Ann Carstens1 and Sheelagh Higgerty2 1 University of Pretoria, Onderstepoort, South Africa 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 middle 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 to hand. 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. It is also advisable to have a mounted bone specimen available to assist in determining precisely where one is scanning. 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 either a bony specimen or a radiograph to show where the transducer is positioned. 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 then 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) and the extensor carpi radialis (ECRT) must also be examined, noting the size, echogenicity, and fiber alignment. A bursa of the extensor carpi radialis (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 growths. The abductor digiti 1 longus (ADL) body and insertion can be visualized, but has not been described to be of clinical significance. Figure 4.1 Dorsal carpus illustrating the surface of the dorsal joint spaces and tendons of the common digital extensor and extensor carpi radialis muscles. Longitudinal split screen composite image of the dorsal mid-carpus showing the antebrachiocarpal joint (ACJ), the middle carpal joint (MCJ), the carpometacarpal joint (CMCJ), and the tendon of the common digital extensor (CDE)(*). Proximal is to the left. (C) Longitudinal split screen composite image of the dorsal mid-medial aspect of the carpus showing the ACJ, the MCJ, and the CMCJ, and the tendon of the extensor carpi radialis (ECR)(*) attaching to the medial tuberosity of the proximal MC3; note the hypoechoic villous structures within the ACJ and the anechoic fluid within the MCJ. Proximal is to the left. (E) Transverse images showing the dorsal hyperechoic surfaces of the intermediate carpal bone (ICB) and the radiocarpal bone (RCB) and transverse section through the ECR tendon (ECRT)(**). Lateral is to the left. (F) Transverse image showing the dorsal hyperechoic surfaces of fourth and third carpal bones (C4 and C3) and transverse section through the CDET(**); note the intercarpal ligament between C4 and C3 (*). Lateral is to the left. If pathology in a specific structure, such as a single tendon and its sheath, is suspected, it should be evaluated in full. For example, for a tendon and its sheath, noting the muscular portion as well as the implantation is advised. 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, and the lateral aspects of the ACJ, C4, MCJ, UCB, CMCJ, and MC4 (Figure 4.2). The hypoechoic cartilaginous or partially mineralized C5 may be noted, although uncommonly seen. Further palmarly, the lateral collateral ligament provides passage for the lateral digital extensor (LDET), between its long superficial and short deep parts (Figure 4.2). Figure 4.2 Lateral carpus illustrating the surface of the lateral joint spaces, the lateral collateral ligament, and the tendon of the lateral digital extensor muscle. (A) Anatomical specimen indicating transducer placement. (B) Longitudinal combined split screen images of the lateral carpus with the superficial (sup) and deep parts of the lateral collateral ligament and the lateral digital extensor tendon (LDET) (*) between them. Proximal is to the left. (C) Obliquely orientated longitudinal split screen image of the lateral to dorsolateral carpus of the LDET (*) to just before it joins the common digital extensor tendon between them. Proximal is to the left. ACJ: antebrachiocarpal joint; C4: fourth carpal bone; CMCJ: carpometacarpal joint; MCJ: middle carpal joint; RCB: radiocarpal bone; UCB: ulnar carpal bone. Some of the ligaments associated with the accessory carpal bone can also be visualized scanning palmarolaterally from the accessory carpal bone to C4 and MC4. (Figure 4.3) Figure 4.3 Lateral carpus illustrating two ligaments associated with of the accessory carpal bone. (A) Anatomical specimen indicating transducer placement. (B) Longitudinal image of the lateral carpus showing the accessorioquartal ligament extending from the ACB to C4 (*) and the more superficial accessoriometacarpal ligament extending from the ACB to MC4 (**). Proximopalmar is to the left. C4: fourth carpal bone; MC4: fourth metacarpal bone. Whereas the main insertion of the ulnaris lateralis tendon (ULT) is on the proximal aspect of the accessory carpal bone (ACB), its extension (long part) can be visualized where it inserts on proximal MC4 after running through a groove on the lateral part of the ACB (Figure 4.4). Figure 4.4 Lateral carpus illustrating the ulnaris lateralis tendon. (A) Anatomical specimen indicating transducer placement. (B) Longitudinal split screen image of the lateral carpus indicating the extension of the long part of the ulnaris lateralis tendon (ULT) (**) from the groove on the lateral aspect of the ACB distally to its distal insertion on MC4. Proximal is to the left. ACB: accessory carpal bone; C4: fourth carpal bone; CMCJ: carpometacarpal joint; MCJ: middle carpal joint; RCB: radiocarpal bone; UCB: ulnar carpal bone. 