Friday, May 31, 2013

Reverse total shoulder - baseplate screw dangers

Glenoid screw position in the Encore Reverse Shoulder Prosthesis: an anatomic dissection study of screw relationship to surrounding structures.

These authors inserted reverse total shoulder prostheses in cadaver shoulders using a 6.5-mm central screw and four 5.0-mm peripheral locking screws placed 90° to the baseplate. They set the screw length at 30 mm for the superior screw holes, 28 mm for the inferior screw holes, 13 mm for the anterior screw holes, and 15 mm for the posterior screw holes [NB that in the recommended technique a set screw length is not recommended, instead the length of the screws is determined by a calibrated drill and a depth gauge]. 

They found that the central screw trajectory was through the anterior cortex. The anterior screw trajectory violated the subscapularis belly in all specimens. The posterior screw touched the suprascapular nerve or artery in 3 of 10 specimens.

Comment: It is difficult to know the degree to which screw penetration affects the outcome of reverse total shoulder. Obviously, the big risk is to the axillary nerve from dissection and to the plexus overall from traction or excess lengthening of the arm. In that many shoulders requiring reverse total shoulders have massive cuff defects, the suprascapular nerve is of less functional concern. In any event, however, drill plunges and excessively long screws need to be carefully avoided by good surgical technique and by customizing the length of the screws used. This is especially important because the rotational position of the baseplate may vary, so that the thickness of scapular bone to be traversed by the peripheral screws will vary from case to case.

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