Instability After Shoulder Replacement: Rockin’ & Rollin’

Instability after total shoulder arthroplasty (TSA) is an uncommon but serious complication that often requires further surgery. Several factors are important regarding the evaluation and treatment of the unstable shoulder arthroplasty. In a simplified sense, instability after shoulder arthroplasty can be organized in two ways—by etiology or by direction. Unfortunately, instability after TSA is often multifactorial, involving soft tissue and/or bony insufficiency, soft tissue tensioning, and/or component size or positioning.

In reviewing the literature on shoulder arthroplasty before the reverse prosthesis, the incidence of instability in unconstrained TSA is approximately 5%. Cofield1 summarized a series of previous reports and identified several risk factors in his own patients. He found that risk factors for instability after TSA included rotator cuff disease—especially deficiency, older patient age, soft tissue laxity, a small prosthetic head size that may not have adequately recreated soft tissue tension, and increased preoperative external rotation.1 This increased external rotation, however, may have actually been a marker for subscapularis insufficiency, which was not scrutinized as carefully in the past as today.

Directions of instability include anterior, posterior, superior, and inferior, however, superior instability or escape represents a very different issue than what is traditionally thought of as instability. Briefly, placement of a hemiarthroplasty in a rotator cuff– deficient shoulder, especially with an incompetent coracoacromial (CA) arch, can produce superior instability and escape of the humeral head with contraction of the deltoid2 (Fig. 1). This may be less of a postoperative complication than it is an incorrect indication for surgery. In patients with rotator cuff deficiency it is extremely important to preserve the CA arch to prevent this complication, should the patient subsequently require an arthroplasty. In any case, this is a different situation from anterior or posterior instability, which appear in markedly different settings.

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Figure 1. Anterosuperior escape in a rotator cuff–deficient shoulder with incompetence of the CA arch. (Color version of figure is available online.)

Humeral shortening is common after hemiarthroplasty for fracture or malunion and can lead to inadequate soft tissue tensioning and subsequent inferior instability with decreased ability to elevate the arm. Care must be taken to restore humeral height and tensioning of the soft tissues to decrease instability. Several techniques and devices have been used to help achieve proper prosthetic positioning in these situations.3 These include, but are not limited to, obtaining contralateral radiograph measurements with a ruler or magnification marker, using external guides referenced from the elbow such as the Tournier (Montbonnot, France), using internal anatomic landmarks such as the superior border of the pectoralis major insertion,4 or the jig-saw puzzle technique of reassembling the fracture fragments onto the prosthesis.5 All of these techniques are used to varying degrees to recreate humeral height as well as version, which if improperly done may lead to instability in other directions as well.

Excessive posterior glenoid wear, as seen in osteoarthritis or after anterior stabilization procedures, can lead to placement of the glenoid component in increased retroversion and subsequent posterior instability.6 Glenoid bone defects must be addressed and several techniques have been described, including increased reaming opposite the defect, bone grafting,7 or glenoid neck osteotomy.8 Regarding humeral version, malrotation of the humeral component in excessive anteversion or retroversion is also possible, especially in revision situations or after arthroplasty for fracture or malunion. Malrotation of the humeral component can lead to instability,9 however, this can be minimized with the use of alignment rods in addition to standard cutting guides. Conversely, in patients with preoperative instability, increased humeral version away from the direction of current instability may help achieve stability.10

Figure 2 shows an osteoarthritic shoulder with a typical appearance of posterior humeral subluxation and posterior glenoid wear. On this computed tomography scan the extent of posterior glenoid wear may not at first be obvious, however, several clues can be used to evaluate for this. Note the increased distance from the coracoid to the lesser tuberosity in this image. By considering this relationship the degree of posterior wear present becomes more evident. In this case there is a “biconcave” glenoid. As a result, there is substantial posterior subluxation, which causes stretching of the tissues posteriorly, anterior contracture, and posterior wear. This situation must be recognized preoperatively to avoid anterior perforation when drilling. Additionally, techniques for asymmetrical removal of glenoid bone and asymmetric reaming must be used to correct abnormal wear patterns, to reestablish near normal version.

