➢ Trunnionosis, characterized by corrosion and fretting of the taper, is a well-known entity commonly demonstrated in retrieval specimens. While there have been a number of recent reports regarding the potential for adverse local tissue reactions related to trunnionosis, it remains a relatively infrequent cause for failure of total hip replacement implants.
➢ A number of factors, including both biomechanical and bioelectrochemical factors, have a known impact on the development and severity of trunnionosis. Furthermore, specific implant design and material-related factors have been shown to influence the risk of trunnionosis leading to adverse local tissue reactions.
➢ Retention of a well-fixed femoral stem, in spite of corrosion of the male taper junction, is acceptable in the majority of cases.
➢ A ceramic head, often in combination with a titanium adaptor sleeve, is the most common replacement reported in the current literature to treat trunnionosis.
➢ In patients with modular-neck total hip replacements, revision of the femoral stem is likely required if corrosion at the modular neck-stem junction is encountered.
Total hip arthroplasty has become one of the most successful orthopaedic procedures for the treatment of end-stage arthritis of the hip1. Implant longevity plays a major role in determining the eventual long-term success of total hip arthroplasty and is dependent on various factors, such as the surgical technique, type of fixation, implant design, and implant material. All modular total hip arthroplasty designs allow for the femoral head to be connected to the femoral stem with use of a single head-neck taper junction. Head-neck modularity was adopted to allow intraoperative adjustments of limb length and femoral offset, head diameter, and material. Dual modular total hip arthroplasty designs have an additional taper junction between an interchangeable neck segment and the stem. Neck-stem modularity was introduced to allow further intraoperative flexibility, mainly the adjustment of femoral version. It also was designed to allow partial revision of the hip implant if required. The purposes of the present review are to provide a brief overview of tapers and trunnions, to describe clinical manifestations that can occur as a result of these junctions, to discuss risk factors, and to provide diagnostic and surgical solutions. The primary focus will be tribocorrosion of the trunnion, although dissociation and fractures at modular neck interfaces also have been described2,3.
Tapers and Trunnions
A taper is a simple conical object with a gradual decrease in diameter from one end to another. The trunnion (the male portion) and the bore (the female portion) are both uniformly tapered. When impacted together, the trunnion of the taper exerts force on the bore. The geometry of the opposing surfaces as well as the static forces that are involved allow the head and stem to function as a unit. Tapers can differ in terms of the diameters at the smaller or larger ends and in terms of the axial distance between the 2 ends. A Morse taper is a standard taper, which is roughly 5/8 in/ft (5.2 cm/m) (a relatively narrow taper angle), or 2.98° per taper side. The tapers in total hip arthroplasty implants are commonly referred to as Morse tapers; however, no defined standard is used across all manufacturers. Each implant manufacturer uses tapers with their own specifications, which vary in terms of taper angle, diameter, straightness, roundness, and surface properties. Today, most femoral neck implant tapers are 12 mm/14 mm. However, it is important to note that femoral heads should not be used interchangeably between designs as the cone angle may differ.
The head-neck taper junction is subject to compressive, bending, and rotational loads that result in shear forces and micromotion at the trunnion that can result in fretting and corrosion3. Fretting is damage caused by the mechanical interaction and relative motion between the 2 sides of the taper, which both alters the surfaces and can lead to the development of corrosion products. Corrosion also can occur at a taper junction because metals are not inert. The basic reaction that occurs during corrosion is the loss of electrons from the metal atom to form free metal ions in solution, which can migrate away from the metal surface or lead to the formation of metal oxides, metal chlorides, or organometallic compounds. Oxide layers that develop in vivo or as a consequence of surface treatments protect implants from corrosion. Mechanical factors, such as applied stresses and fretting, can cause the oxide layer to abrade or fracture and expose the underlying metal, making it more susceptible to corrosion. Many modes of corrosion can occur, including crevice and galvanic corrosion; however, a full description of these modes is beyond the scope of this article4.
