Gibbons MC, Sato EJ, Bachasson D, Cheng T, Azimi H, Schenk S, Engler AJ, Singh A, Ward SR.
Rotator cuff (RC) tendon tears lead to negative structural and functional changes
in the associated musculature. The structural features of muscle that predict
function are termed "muscle architecture." Although the architectural features of
"normal" rotator cuff muscles are known, they are poorly understood in the
context of cuff pathology. The purpose of this study was to investigate the
effects of tear and repair on RC muscle architecture. To this end thirty
cadaveric shoulders were grouped into one of four categories based on tear
magnitude: Intact, Full-thickness tear (FTT), Massive tear (MT), or Intervention
if sutures or hardware were present, and key parameters of muscle architecture
were measured. We found that muscle mass and fiber length decreased
proportionally with tear size, with significant differences between all groups.
Conversely, sarcomere number was reduced in both FTT and MT with no significant
difference between these two groups, in large part because sarcomere length was
significantly reduced in MT but not FTT. The loss of muscle mass in FTT is due,
in part, to subtraction of serial sarcomeres, which may help preserve sarcomere
length. This indicates that function in FTT may be impaired, but there is some
remaining mechanical loading to maintain "normal" sarcomere length-tension
relationships. However, the changes resulting from MT suggest more severe
limitations in force-generating capacity because sarcomere length-tension
relationships are no longer normal. The architectural deficits observed in MT
muscles may indicate deeper deficiencies in muscle adaptability to length change,
which could negatively impact RC function despite successful anatomical repair.
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