Sunday, June 12, 2016

When should we use a stepped posteriorly augmented glenoid component?

Total shoulder arthroplasty for glenohumeral arthritis associated with posterior glenoid bone loss: results of an all-polyethylene, posteriorly augmented glenoid component

Between May 2011 and January 2013, 22 shoulders in 19 patients (15 men and 4 women) underwent primary TSA by a single surgeon. In all cases, an all-polyethylene, posteriorly augmented, stepped glenoid component was implanted. 

In these cases preoperative glenoid retroversion measured 15 degrees or greater (14 of these cases had retroversion of 25 degrees or less).

At a mean follow-up of 36 months, 12 shoulders had osseous integration between the central-peg flanges, 6 had bone adjacent to the central-peg flanges but without identifiable osseous integration, and 1 showed osteolysis. 

While the overall clinical results showed improvement, two patients sustained a total of 3 episodes of humeral dislocation, one anterior and two posterior.

The authors point out that " implantation of a stepped glenoid component requires removal of some posterior bone. In glenoids in which there is both retroversion and glenoid medialization, there may be insufficient subchondral bone, volume, and/or density after preparation to support the posterior component." This is shown by the figure below.

Comment: In the osteoarthritic shoulder with posterior wear, the bone and cartilage have failed to hold up under the loads applied by the humeral head. With a posteriorly augmented glenoid component, the hope is that the thickened posterior polyethylene will be able to hold up under these same loads. 

This report shows that achieving prosthetic balance with this prosthesis can be difficult, even in the hands of an experienced surgeon.

The indications for the use of the stepped prosthesis remain to be determined. Many of the shoulders in this series had relatively low amounts of preoperative retroversion that are in the range usually managed with conventional components and soft tissue balancing.

Should this implant fail, the bone defect would be greater than that present before the arthroplasty as diagrammed below.