EPIDEMIOLOGY AND RISK FACTORS FOR INSTABILITY AFTER REVERSE TOTAL SHOULDER ARTHROPLASTY
Multiple factors may contribute to instability following RTSA, either in isolation or combined. In essence, for an RTSA not to dislocate, the components have to be properly oriented in space, the tension of the surrounding soft-tissues needs to be sufficient, and all sources of impingement with the potential to lever out and dislocate the arthroplasty must be eliminated.
Frankle et al have categorized the factors that contribute to dislocation after RTSA into three groups: loss of compression, impingement, and loss of containment (TABLE 35.1)
.4 Loss of compression
describes laxity between the humeral and glenoid components, and it can be secondary to insufficient deltoid and/or rotator cuff tension. Loss of containment
is secondary to gross implant failure (component disassociation or fracture) or advanced polyethylene wear. Impingement
between the humerus and scapula, the implant and the scapula, or with fibrotic soft tissues may also lead to dislocation. Obese patients with a large body habitus may experience dislocation secondary to abutment of the upper arm with the trunk, combined with traction secondary to the heavy weight of their arm.
Reported rates of dislocation after RTSA have varied widely, and at least in primary RTSA, they seem to have decreased over time. At the Hospital for Special Surgery, the dislocation rate after the first 57 RTSAs performed was 16%.5
At Stanford University, the reported rate of dislocation was 9.2%.6
Zumstein et al performed a meta-analysis of studies published between 1995 and 2008 and reported a mean dislocation incidence of 4.7%.7
A similar dislocation rate of 4% was reported by the Hawkins group.8
The Rothman and Rush groups have reported identical dislocation rates of 2.9%.9
However, Dr Zuckerman’s group has reported a dislocation rate of 0.5% in a study on 591 primary RTSA,11
and similarly, in a study from the Mayo Clinic on 1649 primary RTSA implanted between 2009 and 2015, the rate of reoperation for dislocation after reverse shoulder arthroplasty (RSA) was under 0.5%.12
The dislocation rate reported by different studies have been influenced likely by the type of implants used, performing the procedure during the learning curve, the underlying diagnosis, and the mix of primary and revision procedures. For example, when assessing the outcome of RTSA for proximal humerus nonunion, Raiss et al reported a very high dislocation rate of 34%, but many of the shoulders that dislocated had been managed with resection of the tuberosities.13
Interestingly, in one small series, RTSA performed specifically for treatment of glenohumeral instability was not reported to have a higher instability rate than when performed for cuff tear arthropathy, with only one subluxation leading to revision surgery in each group.14
Several studies have tried to identify risk factors for dislocation following RTSA. These can be divided into patient-related factors and procedure-related factors (TABLE 35.2)
. Patient-related factors
identified in the peer-reviewed literature include male gender, obesity, RTSA for proximal humerus nonunion or other fracture sequelae, axillary nerve injury, prior (open) surgery, and revision shoulder arthroplasty,3
although one study did not find a correlation between body mass index (BMI) and RTSA dislocation.16
Similarly, procedure-related factors
associated with a higher dislocation rate have included inadequate soft-tissue tension, impinging heterotopic ossification, polyethylene disassociation or wear, resection of the tuberosities at the time of the procedure, and superior baseplate inclination.13
However, in one study, cuff tear arthropathy was found to be protective against dislocation.9
The relationship between subscapularis repair or integrity and dislocation rates after RTSA continues to be a matter of debate. Scientific analysis of the studies focused on this issue is complicated by the wide range of implants used in various publications as well as little information on the rate of subscapularis healing when repaired.
Chalmers et al10
and Cheung et al6
reported lower rates of dislocation when the subscapularis was repaired, whereas Vourazeris et al20
and Roberson et al21
found no differences in dislocation rates when the subscapularis was repaired or not. A meta-analysis published in 2019 indicated that repair of the subscapularis at the time of RTSA leads to lower dislocation rates for both medialized and lateralized designs (pooled dislocation rates were 4.1% in the nonrepair group and 0.7% in the repair group).22
In addition, when subscapularis repair cannot be performed, or it is not performed intentionally, use of a lateralized implant seems to decrease the dislocation rate.
