How does the fillet relate to reverse total shoulder implant failure?

Not this fillet

but this one, the degree of rounding of the interior corner of a part. A larger radius dispurses the stress at the junction.

Some implant designs have a stem that can be used either for an anatomic (below left) or a reverse (below center) total shoulder. In order to adapt the stem to a polyethylene (PE) cup, some designs use a tray with a trunion that fits into the Morse taper of the stem. The junction between the tray and the trunion is a fillet (arrow below right). 

 

There are two articles with similar findings.

In vivo fracture of a reverse total shoulder replacement humeral tray: A case report

Failure analysis was performed for two humeral tray components in the same patient that fractured in vivo after only 6 and 9 months.

Implant retrieval analysis indicated that the components failed due to fatigue failure initiating from a small radius fillet at the interface of the Ti6Al4v trunnion and tray regions. Finite element simulations revealed the small radius fillet to have resulted in a large stress concentration and confirmed the possibility for fatigue failure in 6 months. The stress concentration caused by both the small radius fillet and the insufficient tray thickness contributed to the premature fatigue failure of the humeral trays.

Fatigue failure of reverse shoulder humeral tray components of a single design.

These authors retrieved 8 humeral trays of nearly identical designs: 4 Ti-6Al-4V (Ti) and 4 CoCrMo (CoCr). The two Ti devices were revised for in vivo fracture. All Ti humeral tray retrievals fractured in vivo or were cracked at the taper fillet.

 Scanning electron microscopy showed cracking in the other 2 Ti trays and no evidence of cracking in the CoCr components. A geometric difference in the CoCr devices resulted in a 25% decreased stress under simulated activities of daily living. Accounting for the tray material properties, the fatigue failure envelope ranged from 1000 to 1 million cycles for Ti and from 30,000 to >10 million cycles for CoCr.

They concluded  that fatigue failure is possible for some reverse shoulder components and is likely exacerbated by fillet radius, tray thickness, and material choice.

Comment: Modularity and convertibility from anatomic to reverse shoulder arthroplasty are common among currently available shoulder implants.   Each of these designs carries with it features that may expose the patient to risks of prosthetic failure. Surgeons need to be thoughtful as they try newer implants with greater number of intercalated elements, each of which carries with it unique benefits and failure modes.

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