Quantification of Deltoid Muscle Elasticity for Reverse Shoulder Arhthroplasty: Feasibility Assessment of Shear
Wave Elastography
Taku Hatta1, 2, Hugo Giambini1, Koji Sukegawa1, Yoshiaki Yamanaka1, John W. Sperling1, Scott P. Steinmann1, Eiji Itoi2, Kai-Nan An1
1
Mayo Clinic, Rochester, MN, USA, 2Tohoku University Graduate School of Medicine, Sendai, Japan
Disclosures: Hatta T (N), Giambini H (N), Sukegawa K (N), Yamanaka Y (N), Sperling JW (N), Steinmann SP (N), Itoi E (N), An KN (N)
INTRODUCTION:
Reverse shoulder arthroplasty (RSA) is a common surgical option in patients with severe shoulder pathologies. Despite its promising results, there exists
variability regarding several clinical variables after RSA. Among the factors affecting outcomes, postoperative properties of the deltoid muscle is important
since this muscle generates glenohumeral elevation. Excessive muscular tension in the deltoid muscle may be associated with related pain, restricted motion,
or acromion fractures. On the contrary, if the deltoid muscle presents insufficient tension after RSA, it may lead to unsatisfactory outcomes including
decreased strength for shoulder motion or postoperative instability. Currently, surgeons determine the appropriate combination of RSA implants based on
their experiences to assess the tension and stability in replaced joints.
Shear wave elastography, an ultrasound technique, is a novel technique to quantitatively assess material properties of muscles and other soft tissues.
Although several studies have shown that this technique enables the assessment of muscle stiffness in various muscular conditions or pathologies, there are
no studies investigating the deltoid muscle. We believe this should be a valuable tool for quantifying the deltoid muscle properties aiding in preoperative or
intraoperative assessment of RSA.
Anatomically, the deltoid muscle has been classically divided into 3 portions:
anterior, middle, and posterior portions. Recently, an anatomical study based on the
distribution of intramuscular tendons enabled the division of the anterior and
posterior portions into other segments (Sakoma et al. J Anat 2011). Therefore, we
attempted to assess each part of the deltoid muscle independently, with prospects for
future usage to assess the activation pattern or mechanical properties of the muscles.
The purpose of this cadaveric study was 1) to determine the feasible placement
of the ultrasound probe for SWE imaging according to the muscle fiber orientation
on the deltoid muscle regions, and 2) to investigate the reliability and validity of this
SWE technique as a tool for identifying deltoid muscle conditions under different
amounts of stretch during RSA procedure.
METHODS:
Eight (8) fresh-frozen shoulders were used after institutional review board
approval from Mayo Clinic. The scapula was disarticulated from the thorax, and the
humerus was cut at the level of the midshaft. The scapula and a fiberglass rod inserted into the humeral medullary canal were both secured in a customdesigned experimental device.
An ultrasound system (Aixplorer; Supersonic Imagine) and a linear array probe (10-2 MHz) were used to perform the ultrasound examinations. Images
for the SWE measurements were obtained from 5 muscular segments based on muscle fiber orientation; anterior (A1, A2), middle (M), and posterior
segments (P1, P2). SWE values for each region were assessed independently on the plane parallel to the muscle fibers (Fig. 1). Intra- and inter-observer
reliability was evaluated on the current SWE technique for measuring the deltoid muscle elasticity. In addition, to assess SWE values variability based on the
tensile stress on the deltoid muscle, we compared SWE values after humerus osteotomy (0 mm) with those under elongated conditions (+5, +10, and +15
mm) generated with an external fixator and simulating muscle stretch during RSA (Fig. 2).
Intra- and inter-observer reliability was examined using intraclass correlation coefficient (ICC; ICC(1,1) and ICC(2,1), respectively). Friedman with
Dunn’s post hoc tests was used to evaluate the difference in SWE values of the deltoid muscles under elongated conditions. The significance level was set to
P < 0.05.
RESULTS:
Intra- and inter-observer reliability was satisfactory for all segments of
the muscle (ICC(1,1) of 0.765- 0.957, ICC(2,1) of 0.718- 0.947). In
particular, high repeatability was observed in A2 and M.
Elongated deltoid muscles with the external fixator caused a progressive
increase in mechanical properties for all muscular segments (Table. 1).
Especially in A2, M, and P1, SWE values increased two-fold at 15 mm
elongation compared to those at the original length.
DISCUSSION:
To our knowledge, this is the first elastographic study focusing on the
deltoid muscle. Previous studies reported that the deltoid muscle consists of
several segments with distinct fiber architectures. Accordingly, our SWE
measurements were based on these distinctions. We believe this SWE
technique could be a reliable and feasible tool for quantitatively assessing
deltoid muscle elasticity.
There are two limitations in this study. SWE data for the deltoid muscle
were obtained from cadaveric shoulders. In addition, this sample size was
small to determine standard values. Using this methodology, therefore,
future investigations including more samples and live subjects would
define SWE patterns and allow for a more robust correlation of
postoperative recovery of deltoid muscle function during RSA procedure.
SIGNIFICANCE:
This study demonstrates a first step towards the assessment of the material properties of the deltoid muscle using SWE. This novel technique may provide
a useful assessment tool to determine optimal deltoid muscle tension in preoperative planning and intraoperative evaluation of RSA.
ACKNOWLEDGEMENTS:
This study was supported by the grant from NIH (NIAMS) R21 AR065550 and T32 AR56950.
ORS 2016 Annual Meeting Poster No. 2021