Background
Atrophy and fatty degeneration of the rotator cuff muscles were often assessed statically using Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). However, the muscles are essentially power sources and their dynamic evaluation is left to be considered. We devised a new method for dynamic assessment of the supraspinatus (SSP) using ultrasonography (US). To establish this new method the validation of the assessment is needed. The objectives of this study were 1) to clarify the anatomical structure shown by US as bright speckles in muscles, and 2) to compare the moving distances of the speckle and located marker detected on US in the passive SSP movement of cadavers.
Method
1) Edible pork (belly) with fat on the surface was observed by US, and the gross findings were contrasted with the US images.
2) Both shoulders of one cadaveric specimen without rotator cuff tears were used. At first, small rotator cuff tears were created in both shoulders. Second, through 5cm skin incision at the midpoint between the acromion and the medial edge of the supraspinous fossa, the trapezius muscle was detached from the scapular spine and the muscle belly of the SSP was exposed. A suture was looped around the muscular tissue of the SSP as a marker of the actual muscle movement. After the trapezius repair the wound was closed, and the US probe was applied in the longitudinal direction of the SSP. The edge of the rotator cuff tear was manually pulled by 0.5 cm and 1.0 cm. The speckle showing the longest moving distance on the US screen was identified and measured its moving length. Then the moving length of the marker was measured and their distances were compared.
Results
1) The fat layer was hypoechoic. Although it was hard to see grossly from the surface, US showed two layers of muscles. After detachment of the fat the two shallow and deep layers of muscle were confirmed grossly. When The US probe was directly applied in the same direction as the muscle bundle, bright stripes were depicted on US. When the probe was moved and the muscle bundles and probe were not perfectly parallel, the stripe pattern easily varied into dotted speckles.
2) The moving distances of the speckle and marker during traction were recorded as follows. The speckle/marker distances for the right shoulder were 0.21cm/0.14cm and for the left shoulder 0.24cm/0.26cm in 0.5cm traction. At traction distance of 1.0 cm, the speckle/marker distances were 0.42cm/0.42 cm for the right shoulder and 0.51 cm/0.52 cm for the left shoulder.
Discussion
In the first study, the stripe pattern on US was thought to be the epimysium surrounding the muscle bundle. When the muscle bundle was not perfectly parallel to the US probe, the epimysium was depicted as dotted speckles. If the muscle moves along the US probe, theoretically the speckle can be followed endlessly. Considering that the muscle should contract in the direction of the muscle bundle and the speckle pattern indicates that its muscle bundle was not parallel to the US probe, the muscle motion is not parallel to the US probe and we cannot follow the speckle endlessly. But the speckle showing the longest moving distance should mean that its muscle bundle is located most parallel to the probe, and the distance should be close to the actual muscle movement.
The second study showed that the distances of the speckle were almost same as the SSP marker during traction of the rotator cuff-edge, therefore, the speckle with maximum movement distance was thought to be as an indicator of the extent of the SSP movement.