Complications and revisions following reverse total shoulder: doing the math.

Summary: Three takeaway points

(1) The commonly used and easy to measure "revision rate" is an inadequate endpoint for evaluating rTSA outcomes. The 8–20 percentage point gap that exists between the complication rate and the revision rate includes a substantial number of patients with failed but unrevised reverse shoulder arthroplasties. These patients are not considered in determining the failure rate when measured by the percentage having revision. Complication-free survival or patient-reported outcome measures should supplement or replace revision rate or "implant survival" as the primary outcome measure for rTSA.

(2) The most common complications of rTSA, acromial and scapular spine fractures, are unique and difficult to solve problems for patients having reverse arthroplasty. In contrast, the most common complications from aTSA - glenoid component loosening and rotator cuff tear - can be effectively managed by revision to a rTSA

(3) The salvage pathway for patients with rTSA failure is poor. Revision rTSA carries a 31% complication rate and 27% re-revision rate, with outcomes that are significantly worse than those for primary rTSA. This contrasts sharply with aTSA-to-rTSA conversion, the outcome of which approaches the outcomes for primary rTSA. 

The details

Complications vs revisions

Meta-analytic evidence reports that anatomic total shoulder (aTSA) has a higher rate of revision compared to reverse total shoulder (rTSA). This observation may have driven much of the shift toward rTSA for patients with cuff-intact arthritis.  However, it may neglect the fact that surgeons and patients can decide against revision of a failed rTSA because of the low rate of success. Thus, the absence of a revision does not indicate a good outcome.

Consider the references below

Complications and further surgery after reverse total shoulder arthroplasty : report of 854 primary cases reported an overall complication rate of 18–22%, yet a revision rate of only ~10%. That 8–12 percentage point gap likely represents patients who are living with a failing implant — either because revision was technically not feasible, or because expected outcomes were too poor to justify reoperation

Incidence, radiographic predictors, and clinical outcome of acromial stress reaction and acromial fractures in reverse total shoulder arthroplasty found 46 acromial stress fractures (5.4%) in 44 patients and 44 acromial stress reactions (5.2%) in 43 patients. The overall union rate was 55% but was significantly higher following operative treatment compared with nonoperative treatment. However, facture consolidation did not result in better clinical outcomes compared with nonunion; this may question the value of attempted fixation.

Comparison of complication types and rates associated with anatomic and reverse total shoulder arthroplasty found that the top 3 complications for rTSA were acromial/scapular fracture/pain ( complication rate 2.5%, revision rate 0.0%), instability (complication rate 1.4%, revision rate 1.0%), pain (1.2%, revision rate 0.2%). Compare the relationship of complications to revisions for rTSA to that for aTSA in the two charts below constructed from the data in this article. Note that a high percentage of the rTSA complications were not associated with surgical revision. Absence of revision ≠ absence of complication,

The Gap Between Revision-Free and Complication-Free Survival 

Long-Term Outcomes of Reverse Total Shoulder Arthroplasty
A Follow-up of a Previous Study The 93% ten-year revision-free survival rate for rTSA did not capture the 29% of patients who experienced complications.

Long term clinical and radiological outcomes of primary reverse total shoulder arthroplasty at a minimum follow-up of 15 years: Norwegian registry data showed 10-year complication-free rates of only 76–80%, despite revision-free survival of 91–95%. That 15–20 percentage point gap represents patients living with failed implants who are invisible in the revision statistics. Again, this gap exists because many rTSA complications—particularly acromial fractures, low-grade infection, and baseplate loosening with severe bone loss—either cannot be addressed surgically or carry such poor expected revision outcomes that conservative management is chosen.

Revision rates alone remain an inadequate measure of rTSA performance; complication-free survival is the more appropriate endpoint.

Outcomes of Revision rTSA

Revision of reverse total shoulder arthroplasty: a scoping review of indications for revision, and revision outcomes, complications, and rerevisions and Revision of failed shoulder arthroplasty: epidemiology, etiology, and surgical options found the most common indications for revision to be instability/dislocation (28–30%), baseplate or glenoid complications (20–25%), and infection (15–23%). Notably low on the list is revision for the most common complication: acromial/spine fractures.

