Thursday, October 25, 2012

Reverse total shoulder arthroplasty for cuff tear arthropathy: the clinical effect of deltoid lengthening and center of rotation medialization. JSES

JSES recently published: Reverse total shoulder arthroplasty for cuff tear arthropathy: the clinical effect of deltoid lengthening and center of rotation medialization. 

This article nicely presents 16+/- 10 month followup on 37 of 49 shoulders having a reverse total shoulder for  cuff tear arthropathy. The prostheses included 27 Zimmer trabecular metal implants, 7 Delta III implants, and 3 Aequalis implants.  The distance between the lateral acromion and the midpoint of the deltoid tubercle was used as a measure of deltoid length. The average lengthening of the deltoid was 2.1 +/- 1.0 cm. Shoulders with lengthening of the deltoid of less than 2 cm had less active forward elevation than those with lengthening of the deltoid of over 2 cm. When the postoperative acromion-greater tuberosity distance was over 38 mm, shoulders usually achieved over 135 degrees of forward elevation; when this distance was less than 38 mm, less than half could reach over 135 degrees. 

The SST scores increased from 2.0 +/- 1.9 before surgery to 7.5 +/- 2.9 after surgery (p<.001).

In a previously published paper "A method for documenting the change in center of rotation with reverse total shoulder arthroplasty and its application to a consecutive series of 68 shoulders having reconstruction with one of two different reverse prostheses.", we noted that before surgery in shoulders with cuff tear arthropathy the center of rotation (COR) was superiorly displaced by 9 +/- mm from a normalized scapular reference. This doubtless explains the shoulder weakness experienced as a result of slackening of the deltoid. Postoperatively the center of rotation was displaced by 12 mm to a position 3 +/- 3 mm below the scapular reference. We found a difference between the two prosthetic designs used: for the Delta design the COR was inferiorly displaced by 2 ± 3 mm from the reference, whereas it was inferiorly displaced by 7 ± 3 mm with the Encore prosthesis (P < .001). The COR was medially displaced by 28 ± 4 mm with the Delta prosthesis and by 19 ± 3 mm with the Encore prosthesis (P < .001).

While the active function of shoulders appears to be better when there is greater lengthening (greater deltoid tension), caution is need. Greater lengthening may be associated with acromial fractures. Greater lengthening has also been associated with an increased prevalence of brachial plexus lesions.

The authors did not find a significant relationship between the center of rotation and the active forward elevation. We suggest that medialization of the center of rotation is more likely to affect the effectiveness of the residual external rotators, which would be slackened by medialization of the tuberosity relative to the scapula. 

Two thirds of the shoulders developed scapular notching. This continues to be an issue for prostheses that medialize the humerus relative to the scapula.

 One patient had a deep infection requiring implant removal and another had baseplate failure requiring conversion to a hemiarthroplasty.

Studies such as this recent publication are critical to understanding the science and art of the reverse total shoulder.


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