Extra- and intramuscular fat accumulation early after rabbit supraspinatus tendon division: depiction with CT

To quantify the accumulation, progression, and distribution of fat separately in and around the supraspinatus muscle from the onset of tendon detachment and to validate computed tomography (CT) for quantification of fat content by using volumetry and histomorphometry as reference standards.

Institutional animal care committee approval was obtained. The supraspinatus tendon of 30 adult female rabbits (3.0 kg) was detached. Rabbits were sacrificed in groups of 10 after 4, 8, and 12 weeks. Fifteen rabbits that did not undergo surgery served as control subjects. Fat accumulations in and around the proximal, middle, and distal supraspinatus muscle were quantified, volumetrically and histologically, and were compared with extramuscular presence and intramuscular attenuation at CT. Weight, volume, histologic findings, and CT determinations of fat were compared by using one-way analysis of variance. The Pearson test was used to correlate intramuscular fat accumulation with CT observations.

Four weeks after tendon detachment significant extra- and intramuscular fat had accumulated (both P < .05) and progressed during 12 weeks, with accumulation of extramuscular fat being three times greater than that of intramuscular fat (both P < .05). An increasing proximal-to-distal gradient existed for both extra- and intramuscular accumulation (both P < .05). CT depicted the onset, progression, and gradient of extra- and intramuscular fat accumulation (all P < .05). CT attenuation correlated strongly with histologic findings (P < .05).

Fat accumulated early, along an increasing proximal-to-distal gradient, and progressed with time after supraspinatus tendon detachment. CT proved to be a valid tool for monitoring the onset, progression, and gradient separately for extra- and intramuscular fat accumulation.