Conversion of an anatomic to a reverse total shoulder, when is a 'platform' prosthesis of benefit? Reprise
Due to the great interest in this subject, we're repeating this post with some additional thoughts.
An anatomic arthroplasty can fail for many reasons, including malposition, instability, delayed cuff failure and pseudo paralysis. In these situations consideration can be given to conversion of the anatomic prosthesis to a reverse total shoulder as shown here. As demonstrated in that post out preferred method for managing a failed anatomic arthroplasty is to completely remove the existing implant, obtain cultures, and then implant the reverse prosthesis. This approach allows full access to the glenoid and optimal positioning of the humeral component of the reverse. Removal of the anatomic implant is almost always possible and is particularly straightforward if it was inserted using impaction grafting.
Recently, there has been the advent of 'platform' prostheses, in which a humeral stem is fixed in the humeral canal that can be attached to either an anatomic or a reverse proximal humeral prosthesis. For examples, see here, here, here, and here.
It is important to recognize that in a reverse, (1) the glenosphere is placed inferiorly on the glenoid face, (2) the proximal humeral part of the reverse is bigger than that of an anatomic humeral arthroplasty and (3) the soft tissue tensioning considerations of a reverse are different from those of an anatomic arthroplasty. Therefore, the proximal-distal positioning of the humeral component needs to be fine tuned to achieve the ideal reverse arthroplasty. While some systems provide various adaptors to adjust the height, inclination and version of the proximal humeral prosthesis, the flexibility in positioning is limited by the use of the 'platform' fixed in the humeral canal.
Fortunately, we now have a clearer understanding of the indications for a reverse total shoulder, so that the needs for convertible prostheses is diminishing. For example, it is becoming evident that proximal humeral fractures in elderly individuals are often best managed by a primary reverse total shoulder - the idea of 'trying' an anatomic arthroplasty that is convertible to a reverse later is not so appealing. Similarly, individuals with arthritis, cuff deficiency, and instability are also best managed by a primary reverse.
One of the aspects lacking in articles about platform and other types of new shoulder prostheses is the incremental cost of the implant. This information is necessary to determine the value (benefit/cost) of the device. The question becomes, for 100 anatomic arthroplasties, how many successful conversions to reverses would be necessary to justify the incremental cost of (1) the implant and (2) the learning curve?
In our practice, revision of an anatomic to a reverse prosthesis is required almost exclusively in cases where the index procedure is done elsewhere. Often there are problems with stem fixation or positioning that require stem removal, even if 'in theory' the platform stem is convertible to a reverse.
One of the advantages of fixation of a humeral stem with impaction grafting is that - should conversion to a reverse prosthesis be required - the stem can be easily removed and the reverse stem inserted at the desired height and version.