Posteriorly augmented glenoid components are increasingly being advocated for use in anatomic total shoulder arthroplasty. As pointed out in Augmented anatomic glenoid components - are they necessary? their value in terms of improving clinical patient outcomes is yet to be demonstrated. In addition to their increased cost, their use may be associated with increased loosening moments and difficulty in fitting the component to the glenoid bone.
Furthermore, the use of stepped, wedged, or half wedged glenoid components assumes that the two concavities in an eroded glenoid lie with one anterior and the other posterior.
However it has become evident that the pathologic concavity need not be confined to the posterior direction and is not infrequently in the posterior superior aspect of the glenoid.
As a result, it may not be well fit by a posteriorly augmented glenoid component.
They analyzed 268 patients with glenohumeral osteoarthritis indicated for total shoulder arthroplasty. Glenoids with superior inclination ≥10˚ and retroversion ≥20˚ were considered to have biplanar deformity. 49% of these shoulders had type B2 pathoanatomy. The direction of the deformity was directly posterior in 57% and posterior superior in 24%.
Their parameters for acceptable glenoid reconstruction included glenoid polyethylene implant position with ≥ 90% seating and less than 20% cancellous bone exposure, no central peg perforation, no more than a single peripheral peg perforation, and maintenance of prosthetic joint line lateral to pathologic joint line.
Accepting less correction increased the rate of satisfactory reconstruction and decreased the planning system's suggested use of augmented glenoid components:
(1) The shoulders were first virtually planned for anatomic TSA attempting correction to neutral inclination and version. Virtual aTSA planning indicated that 41% of the shoulders could not be reconstructed to neutral inclination and version using any implant. Of those that could, the system suggested that 94% have augmented implants.
(2) The shoulders were then virtually planned for anatomic TSA accepting correction to 5˚ superior inclination and 10˚ retroversion. Virtual aTSA planning indicated that 10% could not be reconstructed with any implant. Of those that could, the system suggested that 58% have augmented implants.
Final implant insertion commonly involved removal of substantial amounts of bone, unseating in the posterosuperior quadrant, cancellous exposure in the anteroinferior quadrant, and vault perforation.
As the authors point out despite classical recommendations for 80% as the threshold for adequate glenoid implant seating, recent evidence suggests that even with 84% seating, the risk of glenoid loosening increases by up to 28% compared to full backside support. Seating values below 90% are associated with increases in bone stress and critical cement volumes. Furthermore, the importance of subchondral bone preservation is well-established, but specific thresholds for the amount of cancellous bone exposure are not defined.
Comment: This is an important study. It makes us ask, is it more important to "correct" the glenoid pathoanatomy or to preserve glenoid bone stock and "accept" increased glenoid version and inclination? (see Glenoid version: acceptors and correctors). How should a surgeon optimize version, inclination, cancellous bone exposure, and component seating? Many of the publications attempting to address these questions are based on finite element analysis, simulation modeling, in vitro studies using Sawbones, theory, or studies using outdated components (e.g. those with keels or non-ingrowth pegs). These studies can create a view of a "correct time zero implant position" or "acceptable resurfacing parameters" that may or may not relate to the outcome of the patients we treat.
This study points out that correcting a biplane deformity to neutral version and inclination frequently frequently results in peg perforation, exposure of subchondral cancellous bone and inadequate backside support of the implant.
It may be that the most robust approach to glenoid component placement is to conservatively ream the glenoid face to a single concavity, accepting glenoid version and inclination, preserving maximal bone quality and quantity, and completely seating a standard round backed glenoid component that matches the reamed concavity.
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