Development of an ultrasound-guided proximo-medial approach for injection of the carpal flexor tendon sheath in horses and comparison with the conventional proximo-lateral ´blind` technique.

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The aim of this prospective case-control study was to describe and implement a reliable, accurate, and straightforward ultrasound-guided proximo-medial approach technique for injecting the carpal flexor tendon sheath in horses. The proximo-medial ultrasound-guided approach for intrathecal access to the carpal canal, as described in this study, was firstly developed on ten cadaveric limbs using an undiluted ionic iodinated contrast medium. Landmarks were identified, and the access point was established. Subsequently, the technique was validated in ten horses, and its efficacy was compared with the proximo-lateral ´blind` technique. For each injection, the following was recorded: successful injection (contrast within the carpal canal), number of attempts, and whether a structure other than the carpal sheath was penetrated. The ´blind` technique had a total accuracy of 70 % (7/10; 95 % confidence interval [CI]: CI 35 %-93 %), with inadvertent injection into the dorso-palmar pouch of the radiocarpal joint, carpal canal surrounding soft tissue, and intravenous injection being the most common issues associated with this approach. In contrast, the ultrasound-guided approach achieved an accuracy of 100 % (10/10; 95 % confidence interval [CI]: 69 %-100 %) for intrasynovial administration of the contrast medium. The ultrasound-guided proximo-medial approach for injecting the carpal sheath is an accurate and technically straightforward technique. This approach should be considered for synoviocentesis of the carpal sheath, particularly in horses with no evident effusion.