Sunday, July 6, 2014

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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