**1. Introduction**

The annual number of shoulder arthroplasties increases continuously. In 2017, an estimated number of about 800,000 patients were living in the United States with a shoulder replacement with a prevalence of 0.258%, increasing markedly from 1995 (0.031%) and 2005 (0.083%) [1]. In 2008, 27,000 shoulder arthroplasties (total endoprostheses only) were performed [2]. Although the number of shoulder arthroplasties performed is still far below that of knee or hip arthroplasties, the indications for shoulder arthroplasties are much more varied and range from rheumatoid arthritis, degenerative arthroses and osteonecrosis to posttraumatic osteoarthritis or osteoarthritis [3,4]. Due to the increasing demand for total endoprostheses to the shoulder, the revision of the glenoid is becoming increasingly important and demonstrates the need for basic research and development in this field.

Problems with arthroplasty or reimplantation of glenoid prostheses consist of the large-volume combined with central-peripheral glenoid defects. Due to the destroyed glenoid with bony defects, immediate insertion of a prosthesis is not possible because

**Citation:** Schaffarzick, D.; Entacher, K.; Rafolt, D.; Schuller-Götzburg, P. Temporary Protective Shoulder Implants for Revision Surgery with Bone Glenoid Grafting. *Materials* **2022**, *15*, 6457. https://doi.org/ 10.3390/ma15186457

Academic Editor: Oskar Sachenkov

Received: 11 July 2022 Accepted: 7 September 2022 Published: 17 September 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

primary stability cannot be guaranteed by anchoring the prosthesis. So far, there have been several attempts to correct the bony defects with augmentations, however with moderate success [5–12].

A common problem was identified in connection with the screws used to fix the augmentation material. In one study, complications were reported in 78% of the observed cases of screw fracture, bent screws or metallic abrasion [13]. The main reason given for performing revision surgery is loosening of the glenoid prosthesis in 32% to 39% of cases [14–16], followed by instability in 23% to 30% and periprosthetic fracture in 11% of cases [14,17]. The success rate over shorter periods of time (<5 years) is given as up to 98% to 99%, which drops drastically to between 33% and 51.5% with a follow-up of >10 years [18–21]. Eccentric loads and the resulting high micro-movements at the boundary between bone cement and cortical and trabecular bone play a major role in loosening the glenoid component. Over a longer period, a phenomenon occurs which is referred to in pertinent literature as the "Rocking Horse Phenomenon" [22,23] and can subsequently lead to bone resorption and massive bony defects in the area of glenoid anchorage. When the glenoid is loosened, patients often suffer increasingly from load-dependent pain after an interval that is sometimes very long and symptom-free [24]. In the case of a confirmed loosening of the glenoid prosthesis, there is in principle an indication for a one-stage changes of the glenoid component, provided that stable anchoring is possible. However, the biggest problems with revisions of the glenoid with a new prosthesis (implant) are large bone defects or deficits in which both the cancellous part and the cortical layer are affected [25]. In these cases, insertion of a new glenoid component is often not possible because it cannot be sufficiently attached to the scapula and glenoid.

In addition to other options for augmentation with calcium phosphates, bio-glass, hydrogels, human bone allografts and biocomposites made by bioactive elements [26], there are some experimental studies about cell-instructive bioengineering procedures to support and restore preexisting bone repair and osseointegration [27,28]. An option that is commonly used as a standard is still a two-stage procedure by means of building-up the defective glenoid with autologous bony augmentations [5–7,10,11,29,30]. The defective glenoid component is removed, and large-volume bone deficits are built up with cortigospongious chips from the iliac crest.

In the case of extensive defects and thus large augmentations, it is necessary to fix the augmentation with screws. After sufficient osseous integration of the cortigospongious augmentation material in the glenoid (about 3 months), a new glenoid component is inserted into the bone material [31,32]. A retrospective examination of 16 revision procedures of shoulder arthroplasties with glenoid build-up showed a settlement or atrophy of the augmentation in all cases [25]. The observed shrinkage is 5 mm in 3 patients, 5 to 10 mm in six patients and more than 10 mm in two patients. All four cases of the investigated group in which the augmentation was fixed with screws showed such a large shrinkage in which the screws were exposed and touched the metallic humeral head of the humeral prosthesis. In one case, the screw even broke due to the additional load.

