Instability after Reverse Total Shoulder, a blog commentary on “Instability after reverse total shoulder arthroplasty: risk factors and how to avoid them” by Lorena Pena, Javier Pena, Estaban Lopez-Anglada, Alejandro F. Brana. Acta orthop. Belg., 2022, 88, 372-279.1
Instability after reverse total shoulder arthroplasty (rTSA) is a well-known and well-described complication of the procedure and is one of the most common causes of revision surgery. It has been described to occur at a frequency ranging from 0.5% to 16%2 with earlier studies noting higher rates and as volume of the procedure has increased, comfort with the procedure has improved, implants and techniques have improved, the rate has more recently approached the 0.5-2% range.2 Numerous causes of instability have been described, including soft-tissue tension/imbalance, implant design (medialization/lateralization of glenoid and humerus), bone loss, and deltoid tension.
In this study, the authors review the case series from a single surgeon to analyze cases of instability after rTSA to better understand why this may occur and how to minimize its occurrence. The authors retrospectively reviewed the rTSAs performed by a single surgeon over an approximately 10-year period. They identified a 103 rTSAs performed with 6 cases of instability identified during this time (5.8%). All episodes of instability occurred in the first 2 months with an average of 32 days. Over one half of patients (55%, 57/103) were reverse shoulders performed for either an acute fracture or their sequela. 20 patients had an Exactech Equinoxe prosthesis while 83 patients had the Tornier (Stryker) Aequalis Reverse II prosthesis. A standard technique was used in all cases with stems placed in approximately 10° of retroversion. The subscapularis was repaired when possible. All cases of instability were anterior dislocations, with 3 occurring in patients treated for cuff tear arthropathy and 3 treated for sequela of fractures. 3 out of 6 patients with instability had obesity defined as a BMI greater than 30. All patients were taken to revision due to concerns of how easy the instability occurred. In 3 of the 6 cases that dislocated, the subscapularis had been repaired but had failed with the dislocation. It was able to be re-repaired in two of the revisions. All cases of instability occurred with the Tornier Aequalis implant, though the authors note that this implant was used four times more often and used for the more complex cases though the rationale for this is not stated. Their only explanation for choice of implant was Hospital supply.
A strength of the study is that this represents the work of a medium volume surgeon (approximately 10 per year), which is helpful due to its generalizability and is more indicative of the volume of shoulder arthroplasties performed as studies have shown that most are performed by surgeons who perform 10 or less per year. The authors are to be commended for their honest and introspective analysis of their experience with a known but troublesome complication of rTSA.
As the authors note, some factors known to be associated with instability include condition of the soft tissues and design of the implant. I believe that these 2 issues are interrelated. The role of the subscapularis in instability after reverse total shoulder arthroplasty has long been debated. In a previous blog post3, I analyzed a meta-analysis4 of the role of subscapularis repair and it was very evident that in implants with a medialized humerus and a medialized glenoid component, subscapularis repair significantly reduced instability. However, in systems with a lateralized glenoid or lateralized humeral component utilized, there was no difference in instability regardless of whether the subscapularis was repaired. In this study, all cases of instability occurred with an implant with a medialized glenoid and medialized humeral component. In 3 of the 6 instability cases the subscapularis was not repaired and in the 3 cases where it was repaired that had instability, the repair had failed.
Another important point of this study was that 3 of the 6 instability cases occurred in patients who were treated for sequela of proximal humerus fractures. Eleven total patients in this cohort were treated for sequela of proximal humerus fractures, which makes the instability rate in this population nearly 25%. As noted in the surgical technique, these patients often had significant soft tissue loss or bone loss or resections that precluded subscapularis repair. In the representative radiographs presented in the study of patients with sequela of previous fractures, significant proximal humeral bone loss is present, particularly significant loss of the greater tuberosity. This loss of bone significantly affects the deltoid wrap angle increasing the risk of instability. Deltoid wrapping refers to the wrapping of the middle deltoid around the greater tuberosity, resulting in increased humeral compression into the glenoid. Deltoid wrapping theoretically decreases the deltoid force needed to abduct the arm.5 This emphasizes the need for reconstruction techniques or implants that help restore the deltoid wrap. Two good examples of this are the humeral reconstruction prosthesis (HRP) or the proximal humeral augmentation tray (HAT), both from Exactech. An alternative technique is an allograft-prosthetic composite to help restore this.
Fortunately, the rates of instability after reverse total shoulder arthroplasty continue to decrease as experience with the surgery and knowledge increase. Despite the decreasing rate, it is a clinically significant problem that often results in revision surgery. Knowledge of risk factors can help further reduce this as well as attention to surgical detail and potentially implant selection, particularly for patients at high risk of instability.
- Pena, L et al. “Instability after reverse total shoulder arthroplasty: risk factors and how to avoid them.” Acta Orthop. Belg., 2022, 88, 372-279.
- Sanchez-Sotelo, J. “The Unstable Reverse Shoulder Arthroplasty” in Zuckerman, J. Shoulder Arthroplasty: Principles and Practice. 2022
- Sajadi, KR. “To Repair, or Not to Repair, That is the Question…” https://www.exac.com/to-repair-or-not-to-repair-that-is-the-question/
- Matthewson G, et al. The effect of subscapularis repair on dislocation rates in reverse shoulder arthroplasty: a meta-analysis and systematic review. J Shoulder Elbow Surg (2019) Article in Press. 1-9.
- Hansen ML, et al. The biomechanics of current reverse shoulder replacement options. Ann Joint 2019;4:17.
- Roche, C et al. Impact of Inferior Glenoid Tilt, Humeral Retroversion and Bone Grafting on Muscle Length and Deltoid Wrapping in Reverse Shoulder Arthroplasty. Bulletin for the Hospital for Joint Diseases. Vol. 71(4):284-93. 2013.
- Roche, C. et al. Impact of Humeral Offset on Muscle Length with Reverse Shoulder Arthroplasty. T of the 61st Annual ORS Meeting. 2015.