2.1.1. Stemless Implants

There has been a recent trend in the use of stemless humeral components. Rationale for this shift has to do with the reported complications of stemmed implant designs, such as loss of bone stock during revision arthroplasty, malpositioning of the humeral implant, especially in cases of post-traumatic malalignment, intraoperative and postoperative periprosthetic fractures, and an altered center of rotation [2–4]. Several stemless designs

**Citation:** Leafblad, N.; Asghar, E.; Tashjian, R.Z. Innovations in Shoulder Arthroplasty. *J. Clin. Med.* **2022**, *11*, 2799. https://doi.org/ 10.3390/jcm11102799

Academic Editors: Markus Scheibel, Alexandre Lädermann and Laurent Audigé

Received: 5 March 2022 Accepted: 12 May 2022 Published: 16 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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are now on the market, all with the aim of providing 3-dimensional reconstruction of the humeral head, recreating the humeral center of rotation independent of the shaft axis [5], avoiding additional greater tuberosity osteotomy in post-traumatic cases, and avoiding the above-listed stem-related complications [4]. Advantages of stemless implants also include decreased surgical time, less blood loss, low stress shielding, and lower risk of diaphyseal stress risers [6,7]. As previously mentioned, particularly in the setting of post-traumatic malalignment or deformities of the glenohumeral joint, stemless designs allow the surgeon to recreate the glenohumeral center of rotation independent of the humeral shaft [5].

Two major stemless designs exist—impaction systems and screw-in systems. Based on work by Habermeyer et al. and Krukenberg et al., there does not appear to be a difference in terms of humeral loosening (n = 0 in both designs), humeral osteophytic exostosis (n=0 in both designs), or functional outcomes [8,9]. Radiographic medial calcar resorption occurs more often with the impaction design, but there does not appear to be a clinical implication of this, as both designs result in significantly improved Constant and Subjective Shoulder Value (SSV) scores [8,10].

Short- to mid-term (6 months–5 years) results of stemless implants have been favorable with Constant scores ranging from 65–86 and revision rates of 0%–11% [8,9,11–15]. These results seem to translate to the long-term (8–9 years), with constant scores of 62–69 and revision rates of 7%–10% [4,16]. Radiographic evaluation of 49 stemless shoulder arthroplasties at 9 years revealed upward migration of the humeral head in 14.7%, incomplete humeral "radiolucent line" in 2.3%, and no loosening of the humeral implant. There was incomplete glenoid radiolucent line without loosening in 27.3% of the stemless TSA [4]. Hawi et al. also reported a 6.9% revision rate, with secondary cuff insufficiency representing the most common cause (13.9%), and periprosthetic injection (2.3%) and periprosthetic fracture (2.3%) being less common. Interestingly, the humeral implant-related complication rate was 0% [4].

Comparing 20 stemless to 20 stemmed implants with 5-year follow up, Uschock et al. found that both implants provided consistently good functional outcomes. They reported no humeral-related complications in the stemless group, whereas there was one fracture of the greater tuberosity leading to humeral implant loosening in the stemmed group. The stemless group had one case of glenoid loosening. The overall revision rate in both groups was 13.8% [17].

Though there are many potential advantages of stemless implants, several notable limitations remain [7]. These implants lack a convertible platform and therefore require implant removal in the revision to RSA setting. They are dependent on proximal bone quality and there are also concerns regarding lesser tuberosity osteotomies given the dependence of subscapularis fixation strength. Additionally, they may be associated with increased cost. Mixed methodology between various studies of stemless implants makes results somewhat difficult to compare, and ye<sup>t</sup> while further long-term studies are required, stemless implants appear to be a favorable option for TSA. See Figure 1, an X-ray of a stemless humeral implant.

#### 2.1.2. Short Stem Implants

The potential advantages of the short-stemmed prostheses are that they rely less on proximal bone stock than stemless implants and provide a larger surface area with a porous coating for ingrowth into the proximal humerus, are easy to revise given convertible implant options, and have over 10 years of implementation in Europe. Short- to mid-term results with short stem implants are also favorable with Constant scores around 75 and ASES scores around 80, with 0%–9% revision rates [18–21]. Romeo et al. reported on the outcomes of the Apex short stem and concluded that TSA with this anatomic press-fit shortstem results in improved clinical outcomes without component loosening at 2 year follow up [20]. In the anatomic TSA setting, the Aequalis Ascend Flex (Fa. Wright, Memphis, TN, USA) short-stem implant has a relatively high occurrence of radiographic changes around the stem (26%), most commonly cortical thinning and osteopenia at the calcar as well as

spot welding laterally. Despite these radiographic findings, no stems were found to be loose and short-term clinical outcomes were favorable and comparable to other short-stem systems [19,22].

**Figure 1.** Stemless anatomic TSA. AP X-ray of anatomic TSA with stemless humeral component.

Some have proposed the use of pyrolytic carbon, or pyrocarbon, which has similarities to cortical bone and a low coefficient of friction. There is a theoretical advantage to the less-stiff quality of pyrocarbon, at least in the hemiarthroplasty setting, in that stiff cobaltchrome humeral heads wear against the less stiff cartilage and subchondral bone of the glenoid. Use of pyrocarbon heads may result in less glenoid bone loss, reducing the complexity of revision surgeries [23–25]. Garrett et al. have reported good short-term outcomes with pyrocarbon head short-stem implants in the hemiarthroplasty setting [21]. Whether these results translate to the anatomic TSA setting is ye<sup>t</sup> to be determined.

The senior author's preference is to use an uncemented short stem (Figure 2) or stemless implant. In poor humeral bone, however, the preference is to cement a short stem.
