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Radiography and Radiographic Interpretation of the Tarsus (VET-473) Western Veterinary Conference 2004 Timothy R

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Radiography and Radiographic Interpretation of the Tarsus (VET-473) Western Veterinary Conference 2004 Timothy R Radiography and Radiographic Interpretation of the Tarsus (VET-473) Western Veterinary Conference 2004 Timothy R. O'Brien, DVM, PhD, DACVR University of California, Davis Davis, CA, USA 18276161 I. Introduction Tarsal diseases are a common cause of hind limb lameness in the horse. The clinical signs of tarsal disease often resemble those of stifle disease particularly the response to flexion tests. As a result both the stifle and tarsus are commonly radiographed to identify pathologic change in the horse with a hind limb lameness and during purchase evaluations. It seems prudent for purchase evaluations to radiograph these anatomical areas in both limbs. Radiographic interpretation of the tarsus is considered difficult by most equine practitioners. There seems to be three major causes of this difficulty. These causes relate to the numerous bones comprising the tarsus, variety of shapes of the tarsal bones, and veterinarians tend to be less familiar with the radiographic anatomy and pathology of the tarsus than most other anatomical regions, particularly regions of the forelimb. I am going to make some general statements in an attempt to simplify the radiographic interpretation of the tarsus. These general statements are: Most radiographic findings are located at the dorsum of the tarsus. Pathologic abnormalities of the tarsus tend to be in specific anatomical areas. Soft tissue swelling (STS) is a key for assisting to localize the abnormality. There are only a few radiographic diagnoses for the tarsus. The diagnoses of abnormalities of the tarsus identified with radiology can be classified in the following way: 1. Soft tissue swelling without bony change 2. Secondary joint disease 3. Fractures 4. Infection 5. Osteochondrosis and developmental disorders The radiographic findings of these five conditions will be correlated to gross pathologic findings to explain the radiographic signs and their variations. II. Radiography of the Tarsus A. Standard Radiographic Examination The standard or routine radiographic examination of the equine tarsus should consist of at least four projections. These projections are: 1. Dorso 0° proximal--plantarodistal oblique or dorsoplantar (DPl) 2. Latero 3° proximal--mediodistal oblique or lateromedial (LM) 3. Dorso 45° lateral--plantaromedial oblique or medial oblique (MO) 4. Plantaro 5° disto 60° lateral--dorsoproximomedial oblique or lateral oblique (LO) 1. The Dorsoplantar View Centering the DPl View--The foot is positioned with the toe pointing slightly laterally to assist with positioning the x-ray machine. The beam is centered at the central tarsal bone (middle of the tarsus) and directed from dorso0° proximal to plantarodistal. The DPl view should be taken at 0° proximally because this angulation allows the medial aspect of the joint spaces to be "open" and most signs of secondary joint disease will be located in the medial region highlighted. Proximal angulation of 10-15° results in the lateral aspect of the intertarsal joint spaces "open" with "loss" of the medial aspect of these joint spaces. This information is important if there is change suspected on the lateral side of midline in the tarsus. Angulation can be determined from the radiograph by the location of the head of MT IV relative to the distal intertarsal joint (DIT) space, i.e., the closer the two indicates greater proximal angulation and the further MT IV is from the DIT joint space the closer to 0° proximal angulation. Primary anatomical areas of the tarsus that can be evaluated with the DPl view are: The medial and lateral surfaces and cortices of the distal tibia, tarsus, and proximal metatarsus. Periarticular and subchondral regions of the four major joints of the tarsus. The thickness (proximodistal height) of the central and third tarsal bones and the medial versus lateral parts of each bone (both should be comparable). Location of the head of MT IV relative to the lateral aspect of the DIT space. The intertarsal and TMT joint spaces width on a well-positioned radiograph should be comparable. The proximal tuberosity (a) and distal tuberosity (b) of the talus. Distal region of the lateral ridge of the trochlea of the talus. Plantar region of the central tarsal bone. Area of origin of the suspensory ligament on plantaroproximal MT III. These areas are identified radiographically. 