The primary purpose of baseplate screws is to apply and maintain compression between the baseplate and glenoid interface to avoid micromotion and resultant fibrous tissue formation. Compression provides immediate and short-term baseplate stability while creating an environment for solid osseous formation to the native glenoid for long-term fixation [26–29]. The time frame for this has been reported by Cameron et al. to be as short as four months for porous-coated endoprostheses with initial mechanical fixation [30].

Many rTSA baseplates typically offer four peripheral screws: superior (S), inferior (I), anterior (A), and posterior (P), but no central screw. It is widely accepted that only two or three of these peripheral screws are beneficial for fixation. Specifically, the benefit of the A and P screws has been questioned by James et al. [31] A or P screws are rarely both engaged in the glenoid bone [22, 25] but even when engaged, they have been reported to provide far less fixation strength than S or I screws [32]. A central baseplate screw has been reported to be the most effective screw for fixation strength [5]. The Arthrex Univers Revers™ design offers a comparably large 6.5 mm central screw (available in 15, 20 and 25 mm lengths as compression or compression-locking), along with superior and inferior 4.5 mm variable angle screws (available in 24, 30, 36, 42 and 48 mm lengths as variable angle compression or variable angle compression-locking screws). These three screws are aligned vertically in the central plane of the baseplate to enable optimal position, length and trajectory into the best bone stock in the glenoid vault as recommended by Humphrey et al. [22], DiStefano et al. [26], and Parsons et al. [27] (see Section “Recommended Surgical Steps to Prepare the Glenoid for Improved Fixation and Outcome”).

The Arthrex Univers Revers™ baseplate test results were comparable to systems with four screws when tested according to ASTM F2028 [33, 34] guidelines (Standard Test Methods for the Dynamic Evaluation of Glenoid Loosening or Disassociation), suggesting comparable short-term fixation. Systems with four or more screws have reported good clinical short-term fixation [35]. However, filling the glenoid with an overabundance of screws may lead to stress shielding, adversely affecting long-term implant fixation. Stress shielding is the degeneration of native bone due to reliance on the rigid metal prosthesis for load transfer, discouraging the remodeling of new bone [12]. Stress shielding at the glenoid interface is believed to contribute to suboptimal osseointegration and loosening or worse glenoid neck fracture resulting in challenging revision surgeries with limited options.

Three Arthrex baseplate sizes have been designed to maximize contact area by closely matching the native glenoid’s “pear”-shaped surface (see Fig. 30.3). This presents several potential benefits over typically “round” baseplates, commonly offered in one size only. First, by maximizing the surface area, size for size, the highly localized pressure regions can be distributed more evenly across a larger implant-to-bone surface area (see Fig. 30.3a). Second, the Arthrex baseplates extend vertically to match the superior region of the native glenoid anatomy (see Fig. 30.3b). Standard round baseplates fail to utilize this superior region, which not only enables additional surface area contact for improved long-term fixation but also acts as a buttress, helping to counteract and distribute the torsional forces imposed at the implant-to-bone interface. An even distribution of forces has been reported to encourage optimal conditions for scapular remodeling [12]. Furthermore, in the event of glenoid bone loss, which commonly occurs in the region of the superior glenoid with rotator cuff deficiency [2], the additional surface area of the implant provides the opportunity to better capture bone graft under the implant, encouraging osseointegration of the graft.

Anglin et al. reported that glenoid component loosening generates the greatest concern among surgeons performing rTSA. The authors devised a specific test to analyze different baseplate designs and geometries, concluding that a roughened fixation surface far outperformed a smooth fixation surface and a curved backed surface showed almost half the distraction of a flat backed surface [36]. Many current baseplate designs are smooth and flat or just slightly curved with no protruding or press-fit geometry, subjecting the screws to shear forces applied to the baseplate bone interface. These designs have reported failure rates between 11.7 % and 40 % [37], confirming that bony ingrowth is an important factor for avoiding baseplate failure.

The underside of all Arthrex Univers Revers™ baseplates includes a unique double stepped press-fit geometry, roughened to 0.30 µm with a TPS coating, to minimize micromotion and encourage bone ingrowth [38, 39], and a slow release CaP coating on the outer surface to encourage bone ongrowth [40]. A roughened titanium substrate of higher than 0.10 µm improves anchorage for the CaP and enables increased surface area for CaP and resultant bone coverage [41]. CaP coatings induce more rapid bone apposition onto prostheses, optimizing osseointegration for long-term fixation and increasing bone quality and strength [42, 43]. The critical micromotion threshold has been disputed. However, what is evident is without stable fixation, the formation of fibrous membrane is more prevalent, predisposing shoulders to early loosening and poor clinical outcomes [28, 29]. The Arthrex Univers Revers™ press-fit baseplate geometry has been specifically designed for maximum stability to encourage bone ongrowth in a reduced micromotion environment. The Arthrex baseplate design has been tested in-line with load protocols specified in ASTM F2028 [34] and subsequently verified by results with significantly lower micromotion values than the threshold limits suggested by Cameron et al. of to 150 µm [44].

The Arthrex press-fit baseplate is specifically designed to match the patient’s anatomy and help to maximize the surface area in contact with bone (see Fig. 30.3). The three centrally aligned screws offer optimal trajectories into the native glenoid bone stock to support a stable compressive healing environment. This combination of a macro-press-fit geometry and micro-roughened TPS substrate with a CaP coating encourages stable osseointegration of the prostheses, optimizing the chances for long-term fixation.