In cases where the screws used to fix the augmentation material comes into contact with the metallic humeral head, abrasion may occur, which may subsequently lead to complications such as metallosis [33,34].

In this paper, we propose the application of temporary protection implants for the augmentation of shoulder prosthesis revision procedures. This paper describes the development of two kinds of temporary prosthesis which support the correction of bony defects and shall significantly improve the result of glenoid augmentation. This is intended to ensure a high degree of primary stability during insertion of a glenoid prosthesis, even during revision, and consequently to achieve more effective rehabilitation.

The goals of this study are as follows: to protect screw heads against direct contact with the metal joint ball of the implant (humeral head), to prevent screws from being unscrewed, to form a sliding partner during the healing phase, better "cohesion" of the bone fragments and to provide a more even force introduction and even pressure on the augmentation

material (functional load). In the following sections, the material and methods used in this study are described. augmentation material (functional load). In the following sections, the material and methods used in this study are described.

prosthesis, even during revision, and consequently to achieve more effective

The goals of this study are as follows: to protect screw heads against direct contact with the metal joint ball of the implant (humeral head), to prevent screws from being unscrewed, to form a sliding partner during the healing phase, better "cohesion" of the bone fragments and to provide a more even force introduction and even pressure on the

### **2. Materials and Methods 2. Materials and Methods**

rehabilitation.

GlenoProtect is a glenoid component made of a suitable material that was developed to protect the augmentation in shoulder prosthesis revision procedures. Two variants have been developed to ensure the above-mentioned capabilities. GlenoProtect is a glenoid component made of a suitable material that was developed to protect the augmentation in shoulder prosthesis revision procedures. Two variants have been developed to ensure the above-mentioned capabilities.

### *2.1. Variant 1: Multidirectional Angle-Stable Screw Connection (Rigid Fixation) 2.1. Variant 1: Multidirectional Angle-Stable Screw Connection (Rigid Fixation)*

*Materials* **2022**, *15*, 6457 3 of 16

The screws have a thread in the head area and can be placed at an angle to the implant (multidirectional). To achieve angular stability, the thread cuts into the implant material and thus fixes the screw. The inclined position of the individual screws results in greater stabilization and resistance to dents (medial/proximal) and withdrawal (Figure 1). The screws have a thread in the head area and can be placed at an angle to the implant (multidirectional). To achieve angular stability, the thread cuts into the implant material and thus fixes the screw. The inclined position of the individual screws results in greater stabilization and resistance to dents (medial/proximal) and withdrawal (Figure 1).

**Figure 1.** Rigid fixation of the augmentation material. **Figure 1.** Rigid fixation of the augmentation material.

Resorption is the greatest unknown factor despite the protection of the augmentation. The resorption leads to a reduction of the functional pressure on the augmentation material and possibly to an increased resorption. The original design was adapted and improved in detail in several processing and optimization steps, with the aim of maximizing mechanical stability while reducing the profile cross-section (thickness or height of the implant). For this purpose, the originally designed curved oblong holes for screw connection were replaced by circular holes, as the intended cutting of the thread of the screw heads did not provide sufficient stability in this case. In addition, the radius of the implant surface was adjusted to achieve an even lower implant height. The result of the detailed design can be seen in Figure 2. Resorption is the greatest unknown factor despite the protection of the augmentation. The resorption leads to a reduction of the functional pressure on the augmentation material and possibly to an increased resorption. The original design was adapted and improved in detail in several processing and optimization steps, with the aim of maximizing mechanical stability while reducing the profile cross-section (thickness or height of the implant). For this purpose, the originally designed curved oblong holes for screw connection were replaced by circular holes, as the intended cutting of the thread of the screw heads did not provide sufficient stability in this case. In addition, the radius of the implant surface was adjusted to achieve an even lower implant height. The result of the detailed design can be seen in Figure 2. *Materials* **2022**, *15*, 6457 4 of 16

