numerous investigators have reported improved function , reduced pain , and high rates of implant survivorship at up to 20  years followup [ 4 , 29 ] .
however , concerns about limitations in implant longevity related to the osteolysis associated with metal - on - polyethylene ( m - pe ) bearing surface wear have led to innovation in tribology [ 10 , 15 , 16 ] and the introduction of new tha bearing couples over the past decade .
in particular , second - generation hard - on - hard bearings , including metal - on - metal ( m - m ) and ceramic - on - ceramic ( c - c ) , have been introduced into the us market based on laboratory studies showing lower wear rates when compared with m - pe bearings [ 9 , 10 , 15 ] .
lower wear rates associated with hard - on - hard bearings are likely to have the biggest impact on revision rates in younger , more active patients , who are at higher risk for wear - related failures [ 3 , 12 , 17 , 18 , 27 ] .
the long - term benefits of more costly hard - on - hard bearings are less obvious in older patients , who are at lower risk for wear - related failures
. however , a potential benefit of m - m bearings is the ability to use large diameter femoral heads , which theoretically could result in a lower short - term risk of dislocation when compared with m - pe bearings
. this potential benefit could be particularly advantageous in older patients who are reportedly at higher risk for tha dislocation [ 5 , 25 , 28 ] .
however , that study employed a cross - sectional study design , and due to the nature of the database used ( the nationwide inpatient sample ) , we were unable to compare longitudinal patient outcomes and differences in the risks of complication and revision tha among patients with different tha bearings .
the purpose of this study was to compare the short - term risk of complication and revision tha among medicare patients having a primary tha with m - pe , m - m , and c - c tha bearings .
the 2005 to 2007 100% medicare inpatient claim files were used to perform a matched cohort analysis in three separate cohorts of tha patients ( m - pe , m - m , c - c ) .
patients in each tha cohort were matched by age , gender , and us census region to the comparison or 
m - m hip resurfacing arthroplasty procedures ( which have a separate icd-9-cm procedure code ) were excluded from the analysis .
each m - m patient and c - c patient ( case ) were matched to three m - pe patients ( control ) , while each c - c patient ( case ) was matched to three m - m patients ( control ) .
case patient , the eligible pool of controls consisted of those patients who were within 3  years in age of the case patient and who were at least 65  years old . if more eligible controls were available than the three required per case , controls closest in age were selected first .
eligible controls were also of the same gender and resided in the same us census region .
all cases and controls had to be enrolled in both part a and part b of medicare .
patients who received their medicare health benefits through a health maintenance organization were excluded because their healthcare expenses were not submitted to the centers for medicare and medicaid services ( cms ) for payment and , therefore , claims from these beneficiaries were not available from the database .
primary tha patients were identified using international classification of diseases , 9th rev , clinical modification ( icd-9-cm ) procedure code 81.51 from the inpatient claims records and grouped into three bearing cohorts based on their corresponding icd-9-cm optional procedure modifier bearing codes ( 00.74 for m - pe , 00.75 for m - m , 00.76 for c - c ) . using each patient s unique de - identified beneficiary i d , the patients were tracked longitudinally with revision tha or selected complications as end points .
the selected complications included deep venous thrombosis ( dvt ) , dislocation , infection , and mechanical loosening .
dvt was identified using the icd-9-cm diagnosis codes 451.1 , 451.2 , 451.81 , 451.9 , 453.1 , 453.2 , 453.4 , 453.8 , and 453.9 , while tha dislocation was identified using any occurrence of icd-9 diagnosis codes 718.35 , 835 , or 996.42 .
infection and mechanical loosening were identified using icd-9 diagnosis codes 996.66 and 996.41 , respectively .
an overall cohort of 36,423 tha patients with m - pe bearings were identified , along with 17,789 tha patients with m - m bearings and 2,835 tha patients with c - c bearings , from which the matched cohorts were derived .
the mean age of all tha recipients included in the study was 74.5  years ( sd  =  6.4  years ) .
multivariate cox proportional - hazards models were constructed to compare complication and revision tha risk among cohorts , adjusting for medical comorbidities , race , socioeconomic status , and hospital factors .
the medicare buy - in status was used as an identifier of patients whose medicare premiums and deductibles were subsidized by the state ( eg , medicaid ) and were used as a proxy for the patient s socioeconomic status . to account for the health status of each patient , the charlson comorbidity index was used as a measure of comorbid illness . for this analysis , the charlson index values were grouped into the following previously validated categories : 0 ( none ) , 1 to 2 ( low ) , 3 to 4 ( moderate ) , and 5 or more ( high ) .
among younger medicare patients ( aged 6569  years ) , m - m bearings accounted for 35% of tha bearings and c - c bearings accounted for 8% . among older medicare patients ( older than 80  years ) ,
among female patients , 67% had m - pe bearings , while 29% had m - m bearings , compared to male patients , among whom 60% had m - pe and 35% had m - m bearings ( fig .  2 ) .
the highest frequency of m - pe bearing surface use was in the northeast ( 72% ) , and the highest frequency of m - m bearing usage was in the south ( 37% ) , while c - c bearing usage was fairly consistent across the four us census regions ( 4%5% ) ( fig . 
