The scapulothoracic conundrum in reverse shoulder where do we stand and what is yet to expand?

Authors: Abdelkader Shekhbihi, Philipp Moroder, Pascal Boileau, Winfried Reichert, Arnold J Suda, Markus Scheibel

Abstract

The normal functioning of the shoulder is characterized by the harmonious coordination between the glenohumeral joint and the scapulothoracic complex, a phenomenon commonly referred to as scapulohumeral rhythm (SHR).Reverse total shoulder arthroplasty (rTSA) shoulders exhibit distinct kinematics compared to normal shoulders.Reduced scapulohumeral rhythm (SHR) in rTSA shoulders implies a greater reliance on scapulothoracic motion over glenohumeral motion for arm elevation.Dynamic analyses suggest heightened scapulothoracic movement after rTSA, implying alterations in rotational movements across various planes.Utilization of reliable tools to measure preoperative scapulothoracic motion and forecast postoperative SHR in rTSA may improve functional results.Posture types and scapulothoracic orientation play an important role in optimal implant configuration and positioning, as well as clinical outcome, and should therefore be considered during patient selection, preoperative planning, and implantation of an rTSA.Recognizing the static position and kinematic changes of the scapulothoracic joint is vital for postoperative rehabilitation and optimizing outcomes in rTSA patients.

Introduction

In 1884, Cathcart (1) laid the groundwork for understanding the scapulothoracic joint’s role in normal shoulder complex kinematics, emphasizing the key relationship between scapular and glenohumeral motion. This insight, paramount for comprehending shoulder joint mechanics and pathology, prompted Codman (2) to coin the term ‘scapulohumeral rhythm’ (SHR) to describe this harmonious interplay. Inman et al. (3) later observed an SHR ratio of 1 (2 degrees of humeral flexion/abduction to 1 degree of scapular upward rotation) in healthy subjects, becoming a focal point in subsequent shoulder kinematics research (4, 5, 6, 7, 8). The journey continued with Braman et al. (9), who authored one of the earliest reports on the restoration of SHR in a case of a female patient with advanced glenohumeral osteoarthritis following total shoulder arthroplasty (TSA).

As the incidence of reverse total shoulder arthroplasty (rTSA) rises (10, 11) and reports of various related complications surface (12, 13), recent research endeavors have focused on an often overlooked yet critical component in shoulder girdle kinematics – the scapulothoracic complex. This renewed attention aims to elicit critical insights into the subtle biomechanics of altered ball-and-socket dynamics and the associated scapulothoracic interaction in the context of rTSA.

The authors conducted this literature review with a profound recognition of the significant role the scapulothoracic joint holds within the rTSA population. Despite the extensive focus on glenohumeral configuration in this demographic, there is an apparent shortage of substantial material in the current literature addressing scapulothoracic orientation and dynamics. This paper is intended to serve as a foundational resource paving the way for further investigations into the evolving landscape of scapulothoracic orientation and kinematics (quantitative analysis of scapular movement patterns in relation to the thorax) in rTSA by investigating several key How do alterations in ball-and-socket dynamics following rTSA impact scapulothoracic kinematics?What are the implications of scapulothoracic biomechanical changes for shoulder stability and range of motion in individuals undergoing rTSA?Can advanced imaging techniques provide valuable insights into scapulothoracic orientation and dynamics post-rTSA?How might a deeper understanding of scapulothoracic biomechanics inform surgical decision-making and rehabilitation strategies for rTSA patients?

The Grammont reverse total shoulder prosthesis

In 1985, Paul Grammont (14) introduced a groundbreaking development in shoulder arthroplasty with the Delta shoulder prosthesis, deviating from the constrained designs prevalent in the 1970s. This innovative design, characterized by a lowered and medialized center of rotation, redefines the moment arm of the deltoid (15). Grammont’s inventive approach unveils several crucial biomechanical foremost, the inclusion of a large ball facilitates an extended range of motion and bolsters stability compared to smaller alternatives. Second, the deliberate absence of a neck creates a minimal lateral offset, aligning the center of rotation directly with the glenoid surface and thereby minimizing torque at the glenoid component fixation point, ensuring stability. Third, the medialization of the center of rotation plays a key role in engaging a more substantial portion of deltoid fibers during elevation or abduction, optimizing biomechanical efficiency. Finally, the intentional humerus lowering in the Delta prosthesis design elevates deltoid tension, adeptly compensating for the lack of a functional rotator cuff (14). The triumph of this particular prosthetic design in mitigating pain and reinstating functional outcomes has prompted a broadening of the indications for rTSA to include failed anatomical TSA, pseudoparalysis of the shoulder, irreparable cuff tears, cuff tear arthropathy, complex proximal humerus fractures, and tumors (16).

