The anterior deltoid’s importance in reverse shoulder
arthroplasty: a cadaveric biomechanical study
Daniel G. Schwartz, MD*, Sang Hoon Kang, PhD, T. Sean Lynch, MD, Sara Edwards, MD,
Gordon Nuber, MD, Li-Qun Zhang, PhD, Matthew Saltzman, MD
Department of Orthopaedic Surgery, Northwestern University, Chicago, IL, USA
Background: Frequently, patients who are candidates for reverse shoulder arthroplasty have had prior
surgery that may compromise the anterior deltoid muscle. There have been conflicting reports on the neces-
sity of the anterior deltoid thus it is unclear whether a dysfunctional anterior deltoid muscle is a contrain-
dication to reverse shoulder arthroplasty. The purpose of this study was to determine the 3-dimensional
(3D) moment arms for all 6 deltoid segments, and determine the biomechanical significance of the anterior
deltoid before and after reverse shoulder arthroplasty.
Methods: Eight cadaveric shoulders were evaluated with a 6-axis force/torque sensor to assess the direc-
tion of rotation and 3D moment arms for all 6 segments of the deltoid both before and after placement of
a reverse shoulder prosthesis. The 2 segments of anterior deltoid were unloaded sequentially to determine
their functional role.
Results: The 3D moment arms of the deltoid were significantly altered by placement of the reverse
shoulder prosthesis. The anterior and middle deltoid abduction moment arms significantly increased
after placement of the reverse prosthesis (P < .05). Furthermore, the loss of the anterior deltoid resulted
in a significant decrease in both abduction and flexion moments (P < .05).
Conclusion: The anterior deltoid is important biomechanically for balanced function after a reverse total
shoulder arthroplasty. Losing 1 segment of the anterior deltoid may still allow abduction; however, losing
both segments of the anterior deltoid may disrupt balanced abduction. Surgeons should be cautious about
performing reverse shoulder arthroplasty in patients who do not have a functioning anterior deltoid muscle.
Level of Evidence: Basic Science Study, Biomechanics, Cadaver Model.
? 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.
Keywords: Reverse total shoulder arthroplasty; shoulder reconstruction; deltoid; flexion; abduction;
moment arms; cadaveric study
Reverse shoulder arthroplasty represents an increasingly
useful tool for the treatment of rotator cuff arthropathy,4,7,11,27
complex proximal humerus fractures,18,29and revision
shoulder arthroplasty.6,19,21The reverse total shoulder pros-
thesis medializes the center of rotation of the glenohumeral
joint and in doing so increases recruitment of deltoid muscles
fibers.5,8,10This increases the moment arm and resultant
glenoid component. A functional deltoid is critical to
The anterior deltoid is seen as a vital structure to
maintain during surgical exposures, and its disruption has
been associated with functional weakness.16,20,23A recent
case series of anterolateral deltoid muscle ruptures after
*Reprint requests: Daniel G. Schwartz, MD, 676 N St. Clair, Suite
1350, Chicago, IL 60611, USA.
E-mail address: email@example.com (D.G. Schwartz).
J Shoulder Elbow Surg (2013) 22, 357-364
1058-2746/$ - see front matter ? 2013 Journal of Shoulder and Elbow Surgery Board of Trustees.
reverse total shoulder arthroplasty in a patient population
that had undergone a previous open rotator cuff repairs
demonstrated the importance of the anterior deltoid to
a successful clinical outcome. The patients in this series all
had significant declines in their functional outcome after
anterolateral deltoid rupture occured.28This report seems to
support the concept that the anterior deltoid is vital for
a successful outcome after reverse shoulder arthroplasty.
However, Glanzmann et al reported a successful reverse
shoulder arthroplasty after a previous deltoid muscle flap
transfer,9suggesting that the entire deltoid may not be
necessary for a successful outcome after this procedure.
These diverging reports underscore the importance to
clearly elucidate the role of the anterior deltoid before and
after reverse shoulder arthroplasty.