Particular attention was paid to the design of the fixing holes. The exact geometry of these holes is important to ensure that the screw heads are held securely and that the screw heads do not project beyond the implant surface. The implant or the fixing holes were de-

To compensate for the risk of functional underloading of the augmentation material, a dynamic system is proposed. Instead of screws, pins are used which only have a thread in the head area that cuts into the implant material. The pins should be placed at a right angel to the implant backside (defined by the geometry), as shown in Figures 4 and 5.

**Figure 2.** Result of detailed design of rigid variant: (**a**) three-dimensional view; (**b**) sectional view. (**a**) (**b**) **Figure 2.** Result of detailed design of rigid variant: (**a**) three-dimensional view; (**b**) sectional view.

situations and to increase the mechanical stability of the fixation (Figure 3).

**Figure 3.** Multidirectional screw positioning possibility. *2.2. Variant 2: Dynamic Fixation (Angular Stable Pins)* 

ment Locking Compression Plate (LCP) System (DePuy Synthes Companies, Zuchwil, Switzerland, and Warsaw, IN, USA, https://www.jnjmedicaldevices.com/en-EMEA/companies/depuy-synthes, accessed on 8 July 2022). The screws can be introduced in multiple directions, i.e., at different angles to each other, in order to adapt to the respective anatomical

Particular attention was paid to the design of the fixing holes. The exact geometry of these holes is important to ensure that the screw heads are held securely and that the screw heads do not project beyond the implant surface. The implant or the fixing holes were designed for the use of osteosynthesis screws "Locking Screw 3.5 mm" from the Small Fragment Locking Compression Plate (LCP) System (DePuy Synthes Companies, Zuchwil, Switzerland, and Warsaw, IN, USA, https://www.jnjmedicaldevices.com/en-EMEA/companies/depuy-synthes, accessed on 8 July 2022). The screws can be introduced in multiple directions, i.e., at different angles to each other, in order to adapt to the respective anatomical situations and to increase the mechanical stability of the fixation (Figure 3). Particular attention was paid to the design of the fixing holes. The exact geometry of these holes is important to ensure that the screw heads are held securely and that the screw heads do not project beyond the implant surface. The implant or the fixing holes were designed for the use of osteosynthesis screws "Locking Screw 3.5 mm" from the Small Fragment Locking Compression Plate (LCP) System (DePuy Synthes Companies, Zuchwil, Switzerland, and Warsaw, IN, USA, https://www.jnjmedicaldevices.com/en-EMEA/companies/depuy-synthes, accessed on 8 July 2022). The screws can be introduced in multiple directions, i.e., at different angles to each other, in order to adapt to the respective anatomical situations and to increase the mechanical stability of the fixation (Figure 3). Particular attention was paid to the design of the fixing holes. The exact geometry of these holes is important to ensure that the screw heads are held securely and that the screw heads do not project beyond the implant surface. The implant or the fixing holes were designed for the use of osteosynthesis screws "Locking Screw 3.5 mm" from the Small Fragment Locking Compression Plate (LCP) System (DePuy Synthes Companies, Zuchwil, Switzerland, and Warsaw, IN, USA, https://www.jnjmedicaldevices.com/en-EMEA/companies/depuy-synthes, accessed on 8 July 2022). The screws can be introduced in multiple directions, i.e., at different angles to each other, in order to adapt to the respective anatomical situations and to increase the mechanical stability of the fixation (Figure 3).

