Saturday, January 26, 2013

Reverse total shoulder - baseplate fixation

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

The stated purpose of this study was to determine whether a 4-screw construct provides more baseplate stability than a 2-screw construct.

A flat-backed glenoid baseplate with 4 screw hole options was implanted into 6 matched pairs of cadaver scapulas using standard surgical technique. Within each pair, 2 screws or 4 screws were implanted in a randomized fashion. A glenosphere was attached allowing cyclic loading in an inferior-to-superior direction and in an anterior-to-posterior direction. Baseplate motion was measured using 4 linear voltage displacement transducers evenly spaced around the glenosphere.

There was a difference: at the 30% loading level, with 2-screw fixation, the average central displacement was 0.17 mm with superior loading, 0.21 mm with anterior loading, and 0.22 mm with posterior loading. With 4-screw fixation, the average displacement was 0.12 with superior loading, 0.15 with anterior loading, and 0.14 with posterior loading, but these differences were not significant.

The authors conclude that using fewer screws could potentially lead to a reduction in operative time, cost, and risk, with no significant negative effect on overall implant baseplate motion.

While these findings are of interest, one must be cautious about applying them clinically, in that the quality of bone in these cadaver shoulders may be better than what is found in the typical patient having a reverse total shoulder. Furthermore there may be differences among designs with respect to the security offered by the central screw or peg as well as the use of locking vs. non-locking screws. 

It is of greater interest that there was a significant difference in the central displacement with increasing number of loading cycles and a significant difference in the central displacement with increases in loading magnitude. This cautions us to counsel our patients about loading a reverse total shoulder - it may not be forgiving.

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