Friday, December 12, 2014

Rocking horse loosening in total shoulder arthroplasty

Back in 1988, we described the phenomenon of 'rocking horse loosening' in which eccentric loading of the glenoid component causes failure of glenoid component fixation to the underlying glenoid bone.

These authors suggest that this “rocking horse” phenomenon is the main cause of glenoid component loosening as a result of eccentric loading of the glenoid rim. They aimed to investigate the influence of increasing glenohumeral implant mismatch on bone-implant interface micromotion in a cemented all-polyethylene pegged glenoid biomechanical model.

Five glenoid sizes, 40 mm, 44 mm, 48 mm, 52 mm, and 56 mm were cyclically loaded with a 44 mm humeral head. They state that these combinations represent  +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm glenohumeral mismatches, respectively. There is an apparent math problem here that can only be resolved if the actual surface diameter of curvature of each glenoid is 6 mm greater than its name, e.g. the 40 mm glenoid actually has a diameter of curvature of 46 mm so the mismatch with a 44 mm head is 2 mm. There is another problem in that they state that the authors refer to  "The radius of curvature mismatch for each glenoid size was +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm". If these were actually radial mismatches, the respective diametral mismatches would extend up to 36 mm, i.e. a 44 mm humeral head on a 80 mm diameter of curvature glenoid. 

Our initial recommendation for the amount of diametral mismatch was 6 mm and that many systems now 'name' the glenoid component for the humeral head that is recommended to go with it rather than the actual diameter of curvature of the glenoid (which is 6 mm greater). However, it is important that surgeons understand these relationships for the particular systems they are using in that nomenclature varies. Thus it is important note that some descriptions use radial rather than diametral mismatch, so again it is important that the surgeon know the system.

The recommendation of 6 mm diametral mismatch was based on our original work on the principles of total shoulder arthroplasty which found that range of motion was enhanced if some translation of the humeral head was allowed before it rode up on the lip of the glenoid component - in shoulder arthroplasty this is allowed by a diametral mismatch with the head being of slightly less curvature than the glenoid. On the other hand we recognized that large amounts of mismatch reduced the glenohumeral contact area and increased the stress in the glenoid component to a degree that challenged the yield stress of the polyethylene. More detail can be found on page 192 of the PEMS book, available for free here.

The authors' biomechanical study found, predictably, that greater degrees of mismatch allowed higher degrees of translation and more eccentric loading in a mechanical model. They did not study the effect of the degree mismatch on range of motion or the relation of mismatch to stresses in the polyethylene.

Comment: Our approach to minimizing rocking horse loosening is shown here. We continue to use a 6 mm diametral mismatch, in that this surface anatomy seems to (1) provide sufficient stability, (2) allow some translation before rim loading, and (3) avoids subjecting the polyethylene to excessive stresses. The actual degree of translation in vivo depends, of course, on factors other than the surface anatomy relationships, such as the orientation of the glenoid component and the ability of the shoulder musculature to balance the ball in the socket.


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