Outcomes After Revision

Revision reverse total shoulder arthroplasty: clinical and radiographic outcomes compared to primary reverse total shoulder arthroplasty found a 31% complication rate and 27% re-revision rate of a failed rTSA: over five times these rates after primary rTSA. The most common complications after revision are recurrent instability (22%), fractures (18%), and baseplate issues (12%). Patients requiring revision rTSA experience markedly worse comfort and function in comparison to primary rTSA

The Salvage Pathway Asymmetry:  revision for aTSA Failure compared to revision for rTSA Failure

Conversion of failed anatomic total shoulder arthroplasty to reverse shoulder arthroplasty yields similar pain and functional outcomes to primary reverse shoulder arthroplasty but has a higher baseplate failure rate: a matched cohort study. In contrast to the often grim outcomes for revision of a failed rTSA, revision of a failed aTSA to a rTSA yields outcomes approaching those of primary rTSA. 

Summary: Three takeaway points (again)

(1) The commonly used and easy to measure "revision rate" is an inadequate endpoint for evaluating rTSA outcomes. The 8–20 percentage point gap that exists between the complication rate and the revision rate includes a substantial number of patients with failed but unrevised reverse shoulder arthroplasties. These patients are not considered in determining the failure rate when measured by the percentage having revision. Complication-free survival or patient-reported outcome measures should supplement or replace revision rate or "implant survival" as the primary outcome measure for rTSA.

(2) The most common complications of rTSA, acromial and scapular spine fractures, are unique and difficult to solve problems for patients having reverse arthroplasty. In contrast, the most common complications from aTSA - glenoid component loosening and rotator cuff tear - can be effectively managed by revision to a rTSA

(3) The salvage pathway for patients with rTSA failure is poor. Revision rTSA carries a 31% complication rate and 27% re-revision rate, with outcomes that are significantly worse than those for primary rTSA. This contrasts sharply with aTSA-to-rTSA conversion, the outcome of which approaches the outcomes for primary rTSA. 

Swans reversing course

Trumpeter Swans

Union Bay Natural Area

Seattle

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Surgeon and industry views on digital technology

Digital technology in shoulder arthroplasty: what do surgeons want? What will industry offer? This well-done survey by Warner and colleagues asked 192 shoulder arthroplasty surgeons from three international organizations (ASES, SECEC, and the Codman Shoulder Society) what digital technologies they use and want, and asked six industry leaders about their development priorities. 

The survey found that 

(1)  Ninety-six percent of respondents perform preoperative planning, 82% use digital tools for this, and about a quarter use navigation or mixed reality intraoperatively. Seventy-six percent of respondents perform more than 40 arthroplasties per year; the study did not reflect the usage or views of the low shoulder arthroplasty volume surgeons who perform the majority of these procedures. The technology adoption question looks very different for a surgeon doing 15 arthroplasties a year versus 100. Industry's implicit pitch is often that technology can compensate for lower surgical volume, but that claim is itself unproven and worth calling out directly.

(2) When asked what they wanted from industry, surgeons prioritized artificial intelligence / machine learning, navigation, and improved virtual reality planning. Industry respondents matched these preferences, ranking AI/ML and navigation as their top development priorities. Both groups felt that AI could reduce errors, generate data for best practices, and improve efficiency.

(3) Surgeons emphasized reliability and patient outcome tracking as priorities, which industry did not consider a key focus. In other words, surgeons want to know if their interventions are working for patients, but industry is more interested in the tools themselves than in measuring whether those tools make a difference.

Significantly, the survey did not ask whether surgeons have observed improved patient-reported outcomes (PROs) when using these technologies: “Has your adoption of digital technology resulted in measurably better patient outcomes?” See Navigation, where are we going?

The Discussion section states that preliminary literature shows improved accuracy of glenoid component positioning with navigation, patient-specific instrumentation, and augmented reality versus freehand techniques. The critical question is whether positioning precision within a narrow range translates into better function, less pain, greater durability, or fewer complications for patients. See: How much precision do we need to pay for in shoulder arthroplasty?

The cost question nobody wants to answer

One of the most revealing findings is that industry respondents could not, or would not, articulate a revenue model for these digital technologies. The paper notes that “no consistent business model for AI/ML emerged” and that companies “were not able, or not willing, to clarify revenue models for most digital technologies.”

This is significant. If companies cannot explain how these tools will be paid for, the cost will inevitably be bundled into implant pricing, passed to hospitals, and ultimately borne by patients and the healthcare system. For technologies without demonstrated clinical benefit, this raises serious questions about value. 

There are other costs of which the surgeon needs to be aware: increased preoperative and OR time, increased space requirements for a robot, time for training in the use of the technology, learning how to detect when the technology is pointing toward a suboptimal procedure, knowing when and how to abort the use of the technology, and the influence the vendor of the technology may have over the choice of implants.