The lateralization of rTSA components with a medialized center of rotation, in accordance with the Grammont design, aims to enhance deltoid muscle torque, thereby imparting joint stability by transforming shear forces at the glenoid component–bone interface into compressive forces (15). Alterations in glenoid component inclination, for example, in the superior direction, may theoretically introduce inferiorly directed deltoid muscle forces and subsequent upward scapular elevation, even in the resting position.

On the other hand, the contribution of humeral component distalization to deltoid muscle tension is well established as a key principle in the Grammont prosthesis. However, the precise degree of recommended humeral distalization remains unclear (7). Beyond à priori deductions, the impact of both lateralization and distalization on the SHR in the context of rTSA remains uncharted territory warranting exploration in future investigations.

Ball-and-socket mechanics post-rTSA

Beyond implant the overlooked influence of scapular kinematics on rTSA mobility

Reverse shoulder arthroplasty emerges as a transformative solution for a subgroup of patients previously lacking viable alternatives, offering functional improvements (15, 17, 18). However, despite these advancements, the majority of rTSA patients do not fully regain shoulder elevation, with mean maximal elevation ranging from 105° to 138° in the existing literature (15, 19, 20, 21).

On the other hand, there are patients that received the same implant for a particular pathology and display a full range of motion, even for internal rotation. This discrepancy brings the mobility of the scapula even more into focus.

Scapulothoracic compensation as a key factor in virtual vs actual range of motion disparities

In a recent study conducted by Berhouet et al. (22), a comparison between virtual and actual postoperative ranges of motion in patients undergoing rTSA revealed significant disparities. During clinical assessment, passive forward elevation and abduction, both with and without manually locking the scapulothoracic joint, demonstrated significant differences in shoulder joint elevation and abduction in virtual versus actual motion. This observed disparity was attributed to the preoperative oversight of scapulothoracic motion (movement of the scapula relative to the thorax), which, as indicated by the existing literature, is heightened in rTSA shoulders (23, 24). Addressing this aspect in preoperative considerations may prove instrumental in aligning virtual and actual postoperative outcomes for enhanced patient-specific prognostic accuracy.

Deltoid biomechanics and muscle synergies in rTSA

The existing body of the literature uniformly highlights the deltoid muscle’s essential role in shoulder biomechanics. In a healthy native shoulder, the anterior deltoid primarily functions as a flexor, the middle deltoid as an abductor, and the posterior deltoid as an extensor (25). Similarly, the importance of the deltoid muscle in the context of rTSA is well established; however, there is a remarkable gap in the literature concerning the contribution of scapular motion to shoulder elevation in rTSA shoulders. In contrast to the native shoulder deltoid, in rTSA, the deltoid acts primarily as an abductor, and the spherical glenoid component, known as the glenosphere, functions as a fixed fulcrum, translating the superior pull exerted by the deltoid into arm elevation (26). The inherent restraint on superior migration of the humeral head, determined by the glenosphere, converts the deltoid force into a rotational component (27). In a cadaveric study, Ackland et al. (28) demonstrated that the superior part of the pectoralis major and the anterior deltoid were most effective in flexion, especially at the beginning of movement. Meanwhile, the teres major, latissimus dorsi, and the inferior part of the pectoralis major were identified as adductors and extensors, exhibiting increased torque-producing capabilities after rTSA.

Scapulohumeral rhythm kinematics after rTSA

Relative contribution of the glenohumeral and scapulothoracic joints in rTSA

In a previously reported study, Bergmann et al. (29) demonstrated that the predominant motion in an rTSA shoulder originated from the glenohumeral joint, contributing to approximately two-thirds of humeral tray elevation. They concluded that the combined contribution of the scapulothoracic and glenohumeral joints to arm elevation was comparable to that of healthy subjects. In contrast, Mahfouz et al. (24), utilizing three-dimensional modeling based on fluoroscopic data, observed that scapular rotation played a more substantial role in overall arm elevation in rTSA shoulders.

