*2.2. Implant Characterstics*

HA was performed using a fracture prosthesis (Affinis Fracture or Articula system), which is part of a modular implant system that allows conversion to RSA using the same humeral stem as in HA. For conversion RSA, the head component and the metaphyseal element were replaced with a reverse metaphyseal element (Affinis Fracture Inverse system) (Figure 2). This element can be fixed to the humeral stem with 10 mm of possible height adjustment and free torsion. On the glenoid side, either a 39 or 42 mm glenosphere can be chosen. The reverse metaphyseal element exists in a 0 or 3 mm offset version. If both metaphyseal parts are at the same level, converting HA to RSA would lengthen the humerus by 23 mm and medialize the center of rotation by at least 19 mm (from 4 to 23 or 25 mm lateral offset, Figure 3). This equates to the minimal lengthening in a worst-case scenario, where the central part was placed completely distally during the implantation of the fracture HA and revised with a 39 mm glenosphere and a 0 mm offset reverse metaphyseal element. When placing the reverse metaphyseal element completely proximally during the revision surgery with the largest glenosphere and the highest offset, a maximum lengthening of 38 mm and medialization of the center of rotation of 21 mm can be achieved.

**Figure 2.** Failure of hemiarthroplasty and conversion to reverse shoulder arthroplasty. (**a**) Preoperative anteroposterior radiograph of a hemiarthroplasty with a firmly cemented stem showing insufficiency of the supraspinatus tendon and superior migration of the humeral head; (**b**) Postoperative anteroposterior radiograph of a conversion reverse shoulder arthroplasty with humeral stem retention; (**c**) Four-year postoperative anteroposterior radiograph showing prosthesis in situ.

**Figure 3.** Modular reverse shoulder prosthesis. If both metaphyseal parts are at the same level, conversion leads to 23 mm of distalization and medializes the center of rotation by 19 mm (left). This corresponds to the worst-case scenario if the reverse metaphyseal element was placed most distally during the primary implantation of hemiarthroplasty (middle). The maximum lengthening of the conversion is 38 mm by choosing a 42 mm glenosphere and an offset of 3 mm (right). The length of the anatomic and the reverse prosthesis can be adjusted by 10 mm to reduce distalization. The center of rotation is medialized between 19 mm and 21 mm.