Warner and colleagues have provided a useful snapshot of surgeon and industry attitudes toward digital technology in shoulder arthroplasty. The data on current usage patterns and preferences are informative. They conclude that “further adoption of these technologies will likely be contingent on well done scientific study which demonstrates value.”  Until we have rigorous evidence that these expensive digital tools produce measurably better patient outcomes, we should be cautious about the assumption that more technology equals better care.

The bottom line is that robotics, navigation, and virtual/augmented reality are all methods of transferring a plan made from a preoperative set of images to the patient, without knowing such key characteristics as condition of the soft tissues (that control mobility and stability) and the condition of the bone (that determines quality of fixation).  Such factors loom large in the outcome realized by the patient and often require substantial modifications of the preoperative plan.

Helmuth von Moltke the Elder wrote: “Kein Operationsplan reicht mit einiger Sicherheit über das erste Zusammentreffen mit der feindlichen Hauptmacht hinaus.” i.e., “No battle plan survives contact with the enemy.”

This is not an argument against planning. It’s an argument for: Flexibility over rigidity. Preparation over prediction. Adaptation over perfection. Once real-world complexity hits (fog of war, friction, shoulder variability), reality diverges from the preconceived plan.

The goal isn’t to be precise/accurate to the preoperative plan.
The goal is to be ready to modify it.

See How to make good decisions in shoulder (and other) surgery : Bayesian Thinking - adjusting a prior plan in consideration of new information.

Not infrequently, plans need to be modified,

Cactus Wren

Tucson

2020

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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).

Are we asking the right questions about failed total shoulder arthroplasty?

A recent publication The Rising Incidence and Future Trends of Revision Total Shoulder Arthroplasty sought to determine the epidemiology of revision TSA using a national database. The authors found that the incidence of revision TSA increased from 223 to 1247 cases per 100,000 person-years between 2015 to 2024, a 5.6-fold increase.

These data point to the steadily increasing number of patients having revision for arthroplasty failure, the economic burden of these revisions, and the need for an increasing number of surgeons who are equipped to manage these failures. Of importance is that these data reflect the incidence of revision per 100,000 people (not the rate of revisions for patients having shoulder arthroplasty). In that the number of shoulder arthroplasties performed per 100,000 people is steadily increasing each year, it is intuitive that the number of revisions per 100,000 people would also increase each year. 

It seems that the questions of greatest importance are not addressed in this study:

1. Are patients undergoing TSA today at greater risk for complications than patients a decade ago? If so, is this a consequence of applying shoulder arthroplasty to younger patients, patients with more complex diagnoses, and patients with poorer overall health? Or is this a consequence of less experienced surgeons performing an increasing number of these procedures?

2. What are the specific failure modes driving revisions, and are those modes changing over time? For anatomic TSA, are the rates of glenoid component loosening, instability, rotator cuff failure, and infection getting higher, lower or staying the same? For reverse TSA how are the rates of instability and dislocation, acromial and scapular spine fracture, infection, notching with component loosening, and periprosthetic fracture changing with time?

3. Which patient and surgical factors are associated with each distinct failure mode? Risk factors differ by failure mode; we need to understand these associations as we strive to reduce the risk of each type of failure.  Aggregating across all causes of revision in a single regression model — as most administrative database studies must do — destroys the signal. The predictors of infection-related revision are nutritional status, glycemic control, obesity, prior surgery, and prophylaxis adequacy. The predictors of instability-related revision in rTSA involve the surgeon's choices of component design, version and inclination and approaches to soft tissue tensioning. The predictors of acromial fracture involve bone mineral density, scapular morphology, type of arthritis, prior surgery, rotator cuff status, component lateralization, and deltoid tensioning. 

In that the surgeon is the method, surgeon knowledge, training, and experience deserve special study across all failure modes. These may be the most important modifiable risk factors — yet they receives minimal attention in a literature preoccupied with implant selection and surgical technology.

4. What might be done differently to prevent each type of failure? This is the most important question and the hardest to study, because it requires honest counterfactual analysis rather than statistical association. It asks not merely which factors correlate with failure, but which specific decisions — patient selection, perioperative optimization, surgical technique, rehabilitation protocol — were modifiable and, if modified, would plausibly have prevented the need for revision.

Answering this question at scale will be difficult, requiring mandatory registries with standardized failure mode coding and linkage between primary and revision procedures.