TABLE 35.1 Frankle Classification of Instability After RTSA
Loss of compression
Loss of deltoid contour
Humeral height loss
Loss of containment
Alteration of D/R ratio (humerosocket depth)
Soft-tissue or bony impingement
D/R, depth/radius; RTSA, reverse total shoulder arthroplasty. (Used with permission from Abdelfattah A, Otto RJ, Simon P, et al. Classification of instability after reverse shoulder arthroplasty guides surgical management and outcomes. J Shoulder Elbow Surg. 2018;27(4):e107-e118.)
TABLE 35.2 Risk Factors for Dislocation After RTSA Reported in the Literature
Obesity and large body habitus
RTSA for proximal humerus nonunion and other traumatic sequelae
Prior (open) surgery
Axillary or brachial plexus palsy
Severe cuff insufficiency
Inadequate soft-tissue tension
Impinging heterotopic ossification
Polyethylene dissociation or wear
RTSA, reverse total shoulder arthroplasty.
EVALUATION OF THE UNSTABLE REVERSE SHOULDER ARTHROPLASTY
History and Physical Examination
As with other shoulder conditions, evaluation of patients presenting with instability following RTSA requires a careful history and physical examination. Not uncommonly, patients presenting with dislocation early after surgery are not aware that the prosthesis is dislocated, since many do not experience major pain due to the dislocation episode.10
The main complaint of patients who develop a late dislocation is a sudden loss of motion and function in association with pain.
It is important to determine whether the dislocation has occurred after a primary or a revision RTSA, as well as to record a number of other important details, including the patient’s BMI, the indication for the index surgery, prior surgeries, the chronology of the dislocation, and any indication of possible deep infection or neurological dysfunction after surgery. Careful review of prior operative reports cannot be overemphasized: What implants were used? What was the condition of the rotator cuff? Was the subscapularis intact at the time of exposure? Was the subscapularis repaired? Were any unexpected intraoperative difficulties noted?
Physical examination should be directed to evaluate the patient’s body habitus, assess the condition and status of the skin incision, identify tenderness or deformity along the acromion or spine of the scapula (indicative of a fracture or nonunion) that might contribute to instability, carefully assess the deltoid for integrity and contraction, and perform a detailed neurovascular examination to identify any evidence of injury to the axillary nerve, suprascapular nerve, or brachial plexopathy. Measuring range of motion is typically impractical in the setting of a dislocated RSA. Little attention is paid to the position of the scapula in space; however, it is our opinion that scapular malposition, oftentimes secondary to kyphosis of the thoracic spine, may also contribute to RTSA instability.
Anteroposterior and axillary radiographs will confirm the dislocation and show its direction, with anterior being most common (FIGURE 35.1)
Whenever possible, it is important to review radiographs obtained prior to dislocation to measure global lateral offset and arm lengthening. Obtaining full-length radiographs of both humeri with magnifier markers is particularly important when evaluating these patients (FIGURE 35.2)
Any available radiographs should be assessed for component position and fixation, as well as any indication of component fracture or dissociation. On the humeral side, alignment of the humeral component in excessive varus or valgus will have a direct impact on the polyethylene opening angle. This problem seems to
be much more prevalent now that ultrashort stems are being widely used.24
On the glenoid side, gross malposition in version or inclination may facilitate instability. Inclination can be assessed using the radiographic projection of the floor of the supraspinatus fossa as a reference.25
FIGURE 35.1 Dislocated reverse shoulder arthroplasty.
It is particularly important to assess the condition of the greater tuberosity, especially when RTSA was performed for acute trauma or sequelae of trauma: Is the greater tuberosity resorbed, indicative of cuff insufficiency? Is the greater tuberosity nonunited or malunited in a position where it may create levering impingement? Radiographs should also be scrutinized for heterotopic bone formation, deformity of the acromion or spine with or without fracture or nonunion, notching indicative of impingement between the humeral component and the medial scapular pillar, and humeral or glenoid osteolysis possibly indicative of polyethylene wear.4