Preventing Acromial/Scapular Spine Fractures in Reverse Shoulder Arthroplasty: Defining what the surgeon can control.

I'll start out with a few conclusions:

(1) Acromial and scapular spine stress fractures are clinically important complications of reverse total shoulder arthroplasty (RSA), occuring in 3-11% of cases and often resulting in poor functional outcomes such as persistent pain and limited active motion. 

(2) The principal risk factors - female sex, advanced age, poor bone quality, inflammatory arthropathy, cuff deficiency, corticosteroid use, thinned acromion from prior surgery or erosion, proximal humeral migration - are not under the control of the surgeon, except as they affect the decision to proceed with RSA surgery. 

(3) Surgeons do control humeral and glenosphere component selection and positioning. However the evidence guiding practice is not robust due to the lack of standardized nomenclature and what parameters should be measured in future clinical research. 

Here are a set of four easy to make measurements the importance of which is supported by the review below. These address the problem of uncertainty and inconsistency found in published studies regarding RSA geometry. Such measurements will be important in answering the key questions surgeons have: what component positions provide the best function and which minimize the risk of complications such as scapular stress fractures for my patients?

(1) Acromio-humeral distance measured from the acromion to the greater tuberosity along a line parallel to the bony glenoid face (both post-op and pre-op to post-op change).

(2) The perpedicular distance between the glenosphere center of rotation (COR yellow dot) and the glenoid bony face (yellow line)

(3) The perpendicular distance between the lateral extent of the glenosphere and the glenoid bony face (blue line)

(4) The perpendicular distance between the tuberosity and the glenoid bony face (long black line).

Consistent use of these measurements would address much of the current ambiguity in the literature, as illustrated by the following review.

Glenosphere lateralization 

One of the issues in reviewing the literature on "glenosphere lateralization" is a failure of many articles to define the term. Are the authors talking about lateralization of the center of rotation in relation to the glenoid bone (yellow line) or lateralization of the lateral aspect of the glenosphere in relation to the glenoid bone (blue line)? The former affects the deltoid moment arm and the range of impingement-free range of motion, while the latter contributes to the global lateralization of the humeral tuberosity (black line) which affects the soft tissue tension that is important for stabilizing the articulation through concavity compression. As seen in Know Your Glenospheres these two dimensions can be varied independently by changing the diameter of curvature of the glenosphere. The effect of the humerus on the global lateralization is the difference between the black and blue lines. 

Biomechanical studies

Implant positioning in reverse shoulder arthroplasty has an impact on acromial stresses and The effect of load and plane of elevation on acromial stress after reverse shoulder arthroplasty found that glenosphere lateralization, but not humeral lateralization, increased acromial stress.

Factors Influencing Acromial and Scapular Spine Strain after Reverse Total Shoulder Arthroplasty: A Systematic Review of Biomechanical Studies found glenoid lateralization was consistently associated with increased acromial and scapular spine strain. 

In addition, transection of the coracoacromial ligament resulted in significantly increased strains. Although preserving the integrity of the CAL is not an implant-related factor, it is a surgeon-controlled variable. Its importance is demonstrated in two basic science papers: Scapular Ring Preservation: Coracoacromial Ligament Transection Increases Scapular Spine Strains Following Reverse Total Shoulder Arthroplasty and Coracoacromial ligament integrity influences scapular spine strain after reverse shoulder arthroplasty and finally the clinical study Does Preservation of Coracoacromial Ligament Reduce the Acromial Stress Pathology Following Reverse Total Shoulder Arthroplasty? Transection of the coracoacromial ligament consistently increased scapular spine strain in biomechanical studies and was associated with higher clinical fracture rates in the 265-patient study (29.4% vs 13.2% with CAL section vs. preservation).

Clinical evidence

There is a lack of clinical studies that have actually measured the radiographic glenosphere COR lateralization and correlated it with acromial fracture risk.  

Up to 8 mm of glenoid-sided lateralization does not increase the risk of acromial or scapular spine stress fracture following reverse shoulder arthroplasty with a 135° inlay humeral component examined RSA patients categorized based on implant specifications (metallic offset from baseplate and glenosphere selection). The amount of glenoid-sided lateralization varied from 0 to 8 mm in 2-mm increments. The actual glenosphere COR lateralization with respect to the glenoid bone was not measured radiographically. 

