"Subacromial space" - what does it mean?

Subacromial Space Width: Does Overuse or Genetics Play a Greater Role in Determining It? An MRI Study on Elderly Twins

These authors point out that age and peripheral microcirculation disorders are the main causes of rotator cuff degeneration. They performed an MRI (magnetic resonance imaging) study with the aim of evaluating the acromiohumeral distance in a group of elderly monozygotic and dizygotic twins, and analyzed the data using the twin design to separate the contributions of shared and unique factors.

They identified twenty-nine pairs of elderly twins. On MRI scans, they evaluated the acromiohumeral distance and health status of the rotator cuff tendons. The acromiohumeral distance was calculated in the coronal oblique projection as the distance between the most caudal point of the lower surface of the acromion and the most cranial point of the proximal aspect of the humerus.

They found that monozygotic twins (pairs of individuals who shared 100% of their genes) had a high degree of concordance of the acromiohumeral distance.  The intraclass correlation coefficient was substantially higher for monozygotic than for dizygotic twins. There were no significant differences among subjects in different job categories with different degrees of labor, either in the total cohort or within the monozygotic and dizygotic groups.

Comment: This is an interesting study indicating that the acromiohumeral distance is mainly genetically determined and only marginally influenced by external factors. 

It is important to recognize that the subacromial "space" is not a space at all. Rather, as the MRI below shows, the distance between the upper humeral head and the undersurface of the acromion is occupied primarily by (1) the supraspinatus tendon and (2) the articular cartilage on the superior aspect of the humeral head. When the distance between the upper humeral head and the undersurface of the acromion is narrowed it indicates thinning of the supraspinatus tendon or the humeral articular cartilage or both.

When there is no interposed cartilage or tendon, the two bones come into contact

While in that past it has been stated that "a pathological narrowing of the subacromial space that may compromise the cuff integrity", we now recognize that the narrowing of the distance between the humeral head and the acromion is the result rather than the cause  of cuff degeneration.

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