Metal-on-metal (MoM) hip implants were first introduced in 1966 with the McKee-Farrar prosthesis.1 With improvements in design and fixation, the second generation of implants was developed using cobalt-chromium-molybdenum alloys, which had two theoretic advantages over conventional metal-on-polyethylene total hip arthroplasty (THA) implants. First, MoM articulations produced significantly less volumetric wear; thus, they had the potential to substantially reduce failure rates secondary to wear-induced osteolysis.2 Second, metal acetabular components can be made thinner, allowing for larger-diameter femoral heads that increase stability and range of motion.3
With these proposed advantages, MoM hip implants were widely used for THA and hip resurfacing procedures in the United States. It is estimated that >1 million MoM articulations have been implanted worldwide since 1996.4 However, concerns emerged when national registry data reported significantly higher revision rates (twofold to threefold) with MoM implants than with other contemporary hip implants.5,6 These revision rates have been attributed to the corrosive metal interaction between the cobalt-chromium components of the femoral head and the acetabular liner. The resulting release of metal particles and metal ions into the periprosthetic space creates macroscopic necrosis; osteolysis; large, sterile hip effusions; and periprosthetic solid and cystic masses (ie, pseudotumors). This spectrum of findings is now termed an adverse reaction to metal debris (ARMD).7,8
More recently, similar adverse reactions have been reported with secondary modular metal junctions (with more reports on the femoral neck-stem junction than the head-neck junction), even in the absence of a primary MoM articulation.9,10 Histologic examination of tissues in the setting of a failed implant exhibited a specific lymphocyte-dominated immunologic response, now known as aseptic lymphocyte-dominant vasculitis-associated lesion (ALVAL). The characteristic histopathology was similar to but not exactly the same as a delayed (type IV) hypersensitivity reaction.11 Additionally, several studies have reported systemic effects associated with elevated serum levels of cobalt and chromium; however, the clinical significance of increased metal ion levels is largely unknown.12,13
In early 2011, the US FDA publicly issued concerns regarding adverse reactions to metal ball-and-socket hip components and ordered five manufacturers of MoM total hip arthroplasty systems to perform postmarket surveillance of such events.14 To date, three implant companies that manufacture MoM bearing surfaces have voluntarily recalled specific devices secondary to higher-than-expected revision rates outside the United States, including the metal liners of the R3 Acetabular System (Smith & Nephew) and the ASR XL Acetabular System (DePuy). A temporary recall of the Durom Acetabular Component (Zimmer) was issued because of inadequate instruction on the surgical technique. With increasing potential for litigation and public reporting of outcomes, orthopaedic surgeons must develop sound clinical and surgical approaches to care for this unique patient population. Here, we provide the framework for patient evaluation and treatment after THA with a MoM implant.
History and Physical Examination
Clinical evaluation and routine surveillance of patients’ symptoms following THA with MoM implants remains critical for appropriate diagnosis and treatment. Patients can generally be classified into two broad categories: asymptomatic or symptomatic (Figure 1). Further classification then can be made based on the component position, implant performance (ie, suboptimal or recalled), and the serum ion levels of cobalt or chromium.15 Clinical symptomology and laboratory data remain highly variable; therefore, surgeons must consider a patient’s presentation in terms of a continuous spectrum as opposed to placing the patient into one specific clinical scenario.
A detailed history and physical examination must be obtained and documented accurately. A basic arthroplasty history begins with the index operation date, location, surgeon, indication, and any adverse events or complications during the acute perioperative course. Specific attention should be paid to the episodes and location of pain or limited function as well as the onset, frequency, and severity of such occurrences. A history of delayed wound healing, pain after dental or gastrointestinal procedures, or systemic illness suggests a periprosthetic infection. Each patient also should be questioned about individual or familial history of metal allergy or hypersensitivity. Park et al16 reported that the rate of positive skin patch testing in patients with a MoM hip implant and osteolysis was higher than that in patients with a MoM hip implant without osteolysis, suggesting a delayed hypersensitivity reaction. However, the relationship between an authentic metal allergy and MoM failure has not been fully established.