1the bearing surface frequency in medicare tha patients by age is shown.fig .  2the bearing surface frequency in medicare
3the bearing surface frequency   in medicare tha patients by us census region is shown . the bearing surface frequency in medicare tha patients by age
the bearing surface frequency   in medicare tha patients by us census region is shown .
when comparing the unadjusted risk of complication using the kaplan - meier method , m - m bearings showed an apparent trend toward higher overall risk of dvt ( fig . 
5 ) than c - c bearings . however , these differences were not present after adjusting for other covariates in these matched cohorts ( table  1 ) .
after controlling for patient and hospital factors , m - m bearings were associated with a higher risk of periprosthetic joint infection ( hazard ratio , 3.03 ; confidence interval , 1.029.09 ) when compared with c - c bearings .
there were no other differences among cohorts in the adjusted risk of revision tha or any other complication , including dvt , dislocation , or mechanical loosening ( table  1).fig . 
4this graph shows trends of higher overall rates of dvt for m - m versus c - c bearings ( unadjusted ) .
( t )  =  percentage of patients without the complication ( dvt ) at the given time points.fig . 
5this graph shows trends of higher overall rates of revision for m - m versus c - c bearings ( unadjusted ) .
( t )  =  percentage of patients without the complication ( revision ) at the given time points.table  1risk - adjusted comparison of risk of complication and revision tha between m - pe and m - m bearings , m - pe and c - c bearings , and m - m and c - c bearingscomplicationm - pe ( n  =  54,431)m - m ( n  =  17,789)hazard risk ratio ( compared to m - pe)p valuem - pe ( n  =  8073)c - c ( n  =  2835)hazard risk ratio ( compared to m - pe)p valuem - m ( n  =  7853)c - c ( n  =  2835)hazard risk ratio ( compared to m - m)p valuedvt1.05%1.03%1.010.881.08%0.88%0.590.070.90%0.88%0.900.73dislocation0.80%0.74%0.920.490.68%0.78%1.080.800.57%0.78%1.210.57infection0.49%0.53%0.980.890.37%0.32%0.620.360.59%0.32%0.330.045mechanical loosening0.22%0.20%0.870.560.25%0.39%0.970.970.23%0.39%1.100.89revision1.16%1.12%0.940.511.04%1.02%0.670.141.34%1.02%0.670.09m - pe  =  metal - on - polyethylene ; m - m  =  metal - on - metal ; c - c  =  ceramic - on - ceramic ; dvt  =  deep vein thrombosis .
this graph shows trends of higher overall rates of dvt for m - m versus c - c bearings ( unadjusted ) .
( t )  =  percentage of patients without the complication ( dvt ) at the given time points .
this graph shows trends of higher overall rates of revision for m - m versus c - c bearings ( unadjusted ) .
( t )  =  percentage of patients without the complication ( revision ) at the given time points .
risk - adjusted comparison of risk of complication and revision tha between m - pe and m - m bearings , m - pe and c - c bearings , and m - m and c - c bearings m - pe  =  metal - on - polyethylene ; m - m  =  metal - on - metal ; c - c  =  ceramic - on - ceramic ; dvt  =  deep vein thrombosis .
tha has become one of the most commonly performed operations in medicare patients , with over 160,000 primary and revision tha procedures reported in medicare patients in   2007 .
successful tha allows elderly patients who suffer from disabling hip disease to regain their function and maintain a more active , healthy lifestyle .
nevertheless , increased utilization and rising costs have led to concerns regarding the appropriateness of using 
hard - on - hard bearings are associated with lower wear rates in the laboratory setting [ 15 , 20 ] . however , wear - related failures , such as bearing surface wear , osteolysis , and mechanical loosening , may be less of a concern among medicare patients when compared with younger , more active patients who undergo primary tha .
therefore , the higher cost associated with hard - on - hard bearings may not be justifiable in terms of improved patient outcomes and lower reoperation rates in medicare patients , unless they are associated with fewer short - term complications , such as dislocation .
one example of a potential short - term benefit of m - m bearings that is often cited is the ability to use large diameter femoral heads , which theoretically could result in a lower risk of dislocation when compared with m - pe bearings .
we therefore compared the short - term risks of complication and revision tha among medicare patients having a primary tha with m - pe , m - m , and c - c tha bearings .
our findings are limited by the use of an administrative database , where bearing surface type is an optional modifier code that can be reported in conjunction with the primary procedure code ( primary or revision total hip arthroplasty ) , which introduces a potential source of bias into our study .
however , this limitation is somewhat mitigated by the matched cohort study design , where patients with known bearing types were matched by age , gender , and u.s .