Does isolated glenosphere lateralization affect outcomes in reverse shoulder arthroplasty? compared shoulders with the COR 2 mm lateral to the glenoid bone to those with the COR 6 mm lateral to the glenoid bone. Acromion and spine fractures were found it 3% of the 2 mm group and in 1% in the 6 mm group.

Lateralized versus nonlateralized glenospheres in reverse shoulder arthroplasty: a systematic review with meta-analysis found no difference in acromion/ spine fracture rates between RSAs catagorized as "lateralized" and "nonlateralized". Data on the difference in COR to glenoid bone distance for the two groups are not presented.

The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem did not find a fracture rate difference between lateralized and non-lateralized glenospheres. Data on the difference in COR to glenoid bone distance for the two groups are not presented

Implant-Positioning and Patient Factors Associated with Acromial and Scapular Spine Fractures After Reverse Shoulder Arthroplasty found that "excessive" glenoid-sided and global lateralization were associated with higher fracture rates; "excessive" is not defined. "Total glenoid lateral offset" was defined as the sum of lateralization contributed by the glenosphere, baseplate, and bone graft if present. Data on the relation of the glenosphere COR to the humeral bone are not presented.

Humeral position

The humerus can be moved distally by the glenosphere (inferior positioning on the glenoid bone, inferior tilt, inferior offset) and by the humerus (using an onlay component, high positioning of an inlay component). Humerus distalization can be documented in terms of postoperative position or as the change in preoperative to postoperative position. 

Different methods have been used to characterize humeral distalization making it difficult to compare studies. It seems most intuitive to directly measure acromiohumeral distance: the distance from the lateral acromion to the lateral prominence of the tuberosity along a line parallel to the glenoid face. This approach can be used both before and after RSA.

Influence of implant design and parasagittal acromial morphology on acromial and scapular spine strain after reverse total shoulder arthroplasty: a cadaveric and computer-based biomechanical analysis found that lengthening above 25 mm produced large strains in the acromion and scapular spine.

Up to 8 mm of glenoid-sided lateralization does not increase the risk of acromial or scapular spine stress fracture following reverse shoulder arthroplasty with a 135° inlay humeral component found that the change in acromiohumeral distance (delta AHD) was significantly higher in the stress fracture group. For every 1cm increase in delta AHD, there was a 121% increased risk for fracture. For every 1mm increase in inferior glenosphere overhang, there was a 19% increase in fracture risk.

The risk of postoperative scapular spine fracture following reverse shoulder arthroplasty is increased with an onlay humeral stem found "Increased postoperative distalization is associated with an increased risk of SSF after RSA." While the authors also concluded that  "An onlay stem resulted in a 10 mm increase in distalization compared with an inlay stem, and a 2.5 times increased risk of SSF. " it is apparent that what's important is not only the component design (inlay vs onlay) but also on the amount of distalization, which is influenced by both design and implant position. An onlay component can be inset in the humerus while an inlay component can be placed high with respect to the tuberosity.

Acromial Fractures in Reverse Shoulder Arthroplasty: A Clinical and Radiographic Analysis  greater arm lengthening was more common in the fracture group 

Predictive factors of acromial fractures following reverse total shoulder arthroplasty: a subgroup analysis of 860 shoulders  showed that a significant association of higher postoperative lateralization (by lateralization shoulder angle), lower distalization (by distalization shoulder angle), a lower acromiohumeral distance, and higher age were predictive only for Levy type III fractures. These result are  contradictory to other data, possibly because of small numbers (only 16 Levy III fractures) and the confounder of older age. Notably among the 860 shoulders the fracture types most clearly related to deltoid tension (Levy I and II) showed no association with any measured parameter.

The data on the effect of humeral component lateralization on acromial/spine fractures is inconclusive.

Summary: The current data are incomplete and, in many cases, inconclusive. Humeral distalization beyond 20-25mm appears to increase fracture risk (biomechanical threshold ~25mm; clinical data showing 121% increased risk per 10mm increase in delta acromiohumeral distance). Transection of the coracoacromial ligament consistently increased scapular spine strain in biomechanical studies and was associated with higher clinical fracture rates in the 265-patient study (29.4% vs 13.2% acromial pathology with CAL section vs. preservation).

