The medical technology people came up with the reversal to try to counteract so many torn up rotator cuffs. It was good out of the box thinking. A lot of shoulder replacements happen because there is so much damage to the soft tissue around the shoulder joint, which is a pretty loosey-goosey joint to start with. The ball is distinct enough, but the socket part is more like a saucer than a cup. It lets the shoulder rotate in all directions, but the ball can slip around more, dislocate more easily, and the strain on ligaments and tendons holding the joint together is significant. The slop n the rotator cuff allows arthritis to move in, too. Tears happen frequently, lots of active people suffer shoulder injuries.
So according to my Doc, with the reverse joint, the way they reconstruct it is intended to use what is left of the shoulder structure differently, and when they do it, they rearrange some of the muscles to pull differently. Advantages: a bit quicker recovery period to getting out of the sling and other restraints. Also a working shoulder where you might not have had enough salvageable parts to do a good job otherwise. Disadvantage, according to my Doc is that you cannot reach as high up as you once did. Mine was due to severe arthritis, my rotator cuff was surprisingly not torn up, and he judged that I was a good candidate for seriously doing PT, so he chose an "anatomical" one for me. He said I should be able to get about full range of motion back – and I think I will as long as I stay with the PT exercises. It will take more time and work to know, but so far it is pretty fantastic.
Either way, you really can't start *real* PT for about (a very loooong) 6 weeks, so there you are way behind how long knee rehab takes, which can start within hours of surgery. Six weeks out on my new knee I was doing broken ground hiking and limited scrambling, though pretty carefully. Two months out saw a 5.5 mile round trip hike with survival pack, bear medicine, and a heavy camera into high mountain lakes in grizzly and elk country.
The other thing he said is that reverse shoulders are quicker and easier to install, which makes surgeons favor them – less work, and maybe an extra replacement a day ($$). When they started with them, they did a smaller percentage with reversals, but now the ratio is more like only 20-30% are anatomical shoulder parts, replacing what Mother Nature invented, verses reverse shoulder parts.