Friday, May 13, 2011

Shoulder joint replacement: minimizing risk of loosening with surgical technique

We have seen that the technique with which the polyethylene glenoid component is inserted is a major factor in its durability. One of the special challenges of the shoulder is that as the shoulder is moved, the ball does not stay centered in the socket, but rather translates across its surface. As shown in the figure below.
This translation causes the plastic socket to be loaded eccentrically. Eccentric loading can potentially cause the polyethylene component to wobble, warp and loosen by a mechanisms we have called "rocking horse loosening, diagrammed below.
Shoulder fellow Collins demonstrated that good carpentry in the preparation of the glenoid component can minimize wobble and warp when the shoulder is loaded eccentrically. The results of this study are shown below
The use of glenoid reaming to create the best fit is shown in the figure below.
When the surface of the glenoid bone is properly reamed, rocking horse loosening can be further resisted by a glenoid fixation system that has pegs in front of and behind its center to resist 'lift-off' as shown below on the right in contrast to the keel fixation as shown below on left.
The effectiveness of our approach has been clinically demonstrated by shoulder fellow Lazarus in his article on radiographic evaluation of keeled and pegged glenoid components. He showed that pegged components inserted by an experienced surgeons have a better track record.

Not infrequently in shoulder arthritis, the back of the bony glenoid is worn more than the front, creating a 'bi-concave' glenoid surface. 
When there is a major bi-concavity, we adjust the angle of the reamer to avoid excessive bone removal.

We find this vastly preferable to trying to fill the gap with 'putty carpentry' in which case the wedge of cement may displace leaving the glenoid component unsupported.



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