Total shoulder arthroplasty - is 'new' or 'modularity' necessarily 'improved'?

Does an increase in modularity improve the outcomes of total shoulder replacement? Comparison across design generations.

These authors reviewed  75 second generation modular total shoulders for primary osteoarthritis that were followed for a minimum of 2 years (mean 7.4 years) or until reoperation.

The results were compared with first generation monoblock TSAs and third generation TSAs which offered multiple humeral head shape options to more precisely replicate patient anatomy. All components were  manufactured by Smith-Nephew (Memphis, TN, USA).

Survivorship among the 3 groups was similar at 5 years but was estimated to be higher in the 1st generation group at 7 years. Survivorship for the second generation implants was estimated to be 89.0 % at 10 years. More glenoids were radiographically at risk in the 2nd and 3rd generation groups than in the 1st; however, this did not reach significance.

Seven shoulders experienced clinically apparent postoperative subluxation of the glenohumeral joint with visible translation of the humeral head during active range of motion (anterior 2, posterior 3, superior 2). One patient developed a seroma that required no additional treatment. One patient suffered an intraop non-displaced fracture of an osteopenic greater tuberosity treated with bone grafting. One patient required removal of cement anterior to the glenoid after extrusion during placement of the glenoid component. Seven shoulders required reoperation. Four shoulders underwent revision arthroplasty for glenoid loosening an average of 6.1 years post-operatively (range 4 to 143 months). Two of these shoulders were converted to reverse total shoulder arthroplasties, and two were converted to hemiarthroplasty. Two shoulders underwent rotator cuff repairs in the setting of instability, with one undergoing concurrent humeral stem revision. One shoulder underwent open reduction internal fixation for a periprosthetic fracture.

Comment: This paper points out that (1) newer is not necessarily better and (2) the modularity of prosthesis is not necessarily the major determinant of the outcome. The case of a second generation prosthesis  from the paper demonstrates this point (shown below). This prosthesis was placed in an excessively superior position because of incarceration of the stem in the diaphysis - trying to seat the prosthesis in a more distal position would have risked intraoperative fracture. The head being high seems to have resulted in glenoid component loosening. This situation, however, is not a result of the system being modular or its generation, but rather the technique with which it was inserted.

We're sure the authors are striving to find out why the newer versions of this prosthesis system seem to be yielding inferior results. The answer may lie among the 4 Ps: there may be differences in the patients, the problems (type of pathology and pathoanatomy), the details of the procedures or the providers doing the surgery.

In our practice the major advantage of modularity is the ability to adjust the head thickness and to use eccentricity to improve stability.

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