Dislocation of the reverse total shoulder

Instability and dislocation are major complications of reverse total shoulder arthroplasty (RSA) and are not easily solved by revision.

To help understand reverse shoulder stability and instability I will use some diagrams by Steve Lippitt from the 5th Edition of Rockwood and Matsens' The Shoulder (note that the completely revised 7th edition will be published next year - wait for it!).  Steve was also critical to the understanding of concavity compression and describing the stability ratio (which will be discussed later in this post). See Glenohumeral stability from concavity-compression: A quantitative analysis

The reverse total shoulder is stabilized by conconcavity compression in which the concavity of the humeral polyethylene is pressed onto the glenosphere by the vector sum of muscle action, gravity and other forces (red arrow).

Dislocation can result when the compressive forces or the concavity of the humeral cup are insufficient to manage a displacing load, such as that from pushing one's self up from an armchair. 

Dislocation can result when the vector sum of the forces acting on the humerus is not aligned with the glenosphere,

When unwanted contact occurs between the scapula and humeral component, displacing forces can misalign the compressive force required for stability.

To start, I'd like to direct the reader to several classic articles on this topic: 

2011 Complications in reverse total shoulder arthroplasty

2015 Results of closed management of acute dislocation after reverse shoulder arthroplasty

2016 Revision for a failed reverse: a 12-year review of a lateralized implant

2018 Classification of instability after reverse shoulder arthroplasty guides surgical management and outcomes

2023 Revision for instability following reverse total shoulder arthroplasty: outcomes and risk factors for failure

I'll pick up the story from 2024 to the time of this writing (April 2025).

*Dislocation of the reverse total shoulder continues to be a major and prevalent issue for patients and surgeons

Mitigating the Risk of Instability After Reverse Shoulder Arthroplasty: A Critical Analysis Review of Patient and Surgical Factors  Instability and dislocation after reverse shoulder arthroplasty may occur in up to 31% of patients. Clinical risk factors for instability include younger age, male sex, increased body mass index, preoperative diagnosis of proximal humerus fracture or rotator cuff pathology, history of instability of the native shoulder or after surgery, and a medical history of Parkinson's disease. In patients at a high risk of instability, surgeons should consider a more lateralized prosthesis (particularly in patients with an incompetent rotator cuff), repairing the subscapularis (particularly when using a medialized prosthesis), and upsizing the glenosphere (>40 mm in male and 38-40 mm in female patients). While potentially useful, less evidence exists for the use of a constrained liner.

Midterm outcomes of primary reverse shoulder arthroplasty: a systematic review of studies with minimum 5-year follow-up The rate of shoulder dislocation was 3.7% (0%-20.4%),

Instability after reverse shoulder arthroplasty: a retrospective review of thirty one cases The most frequent etiology for RSA instability was loss of compression, followed by impingement and loss containment.

Revision of reverse total shoulder arthroplasty: A scoping review of indications for revision, and revision outcomes, complications, and re-revisions 22% of the complications were dislocations or instability. 30% of the revisions were for dislocation or instability.

Predictors of dislocations after reverse shoulder arthroplasty: a study by the ASES complications of RSA multicenter research group.  Patients with a primary diagnosis of glenohumeral osteoarthritis with an intact rotator cuff had an overall lower rate of dislocation than patients with other diagnoses (0.8% vs. 2.5%. Patient-related factors independently predictive of dislocation, in order of the magnitude of effect, were a history of postoperative subluxations before radiographically confirmed dislocation (odds ratio [OR]: 19.52), primary diagnosis of fracture nonunion (OR: 6.53), revision arthroplasty (OR: 5.61), primary diagnosis of rotator cuff disease (OR: 2.64), male sex (OR: 2.21), and no subscapularis repair at surgery (OR: 1.95). 

Complications following reverse total shoulder arthroplasty for proximal humeral fractures: a systematic review The most common postoperative complication was prosthetic instability/dislocation: 2.3%

Complications after reverse shoulder arthroplasty for proximal humerus nonunion The most common postoperative complication was prosthetic instability/dislocation: 12%

Poor clinical outcomes and high rates of dislocation after modular reverse shoulder arthroplasty for proximal humeral oncologic resection Dislocations occurred in 40%

Intraoperative repair of functional subscapularis during RSA by deltopectoral approach could improve internal rotation but does not prevent anterior dislocation. In the functional repair group, three shoulders (1.2%) reported subjective instability and 1 (0.4%) dislocated.None occurred in in either the non-functional repair or non-repair groups. 

Reverse shoulder arthroplasty with a 155 degrees neck-shaft angle inlay implant design without reattachment of the subscapularis tendon results in satisfactory functional internal rotation and no instability: a cohort study. One out of 210 prostheses was revised for dislocation within the first month after primary surgery.

Impact of morbid obesity on postoperative outcomes in reverse total shoulder arthroplasty: A national inpatient sample analysis Morbid obesity (BMI >/=40 kg/m(2)) was associated with a periprosthetic dislocation rate of 2.60 % in comparison to 1.59 % in controls

Impact of accumulating risk factors on the incidence of dislocation after primary reverse total shoulder arthroplasty using a medial glenoid-lateral humerus onlay prosthesis. 1.4% of the patients experienced dislocation with a medialized glenoid-lateralized humerus onlay rTSA prosthesis. The greatest risk factors for dislocation were male sex, age <68 years at the time of surgery, patients with body mass index >30, patients who received glenospheres having a diameter >40 mm, and patients who received expanded or laterally offset glenospheres.

Low success rate of closed reductions when treating dislocations after reverse shoulder arthroplasty: a study by the ASES Complications of RSA Multicenter Research Group.  a closed reduction was initially attempted in the majority of patients, but only about one-third were successful and required no further intervention. Unsuccessful closed reductions were associated with higher patient BMI. Revision surgery for dislocations was complicated by a high rate of recurrent dislocations and rerevision surgery.

*The diameter, depth and orientation of the humeral cup affect stability of the reverse total shoulder. However, it must be remembered that the ability of the RSA to resist dislocation depends not only on the shape and orientation of the cup, but also on the direction and magnitude of the net force as shown by the red arrows in the first two diagrams at the start of this post.

From Grammont to a New 135 degrees Short-Stem Design: Two-Hand Lever Test and Early Superior-Lateral Dislocations Reveal Critical Role of Liner Stability Ratio and Stem Alignment

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Illustration of a reverse total shoulder arthroplasty: radius (r) of the glenosphere and concavity depth (d) or jump height of the liner are required to calculate the liner stability ratio (LSR). Yellow area: the extent of the glenosphere covered by the liner; yellow striped line: angle of coverage (degree of glenosphere coverage by the liner).

Patients having receiving RSA had an 8% dislocation rate for standard liners and a 0% dislocation rate for retentive liners. The authors attribute this difference to the jump height for the 36 mm standard implant of 8.1 and a linear stability ratio (LSR) of 152%; whereas the 36 mm retentive liner had a jump height of 10.1 and linear stability ratio of 195 to 202%

For this design, the most stable liner type was the 36 retentive:

They also found that the mean effective neck-shaft angle was 133 degrees (127-144 degrees) for short stems and 135 degrees (129-143 degrees) for long stems. Long stems significantly reduced varus outliers

which may have an increased risk for instability.

Varus-valgus alignment of humeral short stem in reverse total shoulder arthroplasty: does it really matter? The utilization of short humeral stems in reverse total shoulder arthroplasty has gained attention, however, concerns exist regarding the risk of misalignment with implant insertion. In this cadaver study, anterior dislocation forces were considerably lower in the varus group compared to the neutral group.  Valgus positioning did not significantly impact instability compared to the neutral position.

*Know the implants you're using

Large variability in degree of constraint of reverse total shoulder arthroplasty liners between different implant systems There were variations in jump height between rTSA systems at a given size, resulting in large differences in stability ratio. Standard liners exhibited a stability ratio range from 126% to 214% (mean 158% (SD 23%)) and constrained liners a range from 151% to 479% (mean 245% (SD 76%)). The angle of coverage showed a range from 103 degrees to 130 degrees (mean 115 degrees (SD 7 degrees) for standard liners and a range from 113 degrees to 156 degrees (mean 133 degrees (SD 11 degrees )) for constrained liners.

Four arthroplasty systems had constant stability ratios for standard liners (within 5%) across different sizes, while one system showed slight inconsistencies (within 10%), and ten arthroplasty systems showed large inconsistencies (range 11% to 28%). The stability ratio of constrained liners was consistent across different sizes in two arthroplasty systems and inconsistent in seven systems (range 18% to 106%). 

Impact of constrained humeral liner on impingement-free range of motion and impingement type in reverse shoulder arthroplasty using a computer simulation The humeral liner may be changed to a constrained type when stability does not improve by increasing glenosphere size or lateralization with implants, and patients, particularly women with obesity, have risks of periprosthetic instability that may be secondary to hinge adduction on the thorax. This RSA computer simulation model demonstrated that constrained humeral liners led to decreased impingement-free ROM. 

From Dr Stefan Bauer I received the most interesting response below.

 

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link)
Shoulder rehabilitation exercises (see this link).

Understanding the dislocating reverse total shoulder: concavity compression.

Concavity compression is the primary mechanism by which the glenohumeral joint is stabilized in most of its functional positions.

Let's use some diagrams by Steven B. Lippitt to illustrate. In the anatomic shoulder, the action of the deltoid, cuff and other muscles compress the ball into the glenoid concavity

As long as the direction of the sum of all the forces is contained within the concavity, the joint is stable.

However, when the net force is not contained within the concavity, the joint is not stable

The stability of the joint can also be reduced by compromise of the concavity as in a bony Bankart lesion (left) or with cold flow leading to flattening the rim of a polyethylene glenoid (right).

These same concepts apply in the reverse total shoulder. Stability is provided when the direction of the net glenohumeral force is contained within the humeral concavity. The greater the force compressing the humeral cup and glenosphere together, the greater the stability against displacing forces - inadequate compressive force leaves the joint at risk for instability.

When the net force is uncontained, the joint becomes unstable.

Any unwanted contact between the humerus and scapula or interposed soft tissues can misalign the net force, leading to instability.

Instability of the reverse total shoulder can also be caused by compromise of the concavity of the humeral cup from polyethlene wear as shown below

or from displacement if the polyethylene liner from the humeral tray

In summary, stability of the reverse total shoulder requires a strong compressive force aligned with a competent humeral concavity. 

Instability is the most common mode of failure of the reverse total shoulder. The importance of the concavity-compression mechanism in stabilizing the reverse total shoulder (RSA) is emphasized in a recent article Revision for instability following reverse total shoulder arthroplasty: outcomes and risk factors for failure in which the authors sought to identify factors associated with recurrent instability by studying patients undergoing RSA for instability, including 43 surgeries in 36 patients. Overall, 32/36 patients (89%) required 38 revisions to achieve stability at most recent followup. Loss of compression was the most common mechanism for persistent instability. "Loss of containment" means compromise of the humeral concavity. "Impingement" refers to unwanted contact altering the direction of the net force across the articulation.

Comment: Understanding concavity compression can guide the evaluation and management of the unstable biologic and prosthetic glenohumeral joint. 

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link.

Follow on twitter: https://twitter.com/shoulderarth
Follow on facebook: click on this link
Follow on facebook: https://www.facebook.com/frederick.matsen
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).

Reverse total shoulder: instability and dislocation

Instability and dislocations after reverse total shoulder (RTSA) remain among the most common complications of reverse total shoulder. The authors of Instability after reverse total shoulder arthroplasty: risk factors and how to avoid them conducted a study of 103 RTSAs (97 primary, 6 revision). Among these 103 RTSAs, 6 patients had 5 dislocations (3 in primary RTSAs, 3 in revision RTSAs). One dislocation occurred immediately after surgery, 3 from low-energy mechanisms of injury, and 2 without known inciting events. All dislocations were treated in the operating room; no dislocation was successfully treated with simple closed reduction in the clinic. They found that the risk of dislocation is higher for patients with fractures and post fracture deformity, for patients with higher BMI and for patients undergoing revision surgery. 

The authors suggest that in patients with persistent instability or with risk factors for instability, consideration should be given for use of larger glenospheres, increasing the lateral offset, minimizing retroversion, performing peri-glenoid releases and optimizing the tension of the soft tissues.

Comment: Our colleagues from Spain have provided a nice summary of their experience. The case examples shown below are educational.

You can support cutting edge shoulder research that is leading to better care for patients with shoulder problems, click on this link.

Follow on twitter: https://twitter.com/shoulderarth

Follow on facebook: click on this link

Follow on facebook: https://www.facebook.com/frederick.matsen

Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/

Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link).

How to x-ray the shoulder (see this link).

The ream and run procedure (see this link).

The total shoulder arthroplasty (see this link).

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

The smooth and move procedure for irreparable rotator cuff tears (see this link).

Shoulder rehabilitation exercises (see this link).

Dislocation in the reverse total shoulder - the effect of baseplate inclination

The relationship between glenoid inclination and instability following primary reverse shoulder arthroplasty

These authors point out that despite advances in implant design and surgical technique, instability remains the most common early complication and reason for early revision after reverse shoulder arthroplasty (RSA) (see this link).

Their goal was to evaluate the glenoid implant inclination, as measured by the Beta-angle, as an independent risk factor for instability after primary RSA. According to the authors, "the Beta-angle represents the supplementary angle to the angle between the floor of the supraspinatus fossa and superior and inferior margins of the glenoid (or baseplate)." Increasing superior inclination is reflected by a smaller Beta angle.

Reverse shoulders with instability (34) were matched 1:3 with cases without instability (102) using age, sex, body mass index, and baseplate type.

The authors found no statistically significant differences betweenb the study group and controls with  regard to the proportion of shoulders with rotator cuff tear arthropathy, use of retentive liner, and rate of subscapularis repair.

In this series, despite attempting to impart an inferior tilt to the baseplate, 41% and 35% of the unstable and stable shoulders, respectively, had a slight net superior tilt compared with preoperative

radiographs. There was a wide range of postoperative (63 to 100 degrees) and pre- to postoperative change (-16.5  to  +30.5 degrees) in Beta-angles collectively. There was no significant difference in the postoperative Beta-angle (mean, 80.8 vs. 82.7 degrees or the change in b-angle (mean 1.7 vs. 3.4 degrees) between cases and controls, respectively. 

The authors concluded that neither the final prosthetic glenoid inclination or the change in glenoid inclination, as measured by the Beta-angle, significantly influenced the risk of prosthetic instability after primary RSA.

Comment: From this study one can see that a very wide range of Beta-angles (63-100 degrees) are found in stable shoulders. This may be because superior or inferior inclination of the baseplate does not change the fact that the humerus is articulating in the same way with the sphere known as the glenosphere (see below).

Another pertinent observation is that 4 of the 38 dislocations (>10%) in the standard Zimmer reverse total shoulders (excluding 2 RSAs from another arthroplasty system and one in a massive composite allograft arthroplasty) were found to be associated with polyethylene liner dissociation at the time of revision (see this link). Surgeons should be aware of this possibility when attempting close management of a dislocation. 

Achieving stability in a reverse total shoulder requires careful attention to surgical detail: assuring appropriate soft tissue tension and avoiding unwanted contact between the humeral component and soft tissue or bone in front of, below, and behind the glenosphere. 

How you can support research in shoulder surgery Click on this link.

Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link).

How to x-ray the shoulder (see this link).

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

The smooth and move procedure for irreparable rotator cuff tears (see this link).

Shoulder rehabilitation exercises (see this link).