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Wear rates of retentive versus nonretentive
reverse total shoulder arthroplasty liners in an
in vitro wear simulation
Shannon Carpenter, MD
a
, Daphne Pinkas, MD
a
, Michael D. Newton, BS
b
,
Michael D. Kurdziel, MS
b,c
, Kevin C. Baker, PhD
b,c
, J. Michael Wiater, MD
a,c,
*
a
Department of Orthopaedic Surgery, Beaumont Health System, Royal Oak, MI, USA
b
Department of Orthopaedic Research, Beaumont Health System, Royal Oak, MI, USA
c
Department of Surgery, Oakland University William Beaumont School of Medicine, Rochester, MI, USA
Background: Although short-term outcomes of reverse total shoulder arthroplasty (rTSA) remain promising,
the most commonly cited complication remains prosthetic instability. A retentive rTSA liner is commonly
used to increase system constraint; however, no studies have evaluated the rate of polyethylene wear. Our hy-
pothesis was that more constrained retentive liners would have higher wear rates than nonretentive liners.
Methods: Six nonretentive and six retentive rTSA non-cross-linked polyethylene liners were subjected to
4.5 million cycles of alternating cycles of abduction-adduction and flexion-extension motion loading pro-
files. The rTSA liners were assessed for gravimetric wear loss, 3-dimensional volumetric loss by novel
micro-computed tomography analysis, and particulate wear debris analysis.
Results: Volumetric wear rates were significant at 7 specific time points (1.0, 2.0, 2.5, 3.25, 3.75, 4.0, and
4.5 million cycles) throughout testing between nonretentive and retentive liners; however, overall mean
volumetric wear rate was not statistically significant (P¼.076). Total volume loss between liner test
groups was found to be significant starting after 3.5 million cycles of testing. Maximum and mean surface
deviations were found to be larger for retentive liners vs. nonretentive liners by micro-computed tomog-
raphy analysis across the entire articulation surface.
Discussion and conclusion: Retentive liners undergo significantly greater volume loss and greater surface
deviation compared with nonretentive liners, most notably at later time points representing extended im-
plantation times. Additional stability afforded by retentive liners should be balanced against the potential
for increased wear and potential for subsequent polyethylene wear-induced aseptic loosening.
Level of evidence: Basic Science Study, Biomechanics.
Ó2015 Journal of Shoulder and Elbow Surgery Board of Trustees.
Keywords: Retentive humeral liners; reverse total shoulder arthroplasty; wear particle analysis; in vitro
wear simulation; micro-CT surface deviation analysis
Product support for this study was provided by Zimmer (Warsaw, IN,
USA). Funding was provided by OMeGA Medical Grants Association
(#000605) and the Orthopaedic Research and Education Foundation (#11-
105) fellowship research grants. These entities were not involved in the
study design or collection, analysis, or interpretation of data; in the writing
of the report; or in the decision to submit the article for publication.
Institutional Review Board approval is not applicable: Basic Science
Study, Biomechanics.
*Reprint requests: J. Michael Wiater, MD, Beaumont Health, Depart-
ment of Orthopaedic Surgery, Division of Shoulder and Elbow Surgery,
3535 W 13 Mile Rd, Ste 744, Royal Oak, MI 48073 USA.
E-mail address: mwiater@beaumont.edu (J.M. Wiater).
J Shoulder Elbow Surg (2015) 24, 1372-1379
www.elsevier.com/locate/ymse
1058-2746/$ - see front matter Ó2015 Journal of Shoulder and Elbow Surgery Board of Trustees.
http://dx.doi.org/10.1016/j.jse.2015.02.016
The reverse total shoulder arthroplasty (rTSA) was
originally designed to treat elderly patients with rotator cuff
arthrosis.
4
Since Food and Drug Administration approval in
2003, the indications for rTSA have expanded significantly,
and although promising short-term clinical outcomes have
been described for rTSA, a high complication rate persists,
ranging from 0% to 68% in the literature.
8,9,41
The most
common complications include neurologic injury, intra-
operative periprosthetic fracture, hematoma, infection,
scapular notching, prosthesis loosening and failure, post-
operative stiffness, and prosthetic instability.
