Which Pathology Comes First—Bone Loss or Fatty Infiltration of Rotator Cuff?

The relative roles of bones and muscles in joint pathologies are often difficult to tease apart. In the March 7, 2018 issue of The Journal, Donohue et al. report findings from their attempt to identify associations between preoperative fatty infiltration in rotator cuff muscles and glenoid morphology among 190 shoulders that underwent total shoulder arthroplasty (TSA) for glenohumeral osteoarthritis.

The painstaking analysis included orthogonal CT images to determine fatty infiltration, joint-line medialization assessments, direct measurements of glenoid version, and grading of glenoid morphology (from A1 through C2) using a modified Walch classification. Here’s what Donohue et al. found:

• High-grade posterior rotator cuff fatty infiltration was present in 55% of the 38 glenoids classified as B3, compared with only 8% fatty infiltration in the 39 A1-classified glenoids.
• Increasing joint-line medialization was associated with increasing fatty infiltration of all rotator cuff muscles.
• Higher fatty infiltration of the infraspinatus, teres minor, and combined posterior rotator cuff muscles was associated with increasing glenoid retroversion.
• After the authors controlled for joint-line medialization and retroversion, B3 glenoids were more likely than B2 glenoids to have fatty infiltration of the supraspinatus and infraspinatus.

The authors say these findings “support the idea that there is a causal association between rotator cuff muscle fatty infiltration and B3 glenoid morphology,” but they are quick to add that “from this study we cannot conclude [whether] these patterns of rotator cuff muscle fatty infiltration precede the progression of bone pathology, or vice versa.” Either way, these findings may inform patient-surgeon discussions about TSA, because both glenoid morphology and rotator cuff muscle quality are factors in glenoid-component longevity.