How to Take Radiographs of the Metacarpophalangeal/Metatarsophalangeal Joint (Fetlock Joint)

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1 How to Take Radiographs of the Metacarpophalangeal/Metatarsophalangeal Joint (Fetlock Joint) Joseph W. Morgan, DVM, Diplomate ACVS Author s address: 5366 Leestown Road, Lexington, KY ; jmotrackvet@ yahoo.com AAEP. 1. Introduction The fetlock joint is one of the most common radiographed areas of the horse. It is a relatively simple joint made up of the third metacarpal/metatarsal bone (cannon bone), the proximal phalanx (P1), and the paired proximal sesamoid bones. To accurately assess pathology within the joint, quality radiographic images must be obtained. 2. Materials and Methods Currently in the author s practice, digital radiography is used exclusively. The techniques will vary, depending on which system being used (digital, computed, or plain radiography). It is possible to obtain excellent radiographs with any form of image acquisition as long as proper technique is used. In the author s experience for fetlock radiography, the patient is usually not sedated; however, if the patient is difficult, the author will use anywhere from 2.5 mg to 5 mg of detomidine IV. A good handler is very important because they will properly position the horse so that image acquisition is easier. Radiation safety is always practiced by everyone involved wearing lead gowns and thyroid protectors and the plate holder wearing lead gloves. 3. Results Standard views of the fetlock include five views (Figs. 1 4). The standard views taken follow: dorsal to palmar elevated 10 to 15 (DP), standing lateral to medial view (SLM), flexed lateral to medial view (FLM), dorsomedial (30 ) to palmarolateral oblique view, elevated 10 to 15 (DMPLO), and a dorsolateral (30 ) to palmaromedial oblique view, elevated 10 to 15 (DLPMO). Special Views As mentioned earlier, there are some additional views of the fetlock that are taken to highlight specific anatomic areas of concern. Tangential fetlock DP views are used to highlight portions of the articular surface of the distal metacarpus/metatarsus not seen on the standard DP projection. The most common is a flexed DP elevated at 10 to 15 (FDP) of the fetlock (Fig. 5). More extreme tangents are sometimes necessary, such as the dorsodistal-palmaroproximal DP view, which is taken in the weightbearing position 15 below horizontal (Fig. 6). Another specialty view is the palmar 45 proximolateral-palmarodistal medial oblique view (Pal 45 PrL-DiMO, Fig. 7). This view can also be taken from the contra-axial side of the limb. NOTES AAEP PROCEEDINGS Vol

3. Standing lateral to medial view with excellent overlap of sesamoids

Poor-quality DP caused by lack of 15 elevation, which leads to lack of

2 Fig. 1. Dorsal to palmar view of fetlock, elevated 15 to project joint space. Fig. 3. Standing lateral to medial view with excellent overlap of sesamoids and visualization of mid-sagittal ridge. Fig. 2. Poor-quality DP caused by lack of 15 elevation, which leads to lack of projection of joint space. Fig. 4. Poor-quality SLM caused by improper angle in the medial to lateral and proximal to distal direction Vol. 59 AAEP PROCEEDINGS

Fig. 5. Oblique views are similar on the lateral and medial aspects.

Fig. 7. Flexed lateral to medial view straight through the joint, excellent sesamoid overlap, and visualization of mid-sagittal ridge. 4.

the horse, motion on the radiograph, and poor exposure. These examples can lead to a misrepresentation of the horse and an incorrect diagnosis.

Poor positioning can result in hidden pathology because the proper area is not highlighted (Fig. 9). Dirt, mud, Fig. 6.

3 Fig. 5. Oblique views are similar on the lateral and medial aspects. A 30 angle (from lateral) projects dorsal P1 and divides sesamoids; 15 elevation allows visualization of palmar/plantar condyle of cannon bone. Fig. 7. Flexed lateral to medial view straight through the joint, excellent sesamoid overlap, and visualization of mid-sagittal ridge. 4. Discussion Factors Affecting Radiograph Quality Some factors that result in poor-quality radiographs are poor positioning, dirt/mud/water on the horse, motion on the radiograph, and poor exposure. These examples can lead to a misrepresentation of the horse and an incorrect diagnosis. Motion makes the radiograph difficult to interpret accurately (Fig. 8). Poor positioning can result in hidden pathology because the proper area is not highlighted (Fig. 9). Dirt, mud, Fig. 6. Poor quality oblique as the radiograph beam is oriented in too much of a dorsal to palmar/plantar direction. Fig. 8. Poor-quality FLM because the radiograph beam is not straight through the joint, the sesamoids do not overlap, and there is no visualization of the mid-sagittal ridge. AAEP PROCEEDINGS Vol

$This view is used to detect condyle fractures on the palmar aspect that are not seen easily on the$ standard DP view. Image was taken in the non weight-bearing position. Fig. 11. Palmar 45 PrL-DMiO.

standard DP view. Image was taken in the non weight-bearing position. Fig. 11. Palmar 45 PrL-DMiO.

This projection can be used for either sesamoid. Fig. 10.

$plantar) condylar fracture can be seen that was not apparent on the standard DP view. Fig. 12. Motion.$

4 Fig. 9. The flexed DP highlights the distal palmar/plantar condyles of the cannon bone. This view is used to detect condyle fractures on the palmar aspect that are not seen easily on the standard DP view. Image was taken in the non weight-bearing position. Fig. 11. Palmar 45 PrL-DMiO. This view is used to highlight the abaxial surface of medial sesamoid bone. This projection can be used for either sesamoid. Fig. 10. Dorsodistal-palmaroproximal DP of the fetlock. Despite superimposition of the sesamoids over the fetlock joint, a short, incomplete (dorsal to plantar) condylar fracture can be seen that was not apparent on the standard DP view. Fig. 12. Motion. Flexed lateral to medial view with motion. The image becomes blurry, and subtle detail and lesions may be missed Vol. 59 AAEP PROCEEDINGS

Fig. 13. Poor positioning. This oblique is too DP and therefore does not allow proper visualization of the condyles or dorsal P1. Fig. 15. Poor exposure.

Water artifact has the same effect as dirt and could easily be misinterpreted as a lesion; therefore, limbs should be dried before imaging.

Poor exposure can be the result of underexposure (light image) or overexposure (dark image).

5 Fig. 13. Poor positioning. This oblique is too DP and therefore does not allow proper visualization of the condyles or dorsal P1. Fig. 15. Poor exposure. This underexposed image results in a lack of anatomic detail and also highlights the water present on the horse. Water artifact has the same effect as dirt and could easily be misinterpreted as a lesion; therefore, limbs should be dried before imaging. and water on the horse cause artifacts on the radiographs and can make it difficult to differentiate true lesions from artifact (Figs. 10 and 11). Poor exposure can be the result of underexposure (light image) or overexposure (dark image). Digital images can compensate for some exposure problems by altering the brightness and contrast (windowing) (also see Figs ). However, gross inaccuracies in exposure cannot be corrected, especially overexposure. 1 3 References 1. Auer JA, Stick JA. Equine Surgery. St Louis: Saunders- Elsevier; 2006: Butler JA, Colles CM, Dyson SJ, et al. Clinical Radiology of the Horse. Ames, Iowa: Blackwell Science; 2000: McIlwraith CW, Nixon AJ, Wright IM, et al. Diagnostic and Surgical Arthroscopy in the Horse. St Louis: Elsevier Limited; 2005: Fig. 14. Dirt results in an artifact that can resemble pathology if present over an area of concern. It is very important to clean the area of interest before taking radiographs. AAEP PROCEEDINGS Vol

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