3,4,13,16,28,34
Prosthetic instability is the most frequently cited compli-
cation, representing nearly a third of all complications with
an incidence of 0% to 8% in recent studies.
7,16,40
Factors
thought to contribute to rTSA postoperative instability
include distortion of the osseous and soft tissue anatomy by
prior trauma, inadequate soft tissue tensioning, component
malposition, and inappropriate ratio between the central
depth and the diameter of the concave polyethylene
component.
4,14,17,26
One method to improve prosthetic stability is to increase
the system constraint by using a retentive polyethylene
liner. A retentive liner has a slightly deeper concavity and
higher peripheral rim than standard liners, increasing the
humeral socket depth and providing greater congruency
between the liner and the glenosphere. A recent biome-
chanical study by Clouthier et al
11
showed that increased
humeral liner depth enhanced prosthetic stability in all
loading profiles except inferior loading. Gutierrez et al
17
previously looked directly at the hierarchy of stability
factors in rTSA and found that the humeral socket depth
was one of the most important factors, second only to joint
compressive force.
However, the stability of the retentive liner must be
weighed against the risk of potential decrease in range of
motion, leading to possible impingement and increased
scapular notching.
18
In addition, the increased constraint
may escalate polyethylene wear, which could lead to
osteolysis, component loosening, and subsequent
failure.
12,15,27,32,38
To date, no studies have quantified the
wear rate of retentive vs. standard polyethylene liners. The
purpose of this study was to examine and to compare the
wear rates of both retentive and nonretentive polyethylene
liners in an in vitro rTSA model. We hypothesized that
retentive liners, with their increased constraint and articu-
lating surface area, would have an increased rate of poly-
ethylene wear compared with standard nonretentive liners.
Materials and methods
Test groups
Commercially available components (Zimmer Inc., Warsaw, IN,
USA) were used for wear simulation testing. Nonretentive liners
(N ¼6; Trabecular Metal Reverse Shoulder System, 7elevated
rim, 36-mm diameter, þ0-mm thickness) and retentive liners
(N ¼6; Trabecular Metal Reverse Shoulder System, 12elevated
rim, 36-mm diameter, þ0-mm thickness) were subjected to an
in vitro wear simulation protocol matched against 36-mm gleno-
spheres (Fig. 1). All polyethylene liners were non-cross-linked.
Liner components were press-fit into simulator testing fixtures.
In vitro wear simulation protocol and measurement
of wear
With use of a previously developed protocol from our institu-
tion,
31,39
a 12-station hip wear simulator (MTS Bionix, Eden
Prairie, MN, USA) was converted to an rTSA simulator by
modification of the test fixtures, loading profiles, and testing
protocol. A total of 4.5 million cycles of testing were completed,
alternating simulated abduction-adduction and flexion-extension
motion profiles every 250,000 cycles. The abduction-adduction
loading profile simulated 0to 46of arm swing (20-618 N,
90% body weight), and the flexion-extension loading profile
subjected liners to 44to 90of arm swing (20-927 N, 135% body
weight).
Individual components remained immersed in defined bovine
calf serum (21 g protein/L) containing 0.2% sodium azide,
7.0 10
3
g/mL ethylenediaminetetraacetic acid (Ricca Chemi-
cal, Arlington, TX, USA), and deionized water at all times during
testing. Before wear simulation testing, all liners were presoaked
in lubrication fluid to account for fluid adsorption that occurs
during initial liner immersion. One additional liner per group
served as a load-soak control throughout testing to account for
fluid variations during testing. These liners were subjected to axial
loads but no dynamic rotation.
At each 250,000-cycle interval, liners were removed from
testing fixtures, cleaned, and dried with compressed nitrogen gas
according to ISO Standard 14242-2. Liners were then measured
on a precision mass balance (A-200DS; Fisher Scientific, Pitts-
burgh, PA, USA) for determination of mass loss at each interval.
Mass loss was then converted to volume loss using the known
density of ultrahigh-molecular-weight polyethylene (UHMWPE;
0.933 g/cm
3
). Wear was then quantified and reported as volu-
metric wear rate (mm
3
/million cycle [MC]) and total volume loss
(mm
3
) for the duration of the simulation.
