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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Total shoulder arthroplasty - the effect of glenohumeral non-conformity

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

These authors used a 6-degrees-of-freedom computational model of the glenohumeral joint in a single specimen to estimate the muscle forces, joint contact force, and glenohumeral translation for radial mismatches from 1 mm to 20 mm with the shoulder positioned from 20° to 60° of scaption. Posterior glenohumeral translation increased linearly with radial mismatch.

Comment: The title of this paper should indicate that this study was done with a computer model and does not examine 'glenohumeral translation after a total shoulder replacement'.  The authors examined only one plane of motion, elevation in the plane of the scapula, whereas the greatest amount of posterior loading comes with flexion. They did not model the material properties of the polyethylene glenoid nor the effects of repeated poly loading in a conforming joint configuration. The study included a simulation of only one shoulder and did not include scapular kinematics which can affect the direction of the net humeral joint reaction force.

Eccentric loading is a recognized feature of normal and post arthroplasty mechanics. We've shown previously that eccentric loading causes not only rocking of the glenoid but also deformation of the poly.  Rocking horse loosening appears to be a principal mode of glenoid failure. In a completely conforming glenoid, any translational forces exert a rocking moment on the component as well as increased pressure on the posterior glenoid polyethylene that can lead to cold flow. On the other hand, as this study shows, a high degree of mismatch diminishes the effectiveness of the concavity compression mechanism of glenohumeral stability.  

In a clinical study published in 2002, the authors found significant inverse linear relationship between mismatch and the glenoid radiolucency score (p < 0.0001), with significantly lower (better) radiolucency scores associated with radial mismatches of >5.5 mm.

The stability of a total shoulder arthroplasty depends on many factors, including prosthetic design, glenoid prosthetic orientation, soft tissue balance, and muscle coordination. It is not as simple as it may seem.

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