Recognizing that failure modes differ by surgeon and practice, an immediately applicable and practical approach is analogous to aviation crash analysis. Each surgeon analyzes their failures by comparing each revision to matched non-revised controls from their practice. (See How a surgeon can learn from their own adverse outcomes - an example of intrapractice analysis in reverse shoulder arthroplasty.)

A final question: is the most cost-effective approach to reducing the risk of arthroplasty complications and revisions (A) more expensive technology or (B) better surgeon training (see How much precision do we need to pay for in shoulder arthroplasty?)

A risk factor for small bird adverse outcomes in my backyard.

Cooper's Hawk

2021

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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).

Is this subscapularis reconstructable?

A 64 year old female presented with chronic right shoulder dysfunction and pain after an injury playing tennis in 2022, where she hit an overhead ball and felt a pop in her shoulder. She then had a recurrent injury in 2025 with a similar mechanism playing pickleball and was diagnosed with a rotator cuff tear. She had participated in PT and got some functionality back but remained weak, and then had a fall biking which further reduced the function of her shoulder. She complained of anterior shoulder pain and an inability to perform overhead activity due to weakness. Her exam revealed full active motion, but substantial weakness of belly press and lumbar lift-off. Passive external rotation was to 90 degrees. There was a palpable defect in the subscapularis. Plain radiographs were normal. MRI images are shown below.

In spite of our lack of optimism regarding the reconstructability of this chronic tear, the patient asked for an exploration and repair or reconstruction if possible.

At surgery the upper half of the subscapularis was detached of poor quality. With care to protect the axillary nerve, dissection of the lower half of the subscapularis was carried out  freeing it from the inferior capsule, and releasing the anterior capsule from the glenoid.

After this 360 degree release, excellent quality (> 1 cm thick) tendon with subjacent capsule could be easily approximated to the entire footprint at the lesser tuberosity. Six FiberWire sutures were passed through the lesser tuberosity and the tendon. The long head tendon of the bicep was incorporated in the repair. After securing the tendon, the shoulder externally rotated to 30 degrees with a firm endpoint. 

I thought you might find this interesting.

Building Back

Marsh Wren reconstructing nest

Montlake Fill

2010

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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). 

How much precision do we need to pay for in shoulder arthroplasty?

In a recent series of posts, we've discussed the relationship of patient outcomes from shoulder arthroplasty to "component malposition" and to the use of costly technologies directed at precision in executing a preoperative plan (e.g. robotics, navigation, patient specific instrumentation). 

Does component malposition lead to revision of shoulder arthroplasty? points out that in order to understand the relationship of different degrees of malposition to outcome it is necessary to assess not only malpositions in failed arthroplasties but also malpositions in successful arthroplasties.

Humeral and glenoid component malposition in revised shoulder arthroplasty - Part I - what might have been done differently? recognizes that major component malpositions can cause failure, but questions whether precision technologies are necessary to avoid clinically significant malpositions.

Intraoperative navigation for shoulder arthroplasty - where are we going? suggests (1) The question each surgeon must ask is whether intraoperative navigation addresses problems experienced by patients in their own practice and (2)The question the field of shoulder surgery must ask is which specific patient problems (instability? fracture? stiffness? pain?) occur due to positioning errors that: (a) surgeons using standard techniques cannot adequately control, and (b) navigation systems would prevent. 

What should be the role of robotics in shoulder arthroplasty? asks "to what degree will the application of robotics to execute a preoperative plan address the primary causes of shoulder arthroplasty failure - infection, instability, acromial fracture, limited motion, and component loosening?"

 

Recognizing that the use of robotics, navigation and patient specific instrumentation all are based on a preoperative plan created from a 3D CT scan, CT-free planning for reverse total shoulder arthroplasty - why and how to do it shows a non-CT method for preoperative planning that is applicable to a large percentage of cases of reverse total shoulder arthroplasty cases.

Finally The Bad B2 Pandemic and How I perform a kinematic anatomic shoulder arthroplasty : what is the appropriate amount of stuffing? remind us that while image-based preoperative planning provides a useful preview to the surgery, the final choice of implants and positioning needs to be based on intraoperative assessment of soft tissue balance - something that preoperative images cannot reveal.

So, if precision means carrying out a preoperatively-generated plan, how much precision do we need to get the clinical outcome we want for our patient? 

For fun, let's consider a comparison of two watches:

(1) the world's most accurate watch

and (2) one of the world's cheapest watches

Here are some points of comparison

The F-91W $19.61 is less precise than the $7,400 Citizen Caliber 0100: ± 15 sec/month vs ± 1 sec/year (just as the conventional approach to the placement of shoulder arthroplasty components is less precise than technology-based placement). The question is whether the difference is of importance to the wearer (or the patient); does the 377 times increase in price produce a tangible benefit?