Wear particle characterization
Wear particles were isolated from test serum to characterize par-
ticle morphology. Serum samples were selected randomly from 3
nonretentive and 3 retentive testing stations at 4 different time
points throughout testing. The time points were the first and final
abduction-adduction and flexion-extension time points (after
abduction-adduction at 250,000 and 4,250,000 cycles, and after
flexion-extension at 500,000 and 4,500,000 cycles). Each serum
sample was thawed for overnight digestion in 5.0 M NaOH at
37C with constant stirring to digest proteins and filtered through
a 0.2-mm polycarbonate filter.
Filters were sputtered with Au-Pd and subjected to environ-
mental scanning electron microscopy (Quanta FEG40; FEI,
Hillsboro, OR, USA). Ten images of 10,000magnification were
taken from random locations from each filter and analyzed with a
custom MATLAB program (MathWorks, Natick, MA, USA),
rTSA wear simulation of retentive liners 1373
based on a previously described protocol.
31,39
Particles were
characterized by equivalent circle diameter ((4*Area/p)
1/2
), aspect
ratio (d
max
/d
min
), and roundness ((4*Area)/(p*d
max2
)).
Micro-CT surface deviation
Three-dimensional surface deviations between nonretentive and
retentive liners and their respective soak controls were compared
by a micro–computed tomography (micro-CT) technique adapted
from previously described techniques for wear quantification in
the total hip, knee, and disk arthroplasties.
6,21,22,35-37
After
completion of the simulation, each liner was scanned individually
with 45 mm isotropic voxel resolution, 55 kVp, and 450 mA on an
eXplore Locus RS micro-CT scanner (GE Healthcare Pre-Clinical
Imaging, London, ON, Canada) and reconstructed at full resolu-
tion. Three-dimensional meshes were isolated from the recon-
structed scans using DeVIDE software.
5,24,25,29
Briefly, scans were
filtered using median filtering and thresholded to isolate the liner.
The isolated 3-dimensional mask was then meshed using an iso-
surface generator, simplified using quadratic edge collapse deci-
mation, and exported in stereolithography file format.
Liner meshes were then analyzed using open-source MeshLab
software (http://meshlab.sourceforge.net).
10
Liners were pre-
processed using Poisson surface reconstruction to ensure a water-
tight surface, and each worn liner was coaligned with a
corresponding unworn liner using an iterated closest point algo-
rithm. The surface deviation between the worn and control liner
mesh surfaces was calculated at each point on the mesh. These
measurements were used to generate surface deviation maps for
each worn liner. Surface deviation measured with this technique
represents the volume change in response to both material creep and
material loss due to wear at the conclusion of the wear simulation.
Cross-sectional analysis
To quantify regional surface deviation of the worn bearing sur-
faces, a cross-sectional analysis based on a previously published
technique was performed.
35
Micro-CT cross sections were taken
from worn and control liners at 0,45
,90
, and 135, with
0defined as the most inferior pole of the liner and 180at the
peripheral rim. To standardize alignment between liners, the flat
backside of the liner was used as a reference plane, and the center
of rotation was defined as the geometric center of the liner’s 3-
dimensional mesh.
Each pair of cross sections was imported into MATLAB. The
bearing surface was manually selected, and 200 evenly spaced
points were selected from the control surface. Each cross section
was divided into 2 subsets by the center of rotation, yielding a
total of 8 angles at which measurements were taken (Fig. 1). The
Euclidean distance between the worn and control surface, normal
to the control surface, was calculated for each point, and the mean
distance at each angle was measured.
Statistical analysis
All statistical analyses were performed with SPSS Statistical
Software (version 22; IBM SPSS Statistics, Chicago, IL, USA).
Direct comparisons between particle characteristics and surface
deviations were made by the Student ttest with significance set at
P<.05. Volumetric wear rates were compared by a 2-way anal-
ysis of variance with 2 factors: liner type (nonretentive vs.
retentive) and motion profile (abduction-adduction vs. flexion-
extension), with significance set at P<.05 for variables volu-
metric wear rate and total volume loss.
Results
Gravimetric measurement of wear
All 12 stations completed 4.5 million cycles of wear
simulation testing successfully. Volumetric wear rates be-
tween test groups are shown in Figure 2. In taking all time
points into consideration, no significant difference in mean
volumetric wear rates (mm
3
/MC) was found between
nonretentive and retentive liners (88.1 19.1 vs.