The purpose of our study is to investigate the moment
arms in all planes of movement of the glenohumeral joint
(flexion, abduction, and rotation) in both the native
shoulder and following reverse shoulder arthroplasty
utilizing a laterally based glenosphere. Additionally, we
sought to better elucidate the necessity of the anterior
deltoid for function of the reverse shoulder arthroplasty.
Materials and methods
Eight fresh-frozen cadaveric right shoulders, 6 male and 2 female,
with ages ranging between 46 and 68 years were obtained. The
specimens were stored at -20?C and thawed for 24 hours before
testing. Specimens were excluded from use if they were found to
have prior surgery or deltoid muscle compromise.
The skin, subcutaneous, and adipose tissues were removed
around exposing the underlying muscles. The planes dividing the
anterior (clavicular head), middle (acromial head), and posterior
(posterior scapular spine head) deltoid were defined. Each
segment was measured and divided evenly into 2 portions, making
6 total portions of the deltoid that was then sharply removed from
the scapular spine, acromion, and clavicle. The anterior most
portion of the anterior deltoid was labeled anterior deltoid #1, the
next portion anterior deltoid #2, etc. (Fig. 1).
Nylon mesh was sewn into each of the deltoid segments as well
as the latissimus dorsi and pectoralis major with 0 Vicryl suture
(Ethicon, Somerville, NJ, USA). A stainless steel cable of 1.6 mm
in diameter was sutured to the Nylon mesh. The origin of each
deltoid muscle segment was noted, and an eyelet was placed at the
site to guide excursion of the muscle. The cables were threaded
through the eyelets prior to mounting the specimen on the
experimental apparatus. The clavicle was pinned to the acromion
in its anatomic position.
The scapula was firmly mounted to a customized metal plate with
plates, and screws. The plate was placed with the glenoid vertical,
and the center of rotation of the humeral head in line with a 6-axis
force/torque (F/T) sensor (JR3, Inc., Woodland, CA, USA). Metal
struts extended from the sensor to a platform where the humerus
was fixed inside a metal cylinder with screws, which, in turn, was
clamped to the apparatus (Fig. 2). The position of the humerus
relative to the 6-axis F/T sensor center was noted with a digitizer
(Immersion, San Jose, CA, USA) both before (the anatomical or
native specimen) and after surgery (the prosthetic specimen) to
minimize the error due to the possible offset between the center of
rotation and the 6-axis F/T sensor. A 3D Flock of Bird motion
sensor (Ascension Technology, Burlington, VT, USA) provided
the 6 degree-of-freedom positions and angles of the humerus
during the experiment.
The cables sewn into the muscles were then passed through
eyelets at the measured midpoint of each muscle portion’s insertion
and onto pulleys that were connected to free hanging weights. The
weight applied to the each portion of deltoid was approximately
10% of the maximum force, which can be generated by each
portion, estimated by published physiological cross sectional areas
of the deltoid.2,13,15,26A precision linear motor (LinMot, Sprei-
tenbach, Switzerland) was attached to a selected cable to generate
individual dynamic force, up to 60 N, with constant low velocity to
each of the deltoid segments to determine their 3D mechanical
actions. The pectoralis major and latissimus dorsi were each loaded
with 10 N of weight statically throughout the experiment.
After mounting the scapula and humerus onto the apparatus, the
humerus was placed at 0?of abduction and neutral rotation with
respect to the epicondylar axis. Its position relative to the 6-axis
force sensor was digitized. Weight was added to the cables, and
the linear motor sequentially cycled through each segment
(starting with anterior deltoid #1, ending with posterior deltoid #2)
of the deltoid within pre-defined maximum pulling force limit
(the 10% of maximum pulling force of each portion of deltoid)
while the free weight remained on the other portions providing
a physiologic static load to the joint. The 3D moment arm was
acromion; HH, humeral head; S, scapular spine.
Six portions of deltoid labeled. C, the clavicle; Ac, the
358D.G. Schwartz et al.
determined for each deltoid segment. Next, all muscles were
reloaded with free weights, while measuring the changes in the
moments while the anterior most 3 portions of the deltoid (anterior
deltoid #1 and #2, middle deltoid #1) were sequentially unloaded.
