Total hip arthroplasty is an effective treatment option for advanced hip arthritis in elderly patients. Studies in young patients have traditionally shown less durable results. With current implants, surgical technique, and cementless fixation methods, the durability of total hip arthroplasty may now be related to the wear performance of the bearing surfaces. To improve implant longevity, there are several bearing surface choices currently available for this demanding group of patients. Alternatives must be evaluated in terms of the risks and benefits associated with each articulation, and all new technologies must be carefully monitored over the long term.
Key points
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With modern implants, surgical technique, and cementless fixation methods, the durability of total hip arthroplasty may now be related to the wear properties of the bearing surfaces.
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Alternative bearing surfaces may offer a solution to the wear-associated problems that have limited traditional total hip articulations.
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Bearing alternatives to traditional metal-on-polyethylene couples include metal/ceramic on highly cross-linked polyethylene, ceramic on ceramic, metal on metal, and ceramic on metal.
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Articulation choice must be evaluated in terms of the advantages and disadvantages (risks) associated with each bearing couple.
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Metal-on-metal bearings must be used with caution given the evolving understanding of local adverse responses to metal particulate debris.
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Further study in large cohorts of patients with greater follow-up will clarify the usefulness and durability of alternative bearings in total hip arthroplasty in young patients.
Introduction
Several causes may lead to advanced hip arthritis in the young patient. These causes include avascular necrosis, juvenile idiopathic arthritis, prior infection, slipped capital femoral epiphysis, fracture, Legg-Calves-Perthes disease, developmental dysplasia, and femoroacetabular impingement. The common pathway of severe destructive changes within the hip joint may cause disabling symptoms in the young. For advanced hip arthritis, and when medical management and nonoperative strategies fail, total hip arthroplasty (THA) may be performed in this patient population for pain relief and to improve function.
The demands placed on the hip in the young patient, including activity level and repetitive loading, have led to accelerated failure of THA. Increased loading cycles and higher activity over an increased lifespan, rather than time from prosthetic implantation alone, leads to early implant failure in highly active individuals. The historically high failure rates and poor longevity may dissuade surgeons from offering THA as a treatment option to this young population. In light of these results in young patients, THA has traditionally been reserved for selected patients in whom alternative management strategies have failed.
Improvements in prosthetic design and cementless fixation methods have addressed many of the previous limitations in performing joint replacements in young patients. Along with advances in operative techniques, instrumentation, and patient selection, THA is now a more favorable reconstructive option in active and young patients. Early reports of primary uncemented THA showed adequate survivorship using first-generation implants, and survival rates as high as 99% after 10 years have been reported in patients less than 50 years old using second-generation designs. In the ultrayoung patient, THA has proved successful in series of early-term and midterm follow-up, including those specifically studying the use of alternative bearings in cementless THA.
Pooled femoral mechanical rates in a meta-analysis of cementless THA (mean age, 41.4 years) and hip resurfacing arthroplasty (mean age, 46.6 years) were 1.3% at a mean follow-up of 8.4 years after THA and a 2.6% rate of failure at a mean of 3.9 years after resurfacing. Modern-generation cementless femoral fixation has proved dependable across multiple designs. On the acetabular side, cementless cup fixation has shown similar results, including a recent study of 9584 primary THAs implanted from 1984 to 2004 : no cases of aseptic loosening were seen with more recent three-dimensional highly porous metal surfaces.
With advances in fixation on the acetabular and femoral sides, the limiting factor in durability of joint replacements over the past 2 decades has been bearing surface wear. With the success of THA and its applications to the young, it is estimated that, by 2030, patients less than 65 years old will comprise 52% of primary THAs. The demand in patients aged 45 to 54 years for primary THA in 2030 is projected to be nearly 6 times higher than in 2006. Alternative bearing surfaces may offer a solution to the wear-associated problems that have limited traditional total hip articulations. These new surfaces provide attractive tribologic solutions for the young patient with hip pain and advanced arthritis.
Introduction
Several causes may lead to advanced hip arthritis in the young patient. These causes include avascular necrosis, juvenile idiopathic arthritis, prior infection, slipped capital femoral epiphysis, fracture, Legg-Calves-Perthes disease, developmental dysplasia, and femoroacetabular impingement. The common pathway of severe destructive changes within the hip joint may cause disabling symptoms in the young. For advanced hip arthritis, and when medical management and nonoperative strategies fail, total hip arthroplasty (THA) may be performed in this patient population for pain relief and to improve function.
