Abstract

Rotator cuff tear arthropathy (RCTA) is a complex condition characterized by rotator cuff insufficiency, altered shoulder kinematics, and progressive joint degeneration. In advanced stages, it results in pain, loss of active elevation, and significant functional impairment. Reverse shoulder arthroplasty (RSA) is now the best surgical option for advanced RCTA because it restores shoulder elevation by changing how the joint works and making the rotator cuff less important.

The original RSA concept focused on medialization and distalization of the center of rotation to increase the deltoid moment arm, improve joint stability, and allow active elevation in the absence of a functional rotator cuff. Although this design provided clinical improvement, it also revealed biomechanical limitations, including scapular notching and restricted rotational range of motion, among others.

In response, contemporary RSA systems have evolved design modifications, such as lateralization and glenosphere eccentricity, inferior tilt, variable humeral neck-shaft angles, and modular humeral socket configurations. These strategies aim to optimize deltoid efficiency, minimize impingement, and improve range of motion. Despite the availability of numerous implant designs, no consensus exists regarding the optimal combination of components or positioning parameters. A comprehensive understanding of RSA biomechanics associated with modern designs is essential for implant selection and surgical planning in patients with RCTA.

implant design, kinematics, quality of life, scapular notching, shoulder degeneration

Degenerative glenohumeral arthropathy is a clinically significant condition with high incidence, causing chronic pain, limited range of motion, and progressive loss of shoulder function. Eventually it can lead to reducing the quality of life, as it affects both its physical and psychological components.^[1] Activities of daily living and occupation may become limited in younger patients, while the elderly often lose their functional independence.^[2] At the end stage of this condition, shoulder arthroplasty represents the treatment of choice. Based on different established techniques, reverse shoulder arthroplasty has become the most widely accepted and used option. One of the reasons for this procedure is the biomechanical advantage it provides in cases with rotator cuff insufficiency. The surgery involves reversing the native glenohumeral anatomy by placing a glenosphere on the scapular side and a concave polyethylene cup on a humeral stem.^[3] While the original semi-constrained design, introduced by Grammont, provided predictable clinical and functional improvement, it also led to biomechanical limitations, such as scapular notching and restricted rotational movement.

In response, modern RSA systems provide substantial design modifications for optimizing range of motion as well as implant stability and longevity. These include variations for glenosphere size and eccentricity, tilting angle, lateralization of the center of rotation, humeral socket, and neck-shaft angle. Even though there are several implant designs, there is no consensus on the optimal configuration of implant positioning and combination strategy. The purpose of this review is to summarize the biomechanical principles underlying RSA and to critically examine modern implant design modifications, emphasizing their influence on joint mechanics and functional performance.

The rotator cuff (RC) is a structure of four muscles that plays a main role in shoulder function. It creates a stabilizing effect through a concavity–compression mechanism, in which coordinated tendon forces dynamically press the humeral head into the glenoid, preserving joint congruency during motion.^[4] Rotator cuff tears are the most frequently reported upper limb pathology in individuals older than 50 years, which is an increasing clinical challenge in the context of an aging population.^[5] A distinct degenerative shoulder condition, referred to as rotator cuff tear arthropathy (RCTA), occurs in cases of advanced tendon damage. It is defined as rotator cuff failure, superior migration of the humeral head followed by making contact with the acromion, and characteristic degenerative changes evident on radiographic imaging. From a clinical perspective, patients experience an increasing weakness of shoulder forward flexion, abduction, and external rotation, potentially culminating in pseudoparalysis.^[6,7] Multiple hypotheses have been proposed to describe the biomechanical changes associated with RCTA. One influential theory by Neer et al. suggests that the main reason is disruption of muscular force balance and anterior–posterior instability.^[8]

Subsequently, the humeral head migrates superiorly and impinges against the acromion. Meanwhile, progressive joint degeneration arises from the compromised joint mechanics and repetitive microtrauma that follows the loss of stabilizing structures. Glenoid erosion most commonly presents in an eccentric pattern, predominantly affecting the anterosuperior aspect. At its core, RCTA arises from extensive rotator cuff failure accompanied by secondary degenerative changes of the glenohumeral joint.^[8] In contrast to primary osteoarthritis, RCTA is associated with a distinct type of joint wear resulting from the failure of stabilizing mechanisms that normally maintain humeral head centering, allowing proximal subluxation to occur. On imaging, this process is characterized by erosive changes of the superior glenoid, rounding of the humeral head (“femoralization”), and reshaping of the coracoacromial arch (“acetabularization”).^[4] Cuff tear arthropathy is also characterized by fatty degeneration and muscle fiber atrophy, fibrotic changes, and progressive deposition of adipose tissue within and surrounding the affected muscles and tendons.^[9,10]