Alterations in scapulothoracic motion after rTSA

Kwon et al. (23) employed a 3D electromagnetic motion capture system to analyze dynamic movement of the trunk, scapula, and humerus during active bilateral shoulder elevation. Their findings confirmed that although the glenohumeral joint remains the primary contributor to motion, the proportional involvement of the scapulothoracic joint is significantly increased in rTSA shoulders. Using similar noninvasive techniques, Kontaxis and Johnson (30) obtained static scapula position measurements of rTSA shoulders during elevation, revealing a 24% increase in scapular rotation through regression analysis compared to previously measured scapular rotation in normal subjects (5).

Scapulohumeral rhythm in rTSA shoulders

Matsuki et al. (31) demonstrated that the SHR remained consistent between dominant and non-dominant shoulders, with ratios of 2.6:1 and 2.7:1, respectively. However, Walker et al. (4) observed a significantly lower SHR of 1.3:1 in rTSA shoulders during both unweighted and weighted abduction (Fig. 1), suggesting a fundamental change in kinematics following the procedure.

Preoperative kinematics and their role in postoperative outcomes

Numerous studies have indicated that shoulder kinematics undergo alterations in the presence of various pathologies (32, 33, 34). Consequently, the question arises whether the changes in kinematics observed in rTSA shoulders are a result of the anatomical reversal caused by rTSA surgery or if they stem from presurgical adaptations in response to the shoulder pathology. Obtaining preoperative kinematic data in these patients, especially those who struggle to raise the arm before surgery, is challenging.

Potential role of preoperative scapulothoracic motion in rTSA outcomes

Kwon et al. (23) suggested that the preoperative capacity for scapulothoracic motion might serve as a predictor of postoperative outcomes following rTSA. This hypothesis warrants further investigation to determine whether preoperative scapulothoracic mobility could help anticipate postoperative function and guide surgical planning.

Role of imaging in evaluation of scapular dynamics

Initial studies relied on single-plane conventional radiographs to assess shoulder motion (3). However, recently, other modalities such as biplanar fluoroscopy, motion capture systems with surface markers, static two-dimensional imaging, static three-dimensional (3D) and dynamic 3D single-plane fluoroscopic imaging, magnetic resonance imaging, and electromagnetic tracking devices were used by researchers to achieve the same objective (35, 36, 37, 38, 39, 40, 41, 42).

Utilizing EOS imaging, Lindermann et al. (6) observed a decline in SHR during the 90°–120° range of motion in rTSA shoulders, particularly evident at 12 months postoperatively. While EOS provides advantages over conventional computed tomography (CT) scans, such as reduced artifacts and lower radiation exposure, its widespread adoption is hindered by the substantial financial investment required for implementation. Matsuki et al. (31) utilized 3D-to-2D registration techniques to align 3D bone models from CT scans with the bone silhouettes in fluoroscopic images, enabling a comparative analysis of scapular motion changes between the dominant and non-dominant shoulders. The authors indicated that previous studies measuring the SHR utilized unreliable skin markers that demonstrated movement relative to bones, introducing potential inaccuracies in the assessments (8). CT scans (in the supine position with retracted scapulae) would underestimate the altered scapula orientation (i.e. protraction), especially in the older population with thoracic kyphosis. This underestimation may result in implant mismatch and subsequent postoperative joint motion restriction following rTSA (43).

From the authors’ perspective, preoperative upright CT evaluation holds promise not only in detecting abnormal SHR ratios but also in distinguishing between copers (patients with elevated scapulothoracic function and quality of life) and non-copers among those with preoperative SHR impairment.

Implications of the scapulothoracic orientation on outcome and component choice in rTSA

Historically, the Grammont reverse prosthesis is recognized for its distinct medialized and inferiorly shifted center of rotation, resulting in biomechanics that do not only affect shoulder elevation but also rotational movements. rTSA is proven to restore active elevation of the arm; however, multiple studies reported deficits in rotational movements in the horizontal plane (i.e. external and internal rotation). In a biomechanical cadaveric investigation conducted by Stephenson et al. (44), they revealed that maintaining an optimal retrotorsion angle between 20° and 40° is crucial for facilitating an impingement-free range of motion, particularly when the arm is adducted. Another biomechanical study by Berhouet et al. (45) suggested that aligning with the natural humeral retrotorsion angle is the most effective approach to achieving rotational capacity in rTSA. However, both studies primarily assessed impingement and range of motion within the context of a fixed scapular position and did not account for differences in scapula positioning in relation to the thoracic axes.

