The position of the reverse humeral cup can be guided either by the humeral diaphysis or by the humeral metaphysis. In many humeri, a prosthesis that fits snugly in the diaphysis may place the humeral cup in an excessively medial position - in other words the humeral shaft is excessively lateralized. There are at least three potential problems that can result: (1) the humeral cup may not fit in the metaphysis resulting in loss of rotational control of the implant, (2) the lateralized humerus may abut against the undersurface of the acromion risking acromial/spine fractures, and (3) lateralization may excessively tighten the repaired subscapularis and residual rotator cuff.
On the other hand, inserting the prosthesis so that the cup is centered in the metaphysis may require using a stem that does not snugly fit in the diaphysis. In such a case the fit of the stem in the diaphysis can be secured using impaction grafting. This method is detailed in this video. An example is shown in the radiograph below. Note the lateral position of the stem in the canal, Had a canal fitting implant been used, the humeral cup would have been more medial and would not have fit securely in the metaphysis.
The authors of A Method to Facilitate Improved Positioning of a Reverse Prosthesis Stem During Arthroplasty Surgery: The Metaphyseal-centering Technique point out that the relationship between the diaphysis and the metaphysis is not constant among humeri
As shown in the video, the "metaphyseal-centering technique" gives priority to the positioning of the shell portion of the humeral prosthesis rather than to the stem of the implant. To ensure that a centralized shell position is achieved within the proximal humerus, the stem portion of the humeral prosthesis may need to be purposely undersized and positioned eccentrically within the humeral diaphysis. Bone autograft is used in such cases to improve the fit and fixation of the stem within the humeral canal.
They point out in the figure below, that if the stem fits snugly in the canal, the cup may be too medial (left) or too lateral (right).
By contrast, using metaphyseal centering with appropriately placed impaction grafting (red), both proper metaphyseal placement and secure stem fixation can be achieved even if the stem needs to be placed in valgus or varus.
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