Sonoanatomy of the Finger Synovio-Entheseal Complexes

The objectives of this ultrasound and anatomical study were: (1) To evaluate the reliability of ultrasound identification of the enthesis of the central slip of the extensor digitorum tendon (CSET) using cadaver specimens; (2) To assess the concordance of the measurement of the CSET thickness by ultrasound and gross anatomy; (3) To evaluate the variation in ultrasound identification of the CSET enthesis in a cadaveric experimental model of PIP synovitis.

Four rheumatologist ultrasonographers blindly and independently measured by ultrasound the CSET enthesis thickness in the second to fifth fingers of 8 hands from fresh-frozen human cadavers in two rounds. These fingers were dissected and the thickness of the CSET measured by the anatomist. In addition, an artificial synovitis was created in the proximal interphalangeal (PIP) joints of a different cadaveric hand. The ultrasonographic CSET enthesis thickness was measured by the four investigators before and after intra-articular ultrasound guided injection of material.

Intra- and inter-observer reliability of CSET enthesis thickness measurement were good to excellent (ICC 0.93 for intra-observer agreement and 0.89-0.92 for inter-observer agreement). Ultrasound measurements were consistent and only slightly smaller than the anatomical ones (µ = -0.039 mm). The differences between the measurements of CSET enthesis thickness before and after the synovitis model were not statistically significant.

Ultrasound demonstrated high multiobserver reliability and agreement with gross anatomy in identifying the CSET enthesis and discriminated it from the capsular tissue of the PIP. Furthermore, an experimental model of PIP synovitis did not interfere with its identification.