Tuesday, August 4, 2015

Total shoulder arthroplasty for biconcave glenoids - choosing a glenoid component

Posterior augmented glenoid designs preserve more bone in biconcave glenoids.

These authors "virtually implanted" standard glenoid components inserted with reaming to a neutral version, stepped, and wedged components in 10 arthritic scapulae classified as Walch B2 glenoids. They calculated the volume of surgical bone removal, the maximum reaming depth, and the portion of the implant surface in contact with cancellous vs. cortical bone for each implant.
Each glenoid component was then aligned to each scapula body axis (line E-D in the diagram below).

The eroded surface made up an average of 65% ± 12% of the glenoid width. Mean surgical bone volume removed was least for the wedged (2857 ± 1618 mm(3)) compared with the stepped (4307 ± 1485 mm(3); P < .001) and standard glenoid component inserted with reaming to a neutral version (5385 ± 2348 mm(3); P < .001) designs. Maximum bone depth removed for the wedged (4.2 ± 2.0 mm) was less than for the stepped (7.6 ± 1.2 mm; P < .001) and standard glenoid component inserted with reaming to a neutral version (9.9 ± 3.2 mm; P < .001). The mean percentage of the implant's back surface supported by cancellous bone was 18.2% for the standard glenoid component inserted with reaming to a neutral version, 8.8% for the stepped (P = .02), and 4.3% for the wedged (P = .01).

Both augmented components corrected glenoid version to neutral and required less bone removal, required less reaming depth, and were supported by more cortical bone than in the standard implant. The least amount of bone removed was with the wedged design.

This analysis again demonstrates that the common direction of wear is not posterior, but rather posterior inferior as shown below.

From this figure, one can see that (1) this contour could be converted to a single concavity with a small amount of reaming if some retroversion was accepted, (2) substantial bone would need to be removed if the glenoid was reamed asymmetrically to 'correct' the version and  (3) prosthetic glenoids with either a wedged or stepped backside would involve more bone removal, especially if they were oriented so that the posterior augmentation was oriented in a North-South direction.

This article is yet another that is based on the precept that 'correcting' glenoid retroversion will improve the clinical results of total shoulder arthroplasty; however it is yet to be shown that techniques such as asymmetric reaming of the antero-superior high side, bone grafting or posteriorly augmented glenoid components lead to better clinical outcomes than preserving glenoid bone stock by conservative reaming to a single concavity and using anteriorly offset humeral head components and rotator interval plication to manage any tendency for posterior humeral instability. See this related post.


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