Corrosion has long been recognized to occur in total hip arthroplasty, but the local effects of this process have only recently become better understood. This article provides an overview of corrosion at modular junctions, and discusses the various etiologic factors for corrosion and the biologic response to metal debris released from this junction. Algorithms are provided for diagnosis and treatment, in accordance with the best available data.
Key points
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Corrosion can occur at any modular junction in total hip arthroplasty (THA), with the potential for release of metal debris and ions into the surrounding local environment.
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The potential for corrosion at a particular modular junction is multifactorial and can depend on factors such as taper geometry, constituent materials, forces applied to the junction, femoral head size, component offset, and method of assembly.
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Local effects of metal corrosion include adverse local tissue reactions (ALTR), component fracture or failure, instability, and osteolysis and loosening.
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Taper corrosion should be considered in the differential diagnosis of hip pain following THA, and appropriate testing should include serum metal levels and cross-sectional imaging such as magnetic resonance imaging to evaluate the surrounding soft tissues.
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Treatment consists of exchange of the modular part responsible for the ALTR when possible, versus revision of the entire component when not possible.
Introduction
Metal corrosion in total hip arthroplasty (THA) has long been recognized as a theoretical concern that accompanied the introduction of modularity and soon thereafter was documented in numerous retrieval studies of early modular hip components. Although there were rare reports of poor clinical outcomes associated with corrosion, any connections between adverse local effects and corrosion were not firmly established at that time.
As design and manufacturing of modular junctions improved through the 1990s and 2000s, most early concerns surrounding corrosion largely disappeared from the literature. However, there has recently been a renewed concern surrounding corrosion in THA, as many groups have reported adverse local tissue reactions (ALTR) and other local complications in association with modular hip components. As complications arising from corrosion are being recognized with increasing frequency, it is important for the orthopedic surgeons who perform these procedures and manage these patients postoperatively to understand this process.
This article reviews the important points concluded from retrieval analyses, and explores the multifactorial etiology of corrosion. Local effects of corrosion are reviewed, and diagnostic evaluation and treatment options for patients with metal corrosion discussed.
Introduction
Metal corrosion in total hip arthroplasty (THA) has long been recognized as a theoretical concern that accompanied the introduction of modularity and soon thereafter was documented in numerous retrieval studies of early modular hip components. Although there were rare reports of poor clinical outcomes associated with corrosion, any connections between adverse local effects and corrosion were not firmly established at that time.
As design and manufacturing of modular junctions improved through the 1990s and 2000s, most early concerns surrounding corrosion largely disappeared from the literature. However, there has recently been a renewed concern surrounding corrosion in THA, as many groups have reported adverse local tissue reactions (ALTR) and other local complications in association with modular hip components. As complications arising from corrosion are being recognized with increasing frequency, it is important for the orthopedic surgeons who perform these procedures and manage these patients postoperatively to understand this process.
This article reviews the important points concluded from retrieval analyses, and explores the multifactorial etiology of corrosion. Local effects of corrosion are reviewed, and diagnostic evaluation and treatment options for patients with metal corrosion discussed.
Corrosion at modular interfaces in THA
Modularity offers numerous advantages in modern THA. Modular heads and necks offer increased intraoperative flexibility to better match native anatomy, leg length, and offset, all of which affect stability. Modularity also allows numerous options for head size and choice of bearing surface without requiring a substantial increase in implant inventory. Head-neck modularity further allows the head to be removed at the time of future surgery, either for exposure or to change head size or neck length. In addition, certain implants with modular proximal-stem and mid-stem junctions offer advantages in addressing difficult primary or revision cases. However, as already noted, the addition of these modular junctions does not come without a cost.
Historical Retrieval Analyses
Soon after the introduction of head-neck modularity in THA, numerous retrieval analyses began to report fretting and corrosion at the head-neck junction. Early retrievals documented concerns of increased corrosion associated with mixed-metal junctions, resulting from galvanic acceleration between a titanium (Ti) alloy stem and a cobalt-chromium (CoCr) alloy head ; however, this was challenged in later studies. Prevalence of corrosion among retrieved specimens ranged from 0% to 57% at 0.5 to 5.5 years, but was found to be significantly dependent on device and design.
Corrosion in Metal-on-Metal THA
Release of metal wear debris from the metal-on-metal (MoM) bearing surface was originally thought to be the major cause in the development of ALTR after large-head MoM THA, as this was the cause for ALTR following hip-resurfacing arthroplasty. However, recent work from multiple investigators has implicated corrosion at the modular head-neck taper interface to be a major contributing factor. Given these findings, there is growing concern that the modular taper junction plays a significant role in the failure of large-head MoM THA, although the clinical significance of metal loss from this junction remains somewhat unclear. This additional modular junction may be responsible for the greater elevations in serum metal levels and higher failure rates of large-head MoM THA when compared with hip-resurfacing arthroplasties bearing the same design.