The demands placed on the hip in the young patient, including activity level and repetitive loading, have led to accelerated failure of THA. Increased loading cycles and higher activity over an increased lifespan, rather than time from prosthetic implantation alone, leads to early implant failure in highly active individuals. The historically high failure rates and poor longevity may dissuade surgeons from offering THA as a treatment option to this young population. In light of these results in young patients, THA has traditionally been reserved for selected patients in whom alternative management strategies have failed.
Improvements in prosthetic design and cementless fixation methods have addressed many of the previous limitations in performing joint replacements in young patients. Along with advances in operative techniques, instrumentation, and patient selection, THA is now a more favorable reconstructive option in active and young patients. Early reports of primary uncemented THA showed adequate survivorship using first-generation implants, and survival rates as high as 99% after 10 years have been reported in patients less than 50 years old using second-generation designs. In the ultrayoung patient, THA has proved successful in series of early-term and midterm follow-up, including those specifically studying the use of alternative bearings in cementless THA.
Pooled femoral mechanical rates in a meta-analysis of cementless THA (mean age, 41.4 years) and hip resurfacing arthroplasty (mean age, 46.6 years) were 1.3% at a mean follow-up of 8.4 years after THA and a 2.6% rate of failure at a mean of 3.9 years after resurfacing. Modern-generation cementless femoral fixation has proved dependable across multiple designs. On the acetabular side, cementless cup fixation has shown similar results, including a recent study of 9584 primary THAs implanted from 1984 to 2004 : no cases of aseptic loosening were seen with more recent three-dimensional highly porous metal surfaces.
With advances in fixation on the acetabular and femoral sides, the limiting factor in durability of joint replacements over the past 2 decades has been bearing surface wear. With the success of THA and its applications to the young, it is estimated that, by 2030, patients less than 65 years old will comprise 52% of primary THAs. The demand in patients aged 45 to 54 years for primary THA in 2030 is projected to be nearly 6 times higher than in 2006. Alternative bearing surfaces may offer a solution to the wear-associated problems that have limited traditional total hip articulations. These new surfaces provide attractive tribologic solutions for the young patient with hip pain and advanced arthritis.
Alternative bearing options
There is ongoing debate regarding the optimal bearing surface in young patients. Poor THA survivorship in young patients has been attributed to cemented fixation strategies, screw-in acetabular designs, and the wear properties of conventional polyethylene (PE). The osteolysis seen with conventional ultrahigh-molecular-weight PE wear in the active young patient has prompted the resurgence of alternative bearing surfaces. From the National Inpatient Sample database, 39% of primary THA in 2005 to 2006 had a unique modifier for the bearing surface.
Bearing alternatives to traditional metal-on-conventional-PE couples include metal or ceramic heads on highly cross-linked PE, ceramic on ceramic (CoC), metal on metal (MoM), and ceramic on metal (CoM). Each of these bearing couples provides improved wear properties compared with metal on conventional PE ( Table 1 ). The Global Orthopedic Registry (GLORY), incorporating 100 hospitals in 13 countries, provided a breakdown on the choice of bearing surfaces: conventional PE, 36%; highly cross-linked PE, 55%; MoM, 5%; and CoC, 4%. In the United States, 63% of primary THA received cross-linked PE, and 28% received conventional PE.
| Bearing Articulation Materials | Wear Rate (μm/y) |
|---|---|
| Metal on conventional polyethylene | 120–200 |
| Ceramic on conventional polyethylene | 80–100 |
| Metal on highly cross-lined polyethylene | 0–25 |
| MoM | 1–5 |
| CoM | ∼1 |
| CoC | <1 |
Highly Cross-linked Polyethylene
Mechanical loosening accounted for 19.7% of all revision THA in a 2005 to 2006 national sample. Introduced in the late 1990s, the several processes of cross-linking PE seem to offer a promising answer to the issue of wear. Articulations involving PE provide a less radical departure from traditional hard-on-soft bearing options. Furthermore, wear rates may be less sensitive to femoral head size or small changes in component positioning with highly cross-linked PE.
