The geometry of reverse total shoulder (RSA) implants along with the soft tissue tension and balance are important determinants of the outcome for patients having this procedure. Surgeons need to understand the characteristics of the implants they use so they can made good decisions for each of their patients. While humeral component design and placement, soft tissue management, and the preoperative to postoperative change in acromio/humeral distance are critical variables in RSA, here we'll zone in on the glenosphere.
The nomenclature used to describe the geometry of the glenosphere can be confusing and differs among the manufacturing companies.
Here's a simplified approach to characterizing three key elements of glenosphere anatomy that can be applied to any make of glenosphere
(1) Glenosphere diameter of curvature (blue line)
2) Distance from center of rotation (COR) to glenoid bone (yellow line)
Note that this distance needs to include the thickness of the baseplate lying between the glenosphere and the bone, which varies by manufacterer and with the use of augments.
The "names" of the glenospheres may not be accurate descriptions their geometry. For example for the 32 mm diameter implants in one company's system, there are 32 "neutral", 32 -4, and 32 -6. Including a 3 mm thick baseplate, the distances from the COR to the glenoid bone surface are, respectively 13 mm, 9 mm, and 7 mm (not 0, -4 and -6).
The COR lateralization from bone for the 32, 36 and 40 diameter of curvature glenospheres in this system are shown below.
These values for the 32, 36, and 40 mm glenospheres are shown in the right column of the chart below.
Note that the distance from glenoid bone to the lateral aspect of the glenosphere is the same for the 32 -6, the 36 -4 and the 40 -4.
Since even the most sophisticated preoperative planning software cannot predict which glenosphere will provide the best mobility and stability, we have to rely on intraoperative trialing to determine the best glenoid component geometry. A chart like the above is handy to guide our component trialing by pointing out the COR lateralization (which affects deltoid moment arm and range of motion) and total lateralization (which affects soft tissue tension and stability thorough concavity compression) for the different options.
During trialing the surgeon can assess shuck when the humerus is pulled laterally, unwanted bone contact, range of motion (including extension), and stability using a variety of tests including the two hand lever test.
A 32 -6 may be a reasonable starting point in a small patient with a tight shoulder. An average sized patient with cuff tear arthropathy might merit starting with a 32 -4. A large patent needing stability may need a 36 neutral or 40 -4.
There are many important elements of a good reverse total shoulder arthroplasty. Selecting the best glenosphere for the patient is one of them.
Making good choices
Lewis' Woodpecker
Tualatin
2020
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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link)
The total shoulder arthroplasty (see this link)
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link)
Shoulder rehabilitation exercises (see this link).