The position and angle of the humerus was then captured via the
Flock of Bird motion sensor. This entire procedure was repeated at
45?and 90?of glenohumeral abduction equivalent to 135?of
arm abduction when accounting for theoretical scapulothoracic
Next, the humerus was placed in a neutral position and
released from its clamping while weights hung on each segment.
This allowed the humerus to move into a position of equilibrium
that was measured with the Flock-of-Bird motion sensor. We then
sequentially unloaded the anterior deltoid and the anterior portion
of the middle deltoid while measuring the change in position of
the distal humerus.
The senior author (M.D.S.) implanted a reverse total shoulder
(RSP, DJO Surgical, Austin, TX, USA) (32 - 4 glenosphere, 32
standard shell and polyethylene liner) while the scapula remained
mounted to the apparatus to minimize the error because of
remounting. During the surgical approach, the rotator cuff was
excised to simulate the environment of a massive rotator cuff tear
that predominates in this patient population.4,24The surgical
technique of the DJO Encore prosthesis was followed including
cutting the humerus in 30?of retroversion. We attempted to
tension the prosthesis such that the shoulder could be ranged
without instability, but that there was no undue tension on the
deltoid or attached acromion. The aforementioned entire protocol
was repeated with the implanted reverse total shoulder prostheses
in the specimen.
Three-dimensional mechanical actions, including moment arms
in flexion/extension, abduction/adduction, and internal/external
rotation, were estimated from each specimen both before (the
native specimen) and after surgery (prosthetic) using the measured
3D torques and pulling forces. The exact same method for
measuring the moment arms was used in each phase of the study.
To investigate the role of each portion of deltoid, direction of
rotation (DOR) was computed from the measured 3D moments
with respect to a moving coordinate system (Fig. 2) located at the
glenohumeral joint center with the following 3 orthogonal axes: u
axis being the internal rotation axis parallel to long axis of
humerus; w axis the adduction axis always in frontal and sagittal
plane; and v axis, the flexion axis, orthogonal to both u axis and w
axis. By using the DOR, one can clarify the role of a (portion of)
muscle30and aid in comparison of muscle portions. DOR is
computed as follows:
where DCu, DCv, and DCwdenote 3 components of DOR: Mu, Mv,
and Mwdenote internal rotation, flexion, and adduction moment
about the shoulder (Fig. 2), if a portion of muscle only functions as
an abductor,then DORmust be DCu¼ 0,DCv¼ 0,and DCw¼ 1.If
DCu¼ 1, the portion of the muscle’s role is internal rotation with
its rotation axis coinciding with the internal rotation axis and if
close to -1 external rotation. If DCu¼ 0, it has no mechanical
action in internal or external rotation with its rotation axis perpen-
to 1, and an extensor if close to -1; DCwis an adductor if close to 1,
and an abductor if close to -1. Thus, by examining DOR of
each portion of deltoid, one can define the role of each portion of
metal plate. This figure demonstrates the different positions (0, 45, and 90?) that the specimen was tested at. Note the 6-axis force/torque
sensor directly behind the glenohumeral joint that captures the moments about the joint.
Experimental setup for right shoulder. Note H denotes the base of the humeral shaft and SB is the scapular body attached to the
Biomechanical study of the anterior deltoid359
it is possible to make comparisons both between portions of
deltoids and before and after surgery. Practically, we can use the
angle between the rotation axis of a portion of deltoid muscle with
the abduction axis to evaluate its role in abduction.
Statistical analysis using a paired student t test with the level of
significance defined as P < .05 was performed after a normality
test of each set of data for pre-post comparison. During the portion
of the experiment when the anterior deltoid was unloaded, a 1
sample t test with significance level defined as P < .05 was per-
formed for flexion moment change and adduction moment change.
