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]

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]

Avoiding scapular notching is possible by utilizing a more lateralized glenosphere offset and positioning the center of rotation laterally to the glenoid surface. Contemporary glenosphere designs vary in geometry and positioning, allowing the center of rotation to be located either at the glenoid surface or shifted laterally to more closely approximate native shoulder anatomy. Some systems achieve this goal by increasing glenosphere dimensions, providing up to 10 mm of lateral offset. Although the center of rotation remains relatively medial when compared with anatomic shoulder arthroplasty, sufficient medialization is maintained to preserve the deltoid’s mechanical advantage. The principal limitation of this approach is the elevated risk of baseplate loosening from higher torsional stresses, as lateralization increases the moment arm between the joint’s center of rotation and the baseplate–glenoid interface.^[24,25] To address this limitation, Boileau et al. introduced the bony increased-offset (BIO) RSA technique, which achieves lateralization of the glenosphere while preserving the center of rotation at the bone–implant interface. This approach involves interposing a bone autograft, most commonly obtained from the excised humeral head, between the native glenoid and the glenosphere, using a specially designed baseplate with an elongated central peg for fixation. The BIO technique is especially advantageous in both primary and revision RSA cases characterized by substantial glenoid bone loss.^[26]

In a cohort of 146 patients treated with RSA, increased medial positioning of the center of rotation correlated with diminished external rotation but improved pain relief, while greater glenoid lateralization was associated with a lower rate of scapular notching.^[25] These observations are consistent with other reports demonstrating enhanced range of motion following glenoid lateralization.^[25,27]

Eccentric placement of the glenosphere can be done without changing the position of the baseplate but by adjusting the center of rotation. One method is to use a glenosphere with an inferior offset, which lets the sphere itself go below the inferior margin of the glenoid while the baseplate stays in its original place. Both biomechanical modeling and clinical investigations suggest that this inferior offset effectively lowers the center of rotation, which in turn decreases the risk of adduction-related impingement.^[28] In addition, greater abduction has been shown to be associated with an increased acromiohumeral distance. The use of an eccentrically offset glenosphere enlarges this distance, which may help facilitate an improved range of motion. ^[29] Clinical evidence suggests that an inferior overhang of roughly 2.5 mm beyond the glenoid margin provides the most favorable balance.^[30]

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]

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]

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]

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.