In a recent article, Cooper Williams, VMD, DACVSMR, Certified ISELP Instructor, discussed dynamic imaging, taking advantage of the real-time nature of ultrasound combined with elucidation of structural function (Butterfly Equine Ultrasound Tips: Dynamic Functional Ultrasound Imaging). This article will discuss the ultrasound tool of static non-weightbearing or flexed imaging. This tool orients tissues/structures to optimize sonographic evaluation.
There are three different primary categories for use of this tool.
First Category: Joints
The first category involves exposing joint surfaces and associated structures not accessible in the standing limb. One of the best examples is the stifle. In the weightbearing hindlimb, when imaging the cranial aspect of the stifle, this is optimal for evaluating the trochlear ridges of the distal femur (subchondral bone surfaces) and the overlying cartilage surfaces. However, the stifle must be placed in flexion to allow access to the femoral condyles, the overlying cartilage surfaces, the cranial meniscal attachments on the tibial spine, and portions of the cruciate ligaments. It is not possible to scan these structures in the standing limb. The weightbearing portions of the ankle joints can also be oriented for evaluating subchondral bone and overlying cartilage surfaces with flexion. Portions of the weightbearing of the carpal bones can also be accessed in flexion. Obviously, range of motion can affect the amount of accessibility with any joint.
Second Category: Suspensory Ligament
The second category of utilizing the flexed limb involves the suspensory ligament. When imaging the palmar/plantar metacarpal/metatarsal soft tissues, the major soft tissue structures in this region—progressing from the palmar/plantar aspect cranially—are the superficial digital flexor tendon (SDFT), deep digital flexor tendon (DDFT), accessory ligament of the deep digital flexor tendon (AL-DDFT or inferior check ligament), and suspensory ligament (SL). In the standing limb they are essentially oriented in a columnar type alignment. The flexor tendons are not as wide as the cranial structures, and so externally this is the narrowest part of the limb. This creates air interference artifact, essentially preventing evaluation of a significant portion of both the proximal SL and the AL-DDFT. An interesting phenomenon occurs when the carpus or tarsus are flexed. The now relaxed SDFT and DDFT reorient from the columnar palmar-anterior position to a medial-lateral position, the SDFT medial to the DDFT. This now makes them wider than the cranial structures, allowing full visualization of the proximal SL and the associated bone structures. The bone surfaces include: the caudal aspect of the third metacarpus/metatarsus and the axial surfaces of the second and fourth metacarpal/metatarsal bones. The flexor tendons essentially form a natural standoff enhancement for these proximal structures. Ultrasound is the most sensitive tool we have for bone surface detail, so being able to evaluate these surfaces allows us to elucidate enthesopathies, enthesophytes, syndesmophytes, avulsion fractures, sequestra, and exostoses. Practitioners should never use the words “blind splint” again with ultrasound as the “seeing eye dog.” It is the author’s opinion that it is not a complete sonographic examination without non-weightbearing imaging of this region and that it should be considered “standard practice.”
Third Category: Structures Not Under Tension in Standing Limb
The final category for discussion of flexed or non-weightbearing imaging involves soft tissue structures not under tension in the standing limb. These include the following structures: lateral patellar ligament, gastrocnemius tendon, and the deep portions of the medial and lateral collateral ligaments of the tarsus. Because these structures are not under tension in the standing limb, a sonogram will show “sigmoid” fiber patterns that make it challenging to fully elucidate abnormalities. By placing the limb in a partially flexed or non-weightbearing position, these structures will then be under tension, allowing a true evaluation of their fiber pattern and density.
Take-Home Message
In summary, this review elucidates how essential non-weightbearing imaging is to the sonographer’s “tool chest.” If it is used routinely during sonographic surveys, it will become second nature toward a true complete examination.
About the Author
Cooper Williams, VMD, DACVSMR, Certified ISELP Instructor, grew up in Cecil County Maryland, where he learned to play polo and train polo ponies with his family. He ultimately obtained a five goal rating and played professionally for five years. In the summer of 1981, he was a member of the United States polo team that competed throughout England. He coached the Valley Forge Military Academy polo team from 1981 to 1985. In 1984, he graduated with honors from the University of Pennsylvania School of Veterinary Medicine. He accepted a post graduate internship at Delaware Equine Center, where he developed their ultrasound imaging program. In 1987, he moved to Maryland where he worked for two years at Maryland Equine Center, also developing their ultrasound program. In 1989, Williams started his own practice, focusing on diagnostics and sports medicine in equine athletes. After three years of rigorous study, learning in depth about advanced diagnostic imaging, Williams passed his examination and was certified by the International Society of Equine Locomotor Pathology (ISELP) in 2009 and has continued on as a certified instructor with ISELP, teaching other equine veterinarians all over the world. Cooper is one of a small group of veterinarians in the world who is certified by the International Society of Equine Locomotor Pathology in advanced ultrasound imaging. In August 2014, Williams successfully passed his examination and is officially a Diplomate of the American College of Veterinary Sports Medicine and Rehabilitation. There are only a small group of veterinarians in the world that now can claim they are a specialist in sports medicine and rehabilitation. He is an active member of the North American Regenerative Medicine Association as well as being on the veterinary advisory board for ACell, Inc., a tissue engineering company.
Williams is a paid advisor for Butterfly Network.