The Casio F-91W costs $19.61 on Amazon, where it is currently the #1 Best Seller in Men’s Wrist Watches — more than 3,000 sold in the past month, 57,798 customer reviews, 4.6 stars. It has a 7-year battery. It has been in continuous production since 1989. It has no Bluetooth, no Wi-Fi, no radio receiver, no GPS, and no wireless capability of any kind. There are four buttons on the case. That is the entirety of its interface with the outside world.

Its quartz oscillator drifts approximately fifteen seconds per month. Yet it gets its owner to every meeting, every appointment, every surgical case on time. The reason is straightforward: its imprecision is correctable: the wearer presses two of those four buttons while looking at an external time source. Similarly in the operating room we frequently adjust our preoperative shoulder arthroplasty component selection and positioning based on our intraoperative observations, rather than being fixed to our preoperative plan.

 

The Citizen Caliber 0100 costs seven thousand four hundred dollars. Its quartz oscillator vibrates at 8,388,608 Hz, temperature-compensated every sixty seconds. Its certified annual accuracy is ±1 second. It is, by the consensus of the watchmaking world, the most precise autonomous wristwatch ever produced. But it cannot be corrected because its extraordinary precision is entirely self-referential. It measures its own performance against its own internal standard, derived from its own crystal, monitored by its own circuitry. When that internal standard diverges from external reality — due to a subtle manufacturing variation, an unforeseen temperature excursion, a factor its algorithm did not anticipate — the watch has no mechanism to detect the divergence and no capacity to correct for it. There are no buttons to press. There is no external reference to consult. It proceeds with precision, faithfully, in whatever direction it was last pointed.

Technologies such as robotics, navigation and patient specific instrumentation face the same constraint. They strive to execute the preoperative plan with absolute fidelity. The plan was derived from a CT scan acquired days before surgery, segmented by an algorithm, calibrated to a population average, and fixed at the moment the case was planned. In the operating room, the technology cannot read the bone quality beneath the reamer. It cannot sense the soft tissue tension that requires adjustment. It cannot recognize that the patient’s anatomy, once exposed, differs in a clinically relevant way from the CT model it was given. It proceeds with precision, whether or not that precision is directed at the target that will yield the optimal outcome for the patient.

The F-91W, reset by a human being attending to ground truth, is the more honest model of what excellent surgery actually looks like: an imperfect instrument, actively corrected, continuously coupled to the reality it is meant to serve. There needs to be a human in the loop!

How much precision is needed?

Anna's Hummingbird

Matsen Backyard

 2022

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
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). 

Precision without Wisdom.

A must read!

The recent publication of Precision without Wisdom by University of Washington shoulder fellow alumnus Tassos Papadonikolakis argues that contemporary surgical innovation has become systematically decoupled from clinical judgment, long-term evidence, and ethical accountability. In its 98 pages, the book draws on surgical history, philosophy (particularly Aristotle), innovation science, and healthcare economics to make its case. It is organized into nine chapters with an epilogue and afterword, covering the historical arc of orthopaedic innovation, the failure modes of modern technology adoption, the distorting effects of industry funding, the promise and dangers of artificial intelligence, and the ethical responsibilities of surgeons and institutions.

The central thesis is straightforward and well-stated in the preface: the crisis in medical innovation is not a shortage of intelligence, technology, or capital — it is a crisis of orientation. Papadonikolakis frames the book as an argument for balance: between precision and judgment, between progress and restraint, and between enthusiasm and humility.

This is a book for the thoughtful surgeon and for all others who are concerned about optimizing the appropriateness and cost/effectiveness of the technologies we use in caring for our patients.

Follow on twitter/X: https://x.com/RickMatsen
Follow on facebook: https://www.facebook.com/shoulder.arthritis
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). 

Is "correction" of acromial morphology clinically effective in treating posterior shoulder instability?

The Summary

Posterior shoulder instability is being increasingly recognized as an important clinical problem - both in terms of the resulting compromise in the patient's shoulder comfort and function as well as a prelude to shoulder arthritis. In the Walch glenoid classification system, B0 is a pre-arthritic glenoid with posterior humeral instability that precedes the biconcave morphology and bone loss that defines the B2 glenoid. It is not known whether surgical intervention (and what type of surgical intervention) can mitigate the progression to arthritis. 