Conventional radiography serves as the primary imaging modality for the diagnosis of degenerative shoulder disorders. Accordingly, the two most widely applied classification systems for RCTA are based on radiographic findings and are used for grading the degenerative shoulder disease. The Seebauer classification stratifies RCTA into four categories, differentiating cases without persistent proximal migration of the humeral head (IA and IB) from those demonstrating established proximal migration (IIA and IIB).^[9] Each classification category reflects the distinct biomechanical changes that occur. They depend on the degree of humeral head migration relative to the center of rotation, anatomical changes in the glenoid and coracoacromial arch, and the associated instability. The second one is the Hamada classification, which is based on a radiographic evaluation of coracoacromial arch morphology and a decrease in the acromiohumeral interval (AHI). It stratifies massive rotator cuff tears into five stages, reflecting a stepwise progression of disease severity (Fig. 1)^[11]

Figure 1.

Rotator cuff tear arthropathy type IIA, according to Seebauer, because of the proximal humeral head migration with preserved stability, and type II according to the Hamada classification, with acromiohumeral interval ≤5 mm.^[11]

Computed tomography (CT) plays a central role in evaluating glenoid morphology in patients with RCTA. In advanced degenerative disease, posterior bone loss may develop, often resulting in posterior displacement of the humeral head. A detailed CT assessment is mainly used for surgical planning, particularly when reconstruction or bone grafting of the glenoid is being considered, since it is the most useful method for evaluating the glenoid’s morphology.^[12]

Magnetic resonance imaging (MRI) provides detailed evaluation of rotator cuff morphology and is particularly useful when the reparability of a tear is in doubt. MRI also allows characterization of articular cartilage degeneration. Assessment of fatty degeneration may be performed using either CT or MRI, commonly graded according to the system described by Goutallier et al.^[13]

For end-stage degenerative conditions of the shoulder, arthroplasty represents the definitive surgical option. Depending on the pathology and other individual factors, this may involve hemiarthroplasty (HA), with replacement of the humeral head alone; anatomic shoulder arthroplasty (ASA), in which both the humeral and glenoid articular surfaces are replaced; or reverse shoulder arthroplasty (RSA), where the native joint anatomy is inverted by fixing a metallic glenosphere to the scapula and articulating it with a polyethylene socket mounted on a humeral stem.^[14]

In Germany, the incidence of primary shoulder arthroplasty rose at an average annual rate of approximately 14% between 2010 and 2019. This growth was largely due to a nearly fourfold increase in RSA, while ASA showed a more gradual rise, and hemiarthroplasty utilization fell by more than 70%. Forecast models suggest that the rate of primary shoulder arthroplasty may rise by nearly seven times by the year 2040.^[15] Similar trends have been observed in the United States. Wagner et al. documented a 103.7% rise in primary shoulder arthroplasty between 2011 and 2017, driven predominantly by the rapid expansion of RSA (191.3%) compared with anatomic arthroplasty (38.5%), while the utilization of hemiarthroplasty was reduced by 60.9%.^[16,17]

Reverse shoulder arthroplasty

The prosthesis used in reverse shoulder arthroplasty (RSA) consists of three main components: the glenoid baseplate, the glenosphere, and the humeral socket. In this reversed configuration, the baseplate is placed directly on the glenoid, the metal glenosphere is fixed onto it, and the polyethylene humeral socket is mounted onto the humeral stem.

Paul Grammont’s original semi-constrained RSA concept relies on three fundamental biomechanical mechanisms. First, repositioning the center of rotation medially to the glenoid surface decreases shear forces acting on the glenoid fixation, thereby enhancing implant stability. Second, incorporation of an enlarged glenosphere allows controlled joint constraint, making a functional arc of motion possible within the reversed construct. Third, distal displacement of the humerus augments the deltoid lever arm and improves its capacity to generate elevation forces in the absence of an effective rotator cuff (Fig. 2).^[18,19]

Figure 2.

The native center of rotation is at the center of the humeral head. With RSA, it is placed distally and medially, while the lever of the deltoid (a) and its force (b) are increased.

Furthermore, the Grammont design incorporates a humeral stem with a 155° neck–shaft angle, which reorients the line of action of the deltoid muscle and enhances its ability to produce shoulder flexion and abduction.‌^[20,21] Biomechanical studies have demonstrated that RSA decreases the muscular force needed to achieve shoulder abduction by roughly 30%.^[22] Further refinements in implant design, together with favorable clinical results, have led to a broadening of RSA indications beyond cuff-deficient shoulders such as proximal humeral fractures, primary glenohumeral osteoarthritis with preserved rotator cuff function, and revision arthroplasty.^[19]