The topic of trunnion wear has become very popular quite recently. However, one of the first descriptions of an adverse local tissue reaction secondary to a trunnion problem was reported several decades ago. Svensson et al. described the case of a patient who had formation of a fulminant soft-tissue pseudotumor after a total hip arthroplasty without cement5. In that report, the most salient finding on implant retrieval analysis was extensive corrosion of the modular femoral neck taper junction. There have now been several reports describing adverse local tissue reactions in patients with modular femoral components6-9. Although adverse local tissue reactions initially were described to occur in association with metal-on-metal (MoM) articulations, they also have been shown to occur in association with other articulations10-13.
Trunnionosis following total hip arthroplasty has been demonstrated clinically as well as in retrieval studies7,13. However, the factors that potentiate trunnionosis are controversial. Both biomechanical and bioelectrochemical factors have been described14. The concept of mechanically assisted crevice corrosion has also been supported14. Trunnionosis may be enabled by the disruption of the protective oxidative layer on the metal by fretting, potentiating the corrosion of the exposed metal beneath the oxidative layer through an active combination of biochemical and electrochemical methods14. Time in vivo consistently has been shown to be a risk factor for trunnionosis4,15.
The biomechanical argument has been supported by studies examining how increased head length, diameter, and offset affect trunnionosis. Increased head length has been shown, in retrieval studies, to increase the severity of corrosion and fretting16. Head diameter also has been found to be a substantial risk factor for the development of trunnionosis17,18; however, other reports have indicated that head diameter does not contribute to the development of trunnionosis15. Other studies have indicated that trunnionosis may not be due to torque at the trunnion but rather may be due to a toggling effect7. Thus, the role of head diameter is unclear.
The design and flexural rigidity of the trunnion have been demonstrated to have an important role in the development of trunnionosis. A flexible trunnion may allow fretting as well as point loading at the head-neck junction19, and stems that have decreased mechanical rigidity have been associated with higher rates of trunnionosis4. The shorter mating surface of modern trunnions means the stem-trunnion mating surface may end within the head, leading to edge loading7. Edge loading is known to make tribocorrosion more likely to occur. Reports in the literature have indicated that specific trunnion designs have higher rates of fretting and corrosion20. The specific feature or combination of features of these trunnions that play a role in the development of trunnionosis is unclear. The length of head-stem engagement also may play a role in trunnionosis21. The geometry of the taper must not be considered in isolation, but the interface of the trunnion with the femoral head needs to be considered21. Specifically, the presence of any degree of angular mismatch may increase the effect of trunnionosis22. The trunnion finish also has been demonstrated to vary between femoral stems23. Grooves, also referred to as machining ridges, have been added to a number of femoral stems to facilitate the use of ceramic heads. Deformation of the ridges may allow eventual micromotion of the femoral head. It is also difficult to understand the nomenclature, as trunnions from different manufacturers may have very different actual measurements despite having the same listed descriptions. Although the trunnion design, rigidity, geometry, fit, and surface finish all may play a role in the clinical presentation of trunnionosis, how these factors interact to produce trunnionosis has not been fully elucidated.
More evidence exists for how the articular properties of the total hip replacement impact the development and severity of trunnionosis. In MoM articulations, metal debris is undoubtedly produced at bearing surfaces. However, such debris is also produced at the head-neck taper interface, as demonstrated in studies comparing MoM total hip arthroplasty with hip resurfacing6,8. In retrieval studies of MoM total hip arthroplasty designs, the taper has been found to contribute 32.9% of the total wear volume on average, and, in 28% of cases, the taper wear volume was greater than the bearing surface wear24. In clinical studies of metal-on-polyethylene (MoP) total hip arthroplasty designs, metal ion release and adverse local tissue reactions due to trunnionosis have been reported10,25-27. Retrieval studies also have demonstrated substantial fretting and corrosion at the head-neck taper interface of MoP hip arthroplasty designs28,29. The characteristics of the polyethylene also may be important as studies in the literature have demonstrated both increased and decreased surface torque with the addition of vitamin E to the polyethylene; these forces are transmitted to the trunnion and potentially increase the rate of trunnionosis30,31. The articular surfaces remain an important risk factor for trunnionosis.