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. While 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 insertion of the obliquely scanned abductor digiti 1 longus tendon on the head of the MC2 after passing over the tendon of the extensor carpi radialis tendon (ECRT) (Figure 4.5). Figure 4.5 Medial carpus illustrating the medial collateral ligament and the abductor digiti 1 longus tendon. (A) Anatomical specimen indicating transducer placement. (B) Longitudinal split screen image of the medial carpus illustrating the medial collateral ligament in its entirety; note the long (superficial) fibers (*) extending to MC2 and the short (deeper) fibers (**) attaching to the RCB and C2. Proximal is to the left of the image. (C) Longitudinal scan of the dorsomedial aspect of the carpometacarpal joint, showing the insertion of the obliquely scanned abductor digiti 1 longus tendon (arrow) on the head of the MC2, which has an irregular periosteal reaction thereon, as has the dorsum of C2. Proximal is to the left. ACJ: antebrachiocarpal joint; C2: second carpal bone; ECRT: extensor carpi radialis tendon; MC2: second metacarpal bone; RCB: radiocarpal bone; CMCJ: carpometacarpal joint; MCJ: middle carpal joint; RCB: radiocarpal bone. The muscle bellies and tendons of the superficial digital flexor muscle (tendon – SDFT) and DDFT can be best seen 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. 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 carpal canal (CC) and partially providing the insertion of the retinaculum of the carpus (RetC). The CC extends from approximately 20 cm proximal to the ACJ 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 median artery (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 an 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, and palmaromedially by the 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 (flexor carpi ulnaris 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 4.6, 4.7, 4.8, and 4.9). Figure 4.6 Caudal distal radius and proximal palmar carpus illustrating the carpal canal. (A) Anatomical specimen indicating transducer placement. (B) Caudal transverse ultrasound image at level of distal radius; note the median artery (*), the retinaculum of the carpal canal (open arrow head), and the median nerve (white arrow). Medial is to the left. (C) Palmaromedial longitudinal split screen ultrasound image showing superficial and deep digital flexor tendon musculotendinous junctions immediately proximal to the accessory carpal bone (ACB). Proximal is to the left. AL-SDFT: accessory ligament of the superficial digital flexor tendon; CV: cephalic vein; DDFT: deep digital flexor tendon; SDFT: superficial digital flexor tendon; MA: median artery; FCU: flexor carpi ulnaris; RA: radial artery; UL: ulnaris lateralis. Figure 4.7 Carpal canal over the caudomedial distal radius illustrating the accessory ligament of the superficial digital flexor tendon. (A) Anatomical specimen indicating transducer placement (angled slightly craniolaterally. (B) Medial longitudinal ultrasound image showing the median artery and accessory ligament of the superficial digital flexor tendon (AL-SDFT). Proximal is to the left. ACB: accessory carpal bone; MA: median artery; RA: radial artery; RetC: retinaculum of the carpal canal. Figure 4.8 Caudomedial distal radius illustrating the tendons of the SDFT, FCR, FCU, and the AL-SDFT. (A) Anatomical specimen indicating transducer placement (angled slightly craniolaterally). (B) Transverse scan of the caudomedial aspect of the distal radius, showing the tendons of the SDFT, FCR, FCU, and the AL-SDFT (arrow); in this region, the FCU tendon starts to blend with the carpal retinaculum (*). Medial is to the left. AL-SDFT: accessory ligament to the superficial digital flexor tendon; FCR: flexor carpi radialis; FCU: flexor carpi ulnaris; SDFT: superficial digital flexor tendon.
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Ultrasonography of the Carpus
2 Crowthorne Veterinary Clinic, Kyalami, South Africa
Introduction
Anatomy and Scanning Technique
Dorsal Carpus
Lateral Carpus
Medial Carpus
Palmar Carpus
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