**Figure 2.** Result of detailed design of rigid variant: (**a**) three-dimensional view; (**b**) sectional view.

**Figure 2.** Result of detailed design of rigid variant: (**a**) three-dimensional view; (**b**) sectional view.

*Materials* **2022**, *15*, 6457 4 of 16

*Materials* **2022**, *15*, 6457 4 of 16

(**a**) (**b**)

(**a**) (**b**)

**Figure 3.** Multidirectional screw positioning possibility. **Figure 3.** Multidirectional screw positioning possibility. **Figure 3.** Multidirectional screw positioning possibility.

### *2.2. Variant 2: Dynamic Fixation (Angular Stable Pins) 2.2. Variant 2: Dynamic Fixation (Angular Stable Pins)*

*2.2. Variant 2: Dynamic Fixation (Angular Stable Pins)*  To compensate for the risk of functional underloading of the augmentation material, a dynamic system is proposed. Instead of screws, pins are used which only have a thread in the head area that cuts into the implant material. The pins should be placed at a right To compensate for the risk of functional underloading of the augmentation material, a dynamic system is proposed. Instead of screws, pins are used which only have a thread in the head area that cuts into the implant material. The pins should be placed at a right angel to the implant backside (defined by the geometry), as shown in Figures 4 and 5. To compensate for the risk of functional underloading of the augmentation material, a dynamic system is proposed. Instead of screws, pins are used which only have a thread in the head area that cuts into the implant material. The pins should be placed at a right angel to the implant backside (defined by the geometry), as shown in Figures 4 and 5.

angel to the implant backside (defined by the geometry), as shown in Figures 4 and 5.

**Figure 4.** Dynamic fixation using angle-stable pins: (**a**) initial post-operative situation; (**b**) dynamic load results in compression and functional stimulus of the augmented bone.

Only shear and torsional forces in the medial or sagittal plane are absorbed. The entire unit can sink if the bone block atrophies and the dynamic load and thus a functional stimulus is maintained. This is to reduce bone resorption. The dynamic version can be fitted with up to five pins, and the most appropriate pins should be used depending on the specific circumstances.

**Figure 4.** Dynamic fixation using angle-stable pins: (**a**) initial post-operative situation; (**b**) dynamic

Only shear and torsional forces in the medial or sagittal plane are absorbed. The entire unit can sink if the bone block atrophies and the dynamic load and thus a functional stimulus is maintained. This is to reduce bone resorption. The dynamic version can be fitted with up to five pins, and the most appropriate pins should be used depending on

load results in compression and functional stimulus of the augmented bone.

the specific circumstances.

**Figure 5.** Visualization of dynamic variant of GlenoProtect on glenoid/scapula (**a**) digital model for simulation purpose (bone defect, augmentation not shown) and (**b**) analog model for development purpose (foam material simulates the bone augmentation). **Figure 5.** Visualization of dynamic variant of GlenoProtect on glenoid/scapula (**a**) digital model for simulation purpose (bone defect, augmentation not shown) and (**b**) analog model for development purpose (foam material simulates the bone augmentation).

