Quantified Mechanical Properties of the Deltoid Muscle Using the Shear Wave Elastography: Potential Implications for Reverse Shoulder Arthroplasty

Abstract

The deltoid muscle plays a critical role in the biomechanics of shoulders undergoing reverse shoulder arthroplasty (RSA). However, both pre- and postoperative assessment of the deltoid muscle quality still remains challenging. The purposes of this study were to establish a novel methodology of shear wave elastography (SWE) to quantify the mechanical properties of the deltoid muscle, and to investigate the reliability of this technique using cadaveric shoulders for the purpose of RSA. Eight fresh-frozen cadaveric shoulders were obtained. The deltoid muscles were divided into 5 segments (A1, A2, M, P1 and P2) according to the muscle fiber orientation and SWE values were measured for each segment. Intra- and inter-observer reliability was evaluated using intraclass correlation coefficient (ICC). To measure the response of muscle tension during RSA, the humeral shaft was osteotomized and subsequently elongated by an external fixator (intact to 15 mm elongation). SWE of the deltoid muscle was measured under each stretch condition. Intra- and inter-observer reliability of SWE measurements for all regions showed 0.761-0.963 and 0.718-0.947 for ICC(2,1). Especially, SWE measurements for segments A2 and M presented satisfactory repeatability. Elongated deltoid muscles by the external fixator showed a progressive increase in passive stiffness for all muscular segments. Especially, SWE outcomes of segments A2 and M reliably showed an exponential growth upon stretching (R2 = 0.558 and 0.593). Segmental measurements using SWE could be reliably and feasibly used to quantitatively assess the mechanical properties of the deltoid muscle, especially in the anterior and middle portions. This novel technique based on the anatomical features may provide helpful information of the deltoid muscle properties during treatment of RSA.

Fig 1. Schematic of the shoulder experimental custom-made device.

Fig 2. Deltoid muscle anatomy for positioning of ultrasound probe during SWE measurements.

SWE was examined percutaneously and the values were obtained from 5 segments; anterior (A1, A2), middle (M), and posterior (P1, P2).

Fig 3. Experimental elongation of the deltoid muscle.

Tensile strain in the muscle was generated with the external fixator (arrow). SWE probe was placed at the midpoint level of the deltoid muscle. SWE was examined with intact length (0 mm), and under elongated conditions (+5, +10, and +15 mm).

Fig 4

A) SWE images of elongated deltoid muscle. The colored regions represent the SWE modulus map with the scale to the right of the figure. Arrow head represents the osteotomized region without elongation (0 mm). Arrows represent the extent of elongation (+5, +10, and +15 mm). B) A circular region of interest (ROI) was used to obtain SWE values that included the entire thickness of the muscle.

Fig 5. Distribution of SWE values obtained from deltoid muscles with and without elongation.

Overall data for each muscular segments were fit using exponential growth curves with R² = 0.390 for A1, 0.558 for A2, 0.593 for M, 0.421 for P1, and 0.306 for P2.