumor-like tissues from metal-on-metal hips. Clin Orthop Relat Res 2010;468:2321. 11. Restrepo C, Ross D, Restrepo S, et al. Adverse clinical outcomes in a primary modular neck/stem system. J Arthroplast 2014;29:173. 12. Ball ST, Severns D, Linn M, et al. What happens to serum metal ion levels after a metal-on-metal bearing is removed? J Arthroplast 2013;28:53. 13. Gill I, Webb J, Sloan K, et al. Corrosion at the neck-stem junction as a cause of metal ion release and pseudotumour formation. J Bone Joint Surg Br 2012;94:895. 14. Molloy DO, Munir S, Jack CM, et al. Fretting and corrosion in modular-neck total hip arthroplasty femoral stems. J Bone Joint Surg 2014;96:488. 15. Stimac JD, Boles J, Parkes N, et al. Revision total hip arthroplasty with modular femoral stems. J Arthroplast 2014;29:2167.