Objectives. Although many clinical and experimental investigations have shed
light on muscle atrophy and intramuscular accumulation of fat after
rotator cuff disruption, none have reported on their onset in the
absence of muscle retraction. Methods. In 30 rabbits, we detached one supraspinatus (SSP) tendon and
repaired it immediately, thus preventing muscle retraction. The
animals were killed in groups of 10 at one, two and six weeks. Both
shoulders of 15 non-operated rabbits served as controls. We measured
the weight and volume of SSP muscles and quantified the cross-sectional
area of intramuscular fat (i-fat) histologically. Results. There was significant loss of muscle weight and volume after
one week (p = 0.004 and 0.003, respectively), and two weeks (both
p <
0.001) in the experimental group; which recovered to control
values after six weeks. I-fat accumulated one week after immediate repair,
greater than in the control group and statistically significant
at the mid-part of the muscle (mean 2.7% vs 1.5%,
p = 0.008). I-fat continued to accumulate up to six weeks at all sites
of the SSP muscle (all 3, p <
0.001). More fat accumulated closer
to the musculotendinous junction than at the mid-part after two
and six weeks (p = 0.012 and 0.019, respectively). Conclusion. Muscle atrophy and i-fat accumulation occur early after SSP tendon
tear and immediate repair. While early repair benefitted muscle
recovery, it did not prevent fat accumulation. SSP muscle retraction
was not essential to the muscle alterations. The divergent evolution
of muscle and fat points to different
