Sunday, January 19, 2014

Modular reverse total shoulder components - if it can be put together, it can come apart

Unscrewing instability of modular reverse shoulder prosthesis increases propensity for in vivo fracture: a report of two cases

This relatively hidden article has an important message: modular components can come apart. This is particularly the case for reverse total shoulder components in that the loading in modes other than compression is more likely than in anatomic arthroplasty.

The authors point out that reverse total shoulder complications are reported increasingly and include dislocations, infections, hematomas, glenoid loosening, glenosphere unscrewing, scapular notching, polyethylene wear, metallosis, and dissociation of modular humeral components at the proximal metaphysis and distal diaphysis. Importantly in the two cases they reported, both had proximal humeral bone loss, which, of course, removes a substantial amount of protection from the humeral component. In both of these cases the modular parts of the humeral component became unscrewed with extensive metallosis, suggesting that even before fracture, fretting wear at the unstable screw joint was occurring. 

They suggest that a possible mechanisms for in vivo unscrewing was abutment of the tuberosity against the glenoid (see below). Another is simply that the loading of the reverse humeral component by the glenosphere can exert torque on the component that can lead to failure. Interestingly they point out that ASTM International and International Organization for Standardization standards for cyclic fatigue and fretting corrosion of modular joints only exist for hip prostheses; no testing standards exist specifically for reverse total shoulder components. 

Steps to avoid humeral component failure of this type include (1) the use of a single piece (monoblock) component, (2) use of a well fixed proximal humeral allograft in cases of proximal humeral deficiency, and (3) care at surgery to avoid unwanted contact between the humeral component and the glenoid. While monoblock components may seem attractive, they have the disadvantage of reducing the versatility in restoring bone length and soft tissue tension that modular implants provide. 

The true rate of these complications is not known, but as we say here, 'if it has happened once, it can happen again'.