**4. Discussion**

Within the scope of the present paper, two variants of a glenoid protective prosthesis were developed, with the following functions in the foreground:

Protection of the screw heads against direct contact with the joint ball of the metal humeral head prosthesis, prevention of unscrewing or loosening of the screws, formation of a sliding partner during the healing phase, better "holding together" of the bone fragments as well as targeted application of force and uniform pressure on the augmentation material (functional load).

The development was carried out based on specific known problems with the method currently used and potential improvements based on the selected design. The results are prototypes, which were first validated in mechanical tests and then tested in a clinical pilot study. These two variants were developed for the research program for prosthetics, biomechanics and biomaterials research at Paracelsus Medical University, in order to take the second question into account and to enable a direct comparison within the framework of a clinical study. The specific question is whether significantly better results can be achieved with a dynamic system of fixation of the protective prosthesis or the augmentation compared to a rigid fixation. From a biomechanical point of view, it was postulated that the atrophy of the augmentation should be lower with dynamic fixation. This was justified accordingly in the presented research carried out.

Both implant variants were tested for stability and strength by means of FEM simulation. In addition, based on the results of the FEM simulation, it can be assumed that the rigid design of the protective denture also has advantages in terms of osseointegration of the augmentation, since the modulus of elasticity of the implant material used (PEEK) is similar to that of cortical bone tissue and thus exerts a natural load on the augmentation, which in turn is intended to reduce atrophy.

Final mechanical tests confirmed the results previously obtained in the FEM simulations regarding the stability of both implant variants. In addition to checking the stability, the abrasion behavior was also measured during the mechanical tests, since in contrast to the surface loading in a total prosthesis with a form-fitting ball and ball socket, the present protective implant with only a slight concave curvature is subjected to a theoretical point load. Despite the compressibility and elasticity of the material, the contact surface is relatively small, so that an experimental test of the abrasion properties is necessary. Here, too, the result was positive to the extent that the abrasion determined was very low (significantly less than with the material of a commercial glenoid prosthesis measured in comparison).

In addition to the actual development of the glenoid protection prostheses, appropriate documentation was carried out to register and conduct a clinical study to test the implants. The documentation has also been prepared in accordance with applicable standards and guidelines for the development of a medical device in order to facilitate possible approval and marketing.

As mentioned above, a clinical pilot study will be carried out following the discussed activities with the results obtained. The purpose of this study is to demonstrate that the use of a glenoid protective prosthesis can significantly improve the outcome of glenoid revision in shoulder prostheses. Depending on the findings of this study, various further developments would be conceivable. A promising option would be to not solely use a protective prosthesis during the healing period of the augmentation material (still two-stage procedure): The old glenoid component is removed and the glenoid is rebuilt using the protective prosthesis, followed by the insertion of the new glenoid component in a second surgery, but, in addition, a "revision glenoid prosthesis" is inserted which remains permanently implanted.

The theoretical background and the basics for it were determined and corresponding potential suggestions for improvement were implemented in the form of a glenoid protective prosthesis. "In silico" (FEM simulations) investigations and mechanical tests on prototypes served to verify the (bio)mechanical properties of the protective prostheses. A particularly interesting result was the abrasion measurements. It was found that the abrasion of the protective implants made of PEEK is significantly lower than that of the glenoid component of a standard anatomical shoulder prosthesis. This fact opens up the possibility of further developing the proposed protective prosthesis in such a way that it could be used as a permanent revision implant, thus avoiding the need for a second intervention.

Two variants were designed in the development of the protective prosthesis, and the subsequent clinical trial will show whether the dynamic variant has the postulated advantages over the rigid variant.

**Author Contributions:** Conceptualization, D.S. and P.S.-G.; Investigation, D.S. and D.R.; Methodology, D.S.; Resources, K.E. and D.R.; Supervision, P.S.-G.; Validation, D.R.; Writing—original draft, D.S.; Writing—review & editing, K.E. and P.S.-G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable. The presented study did not involve humans or animals. The proposed clinical pilot study was submitted to a competent lead ethics committee (Ethics Committee Salzburg) for evaluation and a positive vote was issued (business number 415- E/1834/8-2014). The study was registered with the competent authority (BASG—Austrian Federal Office for Safety in Heath Care) and the study was granted (proceedings number 9221175). The execution of the study is still pending.

**Acknowledgments:** We would like to express our sincere thanks to Herbert Resch, acting rector of Paracelsus Medical Private University Salzburg at the time of the study and an experienced specialist in traumatology and shoulder surgery, for his kind support.

**Conflicts of Interest:** The authors declare no conflict of interest.