In contrast with previous attempts to improve PE that have resulted in poor clinical results, the lower wear rates with highly cross-linked PE in vivo at 10 years should theoretically lead to less osteolysis in the long term. With these improvements, highly cross-linked PE has emerged as a viable choice for most patients having THA and may be used with either metal or ceramic head combinations. When cross-linked PE liners are used, surgeons have a wide array of intraoperative choices with regard to head size, orientation of elevation, and offset (eg, standard or lateral offset). As cost constraints emerge, it should be recognized that PE bearings are currently less expensive than comparable ceramic or metal bearings and will almost certainly remain so.
However, cross-linking leads to changes in other material properties besides wear, such as lower yield strength and ultimate tensile strength. There is also lower impact strength, elongation to failure, and resistance to crack propagation. Most of the property alterations are small to moderate, but must be carefully evaluated in light of their use in high-demand, young patients. Next-generation techniques to improve cross-linked PE have focused on improving wear resistance with less effect on the mechanical properties associated with postirradiation melting. Production of highly cross-linked PE uses methods to extinguish free radicals, such as vitamin E doping or sequential application of radiation, with annealing after each dose.
CoC
Ceramic-on-ceramic articulations were developed in the 1970s in Europe. In the United States, Food and Drug Administration approval was granted in 2003. Ceramic-ceramic bearing surfaces provide several benefits with regard to wear reduction in vitro and in vivo, making it a reasonable bearing surface in a young patient population. The hardness and resistance to third-body wear allows a low surface roughness over time. This low surface roughness, along with small grain size, creates a low coefficient of friction. Moreover, ceramics have excellent wettability, which leads to a reduction in adhesive wear and better lubrication properties.
The wear rate of ceramic bearing surfaces can be 4000 times less than the traditional metal-on-PE bearing surfaces. The wear debris from ceramic bearing surfaces is also well tolerated, and alumina particles are bioinert compared with metal and PE debris. These decreased wear rates have correlated with lower amounts of osteolysis and better preservation of bone stock, which is an important consideration when performing THA in young patients with long life expectancy and the potential need for future revision surgery. The evolution of ceramic bearings continues with the use of Biolox Delta (CeramTec AG), an alumina matrix composite. Encouraging early results have been reported.
MoM
MoM bearings have been in use for more than 6 decades and their use was first published more than 30 years ago. Problems with early MoM designs included poor manufacturing tolerances and material selection, early impingement, and inadequate clearance. Next-generation MoM implants benefited from improved manufacturing methods and tolerances, which resulted in low wear rates in vitro. A large European clinical experience, along with the ability to use large head sizes to reduce dislocation risk, made MoM implants an attractive option for the young, high-activity patient. In 2010, the National Joint Registry for England and Wales reported that 14% of patients less than 55 years old having hip arthroplasty underwent resurfacing procedures alone. Advantages compared with other hard bearings include no head/liner fracture risk as with ceramics, and that scratches to the metal surfaces could self-heal. Recent concerns include adverse tissue reactions to metal debris generated by some surface replacement and MoM THA articulations, and corrosion at the head-neck junction of some MoM THA systems with large heads.
CoM
Ceramic-metal articulations were designed as another hard-on-hard bearing alternative. Several in vitro, as well as clinical, studies have sought to explore the clinical usefulness and wear characteristics of this bearing couple. MacDonald and colleagues reported on similar metal ion levels between CoM and MoM articulations in vivo.
Clinical data
Highly Cross-linked Polyethylene
Highly cross-linked PE has shown a substantial reduction in wear compared with conventional PE. Reduction in wear of 85% to 90% has been seen with 28-mm and 32-mm femoral heads, respectively. In a randomized study, Thomas and colleagues reported that no patient with a highly cross-linked PE liner had a wear rate greater than the osteolysis threshold (0.1 mm/y). In the conventional PE study arm, 9% of patients went beyond this threshold.
D’Antonio and colleagues found a 58% reduction in wear with second-generation, sequentially annealed, highly cross-linked PE compared with first-generation, annealed, highly cross-linked PE. No osteolysis was reported, and cup inclination did not affect linear wear. Although volumetric wear may be slightly higher with large femoral heads on cross-linked PE, there does not seem to be a difference in linear wear rates. Compared with conventional PE, cross-linked PE has shown significantly lower wear rates against smooth and scratched surfaces.
Ceramic on highly cross-linked polyethylene
Rajaee and colleagues reported that the use of metal on PE increased from 21.6% patients in 2006 to 25.2% in 2009, whereas ceramic on PE (CoP) increased from 13.5% in 2007 to 25.7% in 2009. This trend may be explained by surgeons’ attempts to minimize complications associated with hard-on-hard bearings. The increase in CoP bearings is likely related to a steady-state wear rate 40% lower than that of metal on PE.
