Tuesday, March 8, 2016

Reverse total shoulder components - the price of modularity

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

These authors suggest that modularity in reverse shoulder arthroplasty provides surgical flexibility and facilitates less-complex revision surgery, but that the potential for fatigue fracture is higher with modular components. 

They studied 8 humeral trays of nearly identical designs: 4 made of Ti-6Al-4V (Ti) and 4 made of CoCrMo (CoCr),

Both types of implant are still on the market.

Some of the results of this retrieval study are shown here.

As shown above, two Ti devices were revised for in vivo fracture.

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. Finite element and fatigue analyses predict a 10-fold lifetime increase for the CoCr devices compared with the Ti devices.

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: This report shows how important it is for the surgeon to understand the material and the geometry of the components he or she plans to use.

We've actually put up a post previously on this exact topic; it is reproduced below

Three years ago a patient had a reverse total shoulder. Post operative films shown below

Within the first month the glenosphere dissociated from the baseplate requiring revision as shown below.

With a year or so, he developed pain and clunking in his shoulder and had these radiographic findings (problem is subtle, can you see it?)

 He came to see us at which time we obtained these films.

 Did you notice the dissociation of the humeral tray from the stem. It turns out that this tray was made of Titanium.

 In this cases, the stem of the tray had fatigued and fractured.

We were able to obtain a Chrome-Cobalt humeral tray and used it in our revision, shown below. We also resected the heterotopic bone and cultured the wound for Propionibacterium. There was no growth on these cultures.

In the current wave of enthusiasm for implanting reverse total shoulders, it is important to recognize that this is a technically demanding procedure and that each implant system has its own nuances for fixation of the glenoid base plate, glenosphere, humeral stem, humeral tray and humeral poly. This case also demonstrates that the forces at the articulation are large - enough to result in fatigue fracture of the stem of the tray. It also points out the importance of understand the metallurgy of the implant.

Our practice is to use an impaction grafted non-modular monoblock prosthesis , which avoids the risk of tray failure


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