Anatomic prosthetic shoulder arthroplasties employ a humeral head component. Each of the many options on the market have four independent geometrical characteristics. It is important to understand these characteristics for the arthroplasty system being considered because these variables provide the surgeon with the ability to tailor the arthroplasty based on the intraoperative findings.
(1) Diameter of curvature - the diameter of a circle fit to the articular surface. The diameter of curvature is independent of the thickness of the head component (see below).
(2) Thickness - the distance between the apex of the articular surface and the base of the humeral head component. The thickness is independent of the diameter of curvature (above)
(3) Eccentricity - the offset of the taper in relation to the center of the humeral head prosthesis. Use of an eccentric head component offsets the articular surface in relation to the humeral shaft.
(4) Location of the taper: if the taper is the humeral body (red arrow) it can block access to the glenoid.
Let's consider the three types of anatomic arthroplasty: hemiarthroplasty, the ream and run, and total shoulder arthroplasty
In hemiarthroplasty (without glenoid arthroplasty), the humeral head diameter of curvature is selected to match that of the resected humeral head.
In the ream and run (see this link), the humeral head component is selected with a diameter of curvature that is 2 mm less than the diameter of curvature to which the glenoid has been reamed. For example a 56 mm diameter of curvature would be paired with a glenoid reamed to a diameter of curvature of 58 mm.
In total shoulder arthroplasty (see this link), the humeral head diameter of curvature is chosen in reference to the glenoid component which will articulate with it. We use a glenoid component that covers the prepared bony glenoid surface with a minimal amount of overhang.
We use a humeral head component that has a diameter of curvature 6 mm smaller than the diameter of curvature of the selected glenoid component. Note that in many systems the number on the glenoid component refers to the diameter of curvature of the humeral head that is to be used with it. In a commonly used system, a 6 mm difference in diameter of curvature is recommended. So a 48 diameter of curvature humeral head is paired with a "48 glenoid" that has an actual surface diameter of curvature of 48+6 or 54 mm.
Changing the thickness of the the humeral component adjusts the laxity of the arthroplasty.
Use of an anteriorly eccentric head component can effectively manage excessive posterior translation without overstuffing the joint (see this link).
An example of the stabilizing value of the eccentric humeral head is shown below - compare the preoperative axillary and the five year post operative axillary view.
Many thanks to Steve Lippitt for his great art work!
Follow on twitter: https://twitter.com/shoulderarth
Follow on facebook: https://www.facebook.com/frederick.matsen
Follow on LinkedIn: https://www.linkedin.com/in/rick-matsen-88b1a8133/
How you can support research in shoulder surgery Click on this link.
Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link)
Shoulder arthritis - x-ray appearance (see this link)
The smooth and move for irreparable cuff tears (see this link)
The total shoulder arthroplasty (see this link).
The ream and run technique is shown in this link.
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
