Onlay reverse total shoulder: the complication of polythylene liner dissociation

Glenohumeral instability is one of the most common complications of reverse total shoulder. The authors of Polyethylene Liner Dissociation From Humeral Tray (PDH): Impediment to Closed Reduction of Dislocated Reverse Total Shoulder Replacement pointed out that polyethylene liner dissociation from the humeral tray can be a barrier to closed reduction of a dislocated reversed total shoulder.

They identified 22 patients having dislocations of a reverse total shoulder. Among these, 10 patients (45%) had liner dissociation, all of which occurred with onlay reverse total shoulders in which the polyethylene liner is placed on a humeral tray.






No liner dissociations were noted with inlay designs in which the liner sits in a cup on the humeral component.








Another recent article, Polyethylene Liner Dissociation after Reverse Shoulder Arthroplasty Dislocation: a Case Series, reported four cases of liner dissociation, again all with onlay designs. Overall the dislocation incidence was 4.7%; the rate of polyethylene dissociation for this case series was 15.4% of all dislocations, or 0.7% of all RSA. In each case the radiographs showed malalignment of the humerus and glenosphere, but not a complete dislocation.


In the photograph below, the liner (blue arrow) has flipped completely out of the humeral tray (white arrow)


Comment: While infrequent, liner dissociation is important to recognize in that closed reduction will not be successful. From the examples in these two series, the radiographic appearance of liner dissociation seems to be relatively consistent and diagnostic. Note in the case shown below left, the normal relationship of the humeral component to the glenoid is altered, but the articulation is not completely separated would be expected in the case of complete dislocation (below right).




It is not clear exactly why this dissociation occurs or why it is seen only with onlay components. It is possible that the fixation of the liner on the tray in an onlay system is less secure than the fixation of the liner in the cup of an inlay system (see comparison design figures above). It is also possible that the greater thickness of the onlay liner provides a longer loosening lever arm when eccentric loads are applied by the glenosphere.

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Here are some videos that are of shoulder interest
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The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).