B0 Glenoid

Acromial morphology — specifically a high, flat posterior acromion with PAH >23mm — is  consistently associated with posterior shoulder instability across multiple independent cohorts and imaging modalities. A high, flat acromion predicts posterior shoulder instability better than almost any other imaging finding. 

Extraordinary association is not causation, and strong association does not automatically justify operative intervention. The acromion shares its embryonic developmental origins with every other tissue involved in posterior shoulder stability. The most parsimonious explanation for its remarkable predictive value is that a high, flat acromion is the most visible and measurable expression of a multi-tissue developmental syndrome — a marker, not the mechanism. 

Evidence supporting acromial correction in posterior shoulder instability is lacking: it includes no prospective clinical trials. The only clinical data rests on a single unreplicated 10-patient case series. The biomechanical studies, while mechanistically interesting, were performed under conditions (e.g. normal glenoid anatomy) that do not reflect typical clinical patients.

The clinical problem

Posterior shoulder instability is defined as dynamic, recurrent, and symptomatic partial or total loss of posterior glenohumeral joint contact. Multiple anatomic factors — labral tears, glenoid morphology, capsular laxity, bone loss, and acromial morphology — may contribute in varying combinations.

Relation of acromial morphology to posterior instability

The morphology of the acromion has long been implicated in shoulder pathology, particularly in relation to subacromial impingement and rotator cuff disease. More recently, interest has shifted toward the posterior acromion, with studies examining its potential role in posterior instability

Reproducibility and Relevance of Acromial Morphology Measurements in Shoulder Pathologies: A Critical Review of the Literature reviewed nine studies assessing sagittal acromial tilt, posterior coverage, and acromial height. In posterior instability the acromion is generally described as more horizontally oriented, less covering posteriorly, and positioned higher. Although these trends suggest a possible biomechanical role for the acromion, reported values vary widely between studies, and significant overlap exists between pathological and control groups. Such variability is compounded by differences in imaging modality and definitions of anatomical landmarks.These methodological inconsistencies undermine reproducibility and limit the clinical applicability of posterior acromial parameters. The reduction of a complex three-dimensional structure like the acromion into two-dimensional projections inevitably loses information about the spatial relationship between the acromion and the posterior humeral head. Acromial parameters identify groups at elevated risk, but do not provide the individual-level diagnostic precision needed to justify surgical targeting of the acromion in a specific patient.

Posterior Acromial Morphology Is Significantly Associated With Posterior Shoulder Instability carried out a study of 41 patients with unidirectional posterior instability and a control group of 53 patients with no instability in which the authors compared measurements of acromial morphology. Significantly, concurrent measures of glenoid morphology were not reported. (The article also presented an analysis of patients with anterior instability, but those are not relevant to our interest in comparing patients with posterior instability to controls).

Radiographic acromial characteristics included posterior acromial tilt,  posterior acromial coverage (PAC), and posterior acromial height (PAH).

Posterior acromial tilt is determined by measuring the angle formed by the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) and a line connecting the most posterior point of the inferior aspect of the acromion to the most anterior point of the inferior aspect (white area).

The posterior acromial coverage (PAC) refers to an angle formed by the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) and a line drawn from the intersection of the small arms of the “Y” to the most posterior point of the inferior aspect of the acromion (red area).

To measure the posterior acromial height (PAH), a perpendicular line is drawn from the reference line (connecting the inferior angle of the scapula with the center of the intersection of the small arms of the “Y”) to the most posterior point of the inferior aspect of the acromion. The PAH (green bracket) is then measured as the distance from the center of the intersection of the small arms of the “Y” to the perpendicular line.

Patients with posterior instability had greater mean posterior acromial height (30.9 versus 20.4 mm) and posterior acromial tilt (63.6° versus 55.9°) compared with the control group. Posterior acromial coverage (48.8° versus 61.6°) was lower in the posterior instability group than in the control group. The authors concluded that in shoulders with posterior instability, the acromion is situated higher and is oriented more horizontally in the sagittal plane than in normal shoulders; and suggested that this acromial position may provide less osseous restraint against posterior humeral head translation.

Correlation of Acromial Morphology With Risk and Direction of Shoulder Instability: An MRI Study posterior instability patients had significantly less posterior acromial coverage than in the control group (68.3° vs. 81.7°) and greater posterior acromial height  (11.0mm vs. 0.7 mm). Crucially, this study excluded patients with multidirectional instability, glenoid bone loss >13.5%, or retroversion >10° — thereby studying the "cleanest" possible posterior instability cases, without the most severe osseous co-pathology. This exclusion criterion is worth noting: by removing patients with the most significant glenoid abnormalities, the study actually enhanced the apparent signal from acromial parameters in isolation. Real clinical populations are rarely this clean.