Studies of ceramic on conventional PE are mixed. Migaud and colleagues reported a 46% rate of osteolysis. In contrast with this, Urban and colleagues found no osteolysis about the femoral or the acetabular components after 18 years of follow-up, with mean linear wear rates between 0.03 and 0.1 mm/y. Data regarding ceramic heads against highly cross-linked PE are limited by short-term follow-up. Garvin and colleagues, in a 2-year follow-up study of CoP (oxidized zirconium femoral head with a highly cross-linked PE acetabular liner), reported that true linear wear was 4 μm per year, which corresponds with one of the lowest reports of in vivo wear in the literature.
CoC
Ceramics have had a long and successful history in Europe, with evidence of low clinical wear and osteolysis. In a 2011 systematic review, Zywiel and colleagues found that the 4 level I studies of CoC THA reported survival from 96% at 8 years up to 100% at a mean of 51 months. In a prospective multicenter study, 1709 alumina ceramic hips were reviewed and no cases of osteolysis were reported. Other comparative studies report 1.7% of osteolysis in the CoC group versus 19.4% of the metal-on-PE group. After 2 to 9 years of follow-up in 194 alumina THAs (average age, 49 years), Murphy and colleagues reported that no patients had osteolysis, and survivorship at 9 years was 96%. For those hips without prior surgery, the survivorship was 99.3%. Lee and colleagues reported 99% survivorship at 10 years in a group of patients with a mean age of 41 years. D’Antonio and colleagues compared average 5-year follow-up data for CoC and metal-on-PE (328 total hips): the revision rates were 2.7% (CoC) and 7.5% (metal-on-PE) with no ceramic fractures. The rate of osteolysis was 1.4% for CoC and 14% for metal-on-PE. D’Antonio and colleagues later presented that the 10-year survivorship of CoC implants was significantly higher than that of the metal-on-PE implants (95.9% and 91.3%, respectively; P = .0122) and the 8-year rate of survivorship of the Trident implants (another ceramic group study) was 97.7%. Similar results have been seen in other reports.
Survivorship analysis in a series from the Mayo Clinic showed promising short-term to midterm results (average follow-up 50 months) using CoC bearing surfaces in young patients (mean age, 16.4 years), with a 96% survival rate in 24 uncemented THAs. Nikolaou and colleagues presented the 5-year results of a randomized controlled trial comparing the clinical and radiological outcomes of 102 THAs in patients less than 65 years of age. The three arms were CoC, metal on conventional PE, and metal on highly cross-linked PE. Two hips had been revised, one for infection and one for periprosthetic fracture. At the final follow-up there were no significant differences between the groups for the mean Western Ontario and McMaster Universities osteoarthritis index, Short Form-12, or Harris hip scores. Radiological outcomes revealed no significant wear in the ceramic group. However, comparison of standard and highly cross-linked PE revealed an almost 3-fold difference in the mean annual linear wear rates (0.151 mm/y vs 0.059 mm/y, respectively; P <.001).
MoM
There are positive long-term studies on the clinical outcomes of MoM hip arthroplasty in young active patients, and 84.4% survival at 20-year follow-up. In a 2011 systematic review, Zywiel and colleagues reported on the 4 level I or II second-generation stemmed MoM THA studies. These studies showed a 96% to 100% mean survival at 38 to 60 months’ follow-up. The 2 level I hip resurfacing studies reported 94% and 98% mean survival at 56 and 33 months.
However, several meta-analysis and systematic reviews show no significant difference between MoM and conventional THA with regard to functional outcomes, as measured by Harris hip scores, and radiographic outcomes. Patients with MoM THA showed up to a nearly 4 times greater complication rate in those series. Milosev and colleagues presented survivorships of 69 MoM THAs (mean age, 60 years), 200 metal-on-PE THAs (mean age, 71 years), and 218 CoC THAs (mean age, 60 years). Survival at 10 years with regard to revision for any reason was 0.984, 0.956, and 0.879 for the metal-on-PE, CoC, and MoM groups, respectively. The survival for MoM articulations was significantly worse compared with that for metal on PE. With revision for aseptic loosening as an end point, survival at 10 years was 0.995, 0.990, and 0.894 for the metal-on-PE, CoC, and MoM groups, respectively. Again, with this end point, MoM survival was significantly worse than that for either the CoC or metal-on-PE groups.