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Figure 2. Posterior glenoid wear and subluxation in an osteoarthritic shoulder. (Color version of figure is available online.)

Glenoid component retroversion creates three problems. 1) Native glenoid retroversion can cause anterior perforation when drilling for peg or keel placement. 2) Glenoid retroversion can lead to new or continued posterior instability if version is not corrected before placement of a glenoid component.11 3) Unaddressed glenoid retroversion can lead to abnormal stresses on the glenoid component that may predispose to premature loosening.

In evaluating patients with posterior instability the surgeon must remember that preoperative posterior glenoid wear and subluxation is common in patients with osteoarthritis. As such, many patients will present with static posterior subluxation, laxity of the posterior capsule, and contracture anteriorly. If soft tissue tension is not restored through releases, especially if some glenoid retroversion is accepted, posterior subluxation may continue postoperatively. This is a problem that requires careful preoperative planning and intraoperative evaluation as it is much easier to prevent than to treat.

It is very rare that posterior capsular plication is necessary.12 This is typically only necessary in patients with severe stretching of the posterior capsule, such as occurs with a chronic locked posterior dislocation.13 Additionally, caution must be had with release of the posterior capsule. For routine TSA, our preference is to do a 270° release rather than a 360° release. In this case the release involves the superior, anterior, and inferior capsule, with careful sparing of the posterior capsule. In rare cases of posttraumatic arthritis or avascular necrosis with significant circumferential scarring, a 360° release may be necessary.

In glenoid component revision it is sometimes difficult to figure out which came first, component loosening or instability. Consider that, if the glenoid component loosens, it can tip posteriorly, which may lead to increased effective retroversion and subsequent posterior instability. Conversely, if the humeral head subluxes posteriorly, it may eccentrically load the glenoid and cause loosening. In many cases the etiology is unclear without previous operative records or early postoperative films. In either case, structural bone grafts are rarely needed; the key is to avoid placement of a glenoid component in excessive retroversion to avoid having to revise it later.

Soft tissue deficiency can also lead to instability, most commonly anteriorly with incompetence of the subscapularis.14 This may occur after division of the tendon during or before arthroplasty. Osteotomy of the lesser tuberosity rather than division of the tendon may help improve healing and decrease the incidence of subscapularis insufficiency after shoulder replacement.15, 16 With rotator cuff repair or tuberosity pull off, immobilization after surgery to protect the repair is advised. For chronic rotator cuff insufficiency, muscle transfers can be considered—not so much the latissimus, but the pectoralis major under the strap muscles for anterior stability with subscapularis insufficiency.17 Although this is less helpful for restoring a stomach press or lift off, the goal is stability rather than strength. Repair with allografts or xenografts may also be beneficial, but this is controversial.18, 19, 20 Additionally, appropriate soft tissue balancing using proper releases at the time of surgery should be done in all cases.

It must also be considered that, while component malposition may be evident radiographically, the function of the soft tissues may not. We reported the utility of magnetic resonance imaging in the presence of a metallic implant to investigate the status of the muscles, the continuity of the subscapularis, whether the rotator cuff is intact, and whether placement of a reverse arthroplasty should be considered.21 Recall that, with an irreparable rotator cuff tear, a reverse arthroplasty is an option, but care must be taken when trying to restore stability with an operation whose major complication is instability.22, 23

In summary, the goals in instability surgery are to correct component position and version and to balance the soft tissues, which may include the rotator cuff. Achilles allografts or other soft tissue grafts can be considered anteriorly,24 in addition to pectoralis transfer as mentioned above. Rehabilitation should be adjusted appropriately and bracing used as necessary. Although fusion may appear as a viable option, it is difficult because of extensive bone loss as well as the presence of hardware and cement. It is important to recognize the above factors so that each can be addressed at the time of revision surgery and also to help prevent instability in primary cases. Most cases are complex and preoperative planning should include revision instruments, bone grafts, high-speed drills, fluoroscopic imaging, and the proper extractors. The brachial plexus may also be involved, so planning for this possibility should also be considered.