Basic science data suggest that glenosphere COR lateralization may increase fracture risk, but clinical studies using categorical classifications ('lateralized' vs 'non-lateralized') without actual measurements are inconclusive. 

The effect of humeral lateralization remains unclear. 

Thus for high-risk patients (elderly women, inflammatory arthropathy, prior acromioplasty, thin acromion), limiting the change in acromiohumeral distance (delta AHD) to <20mm,  preserving the CAL, and avoiding excessive glenoid lateralization may be prudent pending better evidence—which will require standardized measurements such as those proposed above.

See also Preventing Scapular Spine Fractures: The Superior Baseplate Screw Question

Fractures

Mt. Rainier National Park

July 2024

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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).

Updated: As many as 25,000 acromial / scapular spine fractures may occur per year after reverse total shoulders. What are we doing about it??

Two recent articles Acromial stress fractures and reactions after reverse total shoulder arthroplasty: a case-control study and Risk Factors for Acromial and Scapular Fractures Following Reverse Shoulder Arthroplasty: A Meta-analysis of Over 100,000 Shoulders  confirm that these fractures are among the most common, most serious and most difficult to treat complications of reverse total shoulder arthroplasty. We see that the rate of acromial / spine fractures is not decreasing and that with the increasing use of reverse total shoulder arthroplasty the number of patients experiencing these fractures each year will continue to rise rapidly, perhaps to as many as 25,000 per year globally.

and 7,000 per year in the US.

Although the factors associated with these fractures are well known: osteoporosis, inflammatory arthritis, female sex, older age and lower BMI, corticosteroid use, rotator cuff deficiency, prior shoulder surgery (especially cuff repair), none of these is modifiable by the surgeon.

While some surgeon-controlled risk factors have been identified (screw placement and coracoacromial ligament preservation), other possible factors (humeral and glenoid component distalization and lateralization, acromio-tuberosity contact in abduction, change in acromio-humeral distance, and the timelineness and vigor of post operative rehabilitation) have not been consistently associated with fracture risk.

In that most rTSAs have successful outcomes, by what means can we learn how to reduce the rising number of acromial-spine fractures?

The Shoulder Arthroplasty Failture Research initiative seeks to learn safety lessons - not by statistical analysis of large case series or registry data  - but rather by considering in each individual fracture case what might have been done differently to avoid the patient experiencing the complication = causal modeling.

NASA has had 178 crewed space flights and two fatal flights. The two failures taught valuable safety lessons that could not have been learned from statistical analysis of the 178 "cases".  

In each of two individual fatal accidents, NASA had to model many possible causes of the tragedy. The results of causal modeling are shown below.

Challenger (STS-51L, 1986) Cause: Failure of an O-ring seal in the right solid rocket booster. All seven astronauts died. Cold weather on launch day made the rubber O-rings brittle. Engineers had raised concerns about launching in freezing weather, but management overrode them under schedule pressure. Richard Feynman placed a piece of the O-ring material into a glass of ice water and showed that the rubber lost its elasticity at low temperatures, failing to spring back quickly. Counterfactual: had a cold-tested O-ring been used, the lives of the seven astronauts may have been spared.

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Columbia (STS-107, 2003) Cause: A piece of foam insulation from the external tank broke off during launch, striking the left wing's leading edge, damaging its reinforced carbon-carbon panels. The crew module was destroyed on re-entry. All seven astronauts died. NASA had a history of foam shedding from the external tank before Columbia, but it was consistently downplayed. Counterfactual: had NASA addressed the prior foam insulation failures, the lives of the seven astronauts may have been spared.

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Consider these two cases of acromial/spine fractures after reverse total shoulder arthroplasty

In the case on the right, most of us would suggest the counterfactual that "if the screw had not been placed in the scapular spine, the fracture would likely have been avoided".

However, in the case on the left, is it likely that the fracture would have been avoided if the surgeon had achieved a lower position of the baseplate, more inferior tilt of the baseplate, more or less humeral distalization, more or less humeral lateralization, by assuring lack of tuberosity-acromion contact, or...? Expert surgeons may have different opinions, but we will only learn by pushing ourselves to answer the causation question, "would the outcome have been different if..?"

