Acromial and spine stress fractures after reverse total shoulder.

As is the case for most stress fractures, acromial and spine stress fractures (ASF) after reverse total shoulder (RSA) result from changes in the magnitude, direction and frequency of loads applied to the bone. The observation that ASF are more common in shoulders with cuff deficiency suggests that an intact cuff may reduce the changes in loads on the acromion and scapular spine by assuming a portion of the humeroscapular forces.

Changes in acromial and scapular spine loading result from surgeon-controlled factors, including implant design and implant placement. Glenoid-sided lateralization can increase shoulder range of motion by reducing abutment between the humerus and the scapula as well as reducing the risk of scapular notching. What are the down-sides of glenoid-sided lateralization?

The authors of "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 degree inlay humeral component" (recipient of the Robert H. Cofield Award) identified 26 ASFs (5.5%) among 470 RSAs. 

In communication with the corresponding author, it was verified that glenoid-sided lateralization in the system used in this study reflects the combination of baseplate offset (0, 2 or 4mm) and glenosphere offset (0 to 8 mm). See red arrow in the figure below.

In this series, glenoid sided lateralization ranged from 0 to 8 mm. The amount of glenoid sided lateralization was not associated with ASF risk: the incidence of fracture did not increase with greater glenoid-sided lateralization (0-2 mm, 7.4%; 4 mm, 5.6%; 6 mm, 4.4%; 8 mm, 6.0).  

However, humeral distalization did increase the risk of ASFs. The pre- to postoperative change in acromiohumeral distance (AHD, measured as shown above) was higher in the stress fracture group (2.0 ± 0.7 cm vs. 1.7 ± 0.7 cm). For every centimeter increase in the change in AHD, there was a 121% increased risk for fracture. 

It can be concluded that for this implant system (a 135 degree inlay humeral component) the change in humeral position relative to the acromion  (whether from inferior tilt of the glenosphere, increased inferior overhang of the glenosphere, as well as from the type and positioning of the humeral implant) can change the magnitude and direction of the forces experienced by the acromion, creating a risk for stress fracture.

Minimizing the surgeon-controlled risk factors - such as avoiding over-lenthening - seems particularly important in shoulders that are intrinsically at increase risk for ASF, such as those with superior displacement of the humeral head relative to the scapula, a thin acromion, osteopenia, inflammatory arthropathy, advanced age and rotator cuff deficiency (see figure below).

Comment to shoulderarthritis@uw.edu

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