As the inert ceramic or oxinium heads have similar mechanical factors to the large MoM total hip replacements, there may be unique biochemical factors that play a role in the development of trunnionosis in large MoM hips that require further investigation. In spite of the inert nature of ceramic heads, retrieval studies have indicated the presence of trunnionosis in association with such heads as well29. McPherson et al. reported the case of a patient in whom a pseudotumor formed at the site of a ceramic-on-polyethylene (CoP) total hip replacement32. Although galvanic corrosion may not have an important role when ceramic heads are used in modular total hip systems, fretting and tribocorrosion are still possible on the stem in isolation. The complex interaction between biomechanical and biochemical factors needs to be examined to further understand the risks of precipitation of trunnionosis.
The bioelectrochemical concept describes crevice corrosion as having an important role in the development of trunnionosis. The crevice is thought to allow a lower-pH condition to exist. This condition may enable the presence of hexavalent chromium, a much more destructive valence than trivalent chromium33. An electrochemical process also may be supported in that the literature has indicated that the alloys of the head and stem impact trunnionosis15. However, it is thought that mechanically assisted crevice corrosion, rather than galvanic corrosion, is the predominant factor in the tribocorrosion of the head-neck junction14. Although fatigue corrosion has been discussed in engineering literature, this concept has not been advanced in tribocorrosion literature34.
For modular-neck systems, the material and design of the modular neck may be important. Reports in the literature have indicated that titanium necks may have higher fretting corrosion than cobalt-chromium modular necks35,36. It is also notable that finite element analysis has indicated that the stresses on the femoral stem are higher than those on the modular neck37. In fact, modeling of this stem demonstrated that the elastic stress limits of the material were exceeded in the regions corresponding with the areas of corrosion in the retrieved implants.
Finally, although the technique of femoral head impaction on the trunnion affects the disassembly force at the head-neck junction, this factor has not been verified to be a clinical concern with regard to the development of trunnionosis in vivo38.
Systemic toxicity due to elevated ion levels following MoM total hip replacement has been reported, although very rarely39. Systemic toxicity due to elevated ion levels due to trunnionosis has not been reported, to our knowledge. However, the possible clinical effects of elevated ion levels, including neurotoxicity, hypothyroidism, and cardiotoxicity, should be considered39.
Trunnionosis can have a negative effect on the local bone and soft tissues, leading to damage and/or loss of both periprosthetic bone and soft tissue (i.e., capsule, muscle, tendons, etc.) surrounding the hip joint. Adverse local tissue reactions due to trunnionosis have been reported in association with MoP systems as well as modular-neck systems3,10,40,41. We are aware of one case report describing a pseudotumor that formed in a patient with a CoP total hip replacement with secondary trunnionosis32. Local manifestations of trunnionosis can be subtle. The presentation of adverse local tissue reactions that occur secondary to corrosion at modular femoral head-neck or neck-stem tapers, regardless of the bearing, is similar to that of adverse local tissue reactions that are seen in patients with MoM resurfacing implants (where there is no taper junction). Trunnionosis and a secondary adverse local tissue reaction may result in unexplained pain, abductor weakness, peripheral neuropathies, osteolysis, implant loosening, recurrent dislocations, and gait abnormalities42.
The surgeon must first establish the diagnosis of trunnionosis and the potential associated adverse local tissue reaction by means of a careful clinical history, physical examination, radiographs (often in combination with special imaging), and metal ion analysis. Care must be taken to rule out more common problems that can occur after hip replacement surgery, such as infection and aseptic loosening43. In the current clinical environment, with the enormous interest in the potential for trunnion corrosion and fretting, there is a substantial risk of overdiagnosing this relatively rare entity and thereby neglecting more common clinical problems. Elevated serum metal levels, particularly a differential elevation of serum cobalt with respect to chromium levels, can be helpful for establishing this diagnosis. Although there are guidelines for clinically important serum ion levels in patients with MoM total hip replacements, guidelines for suspected trunnionosis do not exist44. However, chromium levels may be relatively lower than cobalt levels in cases of trunnionosis compared with cases of failed MoM total hip replacements45. Multiple studies have recently evaluated possible diagnostic criteria as well as intraoperative findings associated with pain at the site of a total hip replacement secondary to corrosion of the taper junction10,42. Imaging of soft-tissue masses or fluids with ultrasound or MARS-MRI (metal artifact reduction sequence magnetic resonance imaging) can often identify large encapsulated soft-tissue masses, so-called pseudotumors. Sometimes, the diagnosis is one of exclusion.
