Monday, December 31, 2012

Stability of the reverse total shoulder

Reverse total shoulder arthroplasty for the management of failed shoulder arthroplasty with proximal humeral bone loss: is allograft augmentation necessary?
The demands on the rotational stability of a reverse total shoulder are much greater than in an anatomic shoulder for at least three reasons. First the constrained nature of the reverse total shoulder articulation enables the glenoid to 'grab' the humeral component and exert torque on the prosthesis and its fixation into the humerus much more so than the unconstrained glenoid of an anatomic shoulder. Second, as this illustration from the Practical Evaluation and Management of the Shoulder shows, the center of rotation of the humeral head articular surface lies close to the center of the 'orthopaedic axis' i.e. the center of the meduallary canal; as a result the torque lever arm for rotation of the stem in the bone is small.
By contrast, in a reverse total shoulder, the glenoid applies rotational forces to the humeral cup, which is offset in relation to the orthopaedic axis, as diagrammed below.

Finally, as emphasized in this article, the absence of the tuberosities encountered in many cases of revision reverse total shoulder arthroplasty removes the normal stabilizing effect of the proximal humeral bone structure.

Against this background, the authors of this article ask whether allograft augmentation of the deficient proximal humerus encountered in the management of failed shoulder arthroplasty is of benefit. 

Among 251 patients enrolled in a study of reverse total shoulders,  significant humeral bone loss was found in 15 of 56 shoulders undergoing revision for failed arthroplasty without allograft. The average bone loss measured 38.4 mm (range, 26-72 mm). Allograft was avoid out of their concern for the cost of the allograft, increased risk of infection, increased operative time and complexity, graft resorption, and failure of allograft incorporation. A variety of prostheses were used, two were inserted using a press-fit technique and 13 were inserted with cement.

At a minimum of two years, radiographs demonstrated no humeral subsidence or loosening. One modular prosthetic stem fractured. Seven of 15 patients experienced complications: an intraoperative periprosthetic fracture, a case of recurrent instability, a fracture of the humeral stem,  1 deep venous 
thrombosis, 3 transient nerve palsies, and 1 patient experienced painful cerclage cables that required hardware removal.

Considering the complex nature of the shoulders being treated, these results are remarkable, especially with respect to the lack of humeral component loosening. It would be of interest to know more details of their methods for humeral component fixation.


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