Direction of rotation (DOR)
The 3D mechanical action measured by DOR for each
segment of the deltoid before and after the surgery is listed in
Table I. The most inferior portion of the anterior deltoid
(anterior deltoid#1) changed fromanadductortoanabductor
after implantation of the reverse shoulder prosthesis at 0?of
glenohumeral abduction. There were no other significant
is an important flexor of the glenohumeral joint.
Although the DOR remained unchanged for most segments
of the deltoid, increases in several moment arms were
noted (Fig. 3, Table II). The anterior deltoid #1 increased
its abduction moment (P < .01) and flexion moment (P <
.01) in the postsurgical specimen (Table II). At 45?of
glenohumeral abduction, all portions of the anterior and
half of the middle deltoid experienced significant increases
in their abduction moment arms (P < .05). At 90?of
glenohumeral abduction, increases in moment arms were
Isolation of the anterior deltoid
Flexion and abduction moment change were examined
under 3 different conditions: the first portion of the anterior
deltoid unloaded, the entire anterior deltoid unloaded, and
the anterior and first portion of the middle deltoid unloaded.
This allowed us to determine the contribution of each
portion of the anterior deltoid to flexion and abduction
The flexion moment decreased significantly after the first
portion of the anterior deltoid was unloaded, and all other
portions of the deltoid remained loaded at 0?and 45?of
abduction in all pre- and postsurgical specimens (P <.05). In
the native specimen with the entire anterior deltoid and
a portion of the middle deltoid unloaded, the flexion moment
decreased (P < .05) due to the absence of important
contributors (anterior and a portion of the middle deltoid) to
flexion. After placement of the reverse total shoulder
arthroplasty at 0?and 45?, when either a portion of the
anterior deltoid or the entire anterior deltoid was unloaded,
the flexion moments significantly decreased (P < .01).
Similar results occur when examining abduction moment
arms. After placement of the reverse total shoulder arthro-
plasty, abduction was significantly decreased when any
segment of the deltoid was unloaded at 0?. At 45?, no signif-
icant decrease in abduction occurred when only 1 portion
of the anterior deltoid was unloaded, but became significant
(P < .01) when the entire anterior deltoid was unloaded. At
90?, there were no significant changes in flexion; however,
abduction decreased significantly when a portion of the ante-
rior deltoid was unloaded (P < .05) and when the entire
anterior and middle deltoid were unloaded (P <.01).
Shoulder angle (deg)
Direction of rotation before and after surgery
Portion of deltoidPreoperativePostoperative
Small font size means weak action as an internal/external rotator, flexor/extensor, or adductor/abductor. Similarly, middle and large font sizes represent
middle and strong action as an internal/external rotator, flexor/extensor, or adductor/abductor as defined by the DOR.
360D.G. Schwartz et al.
The reverse shoulder prosthesis has revolutionized the way
we treat cuff tear arthropathy, failed shoulder arthroplasty,
and fractures in cuff deficient shoulders. This prosthesis
functions by recruiting deltoid muscle fibers to initiate
glenohumeral abduction and rotation. Unfortunately, many
patients who have these problems have had prior surgery
that may compromise the integrity of the anterior deltoid
muscle. Thus it is important to not only understand how
reverse shoulder arthroplasty changes the moment arms of
the various segments of the deltoid but to also determine if
the anterior segments of the deltoid must be functional for
successful reverse shoulder arthroplasty.
Ackland et al have reported previously on the increased
moment arm of anterior and middle deltoid moment arms
after placement of a reverse shoulder prosthesis.1These
authors utilized a Grammont-style reverse prosthesis that
medializes and lowers the glenohumeral center of rotation.
Our study differs from theirs in that we utilized a prosthetic
design with a more lateralized center of rotation (by 6 mm).
Like Ackland et al, we found that the anterior deltoid
functions as an abductor and flexor of the glenohumeral
joint after implantation of a reverse shoulder prosthesis.
