INNOVATIVE IMAGING TECHNIQUES TO ENHANCE THE QUALITY OF YOUR PRACTICE


maximum echogenicity when examining the palmar metacarpal flexor tendons in the transverse plane. When the US beam is truly perpendicular to the longitudinal axis of linear fibers within a normal tendon, maximum echogenicity is created. When the US beam is not perpendicular to a normal ten- don we can create decreased echogenicity in that structure. In contrast to normal tendon or liga- ment fibers, the echogenicity of fat and, to a lesser degree, muscle is not dependent on the angle of the US beam. The difference between the echogenicity of SL fibers versus the regions of fat and muscle becomes very apparent when the US beam is not perpendicular to the longitudinal axis of the SL fi- bers. This change in the position of the probe can be used to identify regions of fibers versus regions of fat and muscle in the SL. Normal SL fibers will be echogenic when the US


beam is perpendicular to the longitudinal axis of the fibers and will become hypoechogenic when the US beam is no longer perpendicular to the fibers. In contrast, regions of fat and, to a lesser degree, mus- cle will remain echogenic regardless of beam angle. Therefore, comparing the appearance of the SL with the beam both perpendicular and not perpendicular to the ligament allows identification of fibers versus regions of fat and muscle. Therefore, regions of mottling or decreased echogenicity identified in the SL with US can then be further investigated using changes in beam angle to determine if the source of the decreased echogenicity is ligament fibers or re- gions of fat and muscle. This technique provides a method for determining the actual tissue source, which allows us to determine if this is part of a normal pattern of the SL or a region of injury. Similar to the areas of fat and muscle, the echo- genicity of the connective tissue surrounding the SL is not beam angle dependent (remains echogenic, regardless of beam angle). Placing the US beam slightly oblique to the SL allows differentiation of the ligament margins from the surrounding echo- genic connective tissue.


7. Discussion


Ultrasound is an extremely useful tool for the diag- nosis of SL injury. Due to the complicated anatomy and location deep within the limb, ultrasound exam- ination of the SL often requires the clinician to go above and beyond the techniques that can be used to provide a diagnosis for the flexor tendons. In cer- tain cases, the mid to distal aspects of the SL body must be imaged from the plantar aspect of the limb, because at this location the medial aspect of the limb does not provide an adequate contact surface to al- low visualization of the ligament, even with a stand off pad. This is especially true when the ligament is focally affected with areas of enlargement and/or fiber abnormalities. As discussed above, the PSL is located adjacent to the fourth metatarsal bone. By using the DDFT as a window and directing the US beam dorsal and lateral, the entire SL can be


346 2014  Vol. 60  AAEP PROCEEDINGS


visualized as opposed to the planter technique with the limb in a weight bearing position where the ligament is partially obscured. However, the beam angle required for this technique is not ideal for the ligament-bone interface and for imaging the dorsal aspect of the ligament. The addition of the plantar approach with the limb in a nonweight bearing po- sition complements the medial approach with in- creased detail and places the ultrasound beam perpendicular to the dorsal aspect of the ligament as well as the bone ligament interface. These are im- portant advantages when using ultrasound to diag- nose SL injury. The hind SL changes shape more significantly


than the front SL. In addition, subtle changes in shape as a result of ligament injury are often visible before fiber abnormalities are detected. Therefore, comparison to the opposite limb is imperative. In addition, the size and shape of the fourth meta- tarsal bone changes dramatically at the level of the PSL. The size and shape of the fourth metatar- sal bone and its relationship to the third metatarsal bone can be used to ensure that the comparisons between the right and left hind limbs are being made at the same level. Abnormalities in the size, shape, margin, and echogenicity can all indicate abnormalities in the SL. Focal variations in the ligament echogenicity identified using the standard technique require fur- ther investigation with the limb in a nonweight bearing position. Re-examining these regions with the limb in a nonweight bearing position and vary- ing the US beam angles will establish if they are associated with SL fibers or regions of fat and mus- cle. Regions of marked injury in the SL fibers will have decreased echogenicity, regardless of beam an- gle. They will appear as decreased echogenicity with the beam angle not perpendicular to the fibers. Their appearance will remain unchanged as the beam is moved perpendicular to the longitudinal axis of the SL and the surrounding normal fibers become echogenic. In contrast, scarring will be echogenic, regardless of beam angle and can have associated ligament enlargement. The echogenic- ity of mature fibrous tissue is independent of the beam angle, creating its echogenic appearance de- spite changes in the beam angle. Often, injury to the SL alters the fat and muscle distribution and creates indistinct margins between regions of fibers and areas of fat and muscle. The regions of fat and muscle can become less evident with diffuse enlargement of the ligament and with focal regions of fiber injury and enlargement. Loss of the normal fat-muscle distribution in combination with abnormal fibers is often seen in conjunction with an SL injury. The margins of the SL should be closely examined.


This is best done with the limb in a nonweight bearing position and the US beam not perpendicular to the SL. Dorsal margin fraying or tears and focal areas of enlargement along the dorsal border of the


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