Treatment of Trunnionosis
Problems related to trunnionosis can occur either at the head-neck taper junction or at the modular neck-stem interface, or they can coexist at both locations in patients with modular-neck total hip replacement designs. Clinical problems related to the head-neck taper of recent MoM total hip replacements have been well documented by several authors6,8. More recently, trunnionosis and associated adverse local tissue reactions have been associated with MoP total hip replacements10,25-27 (Fig. 1). Although a case of trunnionosis in a patient with a CoP total hip replacement has been reported32, a case series or patient cohort has not been reported, to our knowledge. Undoubtedly, clinical problems related to taper corrosion have occurred most commonly in association with certain modular-neck designs37,46,47 (Figs. 2-A and 2-B).
An algorithm outlining the indications for the surgical treatment of trunnionosis has not been clearly elucidated. Systemic toxicity with organ damage due to elevated metal ion levels would be an absolute indication, but this finding has not yet been reported secondary to trunnionosis, to our knowledge. A pseudotumor that has developed secondary to trunnionosis may be surgically treated or observed with serial imaging, depending on the size of the lesion and the clinical scenario (i.e., the presence or absence of symptoms). However, the presence of a pseudotumor in conjunction with abductor weakness, osteolysis, neurological findings, or substantial pain more strongly indicates the need for surgical treatment. As with the investigation of MoM total hip replacements, the role of metal ions, taken in isolation, is not clear in determining the need for surgery.
Femoral Head-Neck Trunnionosis
It is important to note that most of the treatment algorithms to deal with head-neck trunnionosis (and the associated adverse local tissue reactions) are derived from relatively small case series. In a review article, Engh et al. highlighted the fact that the current treatment protocols for taper corrosion originated from 8 case reports and 1 Level-IV study48. Therefore, there is no high-quality (Level-I or II) evidence to support the proposed algorithms for the treatment of trunnionosis.
Most of the regimens designed for the treatment of adverse local tissue reactions associated with head-neck trunnionosis have originated from the literature on MoM total hip replacement6,49,50. Soft-tissue reactions to metallic debris have been far more common after MoM total hip replacement, and several authors have stressed the importance of a thorough debridement of the pseudotumor material, often requiring a subtotal capsulectomy6,49,50. Therefore, the surgeon should attempt to remove as much of the adverse local tissue reaction material as possible while being cautious to protect the neurovascular structures around the hip capsule. Furthermore, performing a radical debridement with or without capsulectomy can lead to instability following revision surgery, particularly when using a posterior approach49. In summary, a thorough debridement of the abnormal tissue and, when required, a subtotal capsulectomy is often the first stage of treatment of adverse local tissue reactions resulting from head-neck trunnionosis.
The principles for dealing with potential bone loss and/or osteolysis associated with trunnionosis are similar to those for dealing with any osteolytic process resulting from wear debris of the bearing surface of either an MoP or MoM articulation. While the use of bone graft or bone-graft substitutes has been suggested as a way to deal with osseous defects51, there is no clear evidence to support the long-term benefits of filling these cavitary defects. Ultimately, the most important step to provide cessation of the ongoing periprosthetic bone loss is to remove the wear debris generator, in this case the ongoing trunnionosis.