Thus it appears that these biomechanical advantages can be
obtained with either a medial or lateral offset reverse
Reverse shoulder prosthesis designs with a more later-
alized glenosphere have been introduced in an effort to
obtain a more anatomic offset of the center of rotation and
to reduce scapular notching.3,5,7,14,22While our study did
not directly compare a Grammont style prosthesis to
with the prosthesis (post). (A) and (B) are the changes in the rotational moment arms. (C) and (D) are the changes in the flexion/extension
moment arms. (E) and (F) are the changes in the abduction/adduction moment arms.
(A)-(F) represent the changes in moment arms of the 6 deltoid portions in the native specimens (pre) and specimens implanted
Biomechanical study of the anterior deltoid361
a lateralized offset prosthesis, the center of rotation with
lateral offset designs has been shown in the literature to be
significantly different from Grammont style prostheses.25
This change in center of rotation affects the tension on
the deltoid as well as any remaining rotator cuff muscula-
ture. Our data confirm that the anterior and middle deltoid
moment arms are increased with a lateral offset design
Active rotation of the glenohumeral joint is difficult to
consistently improve following reverse shoulder arthro-
plasty.4,7,11,27While limited to the design of a cadaveric
study, to our knowledge, the present study is the only work
to study all 3 degrees of freedom (flexion-extension,
abduction-adduction, and internal-external rotation) follo-
wing reverse shoulder arthroplasty. This information is
potentially useful as physiologic motion occurs in the 3D
Whatley et al28reported on a series of patients that
experienced anterior deltoid rupture following reverse total
outcomes. In our model, when the anterior deltoid was
isolated and unloaded, significant decreases in flexion and
abduction were demonstrated. Thus our biomechanical data
support the clinical findings of Whatley et al, namely that
the anterior deltoid is indeed a vital structure to a functional
reverse total shoulder arthroplasty.
There are implicit limitations to our data. This is
a cadaveric model that clamped the scapula to our experi-
mental apparatus in one plane with the glenoid vertical
and in line with biomechanical sensor. This setup does
not account for the scapula’s normal 3D rotation during
shoulder movement.17Additionally, we performed our
experiment in 0, 45, and 90?of abduction. Thus our find-
ings may not truly reflect what occurs during physiologic
motion that occurs in a continuous plane, and may not be
accurate physiologic expectations in a clinical setting when
the extremes of abduction may not be obtainable. While
flexion and abduction moments decreased significantly
when the deltoid was unloaded, further study is needed to
investigate the relationship between this biomechanical
Moment arm changes (mm)
Ru (int/ext rotation) Rv (flexion/extension)Rw (adduction/abduction)
(Preoperative) (Postoperative)(Preoperative) (Postoperative) (Preoperative) (Postoperative)
Bold text indicates significant results.
* denotes P < .05; ** denotes P < .01 (paired t test).
Moment change (Nm)
Shoulder angle (deg)Flexion moment changeAdduction moment change
0ant 1 unloaded
ant 1&2 unloaded
ant 1&2 and mid 1 unloaded
ant 1 unloaded
ant 1&2 unloaded
ant 1&2 and mid 1 unloaded
ant 1 unloaded
ant 1&2 unloaded
ant 1&2 and mid 1 unloaded
Bold text indicates significant results.
* denotes P < .05; ** denotes P < .01 (independent t test).
362D.G. Schwartz et al.
change and the clinical significance. Despite these limita-
tions, our biomechanical data suggest that the anterior
deltoid is crucial to good function following reverse
Three-dimensional moment arms (internal-external rota-
tion, flexion-extension, and adduction-abduction) of 6
portions of deltoid before and after reverse total arthro-
plasty were evaluated. After arthroplasty, abduction and
moment arms, it was confirmed that the anterior deltoid is
vital to flexion and abduction following reverse total
that surgeons should cautiously use reverse total shoulder
arthroplasty in patients with anterior deltoid insufficiency.
There was no source of external funding for this project.
The implanted prostheses used were donated from DJO
Surgical, Austin, TX, USA. The authors, their imme-
diate families, and any research foundations with which
they are affiliated did not receive any financial payments
or other benefits from any commercial entity related to
the subject of this article.
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