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Beth Israel Deaconess Medical Center Harvard Medical School Boston, MA 02215
The informative article by Tung and Hou [1] concludes that "clinical posterior instability is associated with excessive posterior humeral translation." The authors show that on axial MR images of patients with posterior labrocapsular tears, the position of the humeral head relative to the glenoid fossa is displaced posterior to the plane of the scapular body. A normal labrocapsular apparatus and rotator cuff would be expected to overcome the small gravitational forces displacing the humeral head posteriorly in the supine position.
This sign of glenohumeral subluxation on MR arthrography described by Guntern et al. [2] is presumably less accurate if the examination is performed immediately after injury. In these circumstances muscle tone can be temporarily diminished, as shown by electromyographic measurements and by the inferior humeral head subluxation on standing shoulder radiographs that is commonly shown after a proximal humeral fracture.
The dynamic assessment of glenohumeral laxity at fluoroscopy during shoulder arthrography is far more sensitive than is the passive MR imaging described by Tung and Hou [1]. My experience is that fluoroscopic spot radiographs obtained at preliminary arthrography show lesions of the articular cartilage of the humeral head better than MR imaging does [2]. Because most arthrograms are obtained under fluoroscopic guidance, with intraarticular position documented by the injection of positive contrast medium and gadolinium, it is a simple matter to make these observations, as well as the more traditionally arthrographic assessments of extravasation and joint capacity.
In summary, MR arthrographic examinations are optimally interpreted in conjunction with the preliminary fluoroscopic arthrographic examination. Obviously, this correlation is facilitated in a PACS (picture archiving and communication system) environment and when both examinations are interpreted by the same person or persons.
References
Brown University Rhode Island Hospital Providence, RI 02903
I thank Dr. Hall for his interest in our work [1]. Although we correlated findings on MR arthrography with the shoulder physical examination for posterior instability, we did not compare findings on any dynamic examination of the shoulder joint at the time of contrast instillation with findings at subsequent MR imaging. Furthermore, I am not aware of any publication that specifically does so, and therefore I cannot comment about the relative sensitivity of these two assessments.
For many radiologists performing MR arthrography, the procedure of instilling contrast medium into the joint is intended only to deliver the dilute gadolinium chelate contrast solution into the joint space as a prelude to MR imaging. However, Hall makes an important point: shoulder MR arthrography allows an excellent examination of joint anatomy and provides an opportunity to dynamically evaluate glenohumeral joint stability as well.
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  F. M. Hall, B. Mengiardi, and M. Zanetti Frozen Shoulder * Drs Mengiardi and Zanetti respond: Radiology, May 1, 2005; 235(2): 713 - 714. [Full Text] [PDF]
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