Friday, November 23, 2012

Are Shoulders with A Reverse Shoulder Prosthesis Strong Enough? A Pilot Study CORR

Are Shoulders with A Reverse Shoulder Prosthesis Strong Enough? A Pilot Study CORR

This is an important study that attempts to correlate function with strength after a Reverse total shoulder arthroplasty. The study population was 37 shoulders in 33 patients having 21 primary and 16 revision Tornier reverse total shoulders. The shoulders were evaluated a 4 - 63 months after surgery. The postoperative SST for primary reverse total shoulders was 8 ± 4 in contrast to 4 ± 3. 

While the average shoulder could generate 15 Nm of abduction/adduction torque, only 14 shoulders could be tested for internal and external rotation torque; for those the average external rotation torque was 9.3 Nm. Interestingly the authors correlated measured maximum torque at 60 degrees/second with the Dutch version of the Simple Shoulder Test. Their results are shown below, showing that the strongest correlation of the SST was with external rotation torque:

By design, the reverse total shoulder displaces the center of rotation medially and distally to optimize the deltoid moment arm for elevation. However, this medial displacement puts the anterior and posterior deltoid and any residual rotator cuff at a mechanical disadvantage for rotation.

In reverse total shoulder arthroplasty, design does matter. As we pointed out in a recent post and here,  different component designs have different effects on the position of the center of rotation - both superiorinferiorly (which affects deltoid tension) and mediolaterally (which affects stability, the proximity of the medial aspect of the humeral component to the glenoid, and the moment arms for rotation).

This study suggest that strategies be developed for optimizing the abduction and rotator strength of shoulders with reverse total shoulders. Such strategies may include prosthesis design and placement, muscle transfers and special rehabilitation protocols.


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