The first dilemma facing the surgeon is whether to revise the well-fixed stem in order to remove the potentially damaged male taper junction. In the vast majority of cases, retention of the stem and the male taper has become the standard of care, as supported in a number of review articles10,40,41,48,52-54. In particular, the retrospective review by Goyal et al. demonstrated that there was no difference in survivorship between total hip replacements that were revised to a new metal femoral head in the presence of either high-grade or low-grade head taper corrosion, thus providing strong support for leaving the corroded stem taper in place53. In rare instances, removal of a well-fixed stem might be advisable when there is severe taper damage such that a mechanically stable taper linkage cannot be obtained. When the surgeon is concerned about damage of the in situ taper, he or she must weigh the potential benefits of implanting a new taper against the morbidity associated with the removal of a well-fixed stem (i.e., increased length of surgery, the requirement for an extended trochanteric osteotomy, fracture, bone loss, etc.). In the vast majority of cases, the stem is left in situ, the corrosion byproducts are debrided (i.e., wiped off) with use of a moistened or dry gauze or laparotomy sponge, and the existing taper is reused.
Therefore, implantation of a new femoral head, often in combination with a new acetabular bearing surface, is usually performed. Currently, the most common bearing replacement combination is a ceramic head articulating with a highly cross-linked polyethylene insert. When dealing with a failed MoM total hip replacement in which debris may be originating both at the MoM articulation and at the head-neck taper, conversion to an MoP or CoP articulation is required to reduce the volume of metallic wear debris to which the patient is exposed. Most authors have recommended that a ceramic head be used in the setting of trunnionosis, suggesting that the potential for ongoing corrosion and fretting is decreased because of the chemically inert properties of the ceramic material3,10,48,52. The recent retrieval study by Kurtz et al. demonstrated that taper corrosion was mitigated, although not completely eliminated, with the use of ceramic heads as compared with cobalt-chromium heads29. Furthermore, replacement ceramic heads are often combined with a titanium adaptor sleeve in order to decrease the potential for fracture of the more brittle ceramic head articulating with a potentially damaged neck taper. The current practice of using a titanium adaptor sleeve in combination with a ceramic head has been documented by a number of authors and review articles3,10,48,52. However, Carli et al., in a systematic review, pointed out that there is little evidence to support the necessity of a titanium adaptor sleeve54. Conceptually, using a titanium sleeve does introduce another taper, and there may be a difference between a titanium sleeve on a cobalt-chromium taper versus titanium on titanium with respect to corrosion. This additional junction may increase the risk of crevice corrosion and also the possibility of cold-welding of the taper. However, these potential issues have not been examined in other literature sources, to our knowledge.
In addition, oxidized zirconium (so-called ceramicized metal), which has demonstrated reduced taper corrosion compared with cobalt-chromium heads in retrieval studies, may represent a suitable replacement head material for the treatment of trunnionosis55. While this material has a reduced corrosion potential similar to that of ceramic, the fact that it is a metal substrate represents an advantage as it is not susceptible to fracture. However, this material is currently only available from one manufacturer (Smith & Nephew) and therefore can only be used with tapers from the same manufacturer. Along these lines, the review article by Hussenbocus et al. pointed out the importance of avoiding taper mismatch within or between manufacturers40.
In summary, on the basis of current recommendations, the most appropriate treatment for trunnionosis is retention of the male taper and the stem combined with replacement of the femoral head (most often with use of a ceramic head) and acetabular articulation (most often with use of highly cross-linked polyethylene). Removal of a well-fixed stem is rarely required, and the benefits and risks of revising a well-fixed stem to treat trunnionosis need to be carefully considered. While this treatment is suggested by the current literature, there remains a lack of any long-term or high-quality evidence to support the current practice.
In comparison, because of the relatively common occurrence of adverse local tissue reaction (Fig. 3) resulting from certain modular neck-body junction designs, the guidelines for the treatment of modular neck trunnionosis are clearer. A number of authors, reporting on several implant designs, have provided excellent algorithms for the treatment of modular-neck trunnionosis37,46,47,56. Almost invariably, revision of the stem is required as there is little or no role for the reimplantation of another modular neck, particularly in the case of implants that have been recalled because of poor clinical performance. Therefore, following a thorough debridement of the associated adverse local tissue reaction material and possible subtotal capsulectomy, treatment requires removal of the modular stem and replacement with an appropriate nonmodular neck revision stem. The revision of a well-fixed stem can be challenging and may require an extended trochanteric osteotomy. Thereafter, a new femoral head is implanted, often in combination with replacement of the bearing articulation. Commonly, a ceramic-on-highly cross-linked polyethylene articulation is selected, but selecting cobalt-chromium-on-highly cross-linked polyethylene is also reasonable. Numerous series have outlined the not-infrequent complications associated with the treatment of modular neck trunnionosis, the most common of which are dislocation and infection37,46,56. Implants that confer greater stability, such as dual mobility articulations, may be advisable if stability is a concern57.