A careful physical examination can help narrow the differential diagnosis to extrinsic and intrinsic causes of hip pain (Table 1). The skin should be inspected, and previous incisional scars or signs of infection should be noted. The hip should be palpated to identify areas of tenderness or soft-tissue masses. Range of motion may be limited but can be useful to reproduce painful positions, including pain with active hip flexion, which suggests iliopsoas tendinitis. A complete neurovascular examination is also essential to evaluate for vascular or spinal pathology that can mimic hip pain.
Following the history and physical examination, careful review of both initial and serial postoperative radiographs is performed to identify implant type and size, signs of loosening or osteolysis, and component position. Advanced loosening or prominent retroacetabular, ischial, or pubic osteolytic lesions can create more obvious changes on radiography and likely necessitate earlier revision. Larger femoral head sizes also are important to note because the Australian and England/Wales registries have reported higher rates of revision for patients with a larger femoral head than for those with smaller femoral head components.5,6
Careful radiographic analysis is important, especially evaluation of excessively abducted components on the AP view of the pelvis (Figure 2). Increased abduction angles of ≥50° relative to the horizontal axis on an AP view of the pelvis have been shown to be associated with elevated levels of metal ions in serum and joint fluid tests.17 Acetabular version may also be important; however, excessive version alone has not been proven to be associated with increased wear.18 Lee et al19 suggested that wear rates are consistent regardless of bearing size, clearance, or contact mode. Increased wear rates of MoM implants and subsequent higher revision rate may be secondary to the phenomenon of edge loading, which occurs when the head-cup contact points extend over the cup rim, effectively increasing contact pressure and wear rates. Regardless of the failure mechanism, assessing a satisfactory acetabular component position of ≤50° and recognizing poor-performing or recalled implants are key steps in the evaluation algorithm.
Specialized metal artifact reduction sequence (MARS) MRI produces high-resolution imaging of pelvic soft-tissue structures, with minimal image distortion from the metal implants.20 The modification of the MRI pulse sequence facilitates assessment for the presence and extent of pseudotumor formation as well as of the integrity of abductor and gluteal muscles, which may be compromised by adverse soft-tissue reactions (Figure 3, A). However, the prevalence of pseudotumor formation has been shown to be similar in both symptomatic and asymptomatic patients with MoM implants, emphasizing the need to better correlate clinical and imaging data.21 MARS MRI is generally recommended for symptomatic patients or those with elevated levels of metal ions, regardless of symptomatology. Patients with poorly performing or recalled MoM hip implants may also benefit from specialized advanced imaging earlier in the workup.
There has been interest recently in the use of diagnostic ultrasonography to identify fluid collections about the hip more readily than with MRI; ultrasonography is also less costly.22 Aside from a simple screening evaluation, ultrasonography can be used to guide hip aspiration or biopsy. MRI evaluation may be contraindicated or not be feasible in specific patient populations (ie, those with metallic fragments, a foreign body, cardiac pacemaker, claustrophobia/anxiety), which increases the attractiveness of ultrasonography. Nonetheless, given the wide spectrum of MoM THA presentations, there will undoubtedly be increased use of advanced imaging techniques for both diagnostic and preoperative revision planning purposes.
The final part of the initial MoM hip arthroplasty workup includes basic laboratory evaluation of the erythrocyte sedimentation rate, C-reactive protein level, serum metal ion level quantification, and possible hip aspiration. Elevated erythrocyte sedimentation rate and C-reactive protein values have been reported in cases of noninfected adverse soft-tissue reactions.23 Although the relationship between serum ion levels and the extent of implant wear and/or existence of ARMD is unclear, baseline serum metal ion levels should be obtained in all symptomatic patients with MoM hip components or those with radiographic changes.24,25 The cutoff level of ≥7 parts per billion (ppb, or μg/L) for cobalt or chromium has been generally recommended as an indication for advanced imaging. Hart et al26 have suggested lowering the value to ≥5 ppb to increase sensitivity and identify more MoM failures. Concerns exist for adverse local tissue reaction related to a modular femoral neck-body junction, and in a study of 12 titanium-alloy stem and cobalt-chromium-alloy neck implants, serum cobalt levels were higher than chromium or titanium levels.10 However, elevated ion levels are not an absolute indication for revision surgery; rather, they should be viewed as a supplement to other objective clinical findings. Higher levels of metal ions do require more frequent follow-up and often warrant advanced cross-sectional imaging.