Corrosion in Metal-on-Polyethylene THA
As orthopedic surgeons became more accustomed to seeing adverse reactions to metal debris associated with MoM devices, similar reactions were noted in patients with metal-on-polyethylene bearing surfaces. Given that the potential for corrosion at this modular head-neck junction had been well established from the retrieval analyses of metal-on-polyethylene THAs discussed previously, these reactions provided clear evidence of a similar mechanism of metal release from modular head-neck junctions ( Fig. 1 ), regardless of the bearing surface.
Corrosion at Neck-Body Junctions
Numerous designs of modular neck-body stems, also known as dual-taper stems, have been introduced. Similar to modular head-neck junctions, concerns over corrosion at modular neck-body junctions ( Fig. 2 ) were raised soon after these devices were introduced. Corrosion at this junction has subsequently been documented in retrieval analyses, cases of modular neck fracture, and reports of ALTR. Furthermore, multiple stem designs featuring modular neck junctions from several different manufacturers have been either modified, recalled, or removed from the market after these concerns were noted.
Corrosion at Other Modular Junctions
Concerns over corrosion at stem-sleeve or mid-stem modular junctions were also expressed soon after these devices were introduced. Stem-sleeve devices made from Ti alloy have good mid-term and long-term outcomes reported in both primary and revision settings, although recent reports have raised concerns for fretting and corrosion found at retrieval, found in association with stem fracture, and encountered in junctions with impaired intraoperative disengagement at the time of revision surgery. Mid-stem modular junctions can also undergo corrosion, and this process has been found to be more severe at CoCr modular junctions than at Ti modular junctions. A second retrieval study documented severe corrosion in the mating interfaces of mid-stem modular Ti stems with evidence of etching, pitting, delamination, and surface cracking.
In addition, although corrosion is generally thought to be isolated to femoral components, it can also occur in modular acetabular components, and has been correlated with impingement in one recent study.
Etiology of corrosion
Corrosion is fundamentally an electrochemical process of oxidation and reduction reactions. The metals used in THA components are chosen not only because of their high strength, wear resistance, and fatigue resistance, but also because they are highly corrosion-resistant when compared with most other metals because of their ability to form a passive surface film to prevent oxidation. However, owing to the mechanical environment that occurs in vivo, these components are subject to a mechanically assisted corrosion process that can be considerably accelerated when this passive surface film is disrupted through factors such as fretting, micromotion, or other externally applied forces. Furthermore, corrosion is affected by wear through a complex interaction known as tribocorrosion, in which these 2 processes are coupled in a nonlinear manner.
Factors Associated with Corrosion
The following factors have been associated with increased mechanical forces, fretting, or corrosion at modular junctions in THA, and are therefore of clinical relevance and concern:
Head size
Several recent studies by multiple independent groups have concluded that head size has a role in the propensity for corrosion at the modular head-neck interface. Frictional torque at the bearing surface, which is transmitted to modular junctions, has been shown to increase with increasing head size from 22 mm to 40 mm in a biomechanical study. Likely because of these increased torsional forces, a recent retrieval study found significantly increased corrosion scores at the head-neck taper interface of 36-mm heads when compared with 28-mm heads. Another recent finite element study has found increased stress and deleterious wear generation at the femoral head-trunnion interface in head sizes larger than 40 mm. Despite this evidence, however, it is clear that corrosion can occur with any size of head, as has been documented in the literature.
Neck length and offset
Longer modular heads or necks that offer higher offset increase the load seen at modular junctions, and, as a result, have been shown to lead to higher visual fretting damage in multiple in vitro biomechanical studies and retrieval analyses. In addition, a study of large-diameter MoM THAs found that increasing the lever arm through increased offset was the primary factor leading to taper failure.
Head material and bearing surface
Certain head materials are clearly more susceptible than others to fretting and corrosion. CoCr heads have demonstrated greater fretting corrosion when coupled with stainless-steel stems than when coupled with CoCr stems. Furthermore, ceramic heads produce significantly less fretting and corrosion than CoCr alloy heads, and thus have a lower potential for metal release from the modular trunnion. A biomechanical study found significantly greater metal release (11-fold increase in Co and 3-fold increase in Cr) from a CoCr-CoCr interface when compared with a CoCr-ceramic interface. These protective benefits of ceramics also seem to extend to femoral heads made from oxidized zirconium. However, it must be noted that ceramic heads may not be completely protective against corrosion, as multiple retrieval studies have documented corrosion even in the presence of a ceramic head.
In addition, the routine use of highly cross-linked polyethylene (XLPE) may also play a role in increasing the risk of fretting and subsequent corrosion at modular taper junctions, as XLPE has been shown in a biomechanical in vitro study to exert a higher frictional torque to the bearing interface than conventional polyethylene.
