Challenges for Optimization of Reverse Shoulder Arthroplasty Part II: Subacromial Space, Scapular Posture, Moment Arms and Muscle Tensioning

Abstract

In part II of this comprehensive review on the optimization of reverse shoulder arthroplasty (RSA), we focus on three other challenges: 1. "Conservation of sufficient subacromial and coracohumeral space"; 2. "Scapular posture"; and 3. "Moment arms and muscle tensioning". This paper follows a detailed review of the basic science and clinical literature of the challenges in part I: 1. "External rotation and extension" and 2. "Internal rotation". "Conservation of sufficient subacromial and coracohumeral space" and "Scapular posture" may have a significant impact on the passive and active function of RSA. Understanding the implications of "Moment arms and muscle tensioning" is essential to optimize active force generation and RSA performance. An awareness and understanding of the challenges of the optimization of RSA help surgeons prevent complications and improve RSA function and raise further research questions for ongoing study.

Keywords: abduction; biomechanics; lateralization; rotator cuff length; scapula; scapulothoracic; subacromial space.

Figure 1

Influence of the position and lateralization of the glenosphere (GS) on abduction. (A) Superior GS position with lateral offset; (B) Superior GS position without lateral offset; (C) Inferior GS position with lateral offset providing the best abduction-adduction range; (D) Inferior GS position without lateral offset. Figure reused and modified with permission, Gutiérrez et al. [18].

Figure 2

Types of thoracic sagittal balance and scapular protraction and rotation (A–C). (A) Normal scapular protraction and rotation <45°. (B) Increased scapular protraction and rotation. (C) Severe scapular protraction and rotation >45°. (Figure reused from the article of Bauer et al. [4]. Adapted from the article of Moroder et al. [2].

Figure 3

CT images of a clinical case. (A) C-type scapular internal rotation and protraction with (B) Anterior overstuffing after RSA lateralization and (C) Mismatch between the version of the GS, and humeral implant torsion.

Figure 4

Changes of the length of moment arm (blue) according to RSA design compared to normal anatomy. G: Glenosphere. H: Humerus. (A) Medial G—Medial H with increased length of moment arm compared to (B) Lateral G—Medial H and (D) Anatomic state. (C) Medial G—Lateral H provides the longest moment arm. Figure reused and modified with permission, Hamilton et al. [9].

Figure 5

(A) Distalization shoulder angle (DSA) defined as the angle between a line connecting the superior glenoid tubercule and the most lateral part of the acromion and a line connecting the superior glenoid tubercule and the most superior part of the greater tuberosity. (B) Lateralization shoulder angle (LSA) defined as the angle between a line connecting the superior glenoid tubercule and the most lateral part of the acromion and a line connecting the most lateral part of the acromion and the most lateral part of the greater tuberosity [42].

Figure 6

ERO moment arms for rotation from 30° IR to 60° ER at 30° of ABD. Infraspinatus. (b) Teres minor. (c) Posterior deltoid. (d) Same plot of posterior deltoid scaled the same as (a,b). L: Lateral. M: Medial. G: Glenosphere. H: Humerus. Figure reused with permission, Hamilton et al. [9].