Saturday, April 14, 2012

Total Shoulder Arthroplasty: Relationship of glenohumeral curvature mismatch to stability and interface motion.

JSES recently published "Interface micromotions increase with less-conforming cementless glenoid components."
Readers my want to view this study in view of the concept of concavity compression. You should surely treat yourself to the video on this topic prepared by my late partner, Doug Harryman. as well as his article "The effect of articular conformity and the size of the humeral head component on laxity and motion after glenohumeral arthroplasty. " It is important to recognize that normal glenohumeral kinematics include translation of the humeral head on the glenoid surface. Also, you should be aware of the work of David Collins, who did a very similar study many years ago.

A note of caution about nomenclature. Different prosthesis systems use different ways of describing the components. In this study, the authors describe the surface in terms of the RADIUS of curvature. They used a consistent head radius of 20 mm and examined glenoid surfaces ranging from 20 to 26. Some systems, including the one we use here, describe the head curvature in terms of the DIAMETER, 40 mm to 56 mm for example. In the system we use, the glenoid components are all 6 mm in DIAMETER greater than the humeral head: a 6 mm diametric mismatch or a 3 mm radial mismatch. In contrast, Neer's original total shoulder system had no mismatch in diameter between the glenoid and humeral components.

The premise of this article seems to be that cementless bone ingrowth fixation is desirable for glenoid components and that, for that reason, motion at the prosthesis bone-prosthesis interface is to be avoided. We can agree with the second part of this assertion, although evidence for the first part is lacking.

This study shows that the greater the mismatch between the curvature of the humeral head and the glenoid, the more translation allowed when a displacing force is applied to it. Once one understands the concept of concavity compression, it is intuitive that a greater degree of mismatch between the curvatures would allow for more translation per unit applied displacing force. This is exactly what the authors of the JSES paper found.

Much of the remainder of this article attempts to relate glenohumeral diameter mismatch to glenoid-bone micromotions. The problem is that so many variables we encounter clinically - direction of load, magnitude of load, quality of bone, type of fixation, seating of the component, compliance of the glenoid component, etc -  are not emulated in this in vitro study using 'bone substitute'. To appreciate the complexity of the clinical situation, you may like to read "The influence of glenohumeral prosthetic mismatch on glenoid radiolucent lines: results of a multicenter study."

As we pointed out in our book, The Shoulder is A Balance of Mobility and Stability.  In shoulder arthroplasty, many factors need to be considered in achieving this balance. While glenoid component loosening is the major complication of total shoulder arthroplasty, the geometry of the glenoid and humeral joint surfaces are only one of the many variables that need to be considered.

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