2.3.1 Introduction

The creation of shoulder implants has introduced many solutions for the improvement of pain relief and shoulder functions within patients. These implants help patients who suffer from any infection, pain, discomfort, arthritis, or trauma in their shoulder area. Thus, the need for shoulder implants needs to compensate for these problems. Themistocles Gluck was one of the earlier designers of the first shoulder implants in the late 1800s; however, his work was never published [1, 2]. Therefore, the first known, recorded metal shoulder implant was performed by Jules Emile Pean back in the 1800s. He implanted a constrained total shoulder prosthesis that was made of platinum and rubber [3]. The patient, who suffered from shoulder tuberculosis arthritis, had his humerus resected to remove the infected tissue. Initially, the implant was deemed successful as the patient reported to have an increase in strength and range of motion with this implant. However, later, the patient suffered severe infections which resulted in the prosthetic being removed [4]. There were plenty of attempts before and after this to create a functional shoulder implant; however, none of them were deemed safe to use in practice. The next breakthrough was not until the 1950s. Frederick Krueger based his Vitallium prostheses on the acrylic molds of cadaver shoulders (Figure 2.3.1).

This was implanted in the shoulder of a merchant marine to treat avascular necrosis of the humeral head [6]. His technique is still used today due to the conservation of the rotator cuff. Charles Neer II created a widely used prosthesis for the shoulder. He developed the non‐constrained proximal humerus and total shoulder replacement system [7]. The “Neer 1 prosthesis” was also made of a cast cobalt–chrome alloy which has similar mechanical properties to bone. Although successful, the setback with Neer’s discovery was patients with severely torn rotator cuffs. A diminished rotator cuff can result in the implant moving, loosening, and not functioning properly. Multiple prostheses were created to increase the stability and constraints of its components. By the 1970s, Neer discovered reverse shoulder arthroplasty techniques. He went on to create three different prostheses with the assistance of Robert Averill. The first design neglected the rotator cuff entirely. Neer’s philosophy was to reconstruct the cuff around the prosthesis. The second design introduced a more complex prosthesis which focused on the ability to repair the rotator cuff and other tissues around the implant. The decrease in the range of motion forced Neer to develop a third prosthesis. This design allowed for rotation to increase the range of motion from the second design (Figure 2.3.2).

The experiment was a failure resulting in Neer abandoning this design and concluding that the fixed fulcrum technique still did not account for the rotator cuff repair. Finally, in 1985, Paul Grammont introduced the reverse shoulder prosthesis. This semi‐constrained implant design features a polyethylene humeral cup and a cobalt–chromium–molybdenum geosphere [9]. Procedures referring to reverse total shoulder arthroplasty include biomechanical implants. The design has been frequently used on the glenoid and humerus of human anatomy. We use reverse total shoulder arthroplasty (RSTA) to solve issues related to bone loss and tumors. Goals that consisted within this method were defined to medialize the center of rotation leading toward a stable implant, re‐tensioning of the deltoid by destabilizing the humerus, and a semi‐constrained prosthesis with a large arc of motion [10]. This method allows forces across the shoulder to become altered. The movements around the fixed center of rotation convert the shear forces toward a large vector. The minimization ratio of shear to compressive forces at the joints gives a stable prosthesis. A modern RSTA implant is the Grammont implant. It minimizes the shear force and increases compressive forces to create an overall force vector at the bone–glenoid interface [11]. This aids in the tension of the deltoids and, in doing so, provides a mechanical advantage. RSTA has also replaced hemiarthroplasty as the implant for displaced three‐ and four‐part proximal humerus fractures [12].

Bone grafting (genioplasty) achieves sufficient bony fixation by restoring the glenoid posterior augmentation. It also can lateralize the center of rotation to avoid impingement on the coracoid and scapula. Bone grafting is recommended if medialization occurs past the point of the coracoid. It can successfully reconstruct glenoid curvature and depth to restore stability. It is important to note that the correct position and orientation of the bone graft are crucial for this procedure. If a graft is placed too lateral of a position, it will lead to an increased anterior–inferior peak contact pressure [13]. A recessed graft will lead to high‐edge loading [14]. Orientating the coracoid bone graft in an inferior direction will help avoid increased inferior contact pressure. Congruent‐arc modification will allow the reconstruction of larger defects by matching the graft shape to the glenoid [15]. Other methods of stabilization include autograft and allograft. Autografting is used when there is an irreparable rotator cuff lesion [16]. Allografting is used in the absence of a reliable autograft.

Shoulder arthroplasty is a method used to remove damaged areas of the bone and replace them with parts made of metal and plastic implants. Shoulder implants are available in different shapes and arrangement of sizes. Replacement options can be done partially or totally using anatomic or reverse implants. In an anatomic total shoulder replacement, both the ball and socket are replaced using a method where the implant would resemble the natural shape of the bones [17]. In partial shoulder replacement, only the head of the joint is replaced. The recommendations are only present when the ball portion of the joint has received damage. In reverse total shoulder replacement, both the ball and socket are replaced as the implants are reversed [17]. The ball is attached to the shoulder blade and the socket is attached to the upper arm bone. This method is usually preferred when the rotator cuff is severely damaged. Balloon implantations are a proposed method for irreparable lesions to restore glenohumeral contact pressure. Balloon spacers are placed in the subacromial space [18]. This procedure results in the efficient lowering of the humeral head, increased deltoid load, and normalization of articular contact pressure at abduction angles [19]. In long‐term outcomes, biodegradable balloons may be questionable regarding the setting of an irreparable tear [20]. Its benefits may reduce peak pressure and wear on the repair avoiding re‐tear.

2.3.2 Titanium Alloys

Titanium alloys have been found to hold various properties in use in the biomedical industry. Titanium alloys contain properties such as their biocompatibility and corrosion resistance, and their low modulus of elasticity is closely related to bone, which results in low‐stress shielding [21, 22