So.... 

A. for each acromion/spine fracture case, we should consider

(1) a pre-defined set of causal variables that could have been changed by the surgeon

Screw position: trajectories, lengths, whether any screw is outside-in; distance from superior screw tip to scapular spine; posterior screw proximity to suprascapular notch

Scapular ring status: coracoacromial ligament  (CAL) intact vs transected; any deltotrapezial fascia compromise. 

Construct geometry: humeral distalization, humeral lateralization, neck-shaft angle, humeral inlay/onlay, glenosphere lateralization, glenoid  baseplate tilt, inferior  glenosphereoverhang.  Pre to post op change in acromiohumeral distance (ΔAHD)

Rehab intensity & timing: early deltoid loading milestones.

(2) factors that were non-modifiable for that operation, but critical for counterfactual simulation

Patient bone health: DEXA/T-score proxy; steroid use; rheumatoid/inflammatory arthritis. 

Rotator cuff status: Tear, cuff tear arthropathy

B. Pose explicit counterfactuals
Example queries for each fracture case, for example in a specific case ask: 

• “If the superior screw had been omitted or shortened (inferior-only fixation), would fracture probability have dropped?” 

• “If the CAL had been preserved, would modeled spine strain have stayed below fatigue thresholds?” 

• “If humeral distalization (ΔAHD) had been 3–4 mm less, would the risk have decreased?” 

Pair each patient with 2–4 closest non-fracture rTSA controls (same age/sex/diagnosis/cuff status, bone quality) and run a small within-case causal analysis (not just regression): what single change (if any) most reduces predicted risk for this patient?

C. Convert findings into micro-rules (“guardrails”).
Examples that fall straight out of current evidence:

• Avoid outside-in or long superior screws when fixation allows; favor inferior-biased screw strategy. 

• Preserve the CAL unless there’s a compelling reason to release it. 

• Limit humeral distalization; scrutinize ΔAHD and inferior overhang. 

• Create a “spine-at-risk” checklist for osteoporotic, inflammatory arthritis, steroid-using, cuff-deficient, very low-BMI patients—flagging surgeon controlled variables that appear most influential on fracture risk in this group of patients..

D. As the library of cases grows: every 10–20 fracture cases, publish short, anonymized notes summarizing the modifiable factors that appearn to have the greatest preventable impact on fracture risk? 

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About our two example cases

• Right-hand image (with a screw traversing the spine): the counterfactual (“no superior screw / shorter screw / inferior-only fixation”) is very plausible given where many fractures localize. 

• Left-hand image (debate over baseplate height/tilt, lateralization, distalization, tuberosity–acromion contact): literature does not consistently link most of these to fractures, with the notable exception of excess distalization. Capture ΔAHD, CAL status, and screw map; then test those counterfactuals first.

This type of causal modeling is not familiar to most surgeon-scientists, but we should begin learning to put it to use for the good of our future patients.

Our colleague Jon Levy responded to this post:

"Another outstanding post.
Over the past 15 years I have integrated all of these factors into my workflow and planning efforts.
Of the modifiable actions, I strongly believe the greatest reduction in acromion fractures occurred with:
1) inlay humerus — since I philosophically believe in glenoid-based laterization to maximize motion and avoid notching induced osteolysis, implanting the humerus within the metaphysis (rather than above it) had a dramatic reduction in fracture rate
2) preop planning — focus on 3 goals — (a) maximize glenoid fixation; (b) achieve at least 70 degrees of abduction motion before greater to acromion impingement; (c) avoid final position of the humerus being more lateral than preop position.
Preop planning had the greatest impact in year to year variation in my fracture rate."

I asked Jon how he managed the rTSA when the planned reconstruction was too loose. His response was "If after reducing the shoulder, the tension is not right and soft tissue balancing is too loose, I typically will select a +4 semi constrained trial. 

 

With the system I use, constraint is added by going deeper into the poly socket. This essentially equates to a +2.5 but gains additional constraint.

 

I almost never go to a +8"  

Looking for answers

Cooper's Hawk

Matsen Backyard

2021

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).

How can we prevent acromial and spine fractures after reverse total shoulder?

