RSA, often used in multiple operated patients with distorted anatomy, imparts physiological and biomechanical changes that may increase the potential for complications [3]. First, medialization in the center of rotation doubles the lever arm of the deltoid muscle, allowing recruitment of more anterior and posterior deltoid fibers and therefore increasing deltoid abduction efficiency. These phenomena are also responsible for impingement of the medial border of the humeral component on the scapular neck. Repetitive contact between polyethylene and bone may result in polyethylene wear debris, chronic inflammation and osteolysis [18, 19], radiolucency around the glenoid component [20], loosening of the glenoid component [21], presence of an inferior bone spur, and ossification in the glenohumeral space [7, 16, 22]. Second, the fixed nature of the glenosphere places torsional forces on the humerus that may affect humeral component instability [23]. Third, the semi-constrained nature of the prosthesis restores glenohumeral stability which provides the stable fulcrum which is essential for active anterior elevation. Finally and most relevant to this chapter is lengthening of the arm which provides space for range of motion of the proximal humerus, enhances stability, and re-tensions the deltoid. The latter factor is critical due to the semi-constrained design of the prosthesis. The increase in compressive force between the humeral and glenoid components has a stabilizing effect [9]. Under such tension, the reverse glenoid component provides the stable fulcrum essential for shoulder anterior elevation and prosthesis stability [8]. This tension is determined by arm length.

Few techniques of lengthening with RSA have been validated. Measurement techniques can be either radiographic or clinical. Measurements can focus on the length of the arm, humerus, or acromiohumeral distance. Renaud et al. [24] were the first to propose the determination of a “radio-anatomic index.” They measured acromiohumeral distances on anteroposterior radiographs (Fig. 27.4) but did not control for magnification. In cases of superior escape of the contralateral humeral head, the normal position of the humeral epiphysis was estimated using a horizontal line that passed perpendicular to the center of the glenoid. However, the presence of superior glenoid erosion [25] renders this technique inaccurate.

Lädermann et al. [13] recommended a technique to determine arm and humeral length using bilateral plain anterior–posterior radiographs taken under magnification and fluoroscopic control (Fig. 27.5).

A similar technique to assess the amount of lengthening of the humerus was subsequently reported by Greiner et al. [26]. Lädermann et al. [13] compared the lengths of the affected and contralateral humeral shafts to determine whether the contralateral humerus may be reliably used to determine prosthetic height in complex cases with humeral bone loss, or to perform a postoperative assessment in revision cases where preoperative scaled radiographs of the humerus are unavailable. One disadvantage of this technique is the need to perform magnification controlled radiographs of the entire humerus. As the X-ray beam is centered at the middle third of humerus, these radiographs do not provide an accurate depiction of the acromiohumeral interval. Consequently, this technique of measurement accurately reflects humeral length, but accuracy of the acromiohumeral interval measurement is compromised. Moreover, this technique requires drawing an epicondylar reference line that can be difficult if the humerus is not in neutral rotation.

Jobin et al. [27] proposed another technique to evaluate subacromial and deltoid length postoperatively. In their study, complete preoperative and postoperative true anteroposterior radiographs of the glenohumeral joint in neutral rotation were collected. Subacromial length (acromion to greater tuberosity distance) was measured as the distance from the inferolateral acromial tip to the most prominent superolateral aspect of the greater tuberosity (Fig. 27.6). Middle deltoid length was defined as the distance between the inferolateral tip of the acromion and the midpoint of the deltoid tuberosity with the arm in neutral rotation and 0° abduction, as proposed initially by De Wilde et al. [28]. Length was calibrated by the known diameter of the glenosphere and the fixed bony distances of the humeral shaft width (h), as well as the fixed bony distance from the greater tuberosity to the deltoid tuberosity. Since the technique of Jobin et al. calibrates each radiograph to the glenosphere diameter, it is impossible to determine humeral and subacromial length preoperatively. This technique is therefore not useful in preoperative planning of difficult cases such as those with proximal bone loss. Furthermore, the greater tuberosity was selected for the proximal reference point. This anatomic landmark may be absent preoperatively, malunited, or be difficult to visualize because of arm rotation. Moreover, humeral radiolucencies, stem subsidence, radiological signs of stress shielding, and resorption of the tuberosities are common complications after RSA [16] that can compromise the anatomic landmarks used in this technique.

Lastly, Boileau et al. [8] clinically measured the length of the arm postoperatively relative to the opposite side using a specially designed external caliper (Fig. 27.7). This technique is noninvasive and easy to apply clinically but does not differentiate between humeral and arm length and is therefore not useful for preoperative planning.

The typical postoperative lengthening of the arm, humerus, and subacromial space (acromiohumeral) with a Grammont-style prosthesis is summarized in Table 27.1. Mean arm lengthening varies from 15 to 27 mm, and mean humeral lengthening varies from −5 to 5 mm. The humeral cut is typically more aggressive when a transdeltoid surgical approach is performed, but this is compensated for by an increase in thickness of the polyethylene liner.