96.8 21.9 mm
3
/MC; P¼.076). In examining wear rates
at specific time points in the test (1.0, 2.0, 2.5, 3.25, 3.75,
4.0, and 4.5 million cycles), retentive liners had signifi-
cantly greater wear rates compared with nonretentive liners
(P<.05).
Differences in volumetric wear rate between motion
profiles were observed (Fig. 2,B). The flexion-extension
loading profile exhibited significantly greater volumetric
wear rates than the abduction-adduction loading profile
for both nonretentive (102.4 15.2 vs. 73.8 9.1 mm
3
/
MC; P<.001) and retentive liners (113.1 15.8 vs.
80.4 13.1 mm
3
/MC; P<.001). Within each loading
profile, a significant difference in volumetric wear rate
was found between nonretentive and retentive liners in
both abduction-adduction (73.8 9.1 vs. 80.4
13.1 mm
3
/MC; P¼.011) and flexion-extension loading
profiles (102.4 15.2 vs. 113.1 15.8 mm
3
/MC;
P<.001).
No significant difference in total volume loss was noted
up to 3.5 million cycles of simulation; however, at 3.75
million cycles and after, a significant difference between
liner types was noted. At 4.5 million cycles, retentive
liners had significantly greater total volume loss compared
with nonretentive liners (435.5 30.4 mm
3
vs.
396.4 25.7 mm
3
;P¼.037).
Wear particle analysis
A total of 14,138 wear particles from nonretentive liners
and 12,526 from retentive liners were isolated from envi-
ronmental scanning electron microscopy images for char-
acterization. Wear particles isolated from retentive liners
were larger, with a significantly larger equivalent circle
diameter (0.25 0.14 vs. 0.25 0.12; P¼.023) compared
with nonretentive liners. Retentive liners were also more
fibrillar (less round), with significantly lower aspect ratio
(2.82 1.59 vs. 2.92 1.66; P<.001) and higher
roundness (0.32 0.18 vs. 0.31 0.18; P<.001),
compared with nonretentive liners.
1374 S. Carpenter et al.
Micro-CT surface deviation and cross-sectional
analysis
Three-dimensional surface deviation maps created from
micro-CT scans demonstrate increased penetration of the
glenosphere into retentive liners compared with non-
retentive (Fig. 3). The maximum deviation of retentive
liners (0.69 0.03 mm) was significantly higher than that
of nonretentive liners (0.56 0.06 mm; P<.001). Qual-
itatively, 2 distinct wear scars can be seen on the bearing
surfaces of each liner.
Mean surface deviations obtained from cross-sectional
analysis are shown in Figure 4. Elevated surface de-
viations were observed at 135, 180, and 225,withthe
highest deviations occurring at 180(nonretentive,
0.320 0.067 mm; retentive, 0.549 0.036 mm), cor-
responding to the medial side of the bearing surface.
Surface deviations for both nonretentive and retentive
liners at 180were significantly higher than all other an-
gles, and deviations for retentive liners at 135and 225
were significantly higher compared with those at 0,45
,
90, 270, and 315.
Comparison of gravimetric and mesh volumes
The average post-simulation volumes of nonretentive and
retentive liners, determined gravimetrically, were
6634 34 mm
3
and 7376 38 mm
3
, respectively, whereas
the average volumes of the liners determined from the
meshes isolated from micro-CT were 6600 64 mm
3
and
7313 52 mm
3
, respectively. This yielded an average
percentage difference between gravimetric and mesh vol-
umes of 0.98% and 1.06% for nonretentive and retentive
liners, respectively, showing strong agreement between the
2 measurement techniques.
Discussion
Retentive liners are indicated in rTSA for the reduction of
postoperative instability. They add stability by increasing
rTSA system constraint and have generally been associated
with positive clinical outcomes. However, because of the
relatively recent introduction of this rTSA system, there are
few data available comparing the wear performance of
Figure 2 (A) Volumetric wear rates of retentive and nonretentive rTSA UHMWPE liners subjected to 4.5 million cycles of wear
simulation testing alternating abduction-adduction and flexion-extension motion profiles every 250,000 cycles. (B) Volumetric wear rates
were then examined as function of motion loading profile and liner type. (C) Total volume loss for liner type throughout the entire 4.5
million cycles of testing. *Statistical significance between retentive and nonretentive test groups (P<.05).
