As is the case for the normal shoulder and the anatomic total shoulder, the stability of the reverse total shoulder (rTSA) depends on concavity compression. In the rTSA the concavity of the humeral liner is compressed onto the glenosphere by soft tissue tension and active muscle constraction.
The authors of Computational Analysis of Compressive Joint Stability and Acromial StressAssociated with Varied Rotator Cuff Integrity after Reverse Shoulder Arthroplasty used finite element analysis (FEA) to evaluate changes in joint compressive stability and acromial stress with varied rotator cuff integrity, glenoid component lateralization, and humeral distalization after rTSA.
Glenohumeral contact force (compression) decreased with progressive cuff removal. Compared to the intact rotator cuff state, compression decreased 59% when the subscapularis was absent, 11% when the infraspinatus was absent, and 67% when both subscapularis and infraspinatus were absent.
Compression increased with progressive levels of glenoid lateralization. The table below shows compression (Newtons) for various degrees of cuff integrity and various amounts of glenosphere lateralization.
Six mm lateralization increased compression by two-fold when the subscapularis was intact (red arrow).
Six mm lateralization with an inlay stem resulted in a small (0.2% to 1.8%) increase in the proportion of acromial and scapular spine cortical bone regions exceeding the yield stress threshold.
Addition of an onlay humeral stem, though resulting in larger improvements in joint compressive stability, had a considerable (23.0% to 30.4%) increase in the acromial and scapular spine cortical bone regions exceeding the yield stress threshold compared to inlay humeral configurations.
This study illustrates the important role that compression from the rotator cuff plays in the stability from concavity compression with the rTSA.
It also points out that when the rotator cuff is compromised, joint compression can be recovered by increasing glenoid lateralization and humeral distalization. However, the improvement in joint stability comes at the cost of increases in acromial/scapular spine stress.
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I'm taking the liberty of reposting a post from April so that the reader has much of the rTSA stability/instability story in one spot.
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 The Shoulder. Steve was also critical to the understanding of concavity compression and describing the stability ratio. 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).
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.
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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).
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
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.
Maintaining Stability
Harlequin Duck Stabilized Against the Surf
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