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Abstract
BackgroundIt seems appropriate to assume, that for a full and strong global shoulder function a normally innervated and active deltoid muscle is indispensable. We set out to analyse the size and shape of the deltoid muscle on MR-arthrographies, and analyse its influence on shoulder function and its adaption (i.e. atrophy) for reduced shoulder function.MethodsThe fatty infiltration (Goutallier stages), atrophy (tangent sign) and selective myotendinous retraction of the rotator cuff, as well as the thickness and the area of seven anatomically defined segments of the deltoid muscle were measured on MR-arthrographies and correlated with shoulder function (i.e. active abduction). Included were 116 patients, suffering of a rotator cuff tear with shoulder mobility ranging from pseudoparalysis to free mobility. Kolmogorov-Smirnov test was used to determine the distribution of the data before either Spearman or Pearson correlation and a multiple regression was applied to reveal the correlations.ResultsOur developed method for measuring deltoid area and thickness showed to be reproducible with excellent interobserver correlations (r = 0.814–0.982).The analysis of influencing factors on active abduction revealed a weak influence of the amount of SSP tendon (r = −0.25; p < 0.01) and muscle retraction (r = −0.27; p < 0.01) as well as the stage of fatty muscle infiltration (GFDI: r = −0.36; p < 0.01). Unexpectedly however, we were unable to detect a relation of the deltoid muscle shape with the degree of active glenohumeral abduction. Furthermore, long-standing rotator cuff tears did not appear to influence the deltoid shape, i.e. did not lead to muscle atrophy.ConclusionsOur data support that in chronic rotator cuff tears, there seems to be no disadvantage to exhausting conservative treatment and to delay implantation of reverse total shoulder arthroplasty, as the shape of deltoid muscle seems only to be influenced by natural aging, but to be independent of reduced shoulder motion.
Highlights
It seems appropriate to assume, that for a full and strong global shoulder function a normally innervated and active deltoid muscle is indispensable
The analysis revealed an excellent correlation for the measurements of deltoid thickness (r = 0.847 – 0.921), and for the measurements of areas and pars (r = 0.814 – 0.924), with the best correlation for the total deltoid area (r = 0.982)
The analysis of influencing factors on active abduction revealed a weak influence of the amount of SSP tendon (r = −0.25; p < 0.01) and muscle retraction (r = −0.27; p < 0.01) as well as the stage of fatty muscle infiltration (GFDI: r = −0.36; p < 0.01)
Summary
It seems appropriate to assume, that for a full and strong global shoulder function a normally innervated and active deltoid muscle is indispensable. Pseudoparalysis is associated with mostly large rotator cuff tears, it is an inconsistent finding in those. In the rotator cuff deficient shoulder the most obvious influence on glenohumeral motion is the deltoid muscle. Its unimpaired innervation and function is a prerequisite when considering a reverse total shoulder arthroplasty (RTSA) as treatment of chronic rotator cuff deficient shoulders. The rotator cuff supports elevation/abduction and stabilizes the humeral head within the glenoid, creating a fulcrum on which the deltoid can lever to elevate the arm. In cases of massive tears of the rotator cuff, the stabilizing function of the rotator cuff is lost and the deltoid muscle contraction may lead to an anterior-superior humeral head translation and subluxation [3]
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