Demystifying the Equine Digital Tendon Sheath 

Dr. Elizabeth Acutt described how she uses angling and motion to improve her ultrasound images of the digital flexor tendon sheath.
veterinarian palpating horse's digital tendon sheath
Structures adjacent to the digital tendon sheath can also cause effusion within the sheath. | Getty Images

During a Burst Session at the 2023 AAEP Convention in San Diego, Elizabeth Acutt, DVM, Dipl. ACVR-EDI, assistant professor of clinical large animal diagnostic imaging at the University of Pennsylvania School of Veterinary Medicine, described how she uses a bit of angling and motion to improve her ultrasound images of the digital flexor tendon sheath.  

On-Angle and Off-Angle Imaging

“On-angle and off-angle imaging refers to the angle at which the ultrasound beam hits the structures of interest. In this case, the tendons,” she explained. “When you scan with the probe perpendicular to the limb and to the tendons, you produce an ‘on-angle’ image in which the tendons appear bright (hyperechoic). When you angle the beam by tilting the probe slightly upward or downward, this generates an ‘off-angle’ image in which the tendons appear dark (hypoechoic) with clearly defined margins. Angling helps identify subtle changes at the periphery of the tendon, such as the dorsolateral border tears of the deep digital flexor tendon.”  

Visualizing Tears

She added, “The reason that off-angle imaging is good for visualizing manica flexoria tears is because the manica appears hyperechoic on off-angle imaging relative to the deep digital flexor tendon, which it surrounds, making it easier to delineate between the structures and to trace the intact manica around the margins of the deep digital flexor tendon.” 

Another lesion that can be easy to miss with classic imaging techniques is a dorsolateral border tear of the deep digital flexor tendon in the proximal pastern region. These can be difficult to see but are important causes of pain, lameness, and tenosynovitis.  

“Swing your probe out medially and laterally, and interrogate those margins to get visualization of the dorsomedial and dorsolateral borders of the deep digital flexor tendon in the proximal pastern,” Acutt said. 

Digital Tendon Sheath Anatomy

She also advised practitioners to appreciate that there are multiple attachments between the deep digital flexor tendon and the sheath wall. These are not adhesions, she warned, because adhesions are usually more subtle.  

“And remember that structures adjacent to the sheath can also cause effusion within the sheath. Examples include the oblique and straight sesamoidean ligaments as well as the sesamoid bones themselves,” said Acutt.  

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