Surveillance for Trunnionosis
As a general principle, surveillance is most important when following patients with implants in situ that are associated with an increased risk of trunnionosis. In particular, virtually all MoM total hip replacements (principally those with a large head [>36 mm], especially when a head adaptor sleeve has been used) and implants with a modular neck design should be considered high-risk. In the setting of an MoP articulation with a modular head-neck taper, which has been by far the most common form of total hip replacement implanted worldwide over the past 2 to 3 decades, rigorous surveillance for trunnionosis is only required for implants that have been associated with taper issues and adverse local tissue reactions. To date, trunnionosis and the associated adverse local tissue reaction issues most commonly have involved the combination of a cobalt-chromium head on a cobalt-chromium taper from a specific manufacturer10,13.
Algorithms for the adequate follow-up and investigation of adverse local tissue reactions related to MoM articulations, including the role of both metal ion analysis and advanced imaging, have been proposed by several authors58-60. In addition, the article by Kwon et al., which presents a consensus statement by the American Association of Hip and Knee Surgeons, the American Academy of Orthopaedic Surgeons, and The Hip Society, provides an excellent risk-stratification algorithm for the management of patients who have modular-neck total hip replacements56. However, it remains unclear as to what surveillance is required for head-neck taper junctions in the setting of an MoP total hip replacement. Although very topical in academic forums, trunnionosis arising from MoP total hip replacements represents a very small clinical problem. Therefore, routine follow-up, without the need for advanced imaging and/or metal ion analysis, is generally all that is required for an asymptomatic patient with a well-functioning hip and no radiographic abnormalities. When a patient has hip pain of unknown origin (i.e., when other investigations, especially for infection or aseptic loosening, are negative), it might be appropriate to investigate with advanced cross-sectional imaging (i.e., MARS MRI) and/or metal ion analysis, as suggested by numerous authors10,41,52,56,58,59.
The role of design and manufacturing variables in the development of trunnion problems continues to be debated. Over the past decade, several changes in technology, including the use of larger-diameter (≥36 mm) heads and reduced femoral neck and taper dimensions, have potentially contributed to a perceived increase in clinically important taper fretting and corrosion. Surgeon-related factors, especially the greater variability in taper assembly with smaller-incision surgery, and patient-related factors, such as higher taper stresses due to increasing patient weight and/or physical activity, also may have contributed to this phenomenon. In patients presenting with unexplained pain or late recurrent instability who have an MoP or CoP total hip replacement with head-neck modularity, regardless of head size, one must consider an adverse local tissue reaction from corrosion of the head-neck junction as a cause of the pain, even though this phenomenon is relatively rare. Patients presenting with unexplained pain who have modular neck-body implants also should be considered to have an adverse local tissue reaction resulting from corrosion of the neck-stem interface as a potential cause of the pain. Elevation of serum cobalt out of proportion to chromium and the presence of fluid collections and/or periarticular masses on screening ultrasound or MARS MRI can be helpful findings to establish this diagnosis. In most cases of suspected trunnionosis at the head-neck junction, removal of the femoral head, cleaning of the taper, and then replacement with a different femoral head (usually a ceramic head with a titanium adaptor sleeve), represents adequate treatment based on current recommendations. In contrast, in cases involving adverse local tissue reactions associated with modular neck designs, removal of the modular stem and neck is required followed by replacement with a nonmodular neck revision stem.
Investigation performed at the London Health Sciences Center, University Hospital, London, Ontario, Canada
Disclosure: No external funds were received in support of this study. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work and “yes” to indicate that the author had other relationships or activities that could be perceived to influence, or have the potential to influence, what was written in this work.
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