As emphasized in a recent post, acromial and scapular spine fractures continue to be a major cause of poor results after reverse total shoulder arthroplasty.

A couple of examples to kick things off.

Case 1: A 78 year old man had recognized risk factors for these fractures: osteoporosis and the diagnosis of cuff tear arthropathy. He has been treated for his osteoporosis with Alendronate.

His preoperative and post reverse total shoulder radiographs are shown below.

.

A month after surgery, while his arm was still in a sling immobilizer, he developed pain in his lateral shoulder. Examination revealed a localized spot of exquisite tenderness on the lateral acromion. On an axillary view a non-displaced crack is seen in his acromion at the site of his tenderness.

Case 2: An 82 year old woman had symptomatic cuff tear arthropathy and this AP radiograph

A reverse total shoulder was performed as shown below

Two months after an uneventful recovery, she developed pain on use of the arm and point tenderness over the acromion posteriorly. While plain x-rays were unremarkable, a CT scan documented her stress fracture at the point of her tenderness

While these patients' age, sex, diagnoses of cuff tear arthropathy and osteoporosis were not modifiable, the question is whether there are modifiable risk factors, such as the geometry of the prosthetic RSA reconstruction.

A review of much of the current literature on this topic can be found in this post.

A recent publication, Shoulder Geometry After Reverse Total Shoulder Arthroplasty with a Medialized Glenoid and a Lateralized Humerus Predicts Subacromial Notching and Acromial or Scapular Spine Fractures, attempted to assess (1) whether the difference between the acromion to glenosphere center of rotation distance (DA) and the greater tuberosity to glenosphere center distance (DGT) influences the incidence of subacromial notching (SaN) in shoulders following reverse total shoulder arthroplasty (rTSA) and (2) whether this relationship is associated with the incidence of acromion or scapular spine fractures.

They  conducted a retrospective cohort study of 526 patients who underwent RSA with a medialized glenoid and a lateralized humerus.

After propensity score matching, 360 shoulders were analyzed (240 in the DA ≥ DGT group and 120 in the DA < DGT group). Both groups showed similar improvements in clinical outcomes postoperatively.

The DA ≥ DGT group exhibited a significantly lower incidence of SaN (0%) compared to the DA < DGT group (10.8%, P < 0.001). Additionally, the DA ≥ DGT group had a lower rate of acromion or scapular spine fractures (0.4%) compared to the DA < DGT group (5.0%, P = 0.006) [although a larger sample size will be necessary to achieve statistical power]. 

If we go back to Case 1, the 78 year old man with the acromial fracture, his distance to acromion (green arrow) was ≥ distance to greater tuberosity (yellow arrow).

5. If we revisit Case 2, the 82 year old lady, her distance to acromion (green arrow) was ≥ the distance to greater tuberosity (yellow arrow).

These cases remind us that age, diagnosis of cuff tear arthropathy and osteoporosis are more strongly associated with the occurrence of acromion/spine fractures than component design or position. That said, patient demographics (female sex, age, rheumatoid arthritis) and shoulder diagnosis (cuff tear arthropathy, massive irreparable cuff tears with pseudoparalysis) are not modifiable, so we need to continue to research modifiable factors that may reduce the rate of these factors especially in high risk patients. 

Possible candidates to be studied are (1) assuring that osteoporosis is under optimal management, (2) minimizing global lateralization of the humerus in RSA, (3) defining the optimal degree of glenosphere tilt and inferior placement, (4) burring down the lateral aspect of the greater tuberosity to make sure that there is no tuberosity/acromial contact when the arm is abducted and rotated, (5) slowing the return to activity after surgery, (6) prophylactic calcitonin, (7) considering a cuff tear arthropathy prosthesis rather than a RSA in high risk patients.

Jon Levy kindly responded to this post stating that his big three for minimizing the acromial / spine fracture risk are 

(1) optimizing glenoid component fixation

(2) avoiding early arc abduction impingement (he currently uses 70 degrees as his goal post).

(3) avoiding lateralizing the final humerus position more than the preop position.

We need to continue explore better methods for preventing these fractures: they are disabling for the patients that sustain them; they are too common. 

Common Yellowthroat

Montlake Fill, April 2020

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/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).