Wednesday, December 12, 2012

Conversion of an anatomic hemiarthroplasty to a reverse total shoulder

Some arthroplasty systems provide for the conversion of an anatomic humeral component to a reverse total shoulder. This conversion is easy with in vitro models, but, as we found out this week, in vivo it is challenging. Here is the x-ray of a patient who had had a prosthesis for fracture performed elsewhere. After this procedure she had no active elevation and consulted with us about the possibility of a reverse total shoulder. We had the options of removing the cemented stem and performing our usual reverse total shoulder prosthesis or of trying to preserve the stem and do a conversion. We elected the latter after extensive consultation with the patient.
 Our preoperative templating shows the challenge: while putting the reverse humeral tray and cup on the humeral prosthesis seems easy, the templating view shows that it is necessary to move the humerus distally to allow for insertion of the glenoid component...

to achieve reduction.

 The surgery was very difficult because of the challenge of exposing the glenoid and the problems of getting the glenosphere for this system properly seated on the base plate (required meticulous resection of the bone around the base plate - especially inferiorly - and very fiddly positioning of the glenosphere and challenge of assuring that the glenosphere is fully seated). Reduction was challenging, but stable. There we three special issues we encountered. First, as shown below, it is important that there is no bone extending beyond the medial extent of the base plate, otherwise it will block full seating of the glenosphere. It is difficult to see all the way around the baseplate, yet attention to this detail is essential.

Secondly, the base plate is not circular. Thus the glenosphere must be rotated exactly right for seating. This is not an issue with systems with a circular base plate. 
Finally, the glenosphere is not held in place by a Morse taper, only by a single screw. The fatigue properties of this glenosphere fixation in vivo are not known.

Bottom line, it is important to know your reverse system. They are all different.

Here's our post op film.

Happily, all nerves worked at the end of the case. Stay tuned.


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