3.2.1. Stemless Implants

Stemless RSA implants are not currently FDA approved in the United States, however they have been approved and studied in Europe and Canada. The appeal of a stemless RSA implant is similar to their appeal in anatomic TSA, namely preserving proximal bone stock and easier implantation in the setting of altered distal anatomy, as well as potentially

decreasing implant malposition and periprosthetic fractures [77,78]. International literature has found no significant difference in ROM and clinical outcomes scores between stemmed and stemless RSA in early to mid-term results [79,80]. Osteopenia was noted to be a relative contraindication for stemless implantation due to an association with early humeral component loosening [81,82]. Long-term survivorship of these implants is still under investigation.

#### 3.2.2. Inlay vs. Onlay Implants

The original Grammont style implant was designed as a 155◦ inlay prosthesis. With the inlay component, the metaphyseal portion of the stem is inset into the humeral metaphysis. The thought behind this original design was to increase the surface area contact and medialize the humerus. The humeral stem onlay prosthesis was then developed with the metaphyseal tray sitting on top of the humeral cut surface. This allows for more lateralized and distalized humeral designs, preserved proximal bone stock, and the potential for stem conversion between RSA, TSA, and hemiarthroplasty [83–85].

In a biomechanical study by Walch, when compared to 155◦ inlay and 135◦/155◦ onlay stems, only the 145◦ onlay humeral stem restored >50% of the native ROM and maximally lengthened the cuff [83]. Clinical studies have demonstrated improved adduction, extension, and ER with onlay humerus compared to the traditional inlay component [86,87]. In one of these studies, there was no difference in complications, however increased scapular fractures have been noted in other studies of the onlay stems, particularly with distalizing designs [84,87,88]. Ultimately, further clinical trials are needed to fully delineate the outcomes of inlay versus onlay, but the data so far suggests that onlay stems, particularly with lower neck shaft angle, offer improved outcomes and more revision versatility, but with the increased risk of scapular fracture.

The senior author's preference is to use an inlay short stem humeral component with a lateralized glenoid baseplate (Figure 5). If, however, there is significant proximal humeral bone loss, the senior author's preference is to use a standard length or long stem with proximal humeral allograft when necessary.

**Figure 5.** Lateralized RSA. AP X-ray of lateralized glenoid baseplate.

3.2.3. Vitamin E Polyethylene Implants

Vitamin E has become the most common antioxidant used in polyethylene components for all joint replacements including reverse shoulder arthroplasty [89]. It is added as an antioxidant stabilizer to inhibit oxidative degradation in ultra-high molecular weight polyethylene (UHMWPE). Vitamin E enhanced UHMWPE demonstrates more stability than gamma sterilized or high-dose irradiated UHMWPE implants under accelerated aging

conditions [90–92] and more mechanical and fatigue strength [93]. Vitamin E implants that were evaluated in a wear stimulator for shoulder implants demonstrated a significant reduction in wear compared to non-vitamin E enhanced implants [94].