As an implant material, the use of a plastic material should be considered as the most sensible option. Metallic materials are critical due to the sliding pairing with the also metallic humeral head [35,36]. PE (polyethylene) and PEEK (polyetheretherketone) are possible plastics [37–39]. PE is the most commonly used material for primary glenoid components, where ultra-high molecular weight polyethylene (UHMWPE) is used. PE has good sliding properties, is easy to process and is comparatively inexpensive. With PEEK, the raw material is more expensive, however, it has more suitable mechanical properties than PE. Thus, the modulus of elasticity is closer to that of bone. This results in a better or natural distribution of force on the bone [40,41]. PEEK is a high-performance biomaterial suitable for long-term implantation. It is used to manufacture a wide variety of medical devices and human implants (dentistry, orthopedics and traumatology) and is used in a variety of ways in these applications [42–45]. Due to its chemical composition, PEEK is a very pure and inert material. Extensive biocompatibility tests do not provide any evidence of cytotoxicity, systemic toxicity, irritation or acroscopic reactions [31–35]. In addition, the very low levels of residues and extractable metal ions minimize the potential risk of allergic reactions commonly associated with nickel or other metal ions. In addition, PEEK can be sterilized using all common methods. PEEK is suitable for gamma, ethylene oxide and saturated steam sterilization [46]. In order to determine the mechanical strength, corresponding simulations were car-As an implant material, the use of a plastic material should be considered as the most sensible option. Metallic materials are critical due to the sliding pairing with the also metallic humeral head [35,36]. PE (polyethylene) and PEEK (polyetheretherketone) are possible plastics [37–39]. PE is the most commonly used material for primary glenoid components, where ultra-high molecular weight polyethylene (UHMWPE) is used. PE has good sliding properties, is easy to process and is comparatively inexpensive. With PEEK, the raw material is more expensive, however, it has more suitable mechanical properties than PE. Thus, the modulus of elasticity is closer to that of bone. This results in a better or natural distribution of force on the bone [40,41]. PEEK is a high-performance biomaterial suitable for long-term implantation. It is used to manufacture a wide variety of medical devices and human implants (dentistry, orthopedics and traumatology) and is used in a variety of ways in these applications [42–45]. Due to its chemical composition, PEEK is a very pure and inert material. Extensive biocompatibility tests do not provide any evidence of cytotoxicity, systemic toxicity, irritation or acroscopic reactions [31–35]. In addition, the very low levels of residues and extractable metal ions minimize the potential risk of allergic reactions commonly associated with nickel or other metal ions. In addition, PEEK can be sterilized using all common methods. PEEK is suitable for gamma, ethylene oxide and saturated steam sterilization [46].

ried out. The selected implant variants were analyzed using FEM (Finite Element Method), also known as FEA (Finite Element Analysis) [47]. The simulation program Abaqus (Dassault Systèmes, Vélizy-Villacoublay, France, http://www.3ds.com, accessed In order to determine the mechanical strength, corresponding simulations were carried out. The selected implant variants were analyzed using FEM (Finite Element Method), also known as FEA (Finite Element Analysis) [47]. The simulation program Abaqus (Dassault Systèmes, Vélizy-Villacoublay, France, http://www.3ds.com, accessed on 8 July 2022) was used. A material approved for medical applications in the human body and repeatedly proven in implants is PEEK-Optima from Invibio Ltd. (for properties, see Table 1: Material properties PEEK, Invibio Ltd., Lancashire, UK). These material properties were used as a basis for the FEM analysis.


on 8 July 2022) was used. A material approved for medical applications in the human body and repeatedly proven in implants is PEEK-Optima from Invibio Ltd. (for properties, see Table 1: Material properties PEEK, Invibio Ltd., Lancashire, England). These ma-

**Table 1.** Material properties PEEK. **Table 1.** Material properties PEEK.

*Materials* **2022**, *15*, 6457 6 of 16

terial properties were used as a basis for the FEM analysis.

The load and force assumptions for the simulation were selected by Westerhoff and Bergmann [48] according to the measurements at the Julius Wolff Institute. For the FEM analysis, the data set "Lifting a weight of 10 kg" was used, since the measurement results here show the highest values. The data give a maximum value of 1500 N for the force acting normally on the implant surface. After consultation with shoulder surgeons, a maximum realistic force of 500 N is to be assumed for the intended application. The simulation was performed with 1500 N as well as with 500 N. In the FEM simulation, static forces/loads are applied and the resulting stresses, displacements (elastic) or plastic deformations are determined. For this purpose, a so-called substitute model must be created, which represents a section of the entire situation, reflecting the relevant force application. For the calculation of the stresses in the protective implant, the replacement model was in the form of a hemisphere representing the humeral head with an equally distributed load application. The load and force assumptions for the simulation were selected by Westerhoff and Bergmann [48] according to the measurements at the Julius Wolff Institute. For the FEM analysis, the data set "Lifting a weight of 10 kg" was used, since the measurement results here show the highest values. The data give a maximum value of 1500 N for the force acting normally on the implant surface. After consultation with shoulder surgeons, a maximum realistic force of 500 N is to be assumed for the intended application. The simulation was performed with 1500 N as well as with 500 N. In the FEM simulation, static forces/loads are applied and the resulting stresses, displacements (elastic) or plastic deformations are determined. For this purpose, a so-called substitute model must be created, which represents a section of the entire situation, reflecting the relevant force application. For the calculation of the stresses in the protective implant, the replacement model was in the form of a hemisphere representing the humeral head with an equally distributed load application.