second , other variables of interest which may influence patient outcomes , including surgical approach , surgeon experience , and implant design are not accessible from administrative claims , and we were only able to evaluate complications which are captured in administrative claims data , such as dvt , dislocation , infection , mechanical loosening , and revision surgery , rather than pain and patient reported functional outcomes .
third , since the optional bearing surface modifier administrative codes were not introduced until october , 2005 , our study was limited to outcomes and complications that occurred within two years of the index procedure . however , the rationale for our study was that presumably the reason for using alternative bearings in medicare patients is to reduce short - term complication rates ( eg , dislocation ) , rather than long - term complications ( eg , wear / osteolysis ) , which are more of a concern in younger patients than in medicare patients . furthermore , the australian hip registry   and other reports in the literature [ 6 , 11 , 19 , 22 , 23 , 26 ] have raised concerns regarding higher than expected short - term complication and revision rates in patients with hard - on - hard bearings .
nevertheless , further study is necessary to evaluate differences in long - term clinical outcomes and revision rates as additional data becomes available .
we found the adjusted risk of short - term complication and revision tha among medicare tha patients was similar , regardless of bearing surface , with the exception of patients who had m - m bearings , who had a slightly higher risk of periprosthetic joint infection than patients who had c - c bearings .
the reasons underlying the increased risk of infection for m - m bearings compared with c - c bearings , even after adjusting for medical comorbidities and hospital factors , remain unclear .
it is possible that some of these patients may have been misdiagnosed as having a periprosthetic joint infection when in fact they had a local soft tissue inflammatory reaction related to the m - m articulation [ 6 , 11 ] .
the findings in some patients with such reactions mimic periprosthetic joint infection , as reported previously by mikhael et al . .
although the higher risk of infection in the m - m cohort compared with the c - c cohort was significant ( hazard ratio 3.03 , ci  =  1.02  9.09 ) , the clinical importance of this difference is unclear , especially given the relatively low incidence of infection ( 0.59% versus 0.32% , respectively ) .
the relatively high incidence of m - m bearings reported in the medicare population is somewhat surprising , given m - m bearings are thought by some to be primarily indicated for younger , more active patients who are at higher risk for bearing surface wear and osteolysis with m - pe bearings [ 14 , 17 , 20 ] .
however , one theoretical advantage of m - m bearings in older patients is improved stability ( eg , lower risk of dislocation ) due to the ability to use larger - diameter femoral heads .
our data do not demonstrate an advantage of m - m bearings in terms of lower short - term risk of dislocation when compared with either m - pe or c - c bearings .
this could be related to the increasing trend of using 32- and 36-mm heads in m - pe bearings , which may be sufficient to substantially reduce the risk for dislocation .
although the database used in this study did not include information about femoral head size , according to industry sources , the proportion of femoral heads implanted in the us sized 32  mm or greater has increased from approximately 12% to 79% between 1998 and 2008 . in summary , we found medicare tha patients with hard - on - hard ( m - m , c - c ) bearings had a similar risk of complications and revision tha compared to patients who had m - pe bearings during the first 2  years after primary tha .
these findings provide a basis for additional analyses of the comparative effectiveness of tha bearing surfaces in the medicare population .
 backgroundto address the long - term problems of bearing surface wear and osteolysis associated with conventional metal - polyethylene ( m - pe ) total hip arthroplasty ( tha ) , metal - metal ( m - m ) , and ceramic - ceramic ( c - c ) bearings have been introduced . 
 these bearing surfaces are associated with unique risks and benefits and higher costs . 
 however the relative risks of these three bearings in an older population is unknown.questions/purposeswe compared the short - term risk of complication and revision tha among medicare patients having a primary tha with metal - polyethylene ( m - pe ) , metal - metal ( m - m ) , and ceramic - ceramic ( c - c ) bearings.methodswe used the 2005 to 2007 100% medicare inpatient claim files to perform a matched cohort analysis in three separate cohorts of tha patients ( m - pe , m - m , and c - c ) who were matched by age , gender , and us census region . 
 multivariate cox proportional - hazards models were constructed to compare complication and revision tha risk among cohorts , adjusting for medical comorbidities , race , socioeconomic status , and hospital factors.resultsafter adjusting for patient and hospital factors , m - m bearings were associated with a higher risk of periprosthetic joint infection ( hazard ratio , 3.03 ; confidence interval , 1.029.09 ) when compared with c - c bearings ( 0.59% versus 0.32% , respectively ) . 
 there were no other differences among bearing cohorts in the adjusted risk of revision tha or any other complication.conclusionsthe risk of short - term complication ( including dislocation ) and revision tha were similar among appropriately matched medicare tha patients regardless of bearing surface . 
 hard - on - hard tha bearings are of questionable value in medicare patients , given the higher cost associated with their use and uncertain long - term benefits in older patients.level of evidencelevel ii , prognostic study . 
 see guidelines for authors for a complete description of levels of evidence . 
Level of Evidence
Introduction
Patients and Methods
Results
Discussion