Wednesday, November 12, 2014

Glenohumeral radial mismatch and shoulder stability - a computer model.

Nonconforming glenoid increases posterior glenohumeral translation after a total shoulder replacement.

These authors state that the major complication in nonconforming total shoulder replacement (TSR) is glenoid loosening attributed to posteriorly directed humeral head translations.

They used a 6-degrees-of-freedom computational model of the glenohumeral joint to estimate the muscle forces, joint contact force, and glenohumeral translation for radial mismatches  - the difference between the radius of curvature of the humeral articular surface and the radius of curvature of the glenod articular surface - ranging from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of elevation in the plane of the scapula. Their model suggested that as the radial mismatch increased, the contact location of the humeral head moved posteriorly and inferiorly.

Comment: The title of this paper suggests that this was a study of total shoulders, but rather it was a computer modeling study. It is obvious that less joint surface conformity allows more translation - we don't need a computer model to tell us that. However, in a normal shoulder the humeral head does translate on the glenoid - a completely conforming set of humeral and glenoid components will only allow translation if the prosthetic rim is loaded, risking rim wear of the polyethylene, cold flow of the polyethylene, and rocking horse loosening. 

For truly clinical information on the effect of mismatch, see this post and here.

In our practice we use a 3 mm radial mismatch - the glenoid radius of curvature is 3 mm larger than that of the humeral head. 


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