In vitro assessments of reverse glenoid stability using displacement gages are misleading - recommendations for accurate measurements of interface micromotion.

BACKGROUND Baseplate micromotion of the reverse shoulder glenoid component can lead to implant loosening. We hypothesized that a remotely positioned displacement gage measures elastic deformation of the system rather than actual micromotion at the implant/bone interface. METHODS Reverse glenoid components were implanted into polyurethane blocks of 3 different densities. A 700 N compressive load was maintained and a vertical 700 N shear load was applied for 1000 cycles. In addition to the typical gage measurement, a digital image analysis of micromotion at the implant/block interface using high resolution cameras was performed. The measurements were validated on human specimens. A finite element model was implemented to study the isolated effect of block deformation on baseplate displacements. FINDINGS With the gage, typically reported micromotions were measured. Two orders of magnitude lower micromotions were detected using interface image-based analysis. The finite element simulation showed that elastic deformation alone can cause micromotion magnitudes as measured with displacement gages. Polyurethane blocks of 20 and 15 lbs per cubic foot density best reproduced micromotions as measured on human specimens. INTERPRETATION We found considerably less relative micromotion at the implant/bone interface than previously assumed. Gage measurements quantify elastic deformation and not true interface micromotion. High resolution digital imaging at the implant/bone interface is strongly recommended for an accurate assessment of reverse glenoid component micromotion. Tests should further adopt 20 or 15 pcf bone test surrogates. Further studies are required to identify the failure modes encountered in vivo, and a corresponding in vitro testing methodology can then be developed.