Stem and trunnion design
Stem design also has an etiologic role in propensity for corrosion. Fully coated cobalt-alloy cementless stems have been shown to have a higher prevalence of intergranular trunnion corrosion in a retrieval analysis of 246 implants, although questions remain as to whether the increased prevalence of corrosion associated with these designs reflects problems in the particular stem as opposed to simple usage patterns at the institutions reporting these cases.
Probably more influential than stem design is trunnion design, which can have considerable effects on the likelihood of corrosion. Flexural rigidity of the trunnion, which depends on taper geometry and the modulus of elasticity of the given metal alloy from which the stem or neck is made, has been found in a large retrieval analysis be among the most important factors responsible for corrosion at the head-neck junction. As a result, stems with a smaller taper geometry and those made from a more flexible alloy may be more prone to fretting and corrosion than stems with larger and stiffer trunnion designs.
Method of assembly
Assembling modular junctions under clean and dry conditions is also important in reducing the risk of corrosion. Modular junctions assembled under contaminated conditions (with bone chips) were found to exhibit greater micromotion than those assembled with a clean interface in an in vitro study. In addition, in a separate biomechanical study dry taper assembly has been found to raise the onset load to fretting corrosion when compared with assembly under wet conditions.
Local effects of metal corrosion in THA
Metal release from modular junctions through fretting and corrosion can cause elevated serum metal levels and particle deposition within local tissues. More concerning, this process can also produce a range of local effects in and around the hip, including the following.
Adverse Local Tissue Reactions
ALTR, which have also been described as pseudotumors, were initially reported as a complication of MoM hip resurfacing and MoM THA, with histologic examination of periprosthetic tissues demonstrating large areas of tissue necrosis, chronic inflammatory reaction, and perivascular and diffuse lymphocytic aggregates. Because these reactions arise from a reaction to metal debris, they can also arise from corrosion at modular junctions in THA components.
Svensson and colleagues were the first to report an aggressive soft-tissue reaction following metal-on-polyethylene THA. The patient reported severe pain and weakness 3 years following the index surgery, and on exploration was found to have severe fretting and corrosion at the modular head-neck junction and a large necrotic soft-tissue mass. Following multiple debridements and eventual explantation, the patient was eventually left with a completely denervated leg, a chronically draining wound, and both arterial and venous insufficiency. Three years later, Mathiesen and colleagues described 3 additional cases with similarly extensive soft-tissue reactions and macroscopically visible corrosion at the head-neck interface.
Following these early reports, there were no known additional cases of ALTR secondary to corrosion reported for nearly 2 decades. However, since 2010 there have been several new cases of ALTR described in the setting of corrosion at modular head-neck and neck-body junctions. Clinical presentations can be variable, but symptoms typically relate to pain secondary to fluid accumulation and tissue destruction, with many patients also describing muscle weakness or a limp.
Instability
Patients with ALTR may not always present with pain. Local tissue destruction with associated abductor weakness can manifest as instability in the setting of mild to no preexisting hip pain, and has been reported in patients with MoM bearings. This process can also occur in patients with ALTR secondary to corrosion. If the orthopedic surgeon fails to consider ATLR in the differential diagnosis for recurrent instability, a missed diagnosis might follow.
Loosening and Osteolysis
Local inflammation, mediated by wear debris and ions released from corrosion at the modular junction, can play a role in wear-induced periprosthetic loosening. A recent study found that of 114 large-head MoM THAs with corrosion at the modular head-neck junction, 52% demonstrated radiographic evidence of loosening. Other investigators have also reported cases of component loosening in association with corrosion.
Corrosion-Induced Implant Fracture
Neck fractures of monolithic stem designs have been associated with corrosion at modular head-neck junctions ; however, these reports are rare and seem to be of minimal clinical concern.
Much more common and more concerning is the potential for modular neck fracture in dual-tapered stems, which is probably the second most common clinical problem arising from metal corrosion after ALTR. This problem has been reported in multiple stem designs, but only in those with a Ti alloy modular neck. The incidence of this complication is clearly design-specific, but has been reported as high as 2.4% with one specific design from Europe. Although most case reports on modular neck fracture have not performed a retrieval analysis, those that have found evidence of corrosion at the site of fracture initiation.
In addition to neck fractures, fractures of the femoral stem itself have been reported in association with corrosion at modular stem-sleeve junctions in stem designs with proximal body modularity.
Remote and Systemic Effects
Descriptions of systemic manifestations have been sporadically reported in association with elevated metal levels in MoM hip arthroplasty, although causality and etiology remain uncertain at present. The literature on systemic effects of taper corrosion is even less clear, with investigators having described findings of depression, weight gain, hemolytic anemia, and local motor and sensory dysfunction as well as decreased appetite and weight loss in patients who also had ALTR secondary to corrosion. As with reports of systemic effects of metallosis following MoM THA, it is difficult to establish causality based on these nonspecific symptoms.
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