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.
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.
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?
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"
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Cooper's Hawk
Matsen Backyard
2021