For the simulation of the rigid version, a joint ball made of stainless steel, with a diameter of 48 mm, and its support centered on the implant was modelled (Figure 6). For the simulation of the rigid version, a joint ball made of stainless steel, with a diameter of 48 mm, and its support centered on the implant was modelled (Figure 6).

**Figure 6.** Modeling of the load for the FEM analysis. **Figure 6.** Modeling of the load for the FEM analysis.

Fixation of the implant was assumed with four screws. The posterior implant surface was assumed without bone support, so the implant is only held in place by the screws. Thus, the determined displacement (corresponds to "deflection") of the implant can act as a stimulus on the augmentation. At the simulation model for the dynamic variant, the joint ball is resting eccentrically on the implant. The fixation of the implant was assumed at the end of the conical pins. The eccentric loading and clamping of the pin ends result in a displacement and thus a (rotational) moment at the connection points of the pins with the implant main body. This in turn represents the load situation that occurs when the implant is attached to the bone via the pins and "tilts" the implant surface away due to an eccentric load (with corresponding movement of the shoulder). Fixation of the implant was assumed with four screws. The posterior implant surface was assumed without bone support, so the implant is only held in place by the screws. Thus, the determined displacement (corresponds to "deflection") of the implant can act as a stimulus on the augmentation. At the simulation model for the dynamic variant, the joint ball is resting eccentrically on the implant. The fixation of the implant was assumed at the end of the conical pins. The eccentric loading and clamping of the pin ends result in a displacement and thus a (rotational) moment at the connection points of the pins with the implant main body. This in turn represents the load situation that occurs when the implant is attached to the bone via the pins and "tilts" the implant surface away due to an eccentric load (with corresponding movement of the shoulder).

Mechanical testing was performed in order to confirm the results from the simulation and to demonstrate the effective strength of the implant variants and the respective fixing methods. For the mechanical tests, a specific test stand was built at the Center for Medical Physics and Biomedical Engineering of the Medical University of Vienna at the AKH Vienna.

The test bench is used to simulate the movements of the shoulder joint. For this purpose, the test stand was primarily designed as an actuator-controlled pendulum system. Thus, it is possible to perform cyclic movements of a simulated humeral head prosthesis in relation to the glenoid protective implant. The contact force (force normal to the implant surface) and the type of movement of the ball of the humeral head (from rolling on the implant surface to a pure friction movement at one point) are adjustable.

The first mechanical test is intended to confirm the mechanical stability of the combination of the implant itself and the fastening elements (screws in the rigid and pins in the dynamic variant) under static loading. The implant itself as well as the fastening elements and the connection of the implant with the fastening elements must be sufficiently strong. In particular, the connection of the implant with the screws or pins represents a critical point. With a static implant, the screw heads must not protrude beyond the implant surface under load.

Following the static load test, a dynamic load test was carried out. For this purpose, the pendulum frame was oscillated with an angular deflection of ±30◦ . Due to the dynamics of the system and the centrifugal forces that occur when the pendulum frame is loaded with the additional weight, the test was carried out with a maximum load of 1000 N.

Comparative measurements were carried out to determine differences in the abrasion behavior of different materials. The following test materials were examined in Table 2:

**Table 2.** Overview of test objects for abrasion measurement.


To create largely real conditions, the test objects were placed in a shell filled with a physiological solution of H2O, agar-agar and NaCl during the pendulum motion. Agar-agar was added to increase the viscosity of the liquid, as the synovial liquid is also more viscous.

