Reverse shoulder arthroplasty is a great procedure that eliminates pain and improves motion. After years of suffering, many individuals are finally able to use their shoulders normally again, including the practice of sports. Reverse shoulder replacements combine several parts: (1) the humeral component, (2) a polyethylene liner, (3) the so-called “baseplate” -fixed to the shoulder bony socket with screws-, and (4) a hemispherical part (“glenosphere”) fixed to the baseplate. As you try to understand these last two components, think about hanging a heavy painting on a wall: you first affix a holding hook to the wall (the baseplate), then hang the painting from the hook (the glenosphere).
The baseplate is the foundation that provides solid fixation to bone, but it also determines where the glenosphere will land in space. And you know what the challenge is? All of us are different, and our shoulders are very different too!
How are the sockets of the shoulder different on different people?
The socket of the shoulder joint, called the glenoid, is a very special part of our skeleton! The glenoid is at the upper corner of the triangular shoulder blade. It has the shape of a cone, with its base pointing outwards. The base of this cone is the glenoid “face”, whereas the volume of this cone is the glenoid “vault”. Check out the video below to visualize these bones!
Just like we all have different height, weight and body shapes, the glenoid is slightly different in all of us. In some shoulders, the glenoid is fully developed, but in some it is partly underdeveloped. The face of the glenoid points slightly up and back in reference to the shoulder blade, but the magnitude of this orientation up and back can be dramatically different from individual to individual.
To complicate things further, arthritis of the shoulder joint leads to various patterns and severity of acquired bone loss. In wear-and-tear arthritis, bone loss can be symmetric or asymmetric, typically at the back the glenoid. In bad rotator cuff tears with arthritis, bone loss is more commonly on the upper part. Surgeons use various classification systems for all these patterns of glenoid bone loss (B2 and B3 patterns are common in wear and tear arthritis, whereas E2 and E3 patterns are common in rotator cuff tear arthropathy)
How do surgeons manage to anchor a baseplate in all-comers?
Medical device companies manufacture baseplates that will fit the average shoulder. In order to fit a baseplate to the shoulder socket, surgeons traditionally use instruments called reamers: precision sanders attached to a slow-spinning power tool (like a drill) to remove bone. Reamers create a shape in which the baseplate will fit.
However, in sockets that are abnormally shaped, and especially those with bone loss, the worst thing can be to remove even more bone! Alternatives include adding bone graft between the baseplate and the socket, or using off-the-shelf baseplates that come with a metallic build-up or augment. But what if we could manufacture baseplates that will fit the bone, instead of forcing the bone to fit the implant? (Tailored suits typically fit best, right?) Believe it or not, in this age of artificial intelligence and 3D printing, some medical device companies do offer shoulder-matched baseplates.
The planning and manufacturing process
If your surgeon will replace your shoulder with a custom-made shoulder-matched implant (Shoulder iDTM, Stryker, Mahwah, New Jersey), the first step is to obtain a computed tomography (CT) of the shoulder using specific parameters available pretty much everywhere. A CT scan generates multiple computer files coded according to the Digital Imaging and Communications in Medicine (DICOM) standards. These files can be shared on-line or copied on a hard-drive or disc.
Next, your surgeon uploads the DICOM files of your CT into a computer program called BluePrint…, and the “magic” begins! Blueprint incorporates computer vision artificial intelligence algorithms that automatically create 3-dimensional renderings of the shoulder skeleton, provide automatic measurements, allow the surgeon to virtually plan each surgery, and even request (1) use of robotic surgery or (2) manufacture of a shoulder-matched implant, the two most advanced digital enabling technologies currently available for shoulder replacement.
Once your surgeon decides where your implant needs to fit, the implant is ordered on-line (almost like shopping on Amazon!), and a in a few weeks three sterile boxes are delivered to the operating room, containing the custom-made baseplate, a metallic reproduction of the glenoid, and a custom guide to place the custom implant where it belongs.
The surgical procedure
Once your surgeon gains access to the shoulder socket, any residual cartilage (if there is any left) is meticulously scrapped. The surgeon uses images of the BluePrint plan, the metallic reproduction of the glenoid, and the custom-made guide to place a couple of pins that allow creation of a bone tunnel to fit the post of the implant, as well as alignment of the baseplate. As long as it does not bother you to watch recordings of surgery, you can review the details of he surgery in the video below.
Fixation to bone is augmented with screws and the glenosphere is affixed next, providing a perfect reconstruction for every shoulder, every single time!
