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Paper text:
reverse shoulder replacement surgery is increasing in prevalence due to the advent of better performing , longer lasting implant designs .
even with these improvements , the burden of revision surgery is inevitable , as implants become infected , loosen , or wear .
day et al . have previously described the growth in upper extremity arthroplasty procedure rates at 7 - 13% per year , much higher than primary and revision knee arthroplasty ( 6% and 7% , respectively ) , and primary and revision hip arthroplasty ( 4.5% and 2.5% , respectively ) .
notably , they highlight the rapidly growing revision burden for the upper extremity , a concern due to the more complex nature of revision procedures .
one challenge that may face the revision surgeon is when one side of the joint is well - fixed , for example , a well - ingrown glenoid component and a loose humeral component .
intraoperatively , circumstances can arise where the revising surgeon does not have access to a full range of implants from the required manufacturers .
therefore , they may have to remove a well - fixed component to ensure both sides of the bearing couple are from the same implant system or replace only the loose component and mix between implant systems and potential manufacturers .
due to the sphere - in - a - cup nature of reverse shoulder implants , some manufacturers and surgeons anecdotally suggest that this type of system mixing is allowable .
however , to the authors ' knowledge , the true compatibility between different reverse shoulder arthroplasty ( rsa ) systems has not been studied .
therefore , the primary objective of this study was to determine the intersystem geometric variability between similar - sized implants from different rsa systems . as a secondary objective ,
the intrasystem manufacturing variability between polyethylene inserts of the same system was also determined , as this could affect the accuracy of the intersystem measurements .
it was hypothesized that across the range of systems , some implants would demonstrate high compatibility and others would demonstrate low compatibility .
the polyethylene inserts for eight different rsa implant systems were obtained from six manufacturers [ table 1 ] . for each implant system , six polyethylene inserts of the same size were obtained , either from the same or different manufacturing lots . in all cases , the smallest available diameter of the insert was acquired . for six of the implant designs ,
the manufacturers reported diameter was 36 mm , and for the remaining two designs , the diameter was reported to be 38 mm .
list of implant manufacturer , system and sizes evaluated in this study all 48 polyethylene inserts were scanned with a laboratory microcomputed tomography scanner , using a validated protocol that has been widely utilized to study the polyethylene components from hip and knee arthroplasty .
the scans were performed with an isotropic voxel spacing of 50 m , and x - ray energy of 90 kvp and 40 ma . there were 1200 views acquired per scan , with 10 acquisitions averaged per view to reduce noise .
the insert geometries were extracted from the reconstructed scan volumes using isosurface rendering , at the highest surface quality settings , and saved in stereolithography format for further evaluation [ figure 1a  h ] .
the three - dimensional geometries of the polyethylene inserts from eight different reverse shoulder arthroplasty systems reconstructed from microcomputed tomography scanning .
( a ) arthrex , ( b ) biomet , ( c ) depuy , ( d ) exactech , ( e ) tornier fl ex shoulder system reverse , ( f ) tornier aequalis reversed , ( g ) zimmer anatomical shoulder inverse / reverse , ( h ) zimmer trabecular metal reverse shoulder the analysis process is shown visually in figure 2 .
one insert surface geometry for each of the eight different implant systems was imported into polygonal geometry editing software ( geomagic studio , 3d systems inc . ,
this particular geometry , for each model , was then imported into dimensioning software ( geomagic qualify , 3d systems inc . ,
three - dimensional ( 3d ) comparisons were then made between the six 36 mm diameter inserts and the two 38 mm diameter inserts .
intersystem comparisons were conducted by co - registering two inserts at a time , using an iterative closest points algorithm to determine the best fit between the two articular surface geometries . once registered , the 3d deviations between the two surfaces were calculated and mapped across the entire surface using the dimensioning software .
the maximum absolute deviation ( either positive or negative ) between the two surfaces was computationally determined from the deviation maps .
visual representation of the steps performed by analyzing the articular surface variability between systems intrasystem manufacturing variability was determined by comparing the six scanned inserts for each implant system .
the six geometries from each model were co - registered , and the deviations between them mapped , using a custom software utility .
this software , previously validated , functions in a similar manner to the software used to compare the geometric compatibility of the different models , but can better handle larger files and perform comparisons across more than two geometries . from each deviation map ( which displayed the average deviation across the six geometries ) , regions of interest were selected from the articular surface , the rim , and the backside surface , where the maximum amount of deviation was visualized within those structures . from within each region of interest , the maximum deviation was recorded .
descriptive statistics ( mean , standard deviation , and range ) were calculated for the maximum intersystem surface deviation and the intrasystem manufacturing variability .
differences between implants and regions of interest were compared using a one - way anova with tukey post hoc test .
the average maximum deviation demonstrated by the six 36 mm diameter insert systems in the articular surface comparisons [ figure 3a ] ranged from 53  12 m with the zimmer anatomical inverse / reverse to 71  12 m with the arthrex univers revers .
there was no significant difference between any of the implant models ( p = 0.61 ) .
the overall maximum deviation between any two articular surfaces was 60  16 m on average , with a range of 30 - 80 m .
the maximum articular surface deviation between the two 38 mm diameter models was 51 m . the pattern of the deviations across the articular surface varied between each comparison [ figure 4 ] . the measured deviations from the polyethylene inserts tested .
( a ) the deviation in micrometers between the articular surfaces of the different reverse shoulder arthroplasty systems .
( note that the depuy and exactech inserts were the only 38 mm models , so represent a single comparison ) .
( b ) the intrasystem manufacturing variability for the different reverse shoulder arthroplasty systems tested exemplar articular deviation maps , with the torner flex shoulder system reverse insert used as the reference model compared to the 36 mm diameter polyethylene inserts from the other models and manufacturers .
