Ultrasonography of the Carpus
University of Pretoria, Onderstepoort, South Africa
Introduction
Radiography is the modality most often used to evaluate the equine carpus, since most lesions in this area are bony in nature; however, radiologically evident soft tissue swelling is often associated with, and secondary to, bony carpal pathology, i.e. a mid-carpal joint effusion secondary to a distal radiocarpal chip fracture. Additionally, there is also often no radiological evidence of bony changes in the carpus, and ultrasonography lends itself to evaluate soft tissue changes. Cortical abnormalities are also amenable to ultrasonographic evaluation and particularly useful as a preliminary evaluation if radiological equipment is not handy.
Inspection, palpation, and flexion of the carpus are usually adequate to determine the anatomic structure/s affected in the presence of a pericarpal swelling, but perineural or intra-articular blocks may be required if the above are negative or equivocal. The carpus is a complex joint with multiple bones, ligaments, tendons, tendon sheaths, bursae, and three joints with recesses. To scan the entire carpus may be time consuming, and it is suggested that the area or areas identified clinically or with other modalities, such as radiography or scintigraphy, be ultrasonographically evaluated in detail and the rest of the carpus scanned in a more cursory manner if time is not available.
Carpal anatomy should be revised prior to ultrasonographic evaluation, if the ultrasonographer is unfamiliar with the area to be scanned.
A 7.5–13 MHz linear transducer is advised for evaluating the carpus, with or without a standoff pad, depending on the depth of the area to be evaluated. A split screen or C-scape modality can be used to make a composite image on the screen. The figures in this chapter include a view of the joint to show where the transducer is positioned.
Anatomy and Scanning Technique
Dorsal Carpus
The transducer is placed sagittally at the distal aspect of the radius cranially and moved distally, respectively evaluating the bones and superficial structures of the distal radius, the antebrachiocarpal joint (ACJ), the proximal row of carpal bones, the middle carpal joint (MCJ), the distal row of carpal bones, the carpometacarpal joint (CMCJ) and the proximal aspect of metacarpus 3 (MC3) (Figure 4.1). The surface of the bones should be smooth hyperechoic lines with elevations at the dorsal tubercles at implantation of the dorsal intercarpal ligaments. The hyperechoic cortical line will have a distinct interruption where the joint margin starts; angling the transducer will allow visualization of the most dorsal part of the articular surface. The approximately 1 mm thick dorsal intercarpal ligaments can be noted, running transversely immediately deep to the skin surface. Hereafter the transducer can be moved again to the distal aspect of the radius laterally or medially and the movement repeated until the entire dorsum is scanned. This is repeated with the transducer held in a transverse plane. In this way the distal cranial radius, dorsal aspect of the radial (RCB), intermediate (ICB), ulnar (UCB), second (C2), third (C3), and fourth (C4) carpal bones, the third metacarpal bone (MC3) and the three joints can be visualized. The tendons of the common digital extensor (CDET), extensor carpi radialis (ECRT), lateral digital extensor (LDET), and the abductor digiti longus (ADL) must also be examined, noting the size, echogenicity, and fiber alignment. A bursa (ECRB) is present under the implantation of the ECRT, but usually only visible when more than normal synovial fluid is present within. The tendon sheaths and joints are evaluated for the presence, echogenicity, and amount of fluid within and the joint capsule, the synovial membrane, and sheaths themselves are evaluated for thickening, abnormal echogenicity, or abnormal tissue.
A specific structure such as a single tendon and its sheath should be evaluated in full, noting the muscular portion as well as the implantation, if pathology in this structure is suspected.
Lateral Carpus
Again using a sequential sagittal and transverse scanning technique, starting slightly dorsolaterally, the lateral aspect of the carpus is evaluated, identifying the lateral styloid process, the lateral aspects of the ACJ, C4, MCJ, UCB, CMCJ, and MC4 (Figure 4.2). The CDET can be visualized and also the very thin muscle body. The hypoechoic cartilaginous or partially mineralized C5 may be noted although uncommonly seen. Further palmarly, the lateral collateral ligament provides passage for the LDET, between its long superficial and short deep parts. The main insertion of the ulnaris lateralis tendon (ULT) is on the proximal aspect of the accessory carpal bone (ACB). The long part of the ULT can be visualized where it inserts on proximal MC4 after running through a groove on the lateral part of the ACB. Again the size, echogenicity, and fiber alignment and sheaths of the tendons are examined. Further palmarly and proximal to the ACB, the musculotendinous junction of the deep digital flexor tendon (DDFT) can be seen next to the distocaudal radius. If there is an effusion in the palmar recess of the ACJ or very marked effusion in the carpal canal (CC), this may be seen between the radius and the DDFT.
Medial Carpus
Again using a sequential sagittal and transverse scanning technique, starting slightly dorsomedially, the medial aspect of the carpus is evaluated, identifying the medial styloid process, the medial aspects of the ACJ, RCB, MCJ, C2 (possibly also C1), CMCJ, MC2, both the long and short segments of the medial collateral ligament (MCL), and the flexor carpi radialis tendon (FCRT) (Figure 4.3).
The bellies and tendons of the superficial digital flexor muscle (tendon – SDFT) and DDFT can be seen best on the palmar view. The palmar proximal recess of the ACJ as well as the carpal canal may be visualized adjacent to the distal medial radius caudally if there is synovial distension.
Palmar Carpus
This aspect is actually approached as the mediopalmar to palmar aspect since the ACB curves around on the most lateropalmar aspect of the carpus providing the lateral border of the CC and partially providing the insertion of the retinaculum of the carpus (RetC) (Figure 4.4 and palmar images Figures 4.5 and 4.6). The CC extends from approximately 20 cm proximal to the ABCJ and to approximately 10 cm distal to the CMJ. The ULT can be viewed inserting on the proximal aspect of the accessory carpal bone (ACB). Within the carpal canal the muscular bodies of the tendons of the DDFT and SDFT are visualized from the level of the chestnut where they start becoming tendinous. Immediately proximal to the carpus the MA sends off the distal radial artery (DRA) and then enters the carpal canal, seen as an anechoic tubular structure. The accessory ligament of the SDFT (AL-SDFT) is seen as a homogeneous hyperechoic trapezoid structure (in the transverse plane) originating from the caudomediodistal aspect of the radius dorsomedial to the SDFT, thinning further distally and fusing to the SDFT tendon. The AL-SDFT is bordered medially by the FCRT, palmaromedially by the median artery (MA), vein, and nerve. Mediopalmar to the MA and the AL-SDFT is the flexor carpi ulnaris muscle (FCU). Medially to the SDFT is the muscle belly of the ulnar carpal flexor (tendon – FCUT) and caudolaterally to the DDFT is the UL muscle. The cephalic vein (CV) is situated medial to the RetC. The palmar carpal canal should be evaluated transversely and longitudinally. See Figures 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 4.10.
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