Tears or artifacts? – comparison between normal ultrasound and macro-anatomy of the medial femorotibial meniscus
Introduction: Interpretation of transverse hypoechoic lines seen during ultrasonography of medial menisci is challenging as their cause is not fully understood. The aim of this study was to characterize the normal acoustic anatomy of the medial equine meniscus. Materials and Methods: Ten non-lame adult warmblood horses without anamnestic, clinical, radiographic and ultrasonographic signs of lameness associated to the stifle were euthanized for reasons unrelated to the current study. Prior to death medial femorotibial joints were examined with ultrasound in a coronal plane where the medial collateral ligament adheres to the meniscus. Post mortem photographs of corresponding coronal meniscal sections were made. Hypoechoic areas were labelled on digital ultrasound images and laid over corresponding coronal images of menisci specimen. Correlation of hypoechoic regions with structural elements of the menisci was evaluated. Results: On most ultrasound images 2-3 larger, wedge or club-shaped hypoechoic regions were detected distally in the meniscus, increasing in width from medial to lateral. Their medial, narrow origin mostly correlated to prominent T-shaped junctions between the fibrous meniscal capsule and radially orientated tie-fibers formed by loose connective tissue. More laterally anechoic areas became wider than the tie-fibers and did not correlate to their shape. Discussion/Conclusion: Direct comparisons between normal anatomic structures of equine menisci and corresponding ultrasound images have not been made to date. Anechoic linear areas in normal menisci interpreted as artifacts are potentially caused by refraction of ultrasound at the interface between meniscal capsule and radially orientated tie-fibers. Histology is essential to further characterize causative structures.