Reverse total shoulder arthroplasty polyethylene percentage capture: a descriptive analysis of commercially available...

The depth of the polyethylene insert in relation to the glenosphere radius, also referred to as the polyethylene constraint, is an implant-design factor directly related to reverse total shoulder arthroplasty (rTSA) stability. However, little is currently known about the objective levels of constraint of the various commercially available polyethene inserts. As such, the primary objective of this study was to report the "depth-to-radius" (D/r) ratio of polyethylene inserts from several commercially available rTSA implant systems. Additionally, as various reporting metrics exist to describe the constraint of a polyethylene insert, the second objective was to recommend an industry wide reporting standard that would be easily understood by surgeons, a metric termed the "percentage capture". Our hypothesis was that there would be a high degree of variability in the D/r ratios or the percentage captures of the various commercially available polyethene inserts.

Implant design data was collected from the most commonly used rTSA systems worldwide. Collected data included the neck-shaft angle (degrees), glenosphere diameter (mm), polyethylene classification (standard, retentive, or other), the polyethylene depth (mm), and the inlay/onlay classification for various implant systems. The percentage capture was then calculated for each unique polyethylene insert by dividing the polyethylene insert depth by the glenosphere radius and multiplying by 100 to obtain a percentage. The percentage capture was then statistically compared between different implant designs using multiple one-way analyses of variances.

The mean percentage capture of standard polyethene inserts was 48% ± 6% (range, 35%-63%), which was significantly less (P < .001) than the retentive insert group with a mean percentage capture of 61% ± 7% (range, 45%-81%). Inlay systems exhibited a significantly greater percentage capture compared to onlay designs (P = .001), with means of 56% ± 9% and 50% ± 8%, respectively. Neck shaft angle was also found to significantly influence the percentage capture (P < .001), while glenosphere size did not (P = .276).

Large variability exists in polyethylene liner constraints amongst rTSA manufactures. Within the percent capture range of 45%-63%, some companies label their inserts as "standard" and others as "retentive". As such, the present classification of "standard" and "retentive" levels of constraint between manufactures is subjective. We recommend an industry wide standard with objective nomenclature. We introduce and recommend the percentage capture metric, which is both descriptive and objective. Percentage capture is obtained by multiplying the D/r ratio by 100 and represents the percentage of the hemispheric glenosphere contained within the depth of the liner.