CoM
Isaac and colleagues found significantly lower chromium levels in CoM hips compared with MoM hips. Cobalt levels were lower, but the difference was not statistically significant. Explanted CoM components were tested in a wear simulator. The investigators reported that the CoM components showed higher bedding-in but then similar wear rates (although an order of magnitude less) than those of other studies testing MoM bearings. The clinical outliers were related to component malposition. In a double-blinded randomized controlled trial, Schouten and colleagues compared CoM and MoM couplings. They found an equivalent increase in serum metal ion levels. The functional outcome scores up to 1 year after surgery were similar.
Ceramicized Metal on Polyethylene
Oxinium has been introduced as a means of having the low wear characteristics of a ceramic head, but without the risk of fracture. The clinical data that are currently available do not suggest decreased cross-linked PE wear with the use of an Oxinium head compared with a cobalt-chrome head.
Bourne and colleagues found that Oxinium femoral heads reduced wear even in clinically relevant roughened conditions. However, there is also conflicting evidence that surface damage of ceramicized femoral heads could accelerate wear.
Registry Data
In the 2012 National Joint Replacement Registry (NJRR) Report, the Australian Orthopaedic Association presented 6 bearing surfaces with 11-year cumulative revision data: metal and ceramic on both nonmodified (conventional) and modified (cross-linked or vitamin E impregnated) PE, MoM, and CoC. The report also included ceramicized metal on modified PE, with a follow-up of 8 years. The MoM articulation had the highest cumulative revision rate: at 11 years, revision for loosening/osteolysis (4.2%), metal sensitivity (3.8%), and infection (1.8%) were all greater than those rates for metal on PE (2.2%, 0%, and 0.7%, respectively). MoM articulations with head sizes greater than or equal to 36 mm had higher revision rates than those with head sizes less than or equal to 32 mm. Patients less than 65 years of age with a head size greater than 32 mm had a higher rate of revision than patients 65 years or older with smaller head sizes. Modified PE had a lower rate of revision (5.1%) than nonmodified PE (9.1%), with a difference beginning at 6 months; no difference in revision rate with regard to head size was seen within the modified PE subgroup. For the first time, the registry was able to detect this lower rate of revision for all 3 combinations (ceramic, metal, and ceramicized metal on modified PE) versus nonmodified PE groups. The group with ceramicized metal on modified PE had the lowest revision rate; however, this bearing group had limited patient numbers. Also recorded in the registry were 300 ceramic head/metal bearing couples and 391 metal head/ceramic bearings. The metal head/ceramic bearing articulation had a higher rate of revision (at 5 years, 7.1% cumulate revision) compared with most other bearings, whereas the ceramic head/metal bearing revision rate (4.0% cumulative revision at 3 years) did not seem to be markedly different from other bearing couples.
The Australian Orthopaedic Association NJRR first identified an increased revision rate with regard to MoM prostheses in 2008, when the prostheses were used in conventional hip replacement. A relationship to larger femoral head sizes was reported in 2009 and, more comprehensively, in 2010. In the aggregate, larger MoM femoral heads were associated with more than 2 times the rate of revision as that associated with smaller MoM femoral heads (a revision rate of 4.5% for head sizes of ≤32 mm vs a revision rate of 9.4% for sizes of >32 mm). Although the revision rate attributed to dislocation was lower for larger femoral head sizes, the revision rate attributed to loosening, infection, and metal sensitivity was higher for larger sizes.
The 2012 Annual Report of the National Joint Registry for England and Wales highlighted the further decline of the use of stemmed MoM implants. From a peak of 9000 procedures in 2008 (an increase in use attributable to larger diameter heads, of 36 mm and larger), a progressive yearly decline was seen up until the latest data in the 2012 report. Revisions for aseptic loosening and pain (the most common reasons for revision of stemmed MoM prostheses in both men and women) were significantly higher in patients with MoM. A woman aged 60 years had a 5-year revision rate for an uncemented 46-mm MoM prosthesis of 6.1% compared with a revision rate of 1.6% for a hybrid 28-mm metal-on-PE articulation. Hip resurfacing resulted in similar implant survivorship to other surgical options only in men with large femoral heads; inferior implant survivorship was seen in other patient subgroups, particularly women.
