In a French multicenter study of 457 RSAs in 430 patients, the incidence of dislocation was 4.8% (Fig. 23-1).⁶¹ However, all dislocations (22 patients) occurred after the use of the deltopectoral approach, giving an incidence of 6% in that group. Dislocation was more common after revision of failed anatomical arthroplasties (11%) and in post-trauma cases (9%) compared with cuff tear arthropathy (4%). Of the 22 patients suffering from a dislocation, 16 occurred within the first 3 months. In a more recent review of 284 RSAs, Trappey et al. reported a 5% incidence of instability in the primary group compared with 8% in the revision group (P = 0.388).⁵⁹
Subscapularis insufficiency resulted in an increased risk of instability, and repair of the tendon was recommended, if possible.¹⁸,⁵⁹

In the author’s series (AE) of the first 236 Delta RSAs (1995 to 2002), 16 patients went on to suffer dislocation, giving an incidence of 6.7%. Twelve of these dislocations were seen after revision of failed arthroplasty and three were seen in patients with fracture sequelae. Only one dislocation occurred in the cuff tear arthropathy group. Since 2006, we have used the Delta Xtend RSA (DePuy) and three dislocations occurred in 457 consecutive RSAs (0.7%). Thus, with modern implant design and careful surgical technique, the incidence of instability of RSAs can be very low.

The most important stabilizing factor in an RSA is compressive forces in the joint (deltoid tensioning), followed by socket depth.³³ The size of the glenosphere was found to have less importance. Favre et al. studied the effect of version of the components and reported that the stability was improved if the humeral component was put in neutral or slightly in anteversion.²² Retroversion of the humeral component decreased the stability.

Another situation that could cause instability is when the humerus is levered out, after the humeral bone comes into contact with the scapula at different positions. Performing releases, removing scar tissue around the glenoid—particularly inferior—and excising bony fragments around the glenoid are important steps to avoid instability. Furthermore, by using a larger diameter glenosphere at a low position, the risk is further reduced since bony impingement is minimized.

The results after the treatment of RSA instability have been variable. In the French multicenter study, 22 patients with instability were identified.⁶¹ Seven cases were treated with closed reduction and immobilization, and physiotherapy was postponed until 4 to 6 weeks later. Nine cases underwent revision (open reduction, two cases; polyethylene liner change to a more constrained design, two cases; humeral spacer inserted, five cases). The remaining six patients had no further treatment. Seven of these 22 patients presented with a dislocation more than 1 month after surgery. None of these patients underwent revision and all remained unstable at last follow-up.

Gallo et al. reported on nine cases of instability in a series of 57 patients.²⁶ Only three had a stable functioning RSA at follow-up. Edwards et al. treated seven patients with instability. 18 Closed reduction was performed in two patients and open reduction with deltoid tensioning was performed in five cases. All were stable at follow-up.

Since 1995, we have followed a strict protocol for treating instability of an RSA, and surgical factors to avoid instability have been defined. Deltoid tensioning is regarded as the key factor. In addition, all the humeral components are placed in neutral or a few degrees of anteversion to avoid instability. A dislocation occurring within 48 hours of surgery is treated with closed reduction and immobilization for 3 weeks. We believe that such a dislocation may be due to poor control of the deltoid muscle during the early postoperative period and not a reflection of poor tensioning. Dislocations occurring after 2 days are treated with open reduction, scar tissue removal and lengthening of the humeral component, either with a higher insert or a humeral lengthener (Figs. 23-1 and 23-2). Following this algorithm, all the patients were stable at last follow-up. Deeper, more constrained polyethylene inserts should be avoided since these result in increased polyethylene wear and complications. Stability can usually be restored without the need for these deeper constrained devices, which are regarded as salvage components.

Recently, some RSA designs in the market have introduced shallower inserts compared with the standard depth. These designs allow for better range of motion before impingement occurs, but significantly reduce the stability. It should be noted that when potential range of motion is increased, the stability is reduced. Therefore, it is important for the surgeon to be aware of the design features of the specific implant being used.

If there is a malpositioning of the implants, they have to be corrected (Fig. 23-3A, B). A retroverted humeral component may have to be put in neutral or in slight anteversion. From the author’s experience, this is rarely needed. If the glenoid component is placed too high and bone is protruding below the glenosphere, a levering-out effect can be seen, which needs to be corrected during revision. Another option is changing from a small to a larger diameter glenosphere. In all cases of instability, it is important to rule out the infection, since infection can result in instability.²⁶

When the diagnosis of a chronically dislocated RSA is made in our group, the patient is scheduled for surgery within 1 to 2 weeks. We prefer the beach-chair position, and the incision from the previous procedure is used. Antibiotic prophylaxis is given for 3 days because the procedure is a revision. On exposure, the polyethylene insert is usually found to be worn, and it is removed. The glenosphere is inspected and if there are scratches on the surface, which are very common in our experience, it should also be replaced. Then, a release all around the
glenoid is performed and the scar tissue, particularly inferiorly, is removed. Six to eight biopsies are taken for microbiology. In order to increase deltoid tensioning, the humeral component is lengthened by inserting a metal lengthener or a higher insert. Usually, a 6 to 9 mm lengthening is necessary. A trial reduction is now performed.

During trial reduction, the arm is placed in adduction and only a small gap between the components (1 to 2 mm) should be seen. Next, a longitudinal pull on the arm is performed and no separation between the glenosphere and the insert should be apparent (anesthesia without muscle relaxation is used to allow deltoid tensioning). Finally, the arm is put in extension and external rotation. In this position, the shoulder should be stable, but a small opening between the components of 2 to 3 mm anteriorly can be seen. If the stability is not satisfactory, the first step is to further increase the tension by lengthening the humeral component.

Lengthening of the humeral component may not be enough to create stability and the next step is to increase the size of the glenosphere or to put in an eccentric glenosphere, if available, to create more tensioning. Range of motion is then checked to see that enough room has been created, allowing the humeral component to glide over the glenosphere. Compared with an anatomical arthroplasty, more room is necessary posteriorly, anteriorly, and inferiorly.

Postoperatively, the patient wears a sling for comfort 1 to 2 weeks, but active range of motion rehabilitation is started the day after surgery. The patient is not allowed to push up from sitting to standing with the operated arm in the first 6 weeks after surgery.

The mean rate of infection in 14 studies was 3.8%.68 More infections were seen after revision procedures (5.8%) compared to primary RSAs (2.9%).

Low-virulence organisms, such as Propionibacterium acnes, are a common cause of deep infection after RSA.68

In our series, we observed a high rate of infections (4,7%) in the first series of 277 RSA’s between 1995 to 2005. However, since 2006 only 7 infections (1%) have been diagnosed in 695 consecutive RSAs (Delta Xtend, DePuy, Warsaw, IN, USA). Thus, anatomical arthroplasty and RSA are associated with similar rates of infection.