Regarding biomechanics, modern RSA implant designs continue to build upon the foundational principles introduced by Paul Grammont. Shifting the center of rotation medially transforms the forces acting on the glenosphere into predominantly compressive loads, thereby limiting shear stress at the glenoid–implant interface. Nevertheless, excessive medialization can adversely affect shoulder mechanics, leading to a reduction in rotational range of motion and diminished effectiveness of the external rotators, particularly the m. infraspinatus and m. teres minor. Furthermore, pronounced medialization may promote humeroscapular impingement, commonly referred to as scapular notching, caused by contact between the humeral polyethylene insert and the scapular neck during adduction. This mechanical conflict can contribute to polyethylene wear, bone erosion, osteolysis, and eventual glenoid component loosening. To mitigate these complications, various design and positioning strategies have been introduced, including inferior placement of the baseplate, controlled lateralization, and reduction of the humeral neck–shaft angle. Regardless of the chosen strategy, accurate and stable fixation of the baseplate remains essential for achieving durable long-term outcomes.^[18,19,23]

Inferior angulation of the glenosphere

Inferior tilt of the glenosphere has been described as another strategy to mitigate scapular notching. Initially introduced by Sirveaux et al., this technique is typically used in conjunction with inferior placement of the baseplate and is intended to optimize results, particularly in shoulders with predominant superior glenoid erosion (Fig. 3).^[31]

Figure 3.

Postoperative radiograph showing inferior angulation of the glenosphere. 1) baseplate; 2) glenosphere; 3) humeral socket; 4) stem.

When there is an inferiorly offset eccentric glenosphere, adding inferior tilt can lead to asymmetric load transfer across the baseplate and may provoke the so-called rocking-horse phenomenon. This effect has not been described with concentric glenosphere designs. In their analysis of optimal baseplate placement, Gutiérrez et al. reported that an inferior tilt of roughly 15° yielded the most favorable biomechanics for concentric and laterally eccentric glenospheres. Conversely, for inferiorly eccentric configurations, a neutral orientation (0° inclination) was found to be preferable. Importantly, the authors cautioned against superior tilt, as it markedly increases stresses at the bone–implant interface.^[32]

Humeral component

Design changes to the humeral component have likewise been introduced to reduce the risk of implant-related complications. Although the classic Grammont configuration, characterized by a 155° neck–shaft angle, offers well-recognized biomechanical benefits, it can result in excessive deltoid tension. Over time, this increased tension has been associated with acromial stress fractures and a deterioration in deltoid performance during mid- to long-term follow-up.^[33] As a result, modern humeral implants have shifted toward neck–shaft angles that more closely replicate native anatomy, most commonly between 135° and 145°. Both clinical investigations and biomechanical analyses have shown that reducing the neck–shaft angle is associated with improvements in overall range of motion and lower rates of impingement.^[34,35]

Two principal designs of the humeral socket have been described: inlay and onlay configurations. The inlay design, incorporated in the original Grammont-style prosthesis, seats the polyethylene component within the metaphyseal bone, thereby increasing implant–bone contact but necessitating more extensive metaphyseal reaming. Clinical experience has linked this design to an increased incidence of scapular notching and limitations in postoperative external rotation. In response to these drawbacks, onlay designs were developed. These allow the socket to sit on top of the humeral cut, preserving metaphyseal bone and providing greater modularity, including the ability to rotate the socket relative to the stem to fine-tune humeral offset during surgery (Fig. 4).^[36]

Figure 4.

Radiographic images of RSA with (A) an inlay humeral socket and (B) an onlay humeral socket.

It is worth emphasizing that distalization influences the direction and magnitude of forces transmitted through the remaining rotator cuff. This consideration becomes especially significant when RSA is performed for indications other than RCTA, where a meaningful portion of the cuff is still intact and functional, such as in primary glenohumeral osteoarthritis. Consequently, distalization should be viewed as a parameter whose ideal magnitude varies according to the underlying pathology. While a universally accepted approach to optimizing soft-tissue tension has yet to be established, emerging biomechanical evidence indicates that lateralizing the humerus may represent a more effective strategy for enhancing joint stability and restoring more favorable muscle loading patterns.^[37,38] Some systems maintain the center of rotation on the glenoid surface while achieving humeral lateralization through the use of a larger glenosphere (diameters ranging from 38 mm to 46 mm) and an onlay humeral socket. This theoretically improves the deltoid lever arm, allowing a greater range of motion without impingement.^[39]

Currently, over 25 reverse shoulder arthroplasty implant systems are commercially available; however, there remains no consensus-based guidance on how to optimally pair glenosphere and humeral components to maximize functional outcomes.^[40] The main objective of RSA is to balance joint stability, efficient deltoid mechanics, and a functional, impingement-free range of motion. Modern implant designs address these goals through controlled lateralization of both the glenoid and humeral components, combined with inferior glenosphere overhang to limit impingement, enhance external rotation, and maintain deltoid effectiveness without imposing excessive loads on the glenoid baseplate. Additional functional gains are achieved by avoiding superior glenoid inclination and by tailoring inferior tilt to the degree of glenosphere eccentricity. Importantly, a successful reverse shoulder arthroplasty prioritizes pain relief and stability over maximal motion, aiming instead for durable, patient-specific functional restoration.