Figure 1 Profile of nonretentive and retentive humeral liners (left) used in this simulation. The extended medial lip of the retentive liners
can be appreciated. Cross sections used for surface deviation analysis of worn liners (right). Sections were taken at 0,45
,90
, and 135,
and the bearing surface of each cross section was split into 2 subsets at the center of rotation, yielding 8 angles total from which mea-
surements were taken.
rTSA wear simulation of retentive liners 1375
retentive and nonretentive rTSA liners. A significantly
higher incidence of heterotopic ossification and a trend
toward increased scapular notching have been reported in
retentive trabecular metal reverse shoulder systems.
20
Although component loosening has been cited as a major
reason for revision of rTSA components,
15
no clinical or
biomechanical data are currently available for the effect of
retentive liners on the wear rate or incidence of component
loosening in rTSA. In the setting of total hip arthroplasty,
fully constrained acetabular liners are generally associated
with increased rates of polyethylene wear and subsequent
aseptic loosening due to the increased constraint of the
system.
1,2,23
However, whether this trend applies to reten-
tive liners in rTSA is unclear, as retentive liners add a de-
gree of constraint to the system but are not fully
constraining.
During the course of this simulation, retentive liners
experienced greater total volume loss than nonretentive
liners, with the difference increasing gradually and
becoming statistically significant after 3.5 million cycles of
wear testing. The wear rates of both liner types increased
during the initial bedding-in phase; however, significant
increases in the wear rates of retentive liners at later time
points in the simulation suggest that the wear rates of
retentive liners increased at a greater rate during this initial
phase. This may be due in part to the increased contact area
of retentive liners with the glenosphere, resulting in a
greater wear footprint.
Particles from retentive liners were significantly larger
and more fibrillar than those from nonretentive liners. In
general, more fibrillar particles are thought to elicit a
greater inflammatory response from macrophages.
33
Figure 3 Representative surface deviation color maps of worn nonretentive and retentive liners compared with their respective controls
after completion of the wear simulation. The bearing surface is outlined by a dotted white line. Larger surface deviations can be seen in
retentive liners compared with nonretentive. In both groups, increased wear was seen on the medial aspect of the bearing surface. Scar lines
can be seen across the liner bearing surfaces. These are presumably caused by the motion profile arcs used in the simulation.
Figure 4 Cross-sectional analysis of worn liners compared against soak controls. (A) Mean surface deviation of a cross-sectional analysis
of worn nonretentive and retentive liners. Surface deviations at 180were significantly higher than all other angles for both the nonretentive
and retentive liners, and deviations at 135and 225were significantly higher than those at 0,45
,90
, 270, and 315for retentive liners.
(B) Whisker plots of nonretentive and retentive liners at 0/180showing surface deviation of the worn bearing surface from the control.
Mean surface deviation is more severe at 180than at 0, demonstrating that more wear occurred medially on the bearing surface.
*Significant difference between nonretentive and retentive liners at P>.001.
1376 S. Carpenter et al.
However, whereas the differences in particle morphology
were statistically significant, the actual differences are
small and unlikely to have a clinically relevant impact. The
slight changes in particle morphology can most likely be
attributed to the differences in constraint and contact area
between the 2 liners, with the statistical significance being
largely driven by the large sample sizes (14,138 and 12,526
particles for nonretentive and retentive liners, respectively).
The measurements of particle morphology from this study
were comparable to previous rTSA simulations.
31,39
The techniques described for mapping and quantifying
the surface deviation of worn liners were adapted from
previously described techniques for measuring wear on
polyethylene liners from total hip, total knee, and total disk
replacements.
6,21,35-37
Surface deviations were found to be
globally higher on retentive liners compared with non-
retentive liners. This was further supported by significantly
greater maximum and mean surface deviation of the
retentive liners at all angles during cross-sectional analysis.
These findings support the hypothesis that the observed
differences in volumetric wear cannot be attributed solely
to increased surface area and that the increased wear and
wear rates of retentive liners can be attributed to increased
constraint and surface area.
