Monday, January 21, 2013

What to consider in glenoid positioning?

Glenoid implant orientation and cement failure in total shoulder arthroplasty: a finite element analysis

A commonly held belief is that the ideal shoulder arthroplasty corrects glenoid version to neutral.  This is a problem in that most osteoarthritic glenoids are substantially retroverted. Thus to correct this retroversion, the surgeon has to remove precious bone by reaming the anterior bony glenoid. This not only diminishes the amount of bone available to support the glenod component, but also sacrifices the subchondral cortical bone exposing softer cancellous bone.

This article is a followon to a previous study of the effect of reaming. See also

These authors evaluated the risk of cement mantle fracture in computer models models a typical osteoarthritic glenoid in 15 degrees of retroversion for (a) full correction by reaming, (b) full correction with partial bone contact and cement inserted posteriorly to fill in the gap, (c) partial correction (to 5 degrees) with cement to fill in the gap and (d) no correction, full contact.  In a heterogeneous cortical-trabecular bone model they found that complete correction resulted in the highest risk of failure. 

This is an interesting result, supporting the concepts that (1) the more of the prosthesis supported by cortical bone, the less risk of cement failure and (2) one cannot make up for inadequate bony support by sticking cement between the bone and the unsupported posterior component (we refer to this as 'putty carpentry'). 

To optimize the fit of the bone to the component, we developed a prototypical reamer three decades ago. Now most arthroplasty systems include glenoid reamers. Some are very aggressive and can remove a lot of bone quickly. Excessive reaming has been associated with glenoid component failure.

Our practice is to ream conservatively only to the point where the component is fully supported, accepting the patient's retroversion. Furthermore, we do not place cement between the bone and the component to obviate the cement mantle and the risk of 'cement mantle fracture'. Thus we seek 'bone backing' of the component, rather than interposing a thin layer of cement that becomes brittle on curing (try taking a thin piece of cement and seeing how many times you can bend it before it breaks). 

Here's a recent example of a man with a retroverted glenoid and wear on his preoperative axillary.

His postoperative film shows his retroversion remains, but his component is well supported by bone with no 'putty'. He has had no problem with instability after his surgery.

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