Reverse total shoulder arthroplasty (RTSA) was first approved by the Food and Drug Administration (FDA) in 2004 primarily for the treatment of rotator cuff tear arthropathy. RTSA indications have expanded and recently emerged as an alternative to anatomic total shoulder arthroplasty (ATSA) in patients with significant glenoid erosion with an intact rotator cuff. RTSA design principles have also evolved to accommodate these changing indications to allow improved preservation of the rotator cuff during reverse shoulder arthroplasty (RSA). This more “anatomic” reverse replacement with preserved rotator cuff is an emerging concept that utilizes RTSA component designs that lateralize the glenosphere center of rotation (COR) and inlay an anatomically inclined humeral stem to reduce humeral lengthening and achieve total lateralization that allows more normal cuff function. Preservation of the rotator cuff and a more normal excursion during RTSA is commonly achieved with humeral component inlay designs with a more lateralized glenosphere COR than the traditional Grammont style RTSA.¹ This combination of an inlay humeral stem and lateralized glenosphere avoids overlengthening the humerus and achieves RTSA stability while preserving more anatomic rotator cuff excursion and function (FIGURE 24.1).² In this chapter, we present our preferred techniques to perform an RTSA with an inlay humeral system (VIDEO 24.1). We present a case example of a patient with severe glenoid erosion and an intact rotator cuff (FIGURE 24.2) where an RTSA with an inlay humeral system was utilized, and we present our tips and tricks to optimize exposure, manage the subscapularis, prepare the humerus and glenoid, assess joint stability, and avoid complications to obtain a predictably successful outcome.

Our preferred exposure is the deltopectoral approach as this approach is more extensile, has less risk of injury to the axillary nerve on the undersurface of the deltoid, and represents an internervous plane. The anterosuperior approach that splits between the anterior and middle thirds of the deltoid is possible, and we utilize this anterosuperior approach in patients on hemodialysis with an anteriovenous (AV) fistula and increased pressure, size, and flow in the cephalic vein so that it can be avoided during the surgical approach. In our experience, an anterosuperior approach may be more difficult to obtain interior or neutral tilt of the baseplate but is well suited for fracture RTSA as the tuberosities are retracted out of the way and glenoid exposure is not an issue.

The subscapularis is commonly peeled off the lesser tuberosity during RTSA. Tenotomy or lesser tuberosity osteotomy (LTO) are options, but we prefer a peel technique so that a subscapularis repair at the conclusion of the case is under less tension and can easily be medialized due to the peel technique preserving the entire length of the subscapularis tendon. We prefer to repair the subscapularis at the conclusion of the RTSA to increase internal rotation strength and potentially improve stability. In cases with significant external rotation (ER) lag and inability to externally rotate beyond neutral, the subscapularis may be left unrepaired to prevent it from providing an internal rotation tenodesis effect and relatively weakening the already weakened external rotators. An LTO is infrequently performed during RTSA as it may jeopardize the proximal humeral bone stock. This is especially relevant in an inlay humeral stem system in which metaphyseal reaming removes bone leaving an LTO without a medullary healing surface remaining.

With an inlay humeral stem, the metaphyseal portion of the component is usually placed below or at the level of the humeral head resection, and the polyethylene is placed onto the humeral stem. In contrast, with an onlay humeral system, an anatomic neck cut is made, and then a humeral tray is placed on the stem that is flush with the resection. The polyethylene is then placed onto the humeral tray, which creates a “double stacking” of the components, which may add excessive length to the construct (FIGURE 24.3).³ Depending
on the position of the humeral stem implant, the use of an inlay versus onlay design, the neck-shaft angle, the thickness of the polyethylene, differing amounts of deltoid tension, and joint compression can be applied to achieve stability and function. Onlay humeral designs tend to lengthen the humerus to tension the deltoid, while inlay humeral systems tend not to lengthen the humeral side but rather rely on glenosphere lateralization for appropriate joint tension and stability.

Humeral head resection depends on the RTSA system used and the inclination of the stem (FIGURE 24.4). With inlay humeral components, a more anatomic head cut in 20° to 30° of retroversion helps preserve more anatomic rotator cuff excursion and function. The inclination of the head cut should match the stem and should exit superiorly at the junction of the greater tuberosity and articular surface margin (FIGURE 24.5).⁴ A head cut in an anatomic 135° inclination has been shown to have less notching in systems that do not solely utilize glenosphere lateralization to prevent notching.⁵,⁶,⁷

After preparing the humerus for the inlay humeral stem with broaching and/or reaming, surgeons should focus on the integrity of the metaphyseal cortex (FIGURES 24.6, 24.7 and 24.8). Aggressive metaphyseal bone removal may encroach on the cortex and weaken the proximal humerus. A weakened proximal humerus may get crushed and fragmented during glenoid preparation as a result of retraction of the proximal humerus for glenoid exposure. Insertion of the trial stem should ensure proper fit for the future final implantation of the humeral prosthesis (FIGURE 24.9). Leaving the stem trial in place (FIGURE 24.10) or using an osteotomy
surface metal protector helps share the load during glenoid retractor use and prevents damage to the proximal humeral cortex.

When using an inlay humeral system, the humeral lateral offset is often set by the design of the stem and humeral polyethylene socket as inlay humeral designs do not allow adjustment from intercalary modular components. Therefore, lateralization of the inlay RTSA is obtained primarily by baseplate and glenosphere configurations and less so from altering humeral stem positioning.⁸ Humeral lateralization has many theoretic and biomechanical advantages to deltoid efficiency at arm elevation forces; glenosphere lateralization tends to reduce deltoid efficiency⁹ and jeopardize glenoid fixation but may improve rotator cuff function.¹⁰,¹¹,¹² Progressively thicker humeral polyethylene inserts produce both humeral lateralization and distalization which can overlengthen.¹³ Humeral anteroposterior offset is another factor that can affect rotator cuff moment arms and excursion, but during inlay humeral stem usage, there are less options to adjust this factor as the stem position within the humeral metaphysis determines this
anteroposterior offset.¹⁴ Total combined humeral and glenoid offset can significantly affect deltoid lever arm mechanics.¹⁵