The acoustic properties of the supraspinatus tendon insertions of three adult Japanese white rabbits (male, 3.2–3.5 kg) were measured by scanning acoustic microscopy (SAM). After cutting parallel to the supraspinatus tendon fibers, specimens were fixed with 10% neutralized formalin, embedded in paraffin and sectioned at 5 μm thickness. The sound speed and the attenuation constant along the articular surface of the tendon from the insertion site to the musculotendinous junction were measured by SAM. The two-dimensional distribution of the acoustic properties was displayed with color-coded scales and was compared to the histologic structure. Toluidine-blue staining and immunohistochemical staining against type II collagen were performed to determine the presence of fibrocartilage besides a standard Elastica–Masson staining to assess the overall histologic structure. The distribution of the sound speed and the attenuation constant showed an almost identical pattern. Both parameters were lower in the non-mineralized fibrocartilage than those in the tendon proper. The area indicating low sound speed corresponded well to the area of metachromasia and type II collagen. The non-mineralized fibrocartilage indicated softer material properties than the tendon proper, which could be interpreted as an adaptation for the various biomechanical stresses at the insertion site.