Patch testing and lymphocyte tests have been used to determine individual metal hypersensitivity; an estimated incidence of 25% has been reported in both well and poorly performing orthopaedic implants.27 Hallab et al27 noted a dramatically higher incidence (range, 13% to 71%) of dermal metal sensitivity among patients with failed implants that required revision surgery. This discrepancy prompts speculation that immunologic response may contribute to implant loosening and failure. Currently, no accepted or recommended test exists for the clinical determination of metal hypersensitivity to implanted devices.
Aspiration of hip synovial fluid allows clinicians to discern ARMD from periprosthetic infection, especially when clinical suspicion for infection is increased. The gross appearance of metallosis is described as a “dishwater fluid” with watery, yellowish-gray, and hazy characteristics distinct from purulent aspirates.15,28 Synovial fluid should be sent for standard cultures and sensitivities, cell count, and cell differential. The laboratory must perform a manual cell differential count because fibrinous debris from MoM reactions can artificially elevate automated counts. There are no current absolute guidelines on a cutoff cell count for the diagnosis of ARMD; however, aspirates with a higher number of cells and a larger percentage of monocytes are concerning for a more significant reaction and possible early implant failure.
The initial and ongoing evaluation of an asymptomatic patient with a MoM hip implant remains a perplexing and controversial issue. Although most surgeons typically favor annual follow-up for a low-risk patient (ie, asymptomatic, low activity level, well-positioned implant, no radiographic osteolysis or loosening), no clear consensus exists on when it is appropriate to obtain metal ion levels or advanced imaging in such patients. Evidence exists to suggest that metal ion concentrations in blood can be a particularly useful tool to guide the treatment of asymptomatic patients.22 If the concentration is low, the data are reassuring to the patient and surgeon and allow for more judicious use of otherwise expensive cross-sectional imaging. Conversely, a patient may have an asymptomatic pseudotumor without elevated metal ion levels. Bayley et al29 recently reported that a low percentage of patients had elevated ion levels despite having a high prevalence of pseudotumors.
In an asymptomatic patient with a MoM hip implant who presents with known elevated metal ion levels, we prefer to repeat metal ion tests at a 6- to 12-month interval to evaluate trends. With a demonstrated upward trend of levels, cross-sectional imaging can be obtained, especially levels ≥7 ppb and/or onset of new symptoms or radiographic changes. However, it should be noted that the FDA has not yet established the utility of routine screening of metal ion blood testing in patients with a properly functioning hip.14 The FDA does recommend periodic follow-up of asymptomatic patients with MoM implants (typically every 1 to 2 years), including physical examination and routine radiography.14 We strongly recommend dedicated patient education for all diagnostic options available to promote shared decision making.
Revision Hip Arthroplasty
The decision to perform revision THA of a MoM implant is multifactorial and should be based on documented, objective clinical indications. Proposed risk factors associated with early MoM implant failure include female sex, acetabular component malposition, implant design and size, and obesity.30,31 Aseptic loosening, metal hypersensitivity, and infection are common indications for revision THA, and component malposition, instability, iliopsoas impingement and periprosthetic fracture may also necessitate revision.28,32
At the time of revision surgery, the intraoperative appearance of fluid and soft-tissue necrosis can vary from benign-appearing effusion to profound destruction of the hip capsule and abductors (Figure 3, B). The degree of abductor muscle involvement associated with the pseudotumor corresponds with the difficulty of the revision procedure and patient functional outcomes.8,33 Periprosthetic tissues should be sent for frozen section, standard culture, and pathologic examination to detect chronic inflammation or ALVAL (Figure 3, C). Although the pathophysiology and clinical implications of ALVAL are poorly understood, this unique perivascular lymphocytic infiltration reaction from B lymphocytes, plasma cells, and massive fibrin exudation is becoming more frequently recognized.11,28,34
Evidence-based surgical management guidelines have yet to be established. Fabi et al32 developed a classification scheme for MoM implant failure and soft-tissue complications that may help guide surgical treatment options (Tables 2 and 3). Aside from necessary revision of loose or malaligned components, revision of the MoM hip bearing surface to metal-on-polyethylene or ceramic-on-polyethylene is generally recommended. Severely compromised hip abductor function or damaged soft-tissue inherently affects stability; therefore, the use of a constrained liner or salvage procedures may need to be considered.