2. The Lateromedial View Centering the LM View--The heels are superimposed to produce a true LM view where the ridges of the trochlea of the talus are superimposed. The x-ray beam is centered at the level of the central tarsal bone and there is a slight (3-5°) downward angulation of the beam resulting in the accurate descriptive name for this view being a latero5°proximal-mediodistal oblique. Primary anatomical area of the tarsus that can be evaluated with the LM view are: Dorsal and plantar surfaces and cortices of the distal tibia, tarsus, and proximal metatarsus. Soft tissue swelling, especially intracapsular STS of the tarsocrural joint. Medial ridge of the trochlea of the talus. Lateral ridge of the trochlea of the talus. Dorsal region of the intermediate ridge. Contour of the sustentaculum tali. Periarticular and subchondral regions of the four major joints of the tarsus. Dorsal surface to the central and third tarsal bones. An imaginary straight line connecting the periarticular regions of the dorsodistal talus and the dorsoproximal MT III should be envisioned. Bone dorsal to this line on the LM view represents a productive change. Comparable proximodistal thickness of the central and third tarsal bones. These areas are identified radiographically. 3. The Medial Oblique View Centering the MO view--The x-ray beam is centered at the central tarsal bone, and a horizontal beam is used that is described as a dorso45° lateral-plantaromedial oblique. The medial oblique is taken at 45° from the D-Pl plane. This obliqueness highlights the medial region of the tarsus in the region where the cunean tendon passes obliquely over the tarsus to insert on the first tarsal bone. This region must be examined carefully for radiographic signs of secondary joint disease and productive bony change in the attachment the dorsal ligament on the medial aspect of the central and third tarsal bones and proximal MT III and MT II. Primary anatomical areas of the tarsus that can be evaluated with the MO view are: The dorsomedial and plantarolateral surfaces, cortices, periarticular regions, and subchondral bone of the distal tibia, tarsus, and proximal metatarsus. Medial ridge of the trochlea of the talus. Lateral ridge of trochlea of the talus. Dorsum of the intermediate ridge of the tibia. Proximal and distal tuberosities of the talus. Coracoid process of the calcaneus. Fourth tarsal bone, the head of MT IV, and plantar surface of lateral MT III. The dorsomedial contour of the third tarsal bone which appears as a central prominence with a smooth periosteal surface. These areas are identified radiographically. 4. The Lateral Oblique View Centering the LO view--The x-ray beam is centered at the central tarsal bone and the direction is accurately described as plantaro5° disto60° lateral-dorsoproximomedial oblique. The dorsolateral aspect of the DIT joint has a natural configuration different from that seen on the dorsal or dorsomedial aspect. To visualize the dorsolateral aspect of the DIT joint the x-ray beam must be angled 5° distal to proximal. The 60° lateral angulation from the plantar-dorsal plane highlights the dorsolateral area of the intertarsal joints where pathologic changes more commonly occur. The plantarodorsal direction of the x-ray beam is recommended rather than the dorsoplantar direction because it is more convenient for visualizing the anatomy of the tarsus. Primary anatomical areas of the tarsus that can be evaluated with the LO view are: The dorsolateral and plantaromedial surfaces, cortices, periarticular, and subchondral regions of the distal tibia, tarsus, and proximal metatarsus. Dorsum of the intermediate ridge of the tibia. Lateral ridge of the trochlea of the talus. Medial ridge of the trochlea of the talus. Surface and body of the sustentaculum tali. Fused first and second tarsal bones and the head of MT II. These areas are identified radiographically. B. Special Radiographic Projections There are three views taken to supplement the routine examination of the tarsus. These are the flexed-plantaro75° proximal to the plantarodistal oblique (calcaneal skyline), the flexed- lateromedial (f-LM), and the flexed-dorso0° proximal-plantarodistal oblique (f-DPl). As a general rule it is important for the AEP to take the similar special view of the opposite limb. This approach is strongly recommended because the AEP is relatively unfamiliar with the radiographic anatomy, and the abnormal radiographic findings will be more easily recognized when a direct comparison can be made for differences. This latter reason is true because many abnormalities are either unilateral or differ in the degree of severity of the changes when bilateral. 1. Flexed Pl75° Pr-PlDiO or calcaneal skyline view--The indication for this view is to evaluate the tuber calcis, sustentaculum tali, medial ridge of the talus, and the tarsal groove. Different exposure times are required for this view dependent upon whether the tuber calcis or sustentaculum tali is to be evaluated. In practice, the tuber exposure is routinely made unless a sustentacular lesion is suspected. 2. Flexed LM View--The indication for this view is to further evaluate the trochlea of the talus. The LM view allows the dorsum of the trochlear to be evaluated distally but the area of the trochlea articulating with the distal tibia seen on the LM cannot be evaluated completely. The flexed-LM permits this region of the trochlea to be better visualized and interpreted for contour abnormalities and density changes. 3. Flexed DPl View--The indication for this view is in the foal and younger horse. This view allows better assessment of the distal tibial epiphysis, lateral and medial malleoli, and the tarsocrural joint.
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