The themes of higher failure rates with large-head MoM prostheses, as well as higher failure rates in women with large-head MoM prostheses, had been shown previously in both the National Joint Registry for England and Wales and the New Zealand Joint Registry : at 5 years, the rate of revision of large femoral head MoM prostheses was at least twice that of all other cemented, hybrid, and conventional THAs. Women less than 55 years of age with large femoral head MoM prostheses had a higher rate of revision than older women with large femoral head MoM articulations.
Complications and failure modes
Highly Cross-linked Polyethylene
Although highly cross-linked PE has shown encouraging results beyond a decade of clinical use, further long-term data are needed to assess bearing longevity and degree of clinically evident osteolysis. Furthermore, in vitro studies are needed to better understand PE wear mechanisms under more aggressive/realistic testing scenarios, such as scratched femoral heads and third-body abrasives.
The specific failure modes and material strength of PEs are manufacturer dependent, being caused by the degree of cross-linking, amount of free radical formation, method by which free radicals are quenched, and any treatments after cross-linking. The material’s properties after cross-linking carry a risk of liner fracture, and there is a risk of locking mechanism failure, especially with cup malposition. The minimum thickness for cross-linked liners is not known. Although there have been reports of catastrophic liner failure, it is unclear what contributions implant alignment and socket design may have had compared with the decreased mechanical properties of the material alone.
Adverse local tissue reactions can occur in patients with a metal-on-PE bearing secondary to corrosion at the modular femoral head-neck taper. The clinical presentation is similar to the adverse local tissue reactions seen in patients with MoM bearings.
CoC
Because of the brittleness of ceramics, there is a small risk of fracture. The risk of fracture with ceramic components has been reduced with modern technology and manufacturing quality, using smaller grain size, increased burst strength, and proof testing of each implant. From an estimated fracture risk in 1974 of 1 in 300, the estimated fracture risk in 2000 was less than 1 in 2000 to 10,000. Fracture rates range from 0.005% to 0.02% for alumina bearing surfaces. In a series of 500,000 femoral heads reported in 2000, Willmann reported an estimated risk of 0.012%.
The cause of squeaking is unclear and likely multifactorial. The incidence of squeaking has generally been reported as between 2.7% and 7%, but higher incidences have been reported (up to 21% ). Squeaking may be related to ceramic-ceramic lubrication. Noise occurs when the fluid film between the two surfaces is disrupted, likely because of metal transfer, as shown by Trousdale and colleagues. Edge loading can cause fluid film disruption, stripe wear, and an area of localized friction. The energy created through frictional forces may be transformed to a squeaking noise via vibration of the implant. Squeaking may be more commonly reported with certain designs, such as a TMZF femoral component with a V-40 neck diameter. Wear debris generation may cause osteolysis, even in third-generation ceramic couples.
Like other hard-hard bearings, ceramic components are sensitive to cup position. There is a risk of runaway wear with malposition and edge loading, and risks of impingement demand precise implantation of the prosthesis. Ceramics are also limited by reduced intraoperative flexibility related to cup size, neck lengths, and liner options. The failure modes associated with modular ceramic-metal junctions need to be better understood.
MoM
Several problems with MoM technology have manifested, and large registry cohorts, including the Australian NJRR, have identified higher failure rates with MoM articulations. Very high failure rates for certain prostheses, including the articular surface replacement, have led to product withdrawal.
Toxicity from metal ions has raised concerns among surgeons, the public, and governmental agencies. The constellation of findings, including metallosis, effusions/pseudotumor formation, and soft tissue damage ( Fig. 1 ), has been termed adverse local tissue reactions. On histologic analysis, aseptic lymphocytic vasculitis-associated lesions have been characterized as phenomena of MoM bearings. The metallic wear debris and soft tissue responses may lead to early implant failure and soft tissue destruction, including damage of abductor musculature. Metal ions generated from metal-metal articulations may lead to increased serum ion concentrations of chromium and cobalt. The correlation between serum ion concentrations and bearing wear is being elucidated. A study of 37 revised MoM hips, including resurfacing and THA across 5 implant manufacturers, included 65% women. Ten of 37 patients had adverse tissue reaction. Studies measuring metal ion concentrations have found that patients receiving MoM articulations had significantly greater serum and urine metal ion concentrations compared with patients receiving conventional articulations.
