A glenohumeral joint effusion is not usually appreciable on plain film examination unless there is a very large amount of fluid or a lipohaemarthrosis present; ultrasound is the modality of choice for diagnosis and quantification. Given the overlying musculature of the rotator cuff and the absence of readily visible fat planes, abnormal fluid collection is usually only identified by the presence of a pseudosubluxation.
pseudosubluxation secondary to lipohaemarthrosis; hover to reveal annotations
Source: Radiopaedia
'Subluxation' refers to the partial incongruity of the two articular surfaces, as opposed to dislocation which refers to complete incongruity.
'Pseudosubluxation' in the context of shoulder imaging refers to apparent incongruity of the glenohumeral joint secondary to a pathological process (such as effusion) mimicking a true dislocation.
A lipohaemarthrosis of the shoulder appears similar to one of the knee; it indicates an intracapsular bony injury and appears as a fat-fluid line. Do note that the patient must be erect for the fat-fluid line to become evident.
While reporting, the shortest distance between the cortical bone of the inferior aspect of the acromion process and the cortex of the superior humeral head is measured; this is known as the acromiohumeral interval. This measurement relies on a reasonably standard AP projection of the shoulder – lower centred examinations of the humerus may artificially reduce this interval due to the divergent beam. An acromiohumeral interval of greater than 12mm indicates inferior subluxation of the humeral head and should raise the possibility of an effusion.
PRACTICAL POINT: If misalignment of the glenohumeral joint is visible on the AP projection, a second projection MUST be supplied. Pseudosubluxation must be differentiated from actual dislocation and indirect signs on the AP, such as humeral head rotation or empty glenoid are generally not sufficiently reliable to indicate one over the other. If a pseudosubluxation is mistaken for a dislocation, medical staff may erroneously try to relocate a shoulder which is not dislocated, causing unnecessary suffering for the patient and potentially causing further injury.
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