A high-precision galvo scanner (scanner galvanometer) was used for the tactile measurement of the surface [49] to quantitatively determine the abrasion. The galvo scanner has a resolution of 200 nm. This means that any abrasion in the form of "material shrinkage" can be measured at the relevant point. This method does not determine the abrasion as detached particles, but whether there is abrasion on the implant surface. These depressions in the material can be very precisely measured with the galvo scanner and are thus displayed quantitatively.

The test objects were clamped on a cross table. The galvo scanner was fixed via a 3D articulated tripod so that its lever arm rests on the test object (Figure 7). Now the cross table was moved manually, and the output signal was recorded with a DAQ measuring system (DEWE-43 from DEWESoft, Trobvlje, Slovenia, http://www.dewesoft.com, accessed on 8 July 2022).

**Figure 7.** Measuring tip of the galvo scanner for tactile surface measurement. **Figure 7.** Measuring tip of the galvo scanner for tactile surface measurement.

### **3. Results 3. Results**

In this chapter, the results for the simulation (FEM), mechanical testing results and the abrasion measurements are summarized. In this chapter, the results for the simulation (FEM), mechanical testing results and the abrasion measurements are summarized.

### *3.1. Simulation 3.1. Simulation*

The results of the FEM simulation for both implant variants are presented in Table 3. The results of the FEM simulation for both implant variants are presented in Table 3.


**Table 3.** Summary of FEM simulation results. **Table 3.** Summary of FEM simulation results.

With the rigid variant at 1500 N load, stresses greater than 90–100 MPa occur selectively in the material (Figure 8). With the rigid variant at 1500 N load, stresses greater than 90–100 MPa occur selectively in the material (Figure 8).

Since these areas are rather small and surrounded by areas of significantly lower stress, it can be assumed that structural integrity will be maintained. However, it is to be expected that superficial damage may occur (the areas of high tension lie on the surface of the implant in contact with the joint ball). Since these areas are rather small and surrounded by areas of significantly lower stress, it can be assumed that structural integrity will be maintained. However, it is to be expected that superficial damage may occur (the areas of high tension lie on the surface of the implant in contact with the joint ball).

Peak strain values of up to 168 MPa occur selectively in the material of the dynamic variant (see Figure 9).

*Materials* **2022**, *15*, 6457 9 of 16

**Figure 8.** FEM Analysis of the rigid variant: strain at 1500 N, scaling 90 MPa. **Figure 8.** FEM Analysis of the rigid variant: strain at 1500 N, scaling 90 MPa. maximum strain values occurring over a large area are in the range of 100 MPa. However,

this is already at the limits of the material.

Peak strain values of up to 168 MPa occur selectively in the material of the dynamic

occur. It is more realistic to distribute the forces over a larger surface of the pins. The

**Figure 9.** FEM Analysis of the Dynamic variant: strain at 500 N, scaling 168 MPa. **Figure 9.** FEM Analysis of the Dynamic variant: strain at 500 N, scaling 168 MPa.

 The simulation with a load of 1500 N resulted in peak strain values well above 100 MPa. This would mean that the implant would no longer be able to withstand the load. The high stresses occur again in isolated areas and could occur due to the modeling (clamping only at the tip of the conical pins would distribute the dissipation of forces along the pins). However, these areas are very limited and are justified by the fact that the ends of the pins were firmly clamped for the simulation. This is exactly where these high stresses occur. It is more realistic to distribute the forces over a larger surface of the pins. The maximum strain values occurring over a large area are in the range of 100 MPa. However, this is already at the limits of the material.

**Figure 9.** FEM Analysis of the Dynamic variant: strain at 500 N, scaling 168 MPa. The simulation with a load of 1500 N resulted in peak strain values well above 100 MPa. This would mean that the implant would no longer be able to withstand the load. The high stresses occur again in isolated areas and could occur due to the modeling Summary: The results of the FEM analysis of the rigid variant show that there will be no permanent damage (plastic deformation) when the load is 500 N. Under a load of 1500 N, plastic The simulation with a load of 1500 N resulted in peak strain values well above 100 MPa. This would mean that the implant would no longer be able to withstand the load. The high stresses occur again in isolated areas and could occur due to the modeling (clamping only at the tip of the conical pins would distribute the dissipation of forces along the pins).