The glenoid co-develops embryonically with the acromion and other parts of scapular anatomy. What role does it plan in posterior stability?

It is worth noting that the acromion, glenoid, scapular body, humerus, posterior labrum, posterior capsule, and rotator cuff all arise from the same embryonic mesenchymal condensation during upper limb development. A single disruption to this shared developmental program could plausibly produce co-occurring abnormalities across all of these structures simultaneously. Perhaps acromial morphology is not the primary cause of posterior instability but instead is the most radiographically accessible expression of the scapular morphotype that is visible on a scapular Y-view.

Consideration of posterior instability requires consideration of the critical role the glenoid plays in glenohumeral stability through the concavity compression mechanism.  Glenohumeral stability from concavity-compression: A quantitative analysis found the effective glenoid depth and stability ratios were lowest with loading of the humeral head in the posterior direction, meaning that the glenohumeral joint is constitutively most vulnerable posteriorly, especially since the great majority of shoulder functions are in forward flexion, a position that challenges posterior stability.

Glenoid retroversion is a well-established contributor to posterior instability. Glenoid Retroversion Is an Important Factor for Humeral Head Centration and the Biomechanics of Posterior Shoulder Stability found that every 5° increment produces approximately 2.0mm of additional humeral decentralization; retroversion >10-15° significantly affects joint centralization.

Posterior Shoulder Instability but Not Anterior Shoulder Instability Is Related to Glenoid Version found PSI patients averaged vault retroversion of −21° versus controls (−17.8°). Using chondrolabral version, the retroversion of PSI patients averaged −16.6° versus controls ( −9.2°).


The Influence of Glenoid Retroversion on Posterior Shoulder Instability: A Cadaveric Study each 1° increase in retroversion correlated with a 3.5% decrease in resistance to posterior translation; spontaneous dislocation with an intact labrum occurred at a mean of 22.7° of retroversion. A 4–7° difference in retroversion translates to roughly a 14–25% reduction in posterior restraint from osseous anatomy alone, before any soft tissue or acromial contribution is considered.

Acromion Morphology Is Associated With Glenoid Bone Loss in Posterior Glenohumeral Instability demonstrated that acromial morphology is associated with the severity of glenoid bone loss in posterior instability. Patients without glenoid bone loss had a steeper acromial tilt (58.5°) versus those with ≥13.5% bone loss (67.7°). A flatter acromion tracks with more severe glenoid pathology — consistent with both being expressions of the same underlying developmental abnormality of the scapula.

Thinking outside the glenohumeral box: Hierarchical shape variation of the periarticular anatomy of the scapula using statistical shape modeling demonstrated that glenoid inclination and acromial anatomy represented distinct but related components of overall scapular morphology.

Association of the Posterior Acromion Extension with Glenoid Retroversion: A CT Study in Normal and Osteoarthritic Shoulders  In this 3D morphometric study of 31 normal scapulae, glenoid retroversion correlated significantly with posterior acromial extension, characterized by the acromion posterior angle (APA).  Combining the APA with the acromion length angle (ALA) and acromion tilt angle (ATA) helped improve the correlation. 

Anatomical description of the scapular coordinate system (OXYZ), acromion landmarks (AA, AC), trigonum spinae (TS), angulus inferior (AI), posterior extension of the acromion (AAx), acromion posterior angle (APA), acromion tilt angle (ATA), acromion length angle (ALA), and glenoid retroversion angle (GRA). The three axes (x, y, and z) correspond to postero-anterior, infero-superior, and medio-lateral, respectively.

Can modification of the acromion improve posterior stability in cadavers?

The biomechanical evidence that posterior acromial bone grafting can restore stability was generated in cadavers with normal glenoid anatomy — a condition that does not describe typical patients.

Posterior shoulder stability depends on acromial anatomy: a cadaveric, biomechanical study

Eight fresh-frozen human cadaveric shoulders were biomechanically tested in a shoulder simulator in the load-and-shift and Jerk test positions.  

The force needed to displace the humeral head by 50% of the glenoid width decreased between 23% and 60% in moderate to severe acromial malalignment (high and flat acromion) and increased up to 122% following surgical correction of acromial alignment (low and steep acromion) when compared to the native condition. 

