UW Shoulder and Elbow Academy: Reverse total shoulder - how much motion does a patient need?

The goal of reverse total shoulder is to increase the function of affected shoulders by providing the needed range of active motion as well as stability of the articulation.

A recent paper used computer modeling to evaluate the balance between mobility and stability: Characterizing the trade-off between range of motion and stability of reverse total shoulder arthroplasty

For different hypothetical component designs, they considered the variables of glenoid lateralization (GLat), neck-shaft angle (NSA), inferior offset of the center of rotation (CORinf), and humerus lateralization (HLat) for different humeral cup depths in comparison to a commercially available Grammont design (cup depth, 8.1 mm; GLat, 5 mm; NSA, 155; CORinf, 0 mm; HLat, 0 mm).

Stability was characterized as the shear force that would be required to dislocate the joint (force to dislocate [FTD]).

Joint range of motion (ROM) was measured using global circumduction range of motion.

ROM was optimized with

a cup depth of 6 mm with GLat, 16 mm; NSA, 163.6; CORinf, 4 mm; HLat, 0.6 mm) or

a cup depth of 8.1 mm; GLat, 16 mm; NSA, 160.5; CORinf, 4 mm; HLat, 0.2 mm).

Stability was optimized for stability with

a cup depth of 6 mm with GLat of 16 mm, NSA of 170, CORinf of 4 mm, and HLat of 3 mm or

a cup depth of 8.1 mm with GLat of 16 mm, NSA of 170, CORinf of 4 mm, and HLat of 3 mm).

Comment: This is an interesting study. However, a few limitations should be discussed:

(1) While the model included contact between the humeral component and the scapular inferiorly, it did not appear to include contact between the tuberosity and the acromion which can limit range of motion and contribute to the risk acromial/scapular spine fractures

(2) It did not prioritize stability in the positions clinically observed to be of greatest risk for dislocation, such as adduction, internal rotation and extension.

(3) The range of motion metric was global circumduction, without an emphasis on motions that are of greatest functional importance.

Consider the figures below from the book Practical Evaluation and Management of the Shoulder (freely available at this link). These global diagrams were obtained from living human subjects actively moving their arms while the motion of the humerus in relation to the body was tracked using electromagnetic sensors fixed to the humerus and sternum.

The figure below shows the maximum range of motion - note that the equator represents 90 degrees of humerothoracic elevation and the North Pole would be 180 degrees of elevation.

The figure below shows the positions necessary for achieving the functions of daily living included in the Simple Shoulder Test. Note that only one of these functions (placing the hand behind the head) requires nearly full elevation of the arm. All of the others can be accomplished with 90 degrees of elevation or less. Thus the "global circumduction range of motion" may not be the best measure of a functional range.

Furthermore, elevation is not the only motion of importance. The figure below indicates the rotational position of the arm required for performing these activities of daily living. One of the most problematic movements for patients having a reverse total shoulder is posterior hygiene - wiping the bottom - and tucking in the shirt behind. While these activities do not require much elevation, they do require internal rotation (see below) which is often limited in RTSA. This functionally important movement was not prioritized in the study reported above.

Another study of the functional range of motion is: Defining functional shoulder range of motion for activities of daily living These authors found a "full" range of motion is not required for most functions: the average shoulder motions required to perform 10 common tasks were flexion, 121°; extension, 46°; abduction, 128°; cross-body adduction, 116°; external rotation with the arm 90° abducted, 59°; and internal rotation with the arm at the side, 102°.