From a simple mechanical point of view a reduced neck-shaft inclination, an increased glenosphere offset and a reduced inlay cup depth are decisive design parameters to reduce inferior scapular notching.

Given the complexity of the biomechanical system of the glenohumeral joint and clinical experience, these parameters were assessed as sensitive concerning dislocation and implant loosening [4, 12, 13]. In a subsequent study, it was confirmed that reduced neck-shaft inclination and reduced inlay cup depth may result in reduced stability, whereas an increased glenosphere offset may result in an increased incidence of glenoid component loosening due to the “rocking horse” phenomenon [11].

Overall, the benefit–risk analysis showed no justification for modifications of these sensitive design features. Particularly with regard to the clinical and radiographical outcome, an inferior position of the glenoid component is more efficient to prevent inferior notching than changing other design features.

Therefore, the neck-shaft inclination of 155°, the center of rotation at the glenoid bone–prosthesis interface, and the inlay cup depth were defined as unchangeable according to Grammont’s concept.

The main causes of failure that were detected were instability, infections, humeral and glenoid component loosening, component disconnections, and periprosthetic humeral fractures [3, 21].

Further complications such as stress fractures (e.g., acromial fractures), intraoperative glenoid fractures, wound hematoma, and axillary nerve palsy can also be related to implant design or surgical technique and had to be taken into account [3, 21].

The main problem—the glenoid component fixation and positioning in combination with the notching phenomenon of the humeral inlay with the scapula—was the initial challenge for developing the next generation of prosthesis.

The main modifications are a new 2-peg baseplate fixation without an inferior screw and the inversion of the bearing materials.

The modification of the baseplate design resulted from the conflict of goals with the conventional baseplate with one peg and an inferior screw. On the one hand, the baseplate has to be positioned as inferiorly as possible to prevent notching. On the other hand—in this inferior position—the inclined inferior screw is difficult to place in the scapular neck and there is a systematic potential of implant-implant notching between the screw and the humeral inlay. The inferior screw of the Grammont prosthesis was routinely positioned less inclined or parallel to the central peg to prevent implant–implant contact.

Consequently, a second peg with an appropriate stability and length was designed and the inferior screw was eliminated for the primary baseplate. A double coating on the backside consisting of a rough titanium plasma spray, and a bioactive calcium phosphate coating supports the primary and secondary stability.

With the Grammont baseplate with one peg, the anterior and superior screws are positioned rather far outside the center of glenoid and therefore often do not have an appropriate fit in the bone. With the two-peg design, the anterior and posterior lag screws can be positioned closer to the center and therefore have an improved bony anchorage.

With these modifications and a comparable primary anchorage stability to the conventional baseplates, the danger of malpositioned or broken inferior screws as well as implant–implant notching is eliminated.

The common reverse total shoulder arthroplasty (RTSA) has “soft” polyethylene (UHMWPE) inlays and “hard” cobalt–chromium (CoCr) glenospheres.

In inverted bearing reverse total shoulder arthroplasty (IB-RTSA), such as the Affinis Inverse (Fig. 42.1), the humeral inlays are made from metal or ceramic and the glenospheres from PE.

The notching phenomenon of a “hard” and wear-resistant humeral inlay with the scapula results in a different characteristic. Excessive abrasive wear on the scapular bone, respectively, PE-induced osteolysis is eliminated. With the humeral inlay made from metal or ceramic, the characteristic remains with a simple mechanical notching [5].

Concerning the high rates of infection and of glenosphere disassembly, the goal was to optimize the fixation of the glenosphere to the baseplate and to reduce the complexity as well as hollows inside the glenoid module [3, 17, 21]. Thus, the Affinis Inverse glenosphere was designed as a monolithic component, easy to clean and with minimized hollows.

The benefit of having a modular primary stem with the possibility of posterior offset adjustments to reduce proximal humeral bone loss and to optimize tensioning was considered, although a clinical relevance was not proven at this time.

After a benefit–risk analysis, due to the high rates of humeral disassembly, humeral radiolucencies and periprosthetic fractures of modular stems, it was decided to have monolithic stems with a continuous triple taper design [3, 18, 21]. With the inverted bearing and the hard humeral inlay, it was possible to reduce the wall thickness at the humeral components significantly, and therefore, the humeral bone loss could be reduced.

The bearing diameter 36 and 42 mm was proven as minimum and maximum sizes, but often the step between 36 and 42 mm was too big. Hence, a third size in between (39) was introduced to get more options for an improved soft tissue balancing.

In combination with an eccentricity within the baseplate design, an inferior overhang of the glenosphere of 4.0 mm for size 36, 5.5 mm for size 39 and 7 mm for size 42 results, if the inferior rim of the baseplate is positioned flush with the inferior border of the glenoid.

In addition to the primary Affinis Inverse system, a reversed fracture central part was developed (Fig. 42.2). Contrary to delicate modular uncemented stem systems, the increased dimensions of the modular trauma system as well as the cemented stem fixation allow a safe fixation of the implant. Besides using it for primary fracture reverse treatment, it is possible to use the central part to switch from the anatomic fracture prosthesis to the fracture reverse prosthesis by leaving the stem in situ [20, 24].

Due to the fixation mechanism with freely adjustable height and retrotorsion, it is possible to readjust the prosthesis. This is important especially for an adapted retrotorsion during the revision procedure from an anatomic to a reversed prosthesis.

In 2014, with the aim of optimizing wear characteristics, ceramys^® inlays and vitamys^® glenospheres were added to the system.

vitamys^® is a highly cross-linked, wear-resistant polyethylene of the second generation that is permanently protected against oxidization aging through the addition of 0.1 % a-tocopherol (vitamin E).

ceramys^® is a high-performance ceramic comprising a nano-crystalline dispersion of 80 % stabilized zirconium oxide and 20 % aluminum oxide, which lend this type of ceramic exceptionally high resilience and durability.

The wear reduction of the best possible pairing, vitamys/ceramys, versus the CoCr/UHMWPE pairing, is approximately 80 % (see Fig. 42.3 [6, 7]).

In addition to reduced wear, patients can also benefit from a nickel-free option. With stems, baseplates, and screws made from titanium, polyethylene glenospheres, and ceramic inlays, the complete reverse prosthesis system is free of CoCr and hence of nickel.

The controversial debate about the allergic reactions from implants containing nickel (such as CoCr) continues to be waged [22, 23]. The growing demand for ceramic as a material indicates that it is important for both surgeons and patients to avoid allergy as a complicating factor from the beginning. With ceramic, an option in case allergies develop is offered.