A Posterior Acromial Bone Block Augmentation Is Biomechanically Effective at Restoring the Force Required To Translate the Humeral Head Posteriorly in a Cadaveric, Posterior Glenohumeral Instability Model in ten fresh-frozen cadaveric specimens, a posterior acromial bone block (PABB) increased resistance force to humeral head translation compared to the instability state.

Posterior Shoulder Stability Can Be Restored by Posterior Acromial Bone Grafting (Scapinelli) in a Cadaveric Biomechanical Model With Normal Glenoid Anatomy assessed the stabilizing effect of a posterior acromial bone graft (PABG) in moderate and severe acromial malalignment (high and flat).

At 30° of flexion, the force needed to displace the humeral head 50% increased by 659% when a PABG was added to a moderately malaligned acromion and by 1249% when a PABG was added to a severely malaligned acromion.  Experimentally, a PABG provided comparable or superior stability to that achieved with surgical acromial reorientation while representing a technically simpler and potentially less invasive approach. While these biomechanical results for acromial correction in cadavers with normal glenoid anatomy and without soft tissue contribution are striking, they may not translate to clinical application where the glenoid and soft tissues are likely to be abnormal.  

Can modification of the acromion improve posterior stability in patients?

The entire published clinical evidence base for isolated acromial correction in posterior shoulder instability consists of a single case series by Roberto Scapinelli, an Italian surgeon who first performed the procedure in 1970 and published results in 2006. This single series has never been independently replicated in over 50 years. Posterior addition acromioplasty in the treatment of recurrent posterior instability of the shoulder reported an extraarticular surgical procedure for the treatment of recurrent posterior instability of the shoulder by grafting an inverted segment of the scapular spine to the posterior border of the acromion so that it exerts a slight pressure over the infraspinatus muscle pressing the humerus forward. 

The Scapinelli series in plain terms: n=10 patients. One surgeon. 1970-2006. No controls. No validated outcome instruments. No independent replication. No imaging follow-up for glenoid version or labral integrity. This is the weakest possible form of clinical evidence — and it is all we have. I could find no other reports of isolated acromial surgeries in patients. 

A series of biomechanical studies suggested that posterior instability is associated with both glenoid and acromial malalignment and that neither isolated glenoid correction nor isolated acromial correction fully restored normal kinematics — only combined correction of both did. Posterior Stability of the Shoulder Depends on Acromial Anatomy: A Biomechanical Study of 3D Surface Models. Scapular Morphology and Posterior Shoulder Stability: Biomechanical Evidence From an Advanced Cadaveric Shoulder Simulator. Even with both acromial and glenoid correction, these studies showed approximately 20% residual posterior translation compared to intact shoulders. Residual instability may reflect the soft tissue components (labrum, capsule, rotator cuff) that cannot be addressed by osseous correction alone.

I found one preliminary study that combined acromial osteotomy with glenoid osteotomy. Presumably the two procedues were combined because the authors were not satisfied with the acromial osteotomy alone. Scapular (glenoid and acromion) osteotomies for the treatment of posterior shoulder instability: technique and preliminary results reported the outcome after a “scapular (acromion and glenoid) corrective osteotomy" for posterior escape in 9 consecutive patients. One case had persistent subluxation, and osteoarthritis progression. The other 8 patients had improved clinical scores. In 5 patients the humeral head was recentered.

Current thinking regarding the role of acromial surgery in the management of posterior instability may be reflected in Acromion Morphology Is Associated With Failure of Arthroscopic Posterior Capsulolabral Repair finding that while failure of arthroscopic posterior shoulder capsulolabral repair was associated with a higher acromion the authors did not recommend surgical intervention (posterior acromial bone graft or posterior acromial osteotomy) to change acromial anatomy in the treatment of posterior shoulder instability. This direct statement from proponents of the acromial association with posterior instability should help inform clinical decision-making.

Going forward

The strongest clinical application of acromial morphology data may lie in diagnosis and risk stratification, not operative planning. A high, flat posterior acromion on imaging should prompt the clinician to think more broadly about the shoulder's developmental anatomy — to look carefully at glenoid version, labral quality on MRI, posterior capsular volume, and rotator cuff muscle belly cross-section. It is a signal that the whole posterior stabilizing complex may be developmentally underprogrammed, not just the acromion. 

The recognition of the B0 glenoid as a precursor to shoulder arthritis raises the question of whether identifying this developmental syndrome early — high acromion, glenoid retroversion, and posterior subluxation together — should inform decisions about some type of surgical intervention before the shoulder progresses to the arthritic B2. Answering that question will require thoughtful clinical research.

Trying to figure this out.

 Acorn Woodpecker

Tucson

2020

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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).