TECHNIQUE FOR INSERTION OF A REVERSE-PROSTHESIS HUMERAL COMPONENT

Once the inferior capsule is released from the neck of the glenoid as described in Chapter 27, humeral preparation begins. The humeral head retractor is removed, and the humeral head is dislocated by externally rotating and extending the arm. This maneuver is typically easier than in cases of unconstrained arthroplasty because the compromised rotator cuff offers little resistance to proximal humeral dislocation. A Hohmann retractor positioned superior to the coracoid process is moved to the margin of the humeral head, which is often “bald” (devoid of any discernible rotator cuff), and a modified Hohmann retractor is placed inferiorly and medially at the surgical neck of the humerus to complete the proximal humeral exposure (Fig. 28-2).

Figure 28-2 A and B, Completed exposure of the proximal humerus in preparation for insertion of a reverse prosthesis.

In most patients being treated with a reverse prosthesis, minimal if any peripheral humeral head osteophytes are present. In the uncommon situation in which large humeral osteophytes exist, they are removed with a half-inch osteotome and mallet, just as in cases of unconstrained shoulder arthroplasty (see Chapter 11). In contrast to cases of anatomic arthroplasty in which humeral osteophytes are removed to facilitate identification of the anatomic neck of the humerus to guide resection of the humeral head, osteophytes are removed in reverse-prosthesis procedures to avoid mechanical impingement between the osteophytes and the axillary border of the scapula (Fig. 28-3). A starter awl is used to open the humeral canal (Fig. 28-4). It is helpful to refer to preoperative radiographs to select the proper starting point for the awl, which should be in line with the humeral canal (Fig. 28-5). The humeral head can become deformed in patients with long-standing rotator cuff tears and can make identification of this starting point difficult. The humeral cutting guide, which sets resection at a 155-degree angle of inclination, is placed down the humeral canal (Fig. 28-6).

Figure 28-3 A and B, Large proximal humeral osteophytes may cause mechanical impingement after implantation of a reverse prosthesis if not removed.

Figure 28-4 A and B, A starter awl is used to open the humeral canal.

Figure 28-5 The preoperative anteroposterior radiograph is useful to approximate the position of the starting point for the awl.

Figure 28-6 A and B, Insertion of the humeral cutting guide.

The system that we use allows selection of humeral cut version from neutral to 20 degrees of retroversion with the forearm used as a reference (Fig. 28-7). The advantage of resecting the humeral head in neutral is that less potential impingement may take place between the medial aspect of the humeral component and the lateral border of the scapula with internal rotation (Fig. 28-8). The disadvantage of resecting the humeral head in neutral is that in some cases the humeral cut may nearly miss the articular surface because the patient may normally have excessive humeral retroversion (Fig. 28-9). Conversely, the advantage of resecting the humeral head in 20 degrees of retroversion is that the humeral cut appears more nearly anatomic (Fig. 28-10). The disadvantage of resecting the humeral head in 20 degrees of retroversion is that more inferior impingement may take place with internal rotation (Fig. 28-11). As a compromise between these two extremes, we routinely select 10-degree retroversion with the alignment guide (Fig. 28-12). Resection of the humeral head is performed by resecting a small amount of bone by cutting just under the guide (Fig. 28-13).