Summary:

(clamping only at the tip of the conical pins would distribute the dissipation of forces along the pins). Summary: The results of the FEM analysis of the rigid variant show that there will be no perma-The results of the FEM analysis of the rigid variant show that there will be no permanent damage (plastic deformation) when the load is 500 N. Under a load of 1500 N, plastic deformations or material damage can occur on the contact surface with the joint ball, but the overall strength of the implant and the screw connections would still be guaranteed.

nent damage (plastic deformation) when the load is 500 N. Under a load of 1500 N, plastic

The results of the FEM analysis of the dynamic implant variant show that loads of 500 N do not lead to any damage to the implant or that the entire structural integrity is preserved. At a load of 1500 N, the maximum permissible stresses in the material would be significantly exceeded. In such a case, modeling becomes very difficult, and it is very likely that the resulting stresses arise due to the model assumptions. However, since a maximum of 500 N can be assumed as a realistic force, the simulation of the dynamic implant variant also shows that it has sufficient mechanical stability. The results of the FEM analysis of the dynamic implant variant show that loads of 500 N do not lead to any damage to the implant or that the entire structural integrity is preserved. At a load of 1500 N, the maximum permissible stresses in the material would be significantly exceeded. In such a case, modeling becomes very difficult, and it is very likely that the resulting stresses arise due to the model assumptions. However, since a maximum of 500 N can be assumed as a realistic force, the simulation of the dynamic implant variant also shows that it has sufficient mechanical stability.

deformations or material damage can occur on the contact surface with the joint ball, but the overall strength of the implant and the screw connections would still be guaranteed.

*Materials* **2022**, *15*, 6457 10 of 16

### *3.2. Mechanical Testing 3.2. Mechanical Testing*

### 3.2.1. Rigid Variant 3.2.1. Rigid Variant

For the test, the protective glenoid implant was attached at a distance from a test block with three angular stable osteosynthesis screws (Figure 10). This simulates the case where the protective glenoid implant was attached to the scapula above the graft and the graft was already slightly resorbed. This creates a small gap and the forces applied to the implant are transmitted exclusively via the screws into the scapula, which is the worst case from a mechanical point of view. In this case, the force is transmitted in the area of the small thread onto the head of the osteosynthesis screws. The screw heads must remain securely fixed in the implant and not "tear out" so that the screw heads protrude beyond the implant surface. For the test, the protective glenoid implant was attached at a distance from a test block with three angular stable osteosynthesis screws (Figure 10). This simulates the case where the protective glenoid implant was attached to the scapula above the graft and the graft was already slightly resorbed. This creates a small gap and the forces applied to the implant are transmitted exclusively via the screws into the scapula, which is the worst case from a mechanical point of view. In this case, the force is transmitted in the area of the small thread onto the head of the osteosynthesis screws. The screw heads must remain securely fixed in the implant and not "tear out" so that the screw heads protrude beyond the implant surface.

**Figure 10.** Fixing the test implant at the test block (represented is only one screw before insertion, the test implant was fixed with three screws). **Figure 10.** Fixing the test implant at the test block (represented is only one screw before insertion, the test implant was fixed with three screws).

The test implant was loaded with 1300 N. There was no damage to the implant and the threaded connections also withstood the load. This result corresponds to the behavior expected from the FEM simulation (no permanent deformation, maintenance of overall stability up to 1500 N). The test implant was loaded with 1300 N. There was no damage to the implant and the threaded connections also withstood the load. This result corresponds to the behavior expected from the FEM simulation (no permanent deformation, maintenance of overall stability up to 1500 N).

The implant and the threaded connection were also stable in the test with dynamic loads. The implant and the threaded connection were also stable in the test with dynamic loads.