( a ) intact tornier aequalis reversed insert geometry with the articular surface that was isolated for comparison highlighted in red , ( b ) arthrex , ( c ) biomet , ( d ) tornier flex shoulder system reverse , ( e ) zimmer anatomical shoulder inverse / reverse , ( f ) zimmer trabecular metal reverse shoulder .
registrations were centered at the base of the geometry , where the glenosphere would make contact with the polyethylene insert the average maximum deviation between different inserts from the same system ( i.e. , intrasystem manufacturing variability ) ranged from 30  6 m with the depuy delta xtend reverse to 71  24 m with the biomet comprehensive reverse [ figure 3b ] . the only significant difference ( i.e. , one system having significantly higher manufacturing variability ) was between the depuy and biomet systems ( p = 0.03 ) , with all other systems demonstrating the same magnitude of intrasystem manufacturing variability .
the overall mean of the deviations , across all systems , was 49  17 m , with a range of 23 - 99 m . there was no difference ( p = 0.38 ) in deviation magnitude based on the location of the measurement ( articular surface , rim , or backside surface ) .
the articular surfaces of the same - sized polyethylene inserts from the 8 rsa systems tested in this study were found to be highly compatible .
the magnitude of deviations between implant models was on average 60 m , roughly the thickness of a strand of human hair , and there was no appreciable pattern to the deviations .
this is likely due to the hemispherical nature that all systems implement , for articulation with the round glenosphere .
although it is not the purpose of this paper to make surgical recommendations , similarly sized humeral inserts from the different implant systems tested in this study could theoretically be interchanged . in certain circumstances
, this could reduce the complexity and the associated complications of revising a well - fixed implant , which could potentially benefit the patient .
such data in the orthopedic literature have been for applications in which geometric accuracy is a requirement , including wear measurements in retrieved implants and radiographic motion tracking in radiostereometric analysis or fluoroscopy .
previous studies have reported the manufacturing variability for polyethylene inserts used in total knee replacement to be on the order of tens of microns , similar to the findings of this study for reverse shoulder inserts ( 52  17 m ) .
this intrasystem manufacturing variability is likewise on the same order of magnitude as the geometric variability between inserts of different systems ( 60  16 m ) .
therefore , the geometric variability between the articular surfaces of different implant systems is equivalent to the manufacturing variability of those same implants .
these observed tolerances are also relevant to researchers who will undertake retrieval studies of failed reverse shoulder implants , by establishing the measurement uncertainty for estimating the unworn implant geometry . limitations of this study are that only a single size of polyethylene insert ( the smallest ) was studied from each system , and not all implant models or manufacturers were included in the study .
however , it seems likely that the results would be applicable to other polyethylene insert sizes within the systems studied .
the focus of this study was the articular surface , which is the most important aspect of interdesign compatibility .
however , many of the implant systems have different rim geometries [ figure 1 ] , which are an additional variable and could potentially have an effect on edge wear and impingement .
the glenospheres offered by different manufacturers were also not examined , however by their inherent nature the variation between their articular surfaces , as was found with the polyethylene inserts , is likely to be similar .
our method to directly examine the entire articular surface is more thorough than simply measuring the diameter of liners from different manufacturers , which would reveal whether the difference in diameter ( if any ) was consistent throughout the entire geometry . in the case of a slightly narrower liner articulating against a slightly broader glenosphere ,
the curvature of the glenosphere should still enable bearing contact , but with reduced surface area coverage .
in the reverse case of a broader liner and a narrower glenosphere , there would be greater penetration of the glenosphere providing increased surface area coverage .
the articular surfaces of similarly sized polyethylene inserts used in rsa systems examined in this study were found to be equivalent , with a mean intersystem variability of 60  16 m .
this variability is on the same order of magnitude as the intrasystem manufacturing variability of these polyethylene inserts , which was 52  17 m . although it is not the purpose of this paper to make surgical recommendations , our findings suggest that revision surgeons can theoretically interchange same - sized implant components from the different rsa systems tested , when faced with a well - fixed implant on one side and no way to implant a new component from that same system on the opposite member of the bearing couple
background : as the incidence of reverse shoulder arthroplasty ( rsa ) increases , so will the revision burden . at times 
 , the revision surgeon may be faced with a well - fixed component on one side of the joint and revision implants from a different manufacturer . the ability to use glenoid and humeral implants from different manufacturers could simplify the revision procedure . 
 this study hypothesized that across a range of rsa systems , some implants would demonstrate high size compatibility and others would demonstrate low compatibility.materials and methods : six polyethylene inserts each from eight reverse total shoulder arthroplasty systems were examined ( 48 total inserts ) . 
 all inserts were scanned using a laboratory micro - computed tomography scanner at 50 m isotropic voxel spacing , and their surface geometries were reconstructed . 
 the different implant geometries were co - registered , and the three - dimensional ( 3d ) variability between the articular surfaces of the different implant systems was measured . 
 intrasystem manufacturing variability was also determined by measuring the 3d variability of inserts from the same system.results:the intersystem polyethylene articular surface deviations between same - size systems were not significantly different ( p = 0.61 ) and were a mean maximum of 60  16 m ( range : 30 - 80 m ) . 
 intrasystem manufacturing variability was equivalent between all but two models , averaging 49  17 m ( range : 23 - 99 m).discussion : differences in articular geometry between same - size inserts from different systems were on the same scale as intrasystem manufacturing variability , suggesting that different implant systems of the same nominal diameter could potentially be used interchangeably in revision or extenuating circumstances.conclusion:the results of this study suggest that surgeons can theoretically interchange same - sized implant components from the different rsa systems tested when conducting revisions .