Monday, July 18, 2016

The B2 glenoid

Management of the Biconcave (B2) Glenoid in Shoulder Arthroplasty: Technical Considerations.

These authors point out that severely biconcave arthritic glenoid pathoanatomy can create major technical challenges in shoulder arthroplasty and jeopardize the longevity of prosthetic glenoid components.  They review different strategies for managing this anatomy, ranging from eccentric reaming and total shoulder arthroplasty, to posterior glenoid bone grafting, to posteriorly augmented implants and to  reverse shoulder arthroplasty.

Comment: As we consider the management of various types of glenoid pathoanatomy it is important to recognize that there is a continuum in "B" glenoids - between those that have minimal biconcavity to those that have severe biconcavity as shown in the diagram below. In fact as our recent posts show, there are many types and many shades of glenoid pathoanatomy.

In our experience, every shoulder with posterior humeral decentering has some degree of biconcavity, so maybe there is no such thing as a pure B1.

One of the interesting things that can be observed from this paper (Figure 3) is that the position of the humeral head relative to the glenoid articular surface depends on the position of the arm when the image is obtained. Note that in the CT scan below taken with the arm at the side, the head is relatively centered in the glenoid and not resting in the posterior pathologic concavity.

 However, when the arm was abducted to obtain the axillary view, the humeral head falls in to the posterior concavity as shown below.

For that reason we make a point of taking the axillary view when the arm in the functional position of elevation in the plane of the scapula, what we refer to ask the 'truth view'. See this link.

Another interesting observation in this paper (Figure 5) is the possibility of overcorrecting posterior subluxation. The preoperative view shows posterior subluxation into a pathologic posterior concavity.
 The post operative view shows the humeral head to be anteriorly decentered on the glenoid.

An alternative approach to the description of glenohumeral pathoanatomy can be based on three parametric measurements:

(1) The percent of the glenoid surface that has a pathologic biconcavity (33% posterior in the example below).

 (2) The angle of retroversion of the glenoid face (G) in relation to the scapular body (S)

(3) The centering of the humeral head with respect to the glenoid  (the distance between the anterior glenoid lip and the center of glenohumeral contact (C) divided by the distance between the anterior and posterior glenoid lips (G)).  0.5 indicates a centered humeral head.
Using this system, 
the glenoid below would be 50% anterior biconcavity, 10 degree retroversion, and decentering of .25 (subluxation).

the example glenoid below would be 0 biconcavity, 40 degree retroversion, and centering of .5. 

the example  glenoid below would be 0 biconcavity, 15 degree retroversion, and centering of .5. 

Such a system can provide the information necessary for characterizing the pathology and for planning treatment.


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