The importance of thorough documentation of intraoperative findings during revision of MoM implants cannot be underestimated and should include the appearance of periprosthetic soft tissues and joint fluid (if possible, with digital photographic documentation), intraoperative frozen-section examination results, the condition of implant fixation, signs of corrosion at the articular surface or modular junctions, disposition of retrieved implants, and choice of revision components.15 In patients undergoing revision THA, the removed implant should be sent to a facility experienced in retrieval analysis of MoM hip implants.
Limited complication and outcome data are available following revision THA in patients whose index procedure contained MoM bearing surfaces; nevertheless, short-term results are beginning to emerge. Rajpura et al35 documented improved Oxford hip scores in 8 of 12 patients who underwent revision for failed MoM hip implants, with no cases of infection or instability reported. In a larger series of 32 revised hips, Munro et al36 reported a very high rate of major complications (38%), including instability, neurovascular injury, deep infection, reoperation, component loosening, and recurrence of ARMD, with 28% of those complications related to instability. In contrast, a study of 37 hips revised secondary to MoM implant failure reported no episodes of dislocation.37 However, periprosthetic infection was the singular cause of repeat revision in 8.1% of hips. Metal ions tend to decline most dramatically in the first 3 months following revision surgery but do so less predictably in patients with ultra-high pre-revision chromium levels.38 Postrevision complications and outcomes must continue to be monitored as revision of MoM THA becomes more common.
Management of Recalled Implants
Recently, risk stratification groups have been established to optimize clinical treatment of patients with MoM hip implants.39 The frequency of follow-up evaluation needs to be adapted for each patient based on his or her risk and clinical course. Typically, routine surveillance of these patients should be performed on an annual basis, at a minimum. Asymptomatic patients who possess a recalled implant must be counseled that the risk of revision surgery cannot be justified in the absence of objective clinical indications. Again, thoughtful and complete documentation of medical and surgical management decisions is vital. At the time of revision surgery, release of the MoM hip implants to a retrieval analysis laboratory is recommended. The implant retrieval analysis helps provide further understanding of device failure mechanisms in addition to the clinical, radiographic, and pathologic findings—all of which are valuable to improving the care of patients who undergo arthroplasty.15,39
With increasing concerns about high revision rates and adverse reactions after THA with a MoM implant, orthopaedic surgeons must appropriately evaluate and treat these patients using a systematic approach. Aside from metal implant concerns, other potential causes of a painful THA must be explored. The evaluation begins with a comprehensive history, physical examination, and a thorough radiographic assessment. Initial serum cobalt and chromium metal ion levels should be obtained in addition to standard infectious laboratory workup. Advanced imaging techniques, including ultrasonography and MARS MRI, may be valuable for identification of pseudotumors and serve as important preoperative planning tools. For patients undergoing revision surgery, complete documentation of intraoperative findings and surgical decision making must be performed. Evaluation and treatment algorithms provide a step-by-step objective approach to the treatment of patients with MoM hip implants to optimize outcomes and avoid reliance on a single diagnostic tool. Additional research is needed to better understand failure mechanisms, recognize the clinical significance of elevated metal ion levels, validate current diagnostic tools, and monitor outcomes following revision of these implants.
Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, references 2, 20, and 28 are level I studies. References 4 and 38 are level II studies. References 23, 27, 29, 33, and 34 are level III studies. References 1, 3, 7-13, 16-18, 25, 30-32, and 35-37 are level IV studies. Reference 24 is level V expert opinion.
References printed in bold type are those published within the past 5 years.
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. Clin Orthop Relat Res 2014;472(2):523–528.
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. Clin Orthop Relat Res 2014;472(2):509–516.
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Keywords:© 2015 by American Academy of Orthopaedic Surgeons
metal-on-metal; total hip arthroplasty; aseptic lymphocyte-dominant vasculitis-associated lesion; pseudotumor; adverse reaction to metal debris; metal artifact reduction sequence