Moroder et al. (43) investigated and described the importance of accounting for scapulothoracic orientation in the search for ideal component configuration in rTSA. They proposed a classification for different posture types (type A: upright posture with retracted scapulae; type B: intermediate; type C: kyphotic posture with protracted scapulae) in shoulder arthroplasty, with different scapulothoracic orientations in terms of progressive scapular internal rotation, tilt, downward rotation, protraction, and drooping (Fig. 2).

In a first study, they offered theoretical proof that humeral component retrotorsion needs to be adapted to scapula internal rotation to achieve a balanced opposition of the humeral and glenoid components in neutral rotation (43). In a second simulation study, they demonstrated that incorporating scapulothoracic orientation significantly influenced the simulated range of motion across all planes compared to models that did not account for postural variations (46).

However, certain component choices might be able to attenuate the negative effect of poor posture, such as the choice of humeral component retrotorsion, glenosphere size, and eccentricity (47). In a large-scale clinical study including 681 patients with rTSA, Moroder et al. (48) demonstrated that scapulothoracic orientation can significantly impact postoperative clinical outcomes, including measures such as flexion, abduction, shoulder pain and disability index (SPADI), and pain scores. This highlights the importance of considering posture types during patient selection, preoperative planning, and the implantation of an rTSA.

Several studies have emphasized the importance of achieving a neutral to inferior tilt of the glenoid component regarding the biomechanics of the rTSA (49, 50, 51, 52). Contrarily, superior inclination is associated with detrimental consequences (53, 54, 55, 56).

Kahn et al. (57) were the first to analyze glenoid component inclination in the context of resting scapula rotation based on X-ray imaging. They found that implantation of an rTSA leads to a superior rotation of the scapula by 2° in the resting position and that achieving a β-angle (the angle between the line of the base of the supraspinatus fossa and a line from the superior-to-inferior glenoid rim) greater than 85° is likely linked to an inferior or neutral resting radiographic baseplate tilt. However, a potential limitation of this study is the lack of consideration for other patient-specific factors that could influence the spatial position of the scapula, including posture, spine alignment, arm abduction angle, and discrepancies in lower-limb length.

Areas of further research

Future research avenues offer significant potential for advancing our comprehension of scapulothoracic-glenohumeral interplay and optimizing postoperative rehabilitation strategies. A critical aspect involves further investigating the specific impact of altered scapulothoracic orientation and motion on functional outcomes and complications post-rTSA. In addition, incorporating scapulothoracic motion into patient-specific instrumentation during preoperative planning may enhance the range of motion and allow optimal individual implant configurations. In addition, this integration might bridge the gap between the virtual and actual range of motions, providing a more accurate representation. The application of predictive models that incorporate scapulothoracic dynamics, potentially leveraging artificial intelligence-based image analysis techniques, could further refine preoperative planning by offering personalized biomechanical simulations and optimizing implant positioning.

Furthermore, the potential implications of altered shoulder biomechanics on the periscapular muscles following rTSA remain unexplored, warranting further investigation. Longitudinal studies assessing how scapulothoracic adaptations influence long-term functional outcomes would be particularly valuable, with patient-reported outcome measures (e.g. SPADI, Constant-Murley Score) playing a crucial role in tracking these changes over time.

Simultaneously, exploring innovative rehabilitation protocols that specifically target scapular retraining and muscular balance has the potential to improve patient outcomes significantly. Moreover, integrating advanced imaging modalities, such as dynamic three-dimensional analyses, can offer a better understanding of scapular mechanics in the rTSA population, further supporting predictive modeling and personalized treatment strategies.

Conclusion

The current state of knowledge, despite the limited research on the subject matter, highlights the altered interplay between scapulothoracic motion and glenohumeral dynamics in the context of rTSA. The identified changes, including reduced scapulohumeral rhythm (SHR) and static and dynamic scapular tilting and rotation, provide valuable insights into the biomechanical adaptations after rTSA. Advancing our understanding of scapulothoracic motion in rTSA involves a multidisciplinary approach incorporating advanced imaging techniques, biomechanical studies, longitudinal clinical investigations, computer modeling, patient-specific considerations, functional outcomes research, and optimized rehabilitation protocols. By addressing these aspects, shoulder surgeons can gain valuable insights into the dynamic interactions of the scapulothoracic complex following rTSA.

ICMJE Statement of Interest

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.

Funding Statement

This research did not receive any specific grant from any funding agency in the public, commercial, or not-for-profit sector.