Measuring humeral distalization and lateralization in reverse total shoulder

A prior post, Do lateralization and distalization after reverse total shoulder have a clinically significant relationship with patient outcome? pointed out that humeral lateralization and distalization are commonly measured by angles: the LSA (lateralization shoulder angle) and the DSA (distalization shoulder angle) as shown below

It seems curious that distalization (a linear dimension) is being measured as an angle, rather than as a linear dimension (see yellow line) and

that laterialization (a linear dimension) is being measured as an angle, rather than as a linear dimension (see yellow line).

A recent article Reverse shoulder arthroplasty design inlay vs. onlay: does it really make a difference? emphasized the importance of distalization and lateralization to the outcome of reverse total shoulder and showed a similar method for the linear measurement of each of these linear dimensions. 

The authors point out that humeral distalization and lateralization depend on (a) whether the implant is "inlay" or "onlay" in design,

 (b) the placement of the humeral implant in the bone, 

and (c) the position and geometry of the glenosphere.

Fortunately, the readily available PACS (Picture Archiving and Communication System) makes measurement of distalization (white) and lateralization (yellow) on plan films quite straightforward.

These tools can make the same measurements on the preoperative film, so that the change in distalization and lateralization can be quantified. In this example, the reverse total shoulder medicalized the humerus by 1.9 mm and distalized in by 24 mm.

Making these linear measurements in a standard way sets the stage for assessing the effect of humeral lateralization and distalization on patient outcomes.

 Position is everything

Anna's Hummingbird

Anne's garden, Seattle

Spring 2021

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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).

Reverse total shoulder - avoiding excessive humeral lateralization

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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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).

Shoulder rehabilitation exercises (see this link).

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Note that author has no financial relationships with any orthopaedic companies