Cross-sectional analysis of the surface deviation maps
showed the most prominent wear along the 180axis to-
ward the prominent ridge. This trend was common for both
nonretentive and retentive liners. In addition, on most of the
worn components, scar lines along distinct arcs can be
noted crossing the bearing surface. We believe these to be
the result of the 2 distinct motion profiles used in the study
and the focal point of the wear for each profile.
To the authors’ knowledge, this is the first report of a
micro-CT–based technique to characterize UHMWPE wear
in the setting of rTSA and in conjunction with gravimetric
wear analysis. The high agreement between the mesh vol-
umes with the volumes calculated from the final gravi-
metric weights of the liners and the known density of
UHMWPE indicates that the isolation scheme was able to
accurately isolate the liners from micro-CT scans.
Continued research is under way to apply this technique to
retrieved rTSA liners from our institution’s implant
retrieval library to further understand the rate of UHMWPE
wear in vivo. Micro-CT wear mapping of retrieved devices
and correlation with clinical outcomes will help establish
clinically relevant wear rates. Further comparison of clin-
ical results to those from rTSA wear simulations will pro-
vide a vital tool to evaluate and to validate the motion
profiles used for this and other rTSA simulations and to
contextualize the results of previous and future wear sim-
ulations in terms of their potential clinical relevance.
Previous rTSA wear simulations have characterized the
effect of the presence of a hole at the pole of the gleno-
sphere articulating surface, cross-linked polyethylene hu-
meral liners, and inverted bearing materials on rTSA wear
properties.
19,31,39
This simulation adds to a growing body
of knowledge about the effect of device design parameters
on wear rates in rTSA and more broadly the effects of
device design on component wear in joint replacement. The
limitations of this study include the in vitro nature of the
simulation, the use of only 1 model of rTSA component,
and the use of only a single end point for micro-CT anal-
ysis. Continued clinical evaluation of retentive liner device
performance is warranted for complete evaluation of this
device. The main objective of this simulation was to
compare the relative wear rates of retentive and non-
retentive rTSA liners using an established in vitro wear
simulation protocol.
31,39
The increased wear observed in
retentive liners is consistent with existing literature on fully
constrained liners in total hip arthroplasty and suggests that
the added constraint of retentive liners significantly in-
creases wear volume at later time points, potentially
increasing the incidence of polyethylene wear-induced
aseptic loosening during extended implantation periods;
however, this was not evaluated in the current study.
Future research linking wear rates to clinical outcomes
of rTSA systems will help clarify this finding. Only 1
model of humeral liner was tested in this study. Thus,
whereas the trends observed in this study are generalizable
to other retentive rTSA models, which are all character-
ized by a deeper concavity to engage the glenosphere
compared with standard liners, the absolute wear rates
calculated in this study should not be assumed to be
consistent for other models. The micro-CT–based tech-
nique described in this study was performed only after
completion of the simulation. Whereas comparison of
worn to control samples was a valid and repeatable
method of measurement,
30,35
pre-simulation and mid-
simulation scans could be used in future studies to track
the progression of wear within each liner and to account
for the effects of creep deformation.
Conclusion
The results of this in vitro wear simulation indicate that
retentive liners undergo significantly greater volume loss
than nonretentive liners at later time points representing
extended implantation times. The additional stability
afforded by retentive liners should be balanced against
the potential for increased wear and subsequent aseptic
loosening. Surgeons should be aware of this potential
complication when selecting implants during rTSA.
Acknowledgment
The authors acknowledge Basma Khoury, MS, who
facilitated micro-CT scanning of rTSA liners (MicroCT
Core, University of Michigan, Ann Arbor, MI, USA).
rTSA wear simulation of retentive liners 1377
Disclaimer
J. Michael Wiater receives consulting fees from Zimmer,
Synthes Inc., and Tornier Inc.; receives research support
as principal investigator from Zimmer and Tornier Inc.;
and has received payment for presentations and speaking
engagements from Zimmer Inc. All the other authors,
their immediate families, and any research foundations
with which they are affiliated have not received any
financial payments or other benefits from any commer-
cial entity related to the subject of this article.
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