Reverse shoulder arthroplasty glenoid fixation: is there a benefit in using four instead of two screws?

Quantifying the competing relationship between adduction range of motion and baseplate micromotion with lateralization of reverse total shoulder arthroplasty

“…An experimental study performed by James et al (2013) concluded that there were no statistical differences in central displacement of the glenosphere when two (superior and inferior) versus four fixation screws were used, whereas our results indicate that the number of screws does have a significant effect on micromotion (p=0.008); however, the trends are similar. At 0 mm of lateralization in the present study, two screw configurations increased absolute micromotion from 18.3 ± 5.9 µm to 35.0 ± 14.9 µm (87%) with respect to four screws, indicating that two screw configurations provide less rigid primary fixation.…”

“…However, it is unknown whether or not lateralizing the COR would amplify differences in initial fixation due to subject- and surgery-specific factors such as patient-specific bone density distributions or modified fixation strategies. For example, the results of a recent in-vitro biomechanical study with cadaveric shoulders has suggested that the use of two peripheral fixation screws (versus the typical usage of four screws) does not compromise primary stability of the baseplate in the context of baseplate micromotion (James et al, 2013). However, whether or not the same is true when the COR is lateralized has not been reported.…”

The effects of progressive lateralization of the joint center of rotation of reverse total shoulder implants

“…However, the latter study reported in vivo glenohumeral data for an anatomical shoulder replacement where active rotator cuff muscles can generate large compressive loads. As a result, James et al 26 used a relatively small anterior shear loading (100 N), which, according to this study, is not representative of RSA glenoid loading. The significance of AP forces on fixation was also demonstrated by Stroud et al, 35 who showed that a 0.89-Nm bending moment (as a result of AP shear force) was enough to cause micromotion that exceeded the accepted physiologic tolerance of 150 mm.…”

“…James et al 26 used data based on the study of Westerhoff et al 41 to test 2-screw vs 4-screw glenoid fixation. However, the latter study reported in vivo glenohumeral data for an anatomical shoulder replacement where active rotator cuff muscles can generate large compressive loads.…”

A Critical Review on Prosthetic Features Available for Reversed Total Shoulder Arthroplasty

“…There are no studies published that promote the use of variable angle locking screws, although this does seem to be useful for fixation of the baseplate. James et al investigated the number of screws needed for initial stability [38] and decided that only two locking screws provide sufficient stability and extra screws have no added value. These extra screws become more important when the depth of the central peg anchorage is reduced.…”