To the authors’ knowledge, this is the first published report providing description of the high field MRI appearance of the normal dromedary camel tarsus. The signal intensity of the clinically relevant osseous and soft tissue structures of the dromedary camel tarsus was described and the MRI images corresponded well with the gross anatomic sections. The obtained information in the current study appointed that MRI enabled assessment of structures inside the tarsus (soft and bony tissues) that otherwise cannot be imaged by other means and offers the opportunity to diagnose lesions within the tarsus that cannot be investigated through other imaging modalities . MRI permitted viewing of the camel tarsus in three planes and obtaining information of cartilage, cortical bone, subchondral bone, trabecular bone, cancellous bone, ligaments, and tendons which is not all possible with other available diagnostic imaging tools. This enables the clinicians to interpret the tarsus in different angles and accurately detect the problem. The obtained results are in agreement with the conclusions reported earlier, that MRI offers the best evaluation technique of all anatomical structures, particularly soft tissues, of the tarsal joint in horse, cattle, dog and cat [3, 12-16].
The camel tarsus (hock) is a composite joint made up of four articulations: the tarsocrural, proximal intertarsal, distal inter-tarsal, and the tarsometatarsal joints. The tarsal joints in horses, cattle and camels share some similarities; however, numerous intra-articular and peri-articular anatomical variations exist among those species. The hock joint consists of six tarsal bones in camel and horse and five tarsal bones in cattle. However, the pattern of tarsal bone arrangement is variable: in camels, the second and third tarsal bones are fused; in horses, the first and second tarsal bones are fused; and in cattle, the central and fourth and the second and third tarsal bones are fused. Moreover, the distal part of fibula in camels and cattle persists as an isolated bone (malleolar bone), which articulates with the distal extremity of the tibia, while in horses, it is completely fused to the tibia and forms the lateral malleolus. Furthermore, the talus in camels and cattle: bears a trochlea at either end; the proximal trochlear ridges are directed sagittaly; and the distal trochlea is well defined and articulates with the fused central and fourth tarsal bones in cattle and with the central and fourth tarsal bones in camel. In horses, the talus bears one proximal trochlea and the trochlear ridges are orientated obliquely in a mediolateral direction. The calcaneus in camels and cattle articulates with the distal tibia, while in horses, it has no articular surface for the tibia and covers less of the lateral aspect of the talus than in cattle and camels. Regarding the periarticular soft tissues, the tarsus of cattle and camels have more tendons and ligaments than horses: the fibularis longus tendon; an additional bundle of the short medial collateral ligament connecting the talus and the medial metatarsal bone; and the split of the long digital extensor tendon to the common extensor of digits III and IV and the medial digital extensor. In camels also, the long plantar ligament consists of a medial and lateral limbs, unlike cattle and horses in which the ligament is undivided [1, 19-21]. The anatomical differences among camels, cattle and horses are likely to direct the attention towards demonstration of the magnetic resonance appearance of various intra-articular and peri-articular structures of the dromedary camel tarsus.
In this study, the spin-weighted sequences (T1, T2, PD and STIR) provided high anatomic definition and good tissue contrast in the dromedary camel tarsus. The T1 and PD images were appropriate for the detailed anatomic assessment of tarsal structures and the STIR and T2 sequences were valuable for investigation of the synovial fluid [5, 12, 19]. The protocol demonstrated in the present study was designed to optimize evaluation of various structures in the dromedary camel tarsus, although it was longer than that expected in clinical patients. This was to afford comprehensive reference images of the clinically normal camel tarsus to assist interpretation of MRI images in the clinical situations. The used sequences were extensive in every plane so that an optimal protocol could be determined. Under clinical circumstances, shorter protocols are used in order to shorten the acquisition time and subsequently the cost and duration of general anesthesia.
Magnetic resonance imaging is frequently used in horses owing to its ability to produce high-contrast and anatomically detailed tomographic images [9, 23]. The use of MRI in camels is still in its infancy and was limited to cadaveric studies. Cadaver limbs are commonly used for evaluation of MRI anatomy and the signal intensity recorded in cadavers was similar to that reported in live animals  and findings would be generalizable for live animals. The present study provided the first ever conducted detailed description of the normal anatomy of the dromedary camel tarsus on MRI images. By knowing the variation in MRI images in the normal camel, it would be possible to understand the importance of signal intensity changes in the clinical patients. Additionally, description of the high field MRI appearance of the tarsal joint structures can also be useful in the interpretation of radiographs, ultrasound, and computed tomography of the camel tarsus .
Currently, the use of MRI in camels is overburdened by cost, necessity for general anesthesia, and limited availability of equipment. High field MRI scanners are now available at a small number of equine clinics in the Middle East (where camels exist in high populations) and are increasingly available and developing fast for veterinary use. We do believe that the use of MRI in camels is still a new and developing field and would be a useful diagnostic imaging modality in the future for evaluation of various dromedary orthopedic problems, besides that it is currently a valuable imaging modality for investigational purposes. As clinical availability of this modality increases in the future, it is expected that new orthopedic disorders concerning dromedary camels will be identified that have not previously been reported with other imaging modalities. In the present study, the camel tarsus was a good candidate for the high field MRI. Its narrow linear profile and minimal soft tissue coverage allowed the use of the human extremity coil and enabled close apposition of the magnetic field to the tarsus resulting into good signal intensity as reported for the tarsus of horses and cattle [12, 14]. General anesthesia in camels obligates particular approach due to the different anatomical and physiological features of this species . Tracheal intubation is necessary whenever general anesthesia is selected; however, endotracheal intubation in camels is a challenge , especially in males, due to the presence of a diverticulum of the ventral aspect of the soft palate . Hence, a guiding tube and a long laryngoscope are needed to accomplish successful endotracheal intubation . Accordingly, understanding of the anatomical and physiological dissimilarities of camels is indispensable to ensure a successful outcome of anesthesia and MRI examination in this species.
Knowledge of the normal anatomy and signal intensity of various tissues are crucial for correct interpretation of MRI scans obtained from lame patients. In the present study, synovial fluid had high signal intensity in STIR, PD and T2 images and intermediate signal intensity in the T1- weighted images and it was not possible to define a clear limit between the subchondral and cortical bones as both had low signal intensity. Similar findings were reported in the horse [3, 13]. The corticocancellous junction was regular and clearly defined, as mentioned for the bovine tarsus . Cancellous bone had heterogeneous intermediate to high signal intensity where the bone of the trabeculae had low signal intensity and fat in the bone marrow filling the trabecular spaces had high signal intensity. This agreed with those reported in the horse . Articular cartilage had homogenous intermediate signal intensity, on T1 images, adjacent to the low signal of subchondral bone at articular interfaces. Similar findings were reported in the horse . The tarsocrural joint capsule, synovial tissue and synovial fluid had low to intermediate signal intensity similar to those reported in the bovine tarsus . Tendons, collateral ligaments and the long plantar ligament of the dromedary camel had low signal intensity and the inter-tarsal ligaments had heterogeneous signal intensity. Similar findings were reported in horse [10, 28].
Diagnostic imaging continues to play an important role in the assessment of joint injuries and assists investigators to understand the risk factors associated with the onset and progression of the disease condition. Conventional radiography is the simplest, least expensive and most commonly deployed imaging modality over the last decades because of its reproducibility and feasibility to detect structural damage; however, radiography can provide only indirect information on soft tissues and insensitive to early inflammatory bone involvement and bone damage. . Ultrasonography enables real-time imaging of synovial pathology, articular cartilage and cortical erosive changes at relatively low cost but it is an operator-dependent and the physical properties of sound limit its ability to assess deeper structures . Diagnostic arthroscopy is a crucial skill for diagnosing intra-articular disorders as it enables a direct magnified view of the cartilage surface. However, it is an invasive method, diagnosis is based only on subjective visual evaluation and manual mechanical palpation and deeper joint structure are inaccessible due to anatomic limitations . MRI has become a key imaging tool for evaluation of joint pathology thanks to its ability to assess pathologic and biochemical changes within the joint before morphologic changes become evident with conventional imaging tools. In addition, the multi-plane and multi-slice capability of MRI enables visualization of the area of interest in three orthogonal planes. Therefore, MRI has the advantage of providing details concerning bone, articular cartilage and peri-articular structures, which is not shared by any other imaging modalities .
High field MRI provided comprehensive assessment of the dromedary camel tarsus. Results of the present study indicated that signal intensity varied according to the used sequence and investigated structure in the normal dromedary camel tarsus. Interpretation of MRI images is a challenge and obliges a good knowledge of anatomic details and familiarization to the normal MRI appearance of the region of interest in order to diagnose the problem with confidence. Further studies are warranted to determine the clinical application of high field MRI in the dromedary camel tarsus.