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

.

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).

Shoulder instability

Glenohumeral instability is one of the most common disabling conditions of the shoulder. Yet given the great flexibility of the joint and the small, shallow socket provided by the glenoid

it is a wonder that the shoulder remains as stable as it does with everything we ask it to do

In order to understand, evaluate and manage shoulder instability, one needs to understand how the joint is normally stabilized. Our late fellow, Doug Harryman, did a great job explaining shoulder stability in these two videos: Shoulder Mechanics 1 and Shoulder Mechanics 2.  

The principles of shoulder stability are detailed in Chapter 3 of the freely available Practical Evaluation and Management of the Shoulder (see this link.). 

While the knee is stabilized by ligaments that remain isometric through its range of flexion and extension, the shoulder has no isometric structures that limit its range of motion. The surrounding soft tissues remain lax in most functional positions of the shoulder, except those performed at the extremes of shoulder motion (such as in the baseball pitch shown above).

 (N.B.: laxity is not the same thing as instability (see Laxity of the normal glenohumeral ioint: A quantitative in vivo assessment)

While the hip is stabilized by a deep acetabular socket (which limits hip range of motion when its rim is contacted by the femur), the shoulder's socket is shallow, allowing it a great range of impingement-free motion.

The essential mechanism stabilizing the shoulder is concavity compression, a mechanism that functions throughout the range of shoulder motion. Just as the compressive effect of gravity stabilizes the golf ball in the small, shallow concavity of the tee

the glenohumeral joint is stabilized when the net force on the humeral head compresses it into the small, shallow glenoid concavity of the glenoid

As long as the net force acting on the humeral head passes within the glenoid concavity, the joint is stable. The glenoid arc provides a range of positions in which this condition can be met.

The glenoid bone, cartilage and labrum each contribute to the concavity.

The concavity is less in the anteroposterior direction than in the superiorinferior direction.

As a result, the joint's stability from front to back is less than its stability from top to bottom.

If the concavity is compromised, the range of stable positions is reduced.

As pointed out in A Prospective Analysis of Patients With Anterior Versus Posterior Shoulder Instability, while anterior instability is often traumatic, posterior instability more commonly arises from repeated overloading and wear of the posterior supporting structures from pushing and lifting activities that differentially load the back of the joint. 

Recurrent posterior instability may be accompanied by progressive loss of the supportive posterior glenoid cartilage and bone (see Prospective Evaluation of Posterior Glenoid Bone Loss After First-time and Recurrent Posterior Glenohumeral Instability Events).

 

Stability is also compromised if the net force is not directed within the glenoid concavity.

Fortunately, the scapula can usually be positioned so that the concavity is aligned with the compressive force.

For example, if the hand pushes forward while the scapula is retracted, the force on the humerus is not aligned with the glenoid socket and stability is threatened.

However, if the hand pushes forward while the scapula is protracted, the force on the humerus is closely aligned with the glenoid socket, so stability is optimized.

Functional instability has been defined as instability from pathological muscle activation patterns resulting in malalignment of the net humeral force with the glenoid (see Characteristics of functional shoulder instability). 

Often functional instability can be managed by patient education and physical training to strengthen the cuff muscles that provide compression of the humeral head into the glenoid concavity while avoiding positions that risk force malalignment (such as press ups without scapular protraction as shown earlier).  In refractory cases, muscular retraining may be enhanced by using electrical stimulation (see Shoulder-Pacemaker Treatment Concept for Posterior Positional Functional Shoulder Instability: A Prospective Clinical Trial).

The specifics of surgical treatment for glenohumeral instability are beyond the scope of this post. However, understanding the concavity compression mechanism of shoulder stability can help guide practice: 

(1) when instability results from deficiencies in the labrum, cartilaginous lip, or glenoid bone, consider restoring the glenoid concavity through Bankart repair or bony procedures

(2) when instability results from malalignment of the net humeral force with the glenoid, consider restoring the constraints to excessive motion, using capsular repair/shift, rotator interval plication and/or remplissage.  

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
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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).

Instability and dislocation after reverse total shoulder arthroplasty - are we any smarter about preventing them?

Instability is one of the most common complications after a reverse total shoulder.

The 56 authors of Predictors of Dislocations after Reverse Shoulder Arthroplasty: A study by the ASES Complications of RSA Multicenter Research Group identified 6,621 patients from a multicenter database with minimum 3 month (mean 19.4, range 3-84) followup after a primary or revision reverse total shoulder (RSA) performed by one of 24 experienced surgeons. The study population was 40% male with an average age of 71.0 years. The rate of dislocation was 2.1% (n=138) for the whole cohort, 1.6% (n=99) for primary RSAs, and 6.5% for revision RSAs.

Dislocations occurred at a median of 7.0 weeks after surgery; 23.0% followed a trauma.

The risk factors for dislocation identified in this study were non-modifiable:
(1) diagnosis other than glenohumeral osteoarthritis with an intact rotator cuff (e.g. fracture non-union, rotator cuff disease, failed prior arthroplasty).
(2) history of postoperative subluxations prior to radiographically confirmed dislocation,
(3) male sex,
(4) trauma
and
(5) no subscapularis repair.


Comment:
These authors did not identify modifiable risk factors for dislocation, such as implant type, implant size, implant position, distalization, lateralization, unwanted contact between the humeral component and scapula, or rehabilitation.

By contrast, the authors of Dislocation following reverse total shoulder arthroplasty found two modifiable risk factors - inadequate soft-tissue tensioning and bony impingement (especially in adduction) -  among 14 early (less than three months after surgery) and 5 late (more than 3 months after surgery) dislocations. Non-modifiable risk factors included male sex and prior surgery on the shoulder. Other findings associated with dislocation included asymmetric liner wear and mechanical liner failure; these factors may be modified by prosthesis design and surgical technique.

The authors of Classification of instability after reverse shoulder arthroplasty guides surgical management and outcomes and Revision for instability following reverse total shoulder arthroplasty: outcomes and risk factors for failure identified four categories of factors that contributed to instability in 36 patients having revision of a reverse total shoulder for glenohumeral instability. Many of these are related to implant design and surgical technique and are, therefore, modifiable. The most common mechanism leading to persistent instability was loss of compression.

 (D/R ratio is the ratio of the depth of the polyethylene cup divided by the radius of the cup's concavity).

As emphasized in The normal shoulder, aTSA, and RSA are stabilized by concavity compression and in the articles referenced above, stability of the reverse requires a